i

A History of Suction-TypeLaminar-Flow Control

with Emphasis onFlight Research

MONOGRAPHS IN AEROSPACE HISTORY #13

byAlbert L. Braslow

ii

A History of Suction-TypeLaminar-Flow Control

with Empahsis onFlight Research

byAlbert L. Braslow

NASA History DivisionOffice of Policy and Plans

NASA HeadquartersWashington, DC 20546

Monographs inAerospace History

Number 131999

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Table of Contents

Foreword.........................................................................................................................iv

Preface .............................................................................................................................v

Laminar-Flow Control Concepts and Scope of Monograph ...........................................1

Early Research on Suction-Type Laminar-Flow ControlResearch from the 1930s through the War Years ......................................................3Research from after World War II to the Mid-1960s ................................................5

Post X-21 Research on Suction-Type Laminar-Flow ControlHiatus in Research ..................................................................................................13Resumption of Research .........................................................................................13Research from the Mid-1970s to the Mid-1990s ....................................................16Natural Laminar Flow (NLF) on Swept Wings: F-111/TACT and F-14 .....................17Noise: Boeing 757...................................................................................................18Insect Contamination: JetStar .................................................................................20Leading-Edge Flight Test (LEFT) Program: JetStar...............................................21Surface Disturbances: JetStar .................................................................................26Atmospheric Ice Particles: Boeing 747s and JetStar ..............................................28Hybrid Laminar-Flow Control (HLFC): Boeing 757 .............................................29Supersonic Laminar-Flow Control: F-16XL ..........................................................32

Status of Laminar-Flow Control Technology in the Mid-1990s ...................................34

Glossary.........................................................................................................................35

Documents

Document 1 –Aeronautics Panel, AACB, R&D Review, Report of theSubpanel on Aeronautical Energy Conservation/Fuels .........................38

Document 2 –Report of Review Group on X-21A Laminar FlowControl Program .....................................................................................46

Document 3 –Langley Research Center Announcement: Establishmentof Laminar Flow Control Working Group..............................................61

Document 4 –Intercenter Agreement for Laminar Flow ControlLeading Edge Glove Flights, LaRC and DFRC .....................................62

Document 5 –Flight Report, NLF-144, of AFTI/F-111 Aircraft withthe TACT Wing Modified by a Natural Laminar Flow Glove ...............66

Document 6 –Flight Record, F-16XL Supersonic Laminar FlowControl Aircraft ......................................................................................71

Index ..............................................................................................................................76

About the Author ...........................................................................................................78

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Foreword

Laminar-flow control is an area of aeronautical research that has a long historyat NASA’s Langley Research Center, Dryden Flight Research Center, theirpredecessor organizations, and elsewhere. In this monograph, Albert L.Braslow, who spent much of his career at Langley working with this research,presents a history of that portion of laminar-flow technology known as activelaminar-flow control, which employs suction of a small quantity of air throughairplane surfaces. This important technique offers the potential for significantreduction in drag and, thereby, for large increases in range or reductions in fuelusage for aircraft. For transport aircraft, the reductions in fuel consumed as aresult of laminar-flow control may equal 30 percent of present consumption.

Given such potential, it is obvious that active laminar-flow control with suctionis an important technology. In this study, Al covers the early history of thesubject and brings the story all the way to the mid-1990s with an emphasis onflight research, much of which has occurred here at Dryden. This is an impor-tant monograph that not only encapsulates a lot of history in a brief compass butalso does so in language that is accessible to non-technical readers. NASA ispublishing it in a format that will enable it to reach the wide audience thesubject deserves.

Kevin L. PetersenDirector, Dryden Flight Research CenterFebruary 18, 1999

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Preface

This monograph is the result of a contract with the NASA Dryden HistoryOffice to write a brief history of laminar-flow-control research with an emphasison flight research, especially that done at what is today the Dryden FlightResearch Center (DFRC). I approached the writing of this history from theperspective of an engineer who had spent much of his career working on lami-nar-flow-control research and writing about the results in technical publications.I found out that writing history is quite a bit different from technical writing, butI hope that what I have written will explain laminar-flow control to the non-technical reader while at the same time providing historical background to theinterested technical reader.

After completion of the final draft of this technical history in October 1998, Iwas made aware of NASA TP-1998-208705, October 1998, by Ronald D.Joslin, entitled Overview of Laminar Flow Control. Although some overlapexists between this publication and my own, as would be expected from the twotitles, Joslin’s intent was quite different from mine. He provides an extensivetechnical summary for engineers, scientists and technical managers of thecontent of many key papers without much evaluation of the significance ofspecific results over the years.

I would like to express my gratitude to the following DFRC personnel: DavidFisher, Lisa Bjarke, and Daniel Banks for reading the initial draft; Jim Zeitz forreworking the figures; and Stephen Lighthill for doing the layout. My specialthanks go to J.D. (Dill) Hunley, DFRC historian, who patiently guided thistechnical author through the vagaries of historical composition.

Albert L. BraslowNewport News, Virginia19 February 1999

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AHistory ofSuction-TypeLaminar-FlowControl

Laminar-Flow Control Concepts andScope of Monograph

This monograph presents a history ofsuction-type laminar-flow-control re-search in the National Advisory Commit-tee for Aeronautics and its successororganization, the National Aeronauticsand Space Administration, plus selectedother organizations, with an emphasis onflight research. Laminar-flow control is atechnology that offers the potential forimprovements in aircraft fuel usage,range or endurance that far exceed anyknown single aeronautical technology.For transport-type airplanes, e.g., the fuelburned might be decreased a phenomenal30 percent. Fuel reduction will not onlyhelp conserve the earth’s limited supplyof petroleum but will also reduce engineemissions and, therefore, air pollution. Inaddition, lower fuel usage will reduce theoperating costs of commercial airplanesat least eight percent, depending upon thecost of the fuel and, therefore, will curtailticket prices for air travel. Laminar-flowcontrol is also the only aeronauticaltechnology that offers the capability ofdesigning a transport airplane that can flynonstop without refueling from anywherein the world to anywhere else in the worldor that can remain aloft without refuelingfor approximately 24 hours. Theseenormous performance improvementsthat are potentially available for commer-cial or military applications, therefore,have made the concept the “pot of gold atthe end of the rainbow” for aeronauticalresearchers.

A brief review of some of the funda-mentals involved will improve an under-standing of this technological history.When a solid surface moves through afluid (such as the air), frictional forcesdrag along a thin layer of the fluidadjacent to the surface due to the viscos-ity (stickiness) of the fluid. A distin-guished theoretician, Ludwig Prandtl,showed in 1904 how the flow around asolid body can be divided into tworegions for analysis—this thin layer offluid adjacent to the surface, called theboundary layer, where fluid friction plays

an essential part, and the remaining regionoutside the boundary layer where frictionmay be neglected. The boundary layergenerally exists in one of two states:laminar, where fluid elements remain inwell-ordered nonintersecting layers(laminae), and turbulent, where fluidelements totally mix. The frictional forcebetween the fluid and the surface, knownas viscous drag, is much larger in aturbulent boundary layer than in a laminarone because of momentum losses associ-ated with the mixing action. The energyrequired to overcome this frictional forceon an airplane is a substantial part of thetotal energy required to move the airplanethrough the air. In the case of a transportairplane flying at subsonic speeds, forexample, approximately one-half of theenergy (fuel) required to maintain levelflight in cruise results from the necessityto overcome the skin friction of theboundary layer, which is mostly turbulenton current transport-size airplanes.

The state of the boundary layer, in theabsence of disturbing influences, isdirectly related to the speed of the surfaceand the distance along the surface—first,laminar and then changing to turbulent asthe speed or distance increases. Laminarflow is difficult to attain and retain undermost conditions of practical interest, e.g.,on the surfaces of large transport air-planes. Laminar flow is an inherentlyunstable condition that is easily upset, andtransition to turbulent flow may occurprematurely as a result of amplification ofdisturbances emanating from varioussources. Two basic techniques are avail-able to delay transition from laminar toturbulent flow—passive and active.Laminar flow can be obtained passivelyover the forward part of airplane liftingsurfaces (wings and tails) that haveleading-edge sweep angles of less thanabout 18 degrees by designing the surfacecross-sectional contour so that the localpressure initially decreases over thesurface in the direction from the leadingedge towards the trailing edge. Thelaminar flow obtained in this passivemanner is called natural laminar flow(NLF). In the rearward region of well-

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designed wings, where the pressure mustincrease with distance towards the trailingedge (an adverse pressure gradient),1 activelaminar-flow control must be used. Even ina favorable pressure gradient, activelaminar-flow control is required to attainlaminar flow to large distances from theleading edge.

The principal types of active laminar-flow control are surface cooling (in air) andremoval of a small amount of the boundary-layer air by suction through porous materi-als, multiple narrow surface slots, or smallperforations. For highly swept wings thatare usually required for flight at highsubsonic and supersonic speeds, onlysuction can control sweep-inducedcrossflow disturbances that promoteboundary-layer transition from laminar toturbulent flow. The use of suction hasbecome the general method of choice foractive laminar-flow control and has becomeknown as LFC. A combination of LFC (inregions where pressure gradients due to thesweep introduce large destabilizingcrossflow disturbances) and NLF (inregions with low crossflow) is an approachto simplifying the application of LFC and isknown as hybrid LFC (HLFC). Althoughthe potential performance gains due toHLFC are somewhat lower than thoseobtainable with LFC, the gains are still verylarge.

At this point, a brief description of aparameter of fundamental importance isnecessary for the non-technical reader. Thisparameter is called Reynolds number andwas named after Osborne Reynolds who, in1888, was the first to show visually thetransition from laminar to turbulent flowand the complete mixing of the fluidelements in turbulent flow. Reynoldsnumber is non-dimensional and is equal tothe product of the velocity of a bodypassing through a fluid (v), the density ofthe fluid (ρ) and a representative length (l)divided by the fluid viscosity (µ) or v ρ 1 /µ.Engineers select various representativelengths (1) in the formulation of the

Reynolds number for different purposes. Forexample, non-dimensionalized aerodynamicforces acting on a body moving through airvary with the value of the Reynolds numberbased on the body length. This phenomenonis called “scale effect” and is important in thedetermination of the non-dimensionalaerodynamic forces acting on a full-size (full-scale) airplane or airplane component fromdata measured on a small wind-tunnel model.When engineers select the distance from thecomponent’s leading edge to the end oflaminar flow as the representative length, theresultant length Reynolds number (or transi-tion Reynolds number) is a measure of thedistance from the leading edge to the end ofthe laminar flow. For any value of transitionReynolds number, then, that has been experi-mentally determined, the distance to the endof laminar flow on any size airplane compo-nent can be calculated for any stream-flowvelocity, density, and viscosity from theabove Reynolds number formulation. Theattainable value of transition Reynoldsnumber, as previously indicated, is dependentupon the component’s geometrical shape (theprimary controller of the variation of surfacepressure), various disturbances, and the typeand magnitude of laminar-flow control used.

This monograph will review the historyof the development of LFC and HLFC withemphasis on experimentation, especiallyflight research. A sufficient number ofactivities up to 1965, when a 10-year hiatus inU.S. experimental LFC research began, willillustrate the early progress as well as theprincipal problems that inhibited the attain-ment of laminar flow in flight with eitherpassive or active laminar-flow control.Discussion of a resurgence of research onLFC in 1975 will concentrate on the flight-research portion of an American programdefined to solve the technological problemsuncovered during the previous research.Included will be a discussion of the signifi-cance of aircraft size on the applicability ofpassive or active control.

1 A decreasing pressure in the direction towards the trailing edge is called a favorable pressure gradient and an increasingpressure is called an adverse pressure gradient.

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Early Research on Suction-TypeLaminar-Flow Control

Research from the 1930s through theWar Years

The earliest known experimentalwork on LFC for aircraft was done in thelate 1930s and the 1940s, primarily inwind tunnels.2 In 1939, research engineersat the Langley Memorial AeronauticalLaboratory of the National AdvisoryCommittee for Aeronautics (NACA) inHampton, Virginia, tested the effect onboundary-layer transition of suction

through slots in the surfaces of wind-tunnel models. These tests provided thefirst aerodynamic criteria on the design ofmultiple suction slots and obtainedlaminar flow up to a length Reynoldsnumber of 7 million, a phenomenallylarge value at that time. The first LFCflight experiments ever made followedthese favorable results in 1941. Research-ers installed seventeen suction slotsbetween 20 and 60 percent of the chord3

of a test panel (glove)4 on a wing of a B-18 airplane (Figure 1). Maximum airplanespeed and constraint in the length of the

Figure 1. B-18airplane with testglove for naturallaminar flow andlater for activelaminar-flowcontrol. (NASAphoto L-25336)

2 Three citations that provide extensive bibliographies on both passive and active control of the laminar boundary layer are:Dennis M. Bushnell and Mary H. Tuttle, Survey and Bibliography on Attainment of Laminar Flow Control in Air UsingPressure Gradient and Suction (Washington, DC: NASA RP-1035, September 1979); Charles E. Jobe, A Bibliography ofAFFDL/FXM Reports on Laminar Flow Control ( U.S. Air Force: AFFDL-TM-76-26-FXM, March 1976); and Mary H.Tuttle and Dal V. Maddalon, Laminar Flow Control (1976-1991) – A Comprehensive, Annotated Bibliography (Washington,DC: NASA TM 107749, March 1993). Significant references, primarily of summary natures, that were published since theseare included in subsequent footnotes. A sparse number of technical sources already included in the bibliographies are alsorepeated in subsequent notes to assist readers in locating pertinent technical information discussed in the narrative.

3 Chord is the length of the surface from the leading edge to the trailing edge.

4 A glove is a special section of an airplane’s lifting surface, usually overlaying the basic wing structure, that is designedspecifically for research purposes.

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wing glove, however, limited achievementof a length Reynolds number for transi-tion to a value lower than that achieved inthe wind tunnel.

Experimentation in NACA on LFCceased during the years of World War II inorder to develop natural laminar-flowairfoils, the so-called NACA 6- and 7-series airfoils, under the leadership ofEastman N. Jacobs, Ira H. Abbott, andAlbert E. von Doenhoff at the LangleyMemorial Aeronautical Laboratory.5Significant progress in furthering theunderstanding of the boundary-layertransition process, however, continued tobe made in the U.S.A., both analyticallyand experimentally, principally at theNational Bureau of Standards by G.B.Schubauer, H.K. Skramstad, P.S.Klebanoff, K.P. Tidstrom, and Hugh L.Dryden.6 Development of the laminar-flow airfoils was made possible by theintroduction into service of the Low-Turbulence Pressure Tunnel (LTPT) at theLaRC with an exceptionally low air-stream-turbulence level.7 The author andFrank Visconti measured natural laminarflow in the LTPT up to length Reynoldsnumbers on the order of 16 million.8

Researchers in Great Britain obtainedsignificant flight experience in the mid-1940s on natural laminar-flow airfoilswith wing gloves on the British KingCobra and Hurricane military fighters.9

Large extents of laminar flow wereobtained, but only after considerableeffort to attain wave-free and smoothsurfaces. Although attainment of largeregions of laminar flow was not possiblein daily operations, aircraft designers usedlaminar-flow type airfoils with largeregions of favorable pressure gradient onnew aircraft intended for high-subsonic-speed flight because of their superiorhigh-speed aerodynamic characteristics,e.g., the North American P-51 Mustang.

In Germany and Switzerland, effortsto develop LFC technology with suctionwere under way during the war. TheGermans emphasized the analysis oflaminar stability with continuous suctionrather than discrete suction through slots.Walter Tollmien and Hermann Schlichtingdiscovered theoretically that the boundarylayer resulting from continuous suction isvery stable to small two-dimensional typedisturbances (named after them asTollmien-Schlichting waves)10 and that

5 In a later reorganization, the Langley Memorial Aeronautical Laboratory was renamed the Langley Research Center(LaRC), and that name will be used hereafter to avoid possible confusion. An interim name for the Laboratory from 1948to 1958 was the Langley Aeronautical Laboratory.

6 Dryden later became the Director of the NACA and then the first Deputy Administrator of the National Aeronauticsand Space Administration (NASA).

7 A low level of high-frequency airstream turbulence, a condition approximating that in the atmosphere, is required toobtain natural laminar flow. This turbulence, of extreme importance to NLF, contrasts with occasional low-frequencyturbulence in the atmosphere, known as gusts. Gusts affect an aircraft through changes in the relative angle of the aircraftwith respect to the direction of flight (angle of attack).

8 Albert L. Braslow and Fioravante Visconti, Investigation of Boundary-Layer Reynolds Number for Transition on an65(215)—114 Airfoil in the Langley Two-Dimensional Low-Turbulence Pressure Tunnel (Washington, DC: NACA TN1704, October, 1948).

9 See, for example: W.E. Gray and P.W.J. Fullam, Comparison of Flight and Tunnel Measurements of Transition on aHighly Finished Wing (King Cobra) (RAE Report Aero 2383, 1945); F. Smith and D. Higton, Flight Tests on King CobraFZ. 440 to Investigate the Practical Requirements for the Achievement of Low Profile Drag Coefficients on a “LowDrag” Aerofoil (British A.R.C., R and M 2375, 1950); R.H. Plascoff, Profile Drag Measurements on Hurricane II z.3687 Fitted with Low-Drag Section Wings (RAE Report Aero 2153, 1946).

10 Examples of two-dimensional type disturbances are stream turbulence, noise, and surface irregularities having largeratios of width (perpendicular to the stream flow direction) to height, like spanwise surface steps due to mismatches instructural panels.

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the quantity of air that must be removedto achieve this marked stabilizing effect isextremely small. German researchersderived methods for calculating theboundary-layer characteristics and dragreductions resulting from continuoussuction. The Germans also wanted tovalidate their findings experimentally butwere unable to produce a permeablesurface suitable for continuous suctionwith the necessary degree of smoothness.Alternatives were tried, i.e., suctionthrough a perforated plate and suctionthrough multiple slots. Suction throughperforated plates failed due to excessivedisturbances emanating from the edges ofthe holes. Suction through multiple slotspermitted attainment of extensive regionsof laminar flow up to a length Reynoldsnumber of 3.2 million. In Switzerland,Werner Pfenninger was also investigatingthe use of multiple suction slots. Heobtained full-chord laminar flow on bothsurfaces of an airfoil but only up to amaximum chord (length) Reynoldsnumber of 2.3 million. He attributed thelimitation in the maximum attainableReynolds number for laminar flow withLFC to increased airstream turbulence inthe wind tunnel. From more recent results,he and other researchers agree thatincreased disturbances from small irregu-larities in the slot contours could havecontributed.

Research from after World War II tothe Mid-1960s

Release of the German LFC reports

on continuous suction after the wargenerated renewed interest in both theUnited States and the United Kingdom.11

The NACA initiated a series of wind-tunnel tests at the LaRC in 1946, whichculminated in the attainment of full-chordlaminar flow on both surfaces of an airfoilwith continuous suction through a porousbronze surface. The author, Dale Burrows,and Frank Visconti obtained full-chordlaminar flow to a length Reynolds numberof about 24 million, which was limitedonly by buckling of the low-strengthporous-bronze skin.12 Neal Tetervinperformed theoretical calculations indicat-ing that the experimental suction rateswere consistent with values predictedfrom the then-available stability theory tothe largest chord Reynolds number tested.These wind-tunnel results, therefore,provided the first experimental verifica-tion of the theoretical indication that theattainment of full-chord laminar flow withcontinuous suction would not be pre-vented by further increases in Reynoldsnumber, i.e., further increases in airplanesize or speed (at least subsonically).13

Because porous bronze, however, wasobviously unsuitable for application toaircraft (low strength and large weight)and no suitable material was available,work on the simulation of continuoussuction with multiple slots was reacti-vated by the NACA. In the late 1940s,NACA researchers investigated in theLaRC LTPT an NACA design,14 and Dr.Werner Pfenninger, who had come to theNorthrop Corporation from Zurich,Switzerland, investigated a U.S. Air

11 A team of experts from the allied countries, including Eastman N. Jacobs of the NACA, gathered these reports inGermany soon after the end of hostilities.

12 This was the author’s indoctrination into active laminar-flow control research, which followed previous involvementin the development of the NACA natural-laminar-flow airfoils.

13 Albert L. Braslow, Dale L. Burrows, Neal Tetervin, and Fioravante Visconte, Experimental and Theoretical Studies ofArea Suction for the Control of the Laminar Boundary Layer on an NACA 64A010 Airfoil (Washington, DC: NACAReport 1025, 30 March 1951).

14 Dale L. Burrows and Milton A. Schwartzberg, Experimental Investigation of an NACA 64A010 Airfoil Section with41 Suction Slots on Each Surface for Control of Laminar Boundary Layer (Washington, DC: NACA TN 2644, 1952).

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Force-sponsored design.15 In the first case,the researchers obtained full-chordlaminar flow up to a Reynolds number ofabout 10 million (greatly exceeding thatobtained previously in Germany andSwitzerland), but the slot arrangement hadbeen designed for a considerably largerReynolds number of 25 million. In thesecond case, Dr. Pfenninger obtained full-chord laminar flow up to a Reynoldsnumber of 16-17 million for a modeldesigned for 20 million. In both of thesecases with slots, as well as during theprevious continuous-suction tests, anoverriding problem in attainment oflaminar flow was an increased sensitivityof laminar flow to discrete three-dimen-sional type surface disturbances16 or slotirregularities as wind-tunnel Reynoldsnumber was increased. This occurred inspite of the theory, which indicated thatsuction increased the stability of thelaminar boundary layer with respect totwo-dimensional type disturbances. Moreon this subject will be included later in themonograph.

After the war, the first work theBritish did on LFC was to extend theGerman analytical research on continuoussuction. In 1948, Cambridge Universityexperimented on a flat plate in the floor ofa wind tunnel. This was followed in 1951by flight tests on an Anson aircraft ofcontinuous suction from 10- to 65-percent

chord in a flat pressure distribution.17

Researchers obtained experimentalsuction rates very close to theoreticalvalues for a zero pressure gradient up to alength Reynolds number of 3 million andgood agreement with theory in themeasured boundary-layer profiles.18 Theexperiments indicated adverse effects ofroughness.

The British Royal Aircraft Establish-ment (RAE) tested a porous surface on aVampire aircraft19 starting in 1953(Figure 2). Researchers initially employeda rolled metallic cloth for the surface, butroughness picked up in the mesh causedpremature transition from laminar toturbulent flow. With the use of specialprocedures to provide very smoothsurfaces back to 25 percent of chord, full-chord laminar flow was established at alength Reynolds number of 29 million.With candidates not yet available for apractical porous surface, attention wasdiverted to simulation of a porous surfacewith a perforated metal sheet. From 1954to 1957, the RAE investigated variousarrangements of hole size, spacing andorientation, as did John Goldsmith at theNorair Division of the Northrop Corpora-tion in the United States. Some workedand some did not because of differencesin disturbances generated by the suctionflow through the different hole arrange-ments.20

15 Werner Pfenninger, Experiments With a 15%-Thick Slotted Laminar Suction Wing Model in the NACA, Langley Field,Low Turbulence Wind Tunnel (U.S. Air Force Tech. Rep. 5982, April 1953).

16 Three-dimensional type surface disturbances are those with width to height ratios near a value of one.

17 M.R. Head, The Boundary Layer with Distributed Suction (British A.R.C., R.&M. No. 2783, 1955).

18 A boundary-layer profile is the shape of the variation of a boundary-layer characteristic like local velocity or tempera-ture with height above the surface.

19 M.R. Head, D. Johnson, and M. Coxon, Flight Experiments on Boundary-Layer Control for Low Drag (BritishA.R.C., R.&M. No. 3025, March 1955).

20 Significant sources are: John Goldsmith, Critical Laminar Suction Parameters for Suction Into an Isolated Hole or aSingle Row of Holes (Northrop Corp., Norair Division Report NAI-57-529, BLC-95, February 1957); N. Gregory andW.S. Walker, Experiments on the Use of Suction Through Perforated Strips for Maintaining Laminar Flow: Transitionand Drag Measurements (British A.R.C., R.&M. No. 3083, 1958). Northrop Corp., Norair Division reports cited in thismonograph and others related to its laminar-flow research can be found in the files of Albert L. Braslow located in theLangley Historical Archives (LHA) at the Langley Research Center, Hampton, VA.

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From 1951 to 1955, the British firmHandley Page tested, in wind tunnels andin flight on a Vampire trainer, the conceptof suction strips whereby researchershoped to eliminate the structural difficul-ties associated with fully distributedsuction or with the need for precise slots.Tests included both porous strips andperforated strips with single and multiplerows of holes. The best of the perforatedconfigurations consisted of staggeredmultiple rows of holes. Tests of poroussintered-bronze strips in both the windtunnel and flight were troubled by great

difficulty in ensuring sufficiently smoothjoints between the strips and the solidsurface. The joints introduced largeenough two-dimensional type distur-bances to cause premature transition. Withthe final perforation configuration,researchers obtained repeatable laminarflow to 80 percent of the chord on theVampire trainer wing, equivalent to alength Reynolds number of 15 million. Aninability to obtain laminar flow in the last20 percent of chord was attributed to theeffects of a forward sweep of the wingtrailing edge.

Figure 2. Vampireactive laminar-flow-control flightexperiments.

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Previously, in 1951, the RAE hadbeen unable to obtain the design extent oflaminar flow on a natural laminar-flowairfoil employed in a sweptback wing onan AW52 airplane. This led to a series oftests of sweptback surfaces of variousaircraft during which visual records ofboundary-layer transition were obtained.For sufficiently large leading-edgesweepback, transition occurred very closeto the leading edge. Subsequent tests,using a flow-visualization technique,showed closely-spaced striations in theflow on the surface, indicating stronglythat transition took place on sweptsurfaces as a result of formation ofstreamwise vortices in the laminar bound-ary layer.21 Dr. Pfenninger’s boundary-layer research group at the Norair Divi-sion of the Northrop Corporation in the1950s provided a method of analyzing thecross-flow instability due to sweep. It alsoobtained experimental data showing thatthe cross-flow instability could be con-trolled by reasonable amounts of suctioninitiated sufficiently close to the wingleading edge.22

The Northrop group in the 1950s andearly 1960s made many other majorcontributions to the development of theLFC technology. Under a series of AirForce contracts, the group performedrather extensive investigations in severalareas of concern. Although some workwas done on suction through holes, theprincipal efforts were on suction throughslots. In addition to the improved under-standing of laminar-flow stability and

control on swept wings, the Northropresearchers developed criteria in the areasof multiple-slot design, internal-flowmetering, and duct design plus techniquesfor alleviating the adverse effects ofexternal and internal acoustic distur-bances. In addition, Northrop conductedanalytical investigations of structuraldesign methods and construction tech-niques. These were supported by a limitedeffort on construction and test of small-scale structural samples. The results,however, were insufficient to providetransport manufacturers with confidencethat LFC wings for future transports couldbe manufactured to the required closetolerances for LFC with acceptable costand weight penalties.23 An area receivinganalytical attention only was that of thesuction pumping system. Although thesuction pumping system is of significantimportance to overall aircraft perfor-mance, analyses indicated that no radi-cally new mechanical developments wererequired to provide the necessary suction.

Northrop, in a USAF-sponsoredprogram at Muroc Dry Lake (known bothbefore and after this period as Rogers DryLake) in California, also reactivated flightresearch on LFC in the United States withthe use of a glove on an F-94 aircraft.Muroc is today the site of the Edwards AirForce Base and the Dryden Flight Re-search Center (DFRC). Northrop investi-gated three different slot arrangements ona modified NACA laminar-flow airfoil(Figure 3). Essentially full-chord laminarflow was attained on the wing’s upper

21 W.E. Gray, The Effect of Wing Sweep on Laminar Flow (RAE TM Aero. 255, 1952).

22 W. Pfenninger, L. Gross, and J.W. Bacon, Jr., Experiments on a 30 Degree Swept, 12 Percent Thick, Symmetrical,Laminar Suction Wing in the 5-Foot by 7-Foot Michigan Tunnel (Northrop Corp., Norair Division Report NAI-57-317,BLC-93, February 1957).

23 Structural design of airplanes requires consideration of manufacturing procedures, capabilities, limitations, andavailable materials as well as compatibility with in-service inspection, maintenance, and repair while providing a highdegree of reliability and minimization of cost and weight. Airplane weight not only directly affects an airplane’s perfor-mance but also its total life-cycle economics through its effect on construction costs, operating costs, and perhapsmaintenance costs. The incorporation of laminar-flow control by suction imposes unique structural requirements in thatsmooth, substantially wave-free external surfaces are mandatory. Any associated additional weight or cost must notdissipate the advantages of LFC to a degree that the manufacturer or user would judge the remaining advantages insuffi-cient to warrant the increased complexities or risk.

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surface at Reynolds numbers over 30million, the highest attained on a liftingwing. When the F-94 aircraft speed wasincreased to the point where the localMach number24 on the airfoil surfaceexceeded about 1.09, a new potentialproblem appeared. Full-chord laminarflow was lost with the slot configurationtested. This was probably due to the steeppressure rise through the shock waves thatformed. Other data since that time,however, have shown that laminar flow

can be maintained through some shockwaves with a properly designed slotconfiguration. Another most importantpoint is that for the F-94 glove tests, theairfoils were exceptionally well madewith minimum waves and were main-tained in a very smooth condition; evenso, very small amounts of surface rough-ness, for example from local manufactur-ing irregularities or from bug impacts,caused wedges of turbulent flow behindeach individual source of turbulence.25

Figure 3. F-94active laminar-flow-control flightexperiments.

24 Mach number is a measure of airplane speed in terms of the ratio of the airplane speed to the speed of sound at theflight altitude. Airplane speeds up to the speed of sound are termed subsonic, above the speed of sound, supersonic, withthe supersonic speeds greater than approximately Mach 5 (or 5 times the speed of sound) referred to as hypersonic. Theregion between about Mach 0.85 and 1.15 is termed transonic. Because of the cross-sectional curvature of lifting surfaceslike wings, local Mach numbers of the air above the wing exceed the airplane Mach number.

25 W. Pfenninger, E.E. Groth, R.C. Whites, B.H. Carmichael, and J.M. Atkinson, Note About Low Drag Suction Experi-ments in Flight on a Wing Glove of a F94-A Airplane (Northrop Corp., Norair Division Report NAI-54-849, BLC-69,December 1954).

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By this time, there was a betterunderstanding that the use of increased airdensity in some wind tunnels, to moreclosely approximate full-scale flightvalues of Reynolds number, aggravatedthe surface roughness problem in the windtunnel as compared with flight.26 Never-theless, the vast NACA experience in thedevelopment of laminar-flow airfoils inthe late 1930s and early 1940s, the Britishflight tests of natural laminar-flow airfoilson the King Cobra and Hurricane air-planes in the mid-1940s, and the NACAand other previously mentioned tests oflaminar-flow control through poroussurfaces and slots in the late 1940sconvinced the NACA that the inability tomanufacture and maintain sufficientlywave-free and smooth surfaces was theprincipal impediment to the attainment ofextensive regions of laminar flow formost airplane missions then conceived.The primary focus of the NACA (at leastuntil its transformation in 1958 into theNational Aeronautics and Space Adminis-tration [NASA]) was on the business ofadvancing the understanding of aeronauti-cal phenomena and not on solving manu-facturing or operational problems, whichit considered to be the province of themanufacturer and user. The NACA,therefore, turned its attention away fromLFC per se and concentrated its laminar-flow activities on expanding the under-standing of the quantitative effects ofsurface roughness on transition, with andwithout suction. Based on these NACAdata and pertinent data from numerousother researchers, a correlation was

developed with which the permissiblethree-dimensional type surface-roughnessheight can be estimated within reasonableaccuracy.27

NASA became aware in 1960 of arenewed U. S. Air Force (USAF) interestin active laminar-flow control through avisit of Philip P. Antonatos of the USAFWright Air Development Division(WADD) to the author, who was thenhead of the General AerodynamicsBranch of the LaRC Full-Scale ResearchDivision.28 Contemplated Air Forcemissions at that time included a high-altitude subsonic aircraft of long range orendurance, an ideal match with laminar-flow control. Laminar flow was requiredto obtain the long range or endurance andhigh altitude alleviated the adverse effectsof surface protuberances. Any specialoperational procedures needed to maintainthe required surface smoothness in thepresence of material erosion and corro-sion and to cope with weather effects,29

aircraft noise, and accumulation of dirtand insects could only be evaluatedthrough actual flight experience. WADDalso considered it important to provide animpressive flight demonstration ofimproved airplane performance to bebetter able to advocate the advantages ofthe contemplated new aircraft.

WADD proposed use of two WB-66Dairplanes based on minimum cost, highdegree of safety, and short developmenttime. The Northrop Corporation, undersponsorship of the Air Force (with amonetary contribution from the FederalAviation Administration),30 later modified

26 The method of increasing the Reynolds number on small models in wind tunnels involves increasing the air densitythrough an increase in air pressure (higher unit Reynolds number, i.e., Reynolds number based on a unit length). Theminimum size of a three-dimensional type disturbance that will cause transition is smaller on a small model in anairstream of higher density than that required to cause transition on a full-size airplane at altitude (and, therefore, lowerdensity) at the same relative distance from the leading edge.

27 Albert E. von Doenhoff and Albert L. Braslow, “The Effects of Distributed Surface Roughness on Laminar Flow,” inBoundary-Layer and Flow Control - Its Principles and Application, Vol. 2, edited by G. V. Lachmann (Oxford, London,New York, Paris: Pergamon Press, 1961), pp. 657-681.

28 ALB files, LHA, notebook on Norair and LRC Memos re X-21: memo for LaRC Associate Director, 17 June 1960.

29 Weather effects include the effects of icing, precipitation, clouds, and low-frequency atmospheric turbulence.

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Figure 4. One oftwo X-21 activelaminar-flow-controlairplanes.

these airplanes with slotted suction wingsand designated them as experimentalaircraft X-21A and X-21B (Figure 4).Beginning with the first development-engineering review of the X-21A inJanuary 1963, the author acted as a NASAtechnical consultant to the Air Force.31

Northrop began flight research inApril of 1963 at Edwards Air Force Base.Several problems arose early in theproject that consumed significant periodsfor their solution. Principal among thesewas the old surface smoothness andfairness problem32 and an unexpectedseverity of a spanwise contaminationproblem. With respect to the smoothnessand fairness problem, in spite of a con-certed effort to design and build theslotted LFC wings for the two airplanes tothe close tolerances required, the resultinghardware was not good enough.Discontinuities in spanwise wing spliceswere large enough to cause prematuretransition. Putty, used to fair out these

discontinuities, chipped during flight withresulting roughness large enough totrigger transition.

The combination of X-21 winggeometry, flight altitudes, and Machnumbers was such that local turbulence atthe attachment line, e.g., from the fuse-lage or induced by insect accumulation,caused turbulent flow over much of thewing span (spanwise contamination).33 Atabout the same time, British flight tests ofa swept slotted-suction wing mountedvertically on the fuselage of a Lancasterbomber indicated similar results (Figure5).34 Although flight experimentation andsmall-scale wind-tunnel tests by theBritish had previously indicated theexistence of the spanwise-contaminationproblem, its significance had goneunrecognized. With the large-scale X-21flight tests and further wind-tunnel tests,Northrop developed methods for avoid-ance of spanwise contamination. Thephenomenon is now understood but

30 ALB files, LHA, notebook on Norair and LRC Memos re X-21: memo for LaRC Associate Director, 10 December1963.

31 See ALB files, LHA, notebook on Norair and LRC Memos re X-21: memo to Air Force Aeronautical SystemsDivision from Charles J. Donlan, Acting LaRC Director, dated 2 January 1963, and for other memos and programreviews.

32 Surface smoothness is a measure of surface discontinuities like protuberances or steps. Surface fairness is a measureof the degree of waviness of surface contour (shape).

33 On a sweptback wing, the line at which the airflow divides to the upper and lower surfaces is called the attachmentline. If the boundary layer at the attachment line becomes turbulent for any reason and if certain combinations of wingsweep, wing leading-edge radius, and flight conditions exist, the turbulence spreads outward along the attachment lineand contaminates (makes turbulent) the boundary layer on both wing surfaces outboard of the initial turbulence.

34 R.R. Landeryou and P.G. Porter, Further Tests of a Laminar Flow Swept Wing with Boundary Layer Control bySuction (College of Aeronautics, Cranfield, England, Report Aero. No. 192, May 1966).

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Figure 5. Swept,suction-typelaminar-flow-control wingmounted verticallyon Lancasterbomber.

requires careful attention in the design oflarge LFC aircraft.35

Another problem that was uncoveredduring the X-21 flight tests was associatedwith ice crystals in the atmosphere.Researchers noted that when the X-21flew in or near visible cirrus clouds,laminar flow was lost but that uponemergence from the ice crystals, laminarflow was immediately regained. G.R. Hallat Northrop developed a theory to indicatewhen laminar flow would be lost as afunction of atmospheric particle size andconcentration.36 Little statistical informa-tion, however, was available on the sizeand quantity of ice particles present in theatmosphere as a function of altitude,season of the year, and geographiclocation. Therefore, the practical signifi-cance of atmospheric ice particles on theamount of time laminar flow might be loston operational aircraft was not known.By October of 1965, attainment of“service experience comparable to anoperational aircraft,” one of the program’s

principal objectives, had not even beeninitiated because of the effort absorbed bythe previous problems. To proceed withthis initiative, the advisors to the AirForce recommended that a major wingmodification would be needed beforemeaningful data on service maintenancecould be obtained.37 This, unfortunately,was never done because of variousconsiderations at high levels of the AirForce, probably predominantly theresource needs of hostilities in Vietnam.Much extremely valuable information,however, was obtained during the X-21flight program, supported by wind-tunneland analytical studies. At the end of theprogram,38 flights attained laminar flowon a fairly large airplane over 95 percentof the area intended for laminarization.Unfortunately, top management in gov-ernment and industry remembered thedifficulties and time required to reach thispoint more than they did the accomplish-ment.

35 W. Pfenninger, Laminar Flow Control-Laminarization (AGARD Special Course on Concepts for Drag Reduction,AGARD Report No. 654, June 1977).

36 G.R. Hall, “On the Mechanics of Transition Produced by Particles Passing Through an Initially Laminar BoundaryLayer and the Estimated Effect on the LFC Performance of the X-21 Aircraft” (Northrop Corp., October 1964).

37 ALB files, LHA, folder labeled X-21 Tech Reviews: USAF Aeronautical Systems Division X-21 DAG ReviewAgenda and Attendees with Report of Review Group on X-21A Laminar Flow Control Program, 8 November 1965.

38 Special Section, “Laminar Flow Control Prospects,” Astronautics and Aeronautics 4, no. 7 (July 1966): 30-62. Thissection contains articles by several different authors. On X-21, see also document 2 at the end of this monograph.

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Post X-21 Research on Suction-TypeLaminar-Flow Control

Hiatus in ResearchWith the cessation of military support,

a general hiatus in the development ofactive laminar-flow control technologyensued in the United States from the mid-1960s to the mid-1970s. Other interestwas lacking because of two principalreasons: 1) a lack of a contemplated needfor very long-range missions for commer-cial aircraft for which the benefits ofactive laminar-flow control were anecessity and 2) the fact that the price ofjet fuel was then so low that the estimatedfuel-cost savings for commercial trans-ports with ranges of interest was almostoffset by estimated increases in manufac-turing and maintenance costs. Researchersdid perform significant analytical workand conceptual studies during this period,however.

Resumption of ResearchIn 1973, Gerald Kayten, who was

Director of the Transportation ExperimentProgram Office in the Office of Aeronau-tics and Space Technology at NASAHeadquarters, phoned the author with arequest that he prepare a “white paper” onpotential technology advances that mightreduce the use of fuel by commercial airtransports. The request was in response toincreased prices and increasingly insecuresources of petroleum-based fuel resultingfrom the oil embargo imposed by theOrganization of Petroleum ExportingCountries in 1973. NASA, at that time,was pursuing technological improvementsin various aircraft disciplinary areas(identified and evaluated in the AdvancedTransport Technology Systems andDesign Studies)39 to reduce aircraft noise

and pollution, to improve economics, andto reduce terminal-area delays. Theresultant “white paper,” printed December20th of 1973,40 recommended that thetechnological advances identified forthese purposes be pursued with an in-creased emphasis on their potential forfuel reduction. It also identified additionalpossibilities in the aeronautical disciplinesfor fuel conservation. Principal amongthese, with by far the largest potential forfuel conservation of any discipline, wasdrag reduction through active laminar-flow control. Kayten, in a telephoneconversation with the author on 14January 1974,41 called the paper “damngood,” and he strongly urged that we getgoing quickly. He indicated, however, thatthe reception by others at Headquarterswas nothing more than lukewarm. Thesame was true among LaRC researchersin management positions who believedthat the problems previously evident inthe laminar-flow research were so severeas to render the technology impracticaland that any further efforts would onlydetract from the resources available forother research endeavors.

Because of this continued adversereaction from many in positions ofauthority, start of a significant program onactive laminar-flow control was continu-ally deferred. Leaders of various groupsduring the next couple of years, however,initiated tasks to identify and recommendResearch and Technology (R&T) activi-ties that would be required to developpotential fuel-conservation technologies.The following are examples of the studiesthat resulted. In March of 1974, theAmerican Institute of Aeronautics andAstronautics (AIAA) assembled a groupof 91 of its members in a workshopconference. The objective was “to review

39 These studies were made under the Advanced Technology Transport (ATT) Program at LaRC under the direction ofThomas A. Toll.

40 Albert L Braslow and Allen H. Whitehead, Jr., Aeronautical Fuel Conservation Possibilities for Advanced SubsonicTransports (Washington, DC: NASA TM X-71927, 20 December 1973).

41 ALB files, LHA, chronological notebook on Advanced Technology Transport Office (later called Advanced TransportTechnology Office and later changed in emphasis to Aircraft Energy Efficiency Project Office): note dated 1-14-74.

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and discuss the technological aspects ofaircraft fuel conservation methods and torecommend the initiation of those mea-sures having the best prospects for short-term and long-term impact.” One of theresultant conclusions was that advances inassociated technologies since the 1960swarranted a reevaluation of the applica-tion of laminar-flow control in the designof future long-range transport aircraft.42 InNovember of 1974, the Aeronautics Panelof the DOD/NASA Aeronautics andAstronautics Coordinating Board estab-lished a new subpanel on AeronauticalEnergy Conservation/Fuels, cochaired byA. Braslow, NASA/LaRC and A. Eaffy,USAF/Pentagon.43 The task was to“review the on-going NASA and DODprograms and recommend increasedactivities in fuel-conservation technolo-gies where deficiencies were noted.” Thesubpanel supported further research onLFC, including flight-testing.44 Alsorecommended was the need for system-technology studies with fuel conservationas a primary criterion so that the applica-tion of the various technological advancescould both separately and by interactionproduce further significant fuel savings.45

In 1975, NASA sponsored a Task Force ofengineers from within NASA, the Depart-ment of Transportation (DOT), Federal

Aviation Administration (FAA), andDepartment of Defense (DOD) to exam-ine the technological needs and opportu-nities for achievement of more fuel-efficient transport aircraft and recommendto NASA an extensive technologicaldevelopment program. The Task Forcepublished its recommendations on 9September 197546 and the LangleyDirector, Edgar M. Cortright, immediatelyestablished a Laminar-Flow-ControlWorking Group, chaired by the author, “todefine a program of required R&Tactivities.”47 After definition of detailedplans and a process of evaluation, advo-cacy, and approval by NASA manage-ment, the U.S. Office of Management andBudget (OMB) and the U.S. Congress, theTask Force’s recommendations evolvedinto the NASA Aircraft Energy Efficiency(ACEE) Program. The Office of Aeronau-tics and Space Technology (OAST) atNASA Headquarters managed the pro-gram.

The ACEE Project Office was estab-lished at the LaRC48 to define, implementand manage three of six Program ele-ments. The three elements were Compos-ite Structures, Energy Efficient Transport(subdivided into Advanced Aerodynamicsand Active Controls), and Laminar-FlowControl.49 The acceptance of active

42 “Aircraft Fuel Conservation: An AIAA View” (Proceedings of a Workshop Conference, Reston, VA, 13-15 March,edited by Jerry Grey, 30 June 1974).

43 ALB files, LHA, folder labeled Aeronautics Panel, AACB, Energy Conservation/Fuels: Minutes of Special Meeting,NASA/DOD Aeronautics Panel, AACB, 11 November 1974, and Memorandum to Members of the Aeronautics Panel,AACB, 25 November 1974.

44 ALB files, LHA, folder labeled Aeronautics Panel, AACB, Energy Conservation/Fuels: Report of the Subpanel onAeronautical Energy Conservation/Fuels, Aeronautics Panel, AACB, R&D Review, 5 December 1974, sect. 4.1.2. Seedocument 1 at the end of this monograph.

45 Ibid., sect. 3.8.

46 NASA Task Force for Aircraft Fuel Conservation Technology (Washington, D.C.: NASA TM X-74295, 9 September1975).

47 See document number 3 at the end of this monograph.

48 ALB files, LHA, Project Plan, Aircraft Energy Efficiency Program, Langley Research Center, L860-001-0, May1976. Inserted is a page summarizing some key events.

49 Ralph J. Muraca was Deputy Manager for LFC to Robert W. Leonard, ACEE Project Manager in the LaRC ProjectsGroup headed by Howard T. Wright. The author acted as Muraca’s assistant.

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laminar-flow control with suction (LFC)as part of the NASA ACEE program wasbased on the success of the previousexperimental programs in attainingextensive regions of laminar flow on anoperational airplane and more recentadvances in materials and manufacturingtechnology that might make LFC moreeconomically attractive. The principalmotivation was the potentially larger gainin transport-aircraft performance resultingfrom laminarization of the boundary layerover wing and tail surfaces as comparedwith all other technological disciplines.

Formulation of the approved programreceived very extensive input and supportfrom the air-transport industry.50 Animportant example was the active partici-pation of people from the industry in anLFC technology workshop held at theLangley Research Center on 6 and 7April 1976.51 Representatives of theairlines, manufacturers of large aircraftand aircraft engines, and individuals withexpertise in LFC from the industry andgovernment attended.52 Objectives were toreview the state of the art, identify anddiscuss problems and concerns, anddetermine what was necessary to bringLFC to a state of readiness for applicationto transport aircraft. The ACEE ProjectOffice relied heavily on the discussions.

A change in LFC emphasis from theprevious military application to the moredifficult one of commercial transports,where manufacturing and operationalcosts are more important, made the LFCtask even more challenging. The objectiveof the LFC element was to provideindustry with sufficient information topermit objective decisions on the feasibil-ity of LFC for application to commercialtransports. It was expected that the

technology developed would be appli-cable to but not sufficient for very long-range or high-endurance military trans-ports. The focus was on obtaining reliableinformation regarding the ability toprovide and the cost of providing requiredsurface tolerances as well as on the abilityto maintain laminar flow in an airlineoperational environment. Improvementsin computational ability for providing areliable design capability were also ofimportance in the event practicality couldbe established. Implementation of thethree project elements involved a majorchange in Agency philosophy regardingaeronautical research—a judiciousextension of the traditional NACAresearch role to include demonstration oftechnological maturity in order to stimu-late the application of technology byindustry.

The ACEE/LFC project to bringactive LFC from an experimental status to“technology readiness” for actual applica-tion required solutions to many difficulttechnical problems and entailed a highdegree of risk—characteristics thatdictated reliance on government support.A phased approach to require thatprogress in each area be evaluated prior tofunding the next phase was accepted as ameans of controlling the large resourcecommitments required and of alleviatingthe concern about the risk factor. Thisapproach led to considerable heartburn inthe project office in its attempt to com-plete a successful overall development ina timely fashion; a need to wait forsuccessful results on intermediate stepswas required before there could beadequate advocacy for the inclusion ofsubsequent phases in an annual govern-ment budget cycle. The project office

50 ALB files, LHA, notebook labeled Industry Comments: responses from industry top management to letter fromRobert E. Bower, LaRC Director for Aeronautics, requesting response to five specific questions regarding LFC; internalACEE Project Office memos on visits to industry to review detailed program proposals; and personal notes on trips toindustry.

51 ALB files, LHA, Workshop on Laminar Flow Control held at LaRC, compiled by Charles T. DiAiutolo, 6-7 April1976.

52 General chairmen were Adelbert L. Nagel and Albert L. Braslow of LaRC.

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adopted the following guidelines for theLFC part of the program: “technologyreadiness” should be validated by theaircraft industry, and in particular, bythose companies involved in productionof long-range aircraft; the program shouldbe cognizant of technological advances inother disciplines where those advanceswould be of particular benefit to LFC orwhere their application to future turbulentjet transports appeared likely; and theprogram should build on the existing database, in particular, the USAF X-21 flightsand associated programs previouslydiscussed.

Research from the Mid-1970s to theMid-1990s

For various reasons, the ACEE/LFCproject required flight research in thefollowing activities:• Determination of the severity of the

adverse effects of surface contamina-tion by insects on the extent of laminarflow and the development and valida-

tion of an acceptable solution• Evaluation of LFC surface and wing

structural concepts employing ad-vanced materials and fabricationtechniques

• Development of improved aerodynamicand acoustic design tools and establish-ment of optimized suction criteria

• Validation of airfoil and wing geom-etries optimized for LFC

• Validation of high-lift devices andcontrol surfaces compatible with LFC

• Demonstration of predicted achieve-ment of laminar flow and validation ofacceptable economics in the manufac-ture and safe commercial operation ofLFC airplanes.

A few flight programs that investi-gated aerodynamic phenomena associatedwith attainment of natural laminar flow(NLF) provided information that was alsoof importance to active laminar-flowcontrol at high subsonic speeds. These arediscussed in the following subsectionsalong with those that used LFC.

Figure 6. F-111/TACT variable-sweep transitionflight experiment.(NASA photo ECN3952)

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Natural Laminar Flow (NLF) on SweptWings: F-111/TACT and F-14

Of principal significance in NLFflight research done with an F-111 air-plane and an F-14 airplane was quantifi-cation of the adverse effect of crossflowinstability due to wing sweep. Research-ers installed supercritical, natural laminar-flow airfoil gloves on an F-111 aircraft(Figure 6), re-designated as the F-111/TACT (Transonic Aircraft Technology)airplane, and tested it in early 1980 at theDryden Flight Research Center (DFRC)53

through a range of sweep angles.54 These

results were limited by a restrictedspanwise extent of the gloves, an abbrevi-ated test schedule (caused by the requiredreturn to the Air Force of the borrowedaircraft), and limited instrumentation.55

The results,56 however, provided the basisfor a follow-on program with anothervariable-sweep aircraft (an F-14 on loanto NASA from the Navy, Figure 7) thatenabled attainment of a much broader andmore accurate transition database. The F-14 research began in 1984 at the DFRCand was completed in 1987.57

Flush static-pressure orifices and

Figure 7. F-14variable-sweeptransition flightexperiment. (NASAphoto)

53 From 1981 to 1994, Dryden was subordinated to the NASA Ames Research Center as the Ames-Dryden FlightResearch Facility, but to avoid confusion I will refer to it as DFRC throughout the narrative.

54 NASA flight-test participants were: Einar K. Enevoldson and Michael R. Swann, research pilots; Lawrence J. Cawfollowed by Louis L. Steers, project managers; Ralph G. (Gene) Blizzard, aircraft crew chief; and Robert R. Meyer, Jr.,followed by Louis L. Steers, DFRC principal investigators. For an example of a flight report on the F-111 with the NLFgloves, see document 5 at the end of this monograph.

55 ALB files, LHA, folder labeled SASC 1980-81: memo on Natural Laminar Flow Flight Tests At DFRC On F-111Aircraft, August 1980.

56 Boeing Commercial Airplane Company, Preliminary Design Department, F-111 Natural Laminar Flow Glove Flight TestData Analysis and Boundary Layer Stability Analysis (Washington, DC: NASA Contractor Report 166051, January 1984).

57 NASA flight-test participants were: Edward T. Schneider and C. Gordon Fullerton, research pilots; Jenny Baer-Riedhart, project manager; Bill McCarty, aircraft crew chief; Harry Chiles, instrumentation engineer; Robert R. Meyer,Jr., chief engineer; Marta R. Bohn-Meyer, operations engineer; Bianca M. Trujillo, DFRC principal investigator; andDennis W. Bartlett, LaRC principal investigator.

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surface arrays of hot films58 were distrib-uted over gloves with a different airfoilcontour on each wing to determine localwing pressures and transition locations.Data from these sources and associatedflight parameters were telemetered to theground and monitored in real time by theflight-research engineer. Figure 8 presentsresults from the F-111 and F-14 swept-wing flight research along with resultsfrom low-speed wind-tunnel research inthe LaRC Low-Turbulence PressureTunnel (previously mentioned in the EarlyResearch section) and the Ames ResearchCenter 12-Foot Tunnel. The results arepresented as the variation of the maxi-mum transition Reynolds number withwing leading-edge sweep. The researchengineers, after careful consideration ofthe differences in accuracy of the variousdata, have judged that the extent oflaminar flow (a direct function of thetransition Reynolds number) is unaffectedby wing sweep up to a value of about 18degrees. At higher sweep angles, theextent of laminar flow is appreciablyreduced by crossflow disturbances. The

F-14 transition data also provided suffi-cient detailed information to improve theunderstanding of the combined effects ofwing cross-sectional shape, wing sweep,and boundary-layer suction (even thoughsuction was not used on the F-14) on thegrowth of two-dimensional and crossflowdisturbances.59 This improved understand-ing permits a significant increase inmaximum transition Reynolds numberthrough the use of suction in only theleading-edge region of swept wings incombination with an extent of favorablepressure gradient aft of the suction, aconcept called hybrid laminar-flowcontrol (HLFC), to be discussed later.

Noise: Boeing 757Under a NASA contract, the Boeing

Company performed flight research in1985 on the wing of a 757 aircraft (Figure9) to determine the possible effects of theacoustic environment on boundary-layertransition. Because of a lack of sufficientdata on the acoustic environment associ-ated with wing-mounted high-bypass-ratio turbofan engines, a concern about

Figure 8. Maximumtransition Reynoldsnumber as afunction of wingsweep.

58 The hot-film sensors consisted of nickel-film elements deposited on a substrate of polyimide film with an installedthickness of less than 0.007 inch. Electric current is passed through the nickel elements and circuitry maintains a constantelement temperature. The changes in current required to maintain the temperature constant are measured when changesin boundary-layer condition cause changes in cooling of the elements. The difference in cooling between a laminar andturbulent boundary layer and the fluctuating variations during the transition process from laminar to turbulent can thenbe measured and the transition location determined.

59 R.D. Wagner, D.V. Maddalon, D.W. Bartlett, F.S. Collier, Jr., and A.L. Braslow, “Laminar Flow Flight Experiments,”from Transonic Symposium: Theory, Application, and Experiment held at Langley Research Center (Washington, DC:NASA CP 3020, 1988).

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potential adverse effects of engine noiseled to a belief that the engines needed tobe located in an aft position on thefuselage. This location has a potentiallysevere adverse impact on performanceand LFC fuel savings. Boeing replaced aleading-edge slat just outboard of thewing-mounted starboard engine with a10-foot span smooth NLF glove sweptback 21 degrees. Seventeen microphoneswere distributed over the upper and lowersurfaces to measure the overall soundpressure levels, and hot films were used tomeasure the position of transition fromlaminar to turbulent flow. The starboardengine was throttled from maximumcontinuous thrust to idle at altitudes of 25

to 45 thousand feet and cruise speeds ofMach 0.63 to 0.83.

Although this flight research was notexpected to provide answers on noiseeffects for all combinations of pertinentparameters, it did provide importantindications. The most important was thatengine noise does not appear to have asignificant effect on crossflow distur-bances so that if the growth of crossflowdisturbances in the leading edge iscontrolled by suction, large extents oflaminar flow should be possible even inthe presence of engine noise. If, however,in an HLFC application, the growth oftwo-dimensional type disturbances iscomparable to or greater than the growth

Figure 9. 757transport noiseexperiments.

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of crossflow disturbances, engine noisemight be a more significant factor. Theresults were unable to validate theoreticalpredictions of the magnitude of noiselevels at high altitudes and subsoniccruise speeds.60

Insect Contamination: JetStarA major concern regarding the

dependability of laminar flow in flightinvolved the possibility (most thought,probability) that the remains of insectimpacts on component leading edgesduring flight at low altitudes duringtakeoff or landing would be large enoughto cause transition of the boundary layerfrom laminar to turbulent during cruiseflight. As a first step, the LaRC measuredthe insect remains that had accumulatedon the leading edges of several jet air-planes based at the Center. The Langleyresearchers calculated that the insectremains were high enough to cause

transition, even at altitudes as high as40,000 feet.61 (Remember that an increasein altitude alleviates the adverse effect ofsurface roughness in that the minimumheight of roughness that will inducetransition increases as altitude increases.)An observation, however, had been madepreviously by Handley Page in Englandwhere flight tests of a Victor jet indicatedthat insect remains eroded to one-halftheir height after a high-altitude cruiseflight. The Langley researchers, therefore,deemed it necessary to investigate furtherthe possible favorable erosion but, iferosion was determined to be insufficientto alleviate premature transition at cruisealtitudes, to develop and validate anacceptable solution to the insect contami-nation problem.

Researchers at the DFRC and theLaRC used a JetStar airplane at Dryden(Figure 10) in 1977 to investigate theinsect-contamination problem.62 With

Figure 10. JetStaraircraft and re-search team forinvestigation ofinsect contamina-tion . Left to right:back row —Thomas C.McMurtry, test pilot;Kenneth Linn,instrumentationtechnician; Rob-ert S. Baron,project manager;Donald L. Mallick,test pilot; WalterVendolski, aircraftmechanic; John B.Peterson, Jr., LaRCprincipal investiga-tor; front row —Albert L. Braslow,LaRC; James A.Wilson, aircraftcrew chief; WilliamD. Mersereau, flightoperations; DavidF. Fisher, DFRCprincipal investiga-tor. (Private photoprovided by author)

60 Boeing Commercial Airplane Company, Flight Survey of the 757 Flight Noise Field and Its Effect on LaminarBoundary Layer Transition, Vol. 3: Extended Data Analysis (Washington, DC: NASA CR178419, May, 1988).

61 The calculations were based on von Doenhoff and Braslow, “The Effects of Distributed Surface Roughness onLaminar Flow,” pp. 657-681, cited in footnote 27.

62 Dave Fisher was principal investigator at DFRC, and Jack Peterson formulated the program under the direction of theauthor at the LaRC.

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contract support of the aircraft manufac-turer, the Lockheed-Georgia AircraftCompany, they modified the left outboardleading-edge flap. Five different types ofsuperslick and hydrophobic surfaces wereinstalled in the hope that impacted insectswould not adhere to them. In addition,researchers installed a leading-edgewashing system and instrumentation todetermine the position of boundary-layertransition. Dryden research pilots firstflew the airplane with an inactive washersystem on numerous airline-type takeoffsfrom large commercial airports. They flewat transport cruise altitudes and thenlanded at DFRC for post-flight inspection.These early tests indicated that insectswere able to live in an airport noise andpollution environment and accumulatedon the leading edge. The insects thuscollected did not erode enough to avoidpremature transition at cruise altitudes. Itis probable that insect impacts at themuch higher transport takeoff speed, ascompared with the slow takeoff speed ofthe previously mentioned Victor airplane,initially compresses the insects to agreater degree where further erosion doesnot take place. None of the superslick andhydrophobic surfaces tested showed anysignificant advantages in alleviatingadherence of insects. The need for anactive system to avoid insect accumula-tion, then, was apparent.63

Although researchers had consideredmany concepts for such a system over theyears and had tested some, none had beenentirely satisfactory. The results of theflight research using the leading-edgewasher system that had been installed onthe JetStar leading-edge flap showed thata practical system was at hand. The testsshowed that keeping the surface wet whileencountering insects was effective inpreventing insect adherence to the wingleading edge. After insect accumulation

was permitted to occur on a dry surfacespray could not wash the insect remainsoff the leading edge (somewhat akin tothe inability of an automobile windshieldwasher alone to remove bug accumulationfrom the windshield). The pilots, namedin Figure 10, had flown the airplane withthe spray on at low altitudes over agricul-ture fields in an area with a high densityof flying insects in order to give the wet-surface concept a severe test.64 Supportinganalyses at LaRC also indicated anacceptable weight penalty of a washersystem equal to less than one percent ofthe gross weight of an LFC transportairplane.

Leading-Edge Flight Test (LEFT)Program: JetStar

Planning for a flight test program toprovide definitive information on theeffectiveness and reliability of LFC beganat LaRC soon after approval of the ACEEProgram. The Langley ACEE ProjectOffice expended considerable effort inconsideration of candidate flight vehicles.Representatives of the airlines andtransport manufacturers strongly advo-cated the need for a test aircraft equal tothe size of a long-range transport (asindicated in the question and answersession of the 1976 LFC Workshop, citedin footnote 51) to provide meaningfulresults with respect to aerodynamic,manufacturing, and operational consider-ations. Government managers appliedequally strong pressure towards theselection of a smaller size for cost rea-sons. The Project Office eventuallyformulated a satisfactory solution thatfulfilled both requirements. It decided torestrict the tests to the leading-edge regionof a laminar-flow wing suitable for ahigh-subsonic-speed transport airplanebecause the most difficult technical anddesign challenges that had to be overcome

63 David F. Fisher and John B. Peterson, Jr., “Flight Experience on the Need and Use of Inflight Leading Edge Washingfor a Laminar Flow Airfoil,” AIAA Aircraft Systems and Technology Conference, Los Angeles, CA (AIAA paper 78-1512, 21-23 August 1978).

64 Details of these flight tests are included in ibid.

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before active laminar-flow control withsuction could be considered a viabletransport design option were (and still are)embodied in this region. The externalsurfaces at the leading edge must bemanufactured in an exceptionally smoothcondition (smoother than necessary atmore rearward locations) and must bemaintained in that condition while subjectto foreign-object damage, insect impinge-ment, rain erosion, material corrosion,icing, and other contaminants. In addition,an insect-protection system, an anti-icingsystem, a suction system, and perhaps apurge system and/or a high-lift leading-edge flap must all be packaged into arelatively small leading-edge box volume.Most of these problems equally affect theconcept of hybrid LFC and the concept ofactive laminar-flow control with suctionto more rearward positions.

The Project Office then selected thesame JetStar airplane that was previouslyused for the initial insect-contaminationflight research. The test article would bedimensionally about equivalent to theleading-edge box of a DC-9-30 airplane, asmall commercial transport, wheresolution of the packaging problems wouldprovide confidence for all larger HLFCand LFC airplanes with suction to morerearward positions. Its choice, however,did not receive unanimous concurrence.Dr. Pfenninger, who was then employedby the LaRC, strongly objected to selec-tion of an airplane with a leading-edgesweep as high as the JetStar’s (33 de-grees) because he expected greatlyincreased difficulty in handling the largecrossflow disturbances that would beintroduced.65 The Project Office acceptedthe risk, however, after extensive feasibil-

ity studies and technical evaluations ofseveral candidate aircraft.66

Selection of the most promisingapproaches to satisfaction of LFC systemsrequirements for both slotted-surface andperforated-surface configurations wasbased on several years of design, fabrica-tion and ground testing activities.67 TheDouglas Aircraft Co. and the Lockheed-Georgia Aircraft Co. were the majorcontributors to this activity. Unfortu-nately, the Boeing Co. did not participateinitially because of a corporate decision toconcentrate its activities on the develop-ment of near-term transport aircraft.Boeing became active in the laminar-flowdevelopments later. Inasmuch as no clear-cut distinction existed at that time be-tween multiple slots and continuoussuction through surface perforations madewith new manufacturing techniques(although continuous suction had aerody-namic advantages), the Project Officeprudently decided to continue investiga-tion of both methods for boundary-layersuction. The Lockheed-Georgia AircraftCompany installed a slotted configurationon the left wing, and the Douglas AircraftCompany installed a perforated configura-tion on the right wing. The leading-edgesweep of both wing gloves was reducedfrom the wing sweep of 33 degrees to 30degrees to alleviate the crossflow instabil-ity problem somewhat. Figures 11-13present illustrations of the airplane andthe leading-edge configurations.

The design of the slotted arrangementrepresented a leading-edge region for afuture transport with laminar flow on bothsurfaces in cruise flight and included0.004-inch-wide suction slots (smallerthan the thickness of a sheet of tablet

65 ALB files, LHA, pocket-size “Memoranda” notebook: entry dated 2 September 1976.

66 ALB files, LHA, folder labeled LaRC Internal Memos on LFC dated 12/3/75 to 11/16/78: Memo to Distribution fromRalph J. Muraca, Deputy Manager, LFC Element of ACEEPO on Feasibility Studies of Candidate Aircraft for LFCLeading Edge Glove Flight — Request for Line Division Support, 16 November 1978.

67 Albert L. Braslow and Michael C. Fischer, “Design Considerations for Application of Laminar-Flow Control Systemsto Transport Aircraft,” presented at AGARD/FDP VKI Special Course on Aircraft Drag Prediction and Reduction at thevon Kármán Institute for Fluid Dynamics, Rhode St. Genese, Belgium on 20-23 May 1985, and at NASA Langley on 5-8 August 1985, in AGARD Rept. 723, Aircraft Drag Prediction and Reduction (July 1985): 4-1 through 4-27.

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paper) cut in a titanium surface.68 Theslots that encompassed the wing stagna-tion line also served the dual purpose ofejecting a freezing-point depressant fluidfilm for anti-icing and for insect protec-tion. During climb-out, these slots werepurged of fluid and they joined the other

suction slots for laminarization of theboundary layer under cruise conditions.

The design of the perforated arrange-ment represented a leading-edge region

for a future transport with laminar flow onthe upper surface only—an approach thatcan provide future transports with signifi-cant simplifying advantages at the ex-pense of a somewhat higher drag. In thedesign of future transports with upper-surface suction only, the adverse effect of

a loss in lower-surface laminarization willnot be as great as one might expectbecause the skin friction is higher on theupper surface due to higher local veloci-

Figure 11a. JetStarTest-bed aircraft forthe NASA Leading-Edge Flight Testprogram.

Figure 11b. JetStaraircraft and teamfor Leading-EdgeFlight-Test Pro-gram. Left to right:J. Blair Johnson,aerodynamicsengineer; GaryCarlson, aircraftmechanic; MichaelC. Fischer, LaRCprincipal investiga-tor; James Wilson,aircraft crew chief;Donald L. Mallick,test pilot; David F.Fisher, DFRCprincipal investiga-tor; John P. Stack,LaRC instrumenta-tion technician;Edward Nice,aircraft mechanic;Ron Young,instrumentationengineer; EarlAdams, DFRCinstrumentationtechnician; RobertS. Baron, DFRCproject manager;Russell Wilson,aircraft inspector;Richard D. Wagner,LaRC projectmanager; unidenti-fied; Fitzhugh L.Fulton, test pilot.(NASA photo ECN30203)

68 No leading-edge high-lift device was required for the transport aircraft conceptualized by Lockheed for this applica-tion of LFC.

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Figure 13. Leading-Edge Flight-Testprogram slottedtest article.

Figure 12. Leading-Edge Flight-Testprogram perforatedtest article.

ties. A relatively small extension in upper-surface laminarization, therefore, can beused to significantly attenuate the in-creased drag of the lower surface.

The advantages of laminarization ofonly the upper surface include severalfeatures. Conventional access panels towing leading- and trailing-edge systemsand fuel tanks can be provided on thewing lower surface for inspection andmaintenance purposes without disturbingthe laminar upper surface. Laminarizedsurfaces in areas susceptible to foreign-object damage are eliminated. The wingcan be assembled from the lower surfacewith the use of internal fasteners; this is

much preferable to concepts that useexternal fasteners, where the fastenerscould induce external disturbances. Theinitial manufacturing costs and themaintenance costs are reduced. Upper-surface-only laminarization also willpermit deployment of a leading-edgedevice for both high lift and shieldingfrom direct impacts of insects. Deploy-ment, when needed, and retraction intothe lower surface, when not needed, willbe permitted because the need for strin-gent surface smoothness on the lowersurface will be eliminated. The testarrangement used such a device with anauxiliary nozzle to spray freezing-point

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depressant fluid for anti-icing and toprovide conservatism in the elimination ofinsect adherence.69 Finally, Douglas useda system for reversing the flow of airthrough the perforations on the testarrangement to remove possible residualfluid.

Use of electron-beam technologymade possible, for the first time, manufac-ture of holes of a small enough size andspacing to avoid introduction of aerody-namic disturbances as large as those thathad previously caused premature transi-tion in wind tunnels. The successful useof laser “drilling” of holes followed later.The perforations in the test arrangementon the JetStar were 0.0025 inch in diam-eter (smaller than a human hair) and werespaced 0.035 inch apart in a titanium skin(over 4,000 holes per square foot ofsurface area). Only very close inspectionwould reveal a difference between aperforated-wing surface and a solid one.

In general, instrumentation wasconventional but careful attention wasrequired to avoid any adverse interferencewith the external or internal airflows. Anunconventional instrument called aKnollenberg probe (a laser particlespectrometer) was mounted atop a ventralpylon on the fuselage upper surface tomeasure the sizes and quantities ofatmospheric ice and water droplets. Acharging patch, mounted on the pylonleading edge, provided a simple way todetect the presence of atmosphericparticles and an impending loss of laminarflow by responding to the electrostaticcharge developed when ice or waterdroplets struck the aircraft surface. Thepatch was investigated as a possible low-

cost application to future laminar-flowairplanes.

The Dryden Flight Research Centeragain conducted the flight tests.70 Afterinitial tests to check out and adjust allsystems and instrumentation, the principaleffort focused on demonstration of theability to attain the design extent oflaminar flow under routine operationalconditions representative of LFC subsoniccommercial airplanes and on provision ofinsight into maintenance requirements.Simulated airline flights included groundqueuing, taxi, take off, climb to cruisealtitude, cruise for a sufficient time todetermine possible atmospheric effects onlaminar flow, descent, landing, and taxi.Conditions representative of airlineoperations included one to four operationsper day and flight in different geographi-cal areas, seasons of the year, andweather. Also, as in the case of commer-cial airline operations, the airplaneremained outdoors at all times while onthe ground and no protective measureswere taken to lessen the impact of adverseweather or contamination on the testarticles. In order not to increase pilotworkload in the operation of LFC air-planes, the suction system was operated ina hands-off mode (except for on-offinputs).

All operational experience with theLFC systems performance (for bothperforated and slotted configurations)during the simulated-airline-serviceflights was positive.71 Specifically, duringfour years of flight testing from Novem-ber 1983 to October 1987, no dispatchdelays were caused by LFC systems.Laminar flow was obtained over the

69 After the early JetStar flight tests on the effectiveness of wetting the leading-edge surfaces for prevention of insectadherence, analyses and wind-tunnel tests of live-insect impacts were made by both Lockheed and Douglas to developdetailed arrangements of leading-edge-protection methods for their selected LFC configurations.

70 NASA Flight-test participants were: Donald L. Mallick and Fitzhugh L. Fulton, research pilots; Robert S. Baron,project manager; Ronald Young, instrumentation engineer; David F. Fisher followed by M.C. Montoya, DFRC principalinvestigators; and Michael C. Fischer, LaRC principal investigator. For background to the flight testing, see document 4at the end of this monograph.

71 Dal V. Maddalon and Albert L. Braslow, Simulated-Airline-Service Flight Tests of Laminar-Flow Control withPerforated-Surface Suction System (Washington, DC: NASA Technical Paper 2966, March 1990).

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leading-edge test regions as planned afterexposure to heat, cold, humidity, insects,rain, freezing rain, snow, ice, and moder-ate turbulence. Removal of groundaccumulations of snow and ice was nomore difficult than the then-normalprocedures for transport aircraft. Nomeasurable degradation of the titaniumsurfaces occurred. Surface cleaningbetween flights was not necessary. Pilotadjustment of suction-system operationwas unnecessary. The simple electrostatic“charging patch” device appeared to offeran inexpensive and reliable method ofdetecting the presence of ice crystals inflight (more about the atmosphericparticle problem later).

The emergence of electron-beamperforated titanium as a wing surface thatmet the severe aerodynamic, structural,fabrication, and operational requirementsfor practical aircraft applications wasconsidered to be a major advance inlaminar-flow control technology by theprincipal government and industryinvestigators. Fabrication of the slotted-surface test article resulted in a suctionsurface that was only marginally accept-able, resulting in poorer performance.Some further development of slotted-surface manufacturing techniques,therefore, was (and is) still required. Alsoneeded is proof of satisfactory aerody-namic performance of the perforatedsurface at larger values of length

Reynolds number, i.e., to distances greaterthan the end of the leading-edge testarticle. Nevertheless, the simulated-airline-service flights successfully demon-strated the overall practicality of baselinedesigns for leading-edge LFC systems forfuture commercial-transport aircraft, amajor step forward.

Surface Disturbances: JetStarIn 1986 and 1987, the LaRC LFC

Project Office, which had continuedresearch on LFC after termination of theACEE Project,72 took advantage of thecontinued availability of the JetStarairplane at the DFRC to further thequantitative database on the effects oftwo- and three-dimensional-type surfaceroughness and on the effects of suctionvariations.73 The most significant resultsthat were obtained concerned clarificationof the quantitative effects of crossflowdue to sweep on the roughness sizes thatwould cause premature transition. Asindicated many times in this monograph,the adverse effect of surface protuber-ances on the ability to maintain laminarflow was the primary inhibiting factor tothe practicality of LFC. Although anempirical method of determining thequantitative effects of surface roughnesson transition had been developed muchearlier for unswept wings,74 some indica-tions had later become available75 thatwing sweep (crossflow effects) might

72 Richard D. Wagner headed the LaRC LFC Project Office during the 1980s (at first, still under ACEE) and was followedby F.S. Collier, Jr. The author, after his retirement from NASA in 1980, continued to provide significant input into theplanning, analysis and reporting of much of the experimental research and development activities through local aerospacecontractors. Dal V. Maddalon was technical monitor for these contracts. See ALB files, four folders labeled SASC (Systemsand Applied Sciences Corporation) and one folder labeled Analytical Services and Materials, Inc. (April, 1980 through Sept.,1993).

73 Dal V. Maddalon, F.S. Collier, Jr., L.C. Montoya, and C.K. Land, “Transition Flight Experiments on a Swept Wing withSuction” (AIAA paper 89-1893, 1989); Albert L. Braslow and Dal V. Maddalon, Flight Tests of Three-Dimensional SurfaceRoughness in the High-Crossflow Region of a Swept Wing with Laminar-Flow Control (Washington, DC: NASA TM109035, October 1993); Albert L. Braslow and Dal V. Maddalon, Flight Tests of Surface Roughness Representative ofConstruction Rivets on a Swept Wing with Laminar-Flow Control (Washington, DC: NASA TM 109103, April 1994).

74 See von Doenhoff and Braslow, “The Effects of Distributed Surface Roughness on Laminar Flow,” pp. 657-681,cited in footnote 27.

75 Dezso George-Falvy, “In Quest of the Laminar-Flow Airliner—Flight Experiments on a T-33 Jet Trainer,” 9thHungarian Aeronautical Science Conference, Budapest, Hungary (10-12 November 1988).

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exacerbate the roughness effects. Analysisof the additional JetStar data76 indicatedthat the adverse effect of crossflowoccurred for two- rather than three-dimensional type roughness.77

Figure 14 plots a roughness Reynoldsnumber parameter against the ratio ofroughness width to height.78 The symbolsrepresent data for unswept wings with no

crossflow except for a group of threeidentified for swept wings in highcrossflow.79 The vertical bracket indicatesa range of roughness data obtained on thesweptback JetStar in a region of highcrossflow. The horizontal line representsother roughness data obtained on theJetStar in both low and high crossflow.The important conclusions are: 1) forpractical engineering application, thepermissible height of three-dimensional

type roughness (ratios of roughness widthor diameter to height of approximately 0.5to 5.0) located in a high crossflow regionis the same as that previously establishedin zero crossflow; 2) only for more two-dimensional type roughness (roughnesswidth to height ratios equal to or greaterthan approximately 24) will highcrossflow decrease the permissible height

of roughness; and 3) for values of rough-ness width to height ratios equal to orgreater than approximately 30, develop-ment of a different criterion for permis-sible roughness height is required. Infor-mation of this kind is crucial for theestablishment of the manufacturingtolerances and maintenance requirementsthat must be met for surface smoothness.

Figure 14. Com-parison of swept-wing surfaceroughness datawith unswept-wingvon Doenhoff-Braslow datacorrelation.

76 From the second and third sources cited in footnote 73.

77 For any reader interested in a brief summary of the basic two- and three-dimensional roughness effects on laminarflow without crossflow, the discussion on pages 2-4 of the second citation in footnote 73 is recommended.

78 From Figure 7 of the third source cited in footnote 73.

79 See von Doenhoff and Braslow, “The Effects of Distributed Surface Roughness on Laminar Flow,” pp. 657-681,cited in footnote 27.

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Atmospheric Ice Particles: Boeing 747sand JetStar

As indicated in the section on thepost-World War II to mid-1960s period,the practical significance of atmosphericice particles on the amount of timelaminar flow might be lost on operationalaircraft was not known because of a lackof information on particle concentrations.Unanalyzed cloud-encounter and particle-concentration data became available fromthe NASA Lewis Research Center (LeRC)Global Atmospheric Sampling Program(GASP) in the late 1970s. From March1975 to June 1979, NASA obtained datawith instruments placed aboard four 747airliners on more than 3,000 routinecommercial flights that involved about88,000 cloud-encounters.80

With the GASP data, Richard E.Davis of the LaRC estimated averagecloud-cover statistics for several long-

distance airline routes. He then madeconservative estimates of the probableloss of laminar flow on these major airlineroutes by assuming that all cloud encoun-ters cause total loss of laminar flow, i.e.,that the percentage loss of laminar flowon a given flight is equal to the percentageof time spent within clouds on that flight.For further conservatism, he assumed thatpilots would make no attempt to avoidflight through clouds. Figure 15 is anexample of the potential laminar-flow losson some of the major airline routes—LosAngeles-Tokyo, New York-London, andNew York-Los Angeles. The figure alsoincludes a world average. These resultsnow make it apparent that cloud encoun-ters during cruise of long-range commer-cial air transports are not frequent enoughto invalidate the large performanceimprovements attainable through applica-tion of LFC.

Figure 15. Potentiallaminar-flow losson some majorairline routes.

80 William H. Jasperson, Gregory D. Nastrom, Richard E. Davis, and James D. Holdeman, GASP Cloud- and Particle-Encounter Statistics, and Their Application to LFC Aircraft Studies, Vol. I: Analysis and Conclusions, and Vol. II:Appendixes (Washington, DC: NASA Technical Memorandum 85835, October 1984).

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In addition, during the JetStar LEFTprogram, the Dryden flight-test teammeasured the size and concentration ofatmospheric particles encountered at thesame time they measured the degree oflaminar-flow degradation. With theseLEFT measurements, Davis at LaRCprovided some validation of the Halltheory of laminar-flow loss as a functionof atmospheric particle size and concen-tration.81

Hybrid Laminar-Flow Control(HLFC): Boeing 757

The hybrid laminar-flow controlconcept integrates active laminar-flowcontrol with suction (LFC) and natural

laminar flow (NLF) and avoids theobjectionable characteristics of each. Theleading-edge sweep limitation of NLF isovercome through application of suctionin the leading-edge box to controlcrossflow and attachment-line instabilitiescharacteristic of swept wings. Wingshaping for favorable pressure gradientsto suppress Tollmien-Schlichting instabili-ties and thus allow NLF over the wingbox region (the region between the twowing structural spars) removes the needfor inspar LFC suction and greatlyreduces the system complexity and cost.82

HLFC offers the possibility of achievingextensive laminar flow on commercial ormilitary transport aircraft with a system

Figure 16. Possi-bilities of laminarflow on sweptwings.

81 See Hall, “On the Mechanics of Transition,” cited in footnote 36.

82 Examples of additional complexities associated with suction over the wing box include: manufacture of a structuralbox of sufficient strength and light weight with slots or perforations over a much more extensive area of the wing skin;extensive internal suction ducting that decreases the internal wing volume available for storage of airplane fuel; largersuction pump(s) than otherwise needed; an increased difficulty in providing the required surface smoothness and fairnessfor maintenance of laminar flow over inspection panels in slotted or perforated surfaces when laminarization of bothupper and lower surfaces is desired; and a need to avoid hazards due to possible leakage of fuel into the suction ducting.These complexities, along with other special features, increase airplane weight and manufacturing costs as well asmaintenance costs.

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no more complex than that already provenin the LEFT program on the NASAJetStar.

The relative place of HLFC, LFC andNLF in the wing-sweep-to-aircraft-sizespectrum is indicated in Figure 16. On agrid of chord Reynolds number vs.quarter-chord sweep are plotted variousitems. The shaded area indicates theapproximate chordwise extent of naturallaminar flow attainable on a wing withinitially decreasing surface pressures inthe direction towards the trailing edge(upper left plot). Ranges of wing chordReynolds number in cruise for fourcommercial transport airplanes aresuperimposed—for the Douglas DC-10,Lockheed L-1011, Boeing-757 andDouglas DC-9-80 airplanes. For eachairplane, the wing chord Reynoldsnumber decreases along the span fromroot to tip because of a taper in the wingplanform.

The figure indicates that naturallaminar flow can be attained only on

regions of the wings near the wing tips.As the wing-section chord increases(increased Rc) due to either a locationnearer the wing root or an increase inairplane size, the chordwise extent oflaminar flow decreases (due to increasedTollmien-Schlichting instabilities). Also,less laminar flow is attainable as the wingsweep increases (due to increasedcrossflow instabilities). The use of wallsuction, however, permits the mainte-nance of laminar flow to large chordwiseextents at both high sweep and large size(high Reynolds number), as indicated bythe X-21 data point, but at the expense ofcomplexities due to the extensive suctionsystem. A combination of principles foractive laminar-flow control and naturallaminar flow—hybrid laminar-flowcontrol (HLFC)—greatly increases thesize of high subsonic-speed airplanes forwhich large extents of laminar flow canbe obtained as compared with naturallaminar-flow airplanes. For example,compare the chord Reynolds number for

Figure 17. Improve-ment in lift-to-dragratio due to laminarconcepts.

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60-percent chord laminar flow withHLFC on the upper surface with the chordReynolds number for natural laminar flow(of a smaller relative extent) on CitationIII and Learjet airplanes, also plotted inFigure 16.

Figure 17 plots the percentageimprovement of lift-to-drag ratio (L/D)83

for each of the three laminar-flow con-cepts as compared with a turbulentairplane, plotted as a function of airplanewing area. The figure shows a largeimprovement in L/D for HLFC as com-pared with NLF. For the larger airplanes,of course, appreciably larger benefits areobtained with active laminar-flow controlwith suction to positions farther aft. As inthe case of LFC farther aft, the concept ofhybrid laminar-flow control requiressmoothness of surface finish and contouras well as protection from insect residueand ice accumulation in the leading-edgeregion. The systems developed in theLEFT program for the leading-edgeregion are equally applicable for thehybrid laminar-flow control application.

Under a participatory arrangementbetween the LaRC, the USAF, and theBoeing Commercial Airplane Company,Boeing flight tested the effectiveness ofhybrid laminar-flow control on a com-pany-owned 757 airplane in 1990. Figure18 shows an HLFC glove installed on alarge section of the left wing. The systemsin the leading-edge wing box are verysimilar to those flight tested on the JetStarairplane—a Krueger flap84 for insectprotection and high lift; a perforatedtitanium suction surface; and suction tothe front spar with an ability to reverseflow for purging. Rather than use ejectionof a freezing-point depressant, the designencompassed thermal anti-icing, i.e.,reversal of the airflow and expulsion ofheated air through the perforations in theleading-edge region. Boeing pilots flewthe airplane at transport cruise Machnumbers and altitudes.

The primary goal was to establish theaerodynamics of HLFC at Reynoldsnumbers associated with medium-sizetransport airplanes to reduce industry

Figure 18. 757subsonic hybridlaminar flow controlflight experiment.(NASA photo L-90-9549)

83 L/D is a significant measure of aerodynamic performance.

84 “Krueger” designates a specific type of leading-edge high-lift device (flap) that retracts into the wing lower surface.When used for an active laminar-flow control application, the flap also shields the wing from insect impacts duringtakeoff and landing and when retracted under the leading edge for cruise, does not interfere with the upper-surfacelaminar flow.

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risks to acceptable levels. Results werevery encouraging. Transition location wasmeasured several feet past the end ofsuction and with less suction than esti-mated. The Krueger leading-edge flapproved effective as the insect shield.Existing manufacturing technologypermitted construction of the leading-edgebox to laminar-flow surface-qualityrequirements. All necessary systemsrequired for practical HLFC were suc-cessfully installed into a commercialtransport wing.85

Research engineers at the LaRCcalculated the benefits of the applicationof hybrid laminar-flow control to a 300-passenger long-range twin-engine sub-sonic transport.86 With what appear to bereasonable assumptions of 50-percentchord laminar flow on the wing uppersurface and 50-percent chord laminar flowon both surfaces of the vertical andhorizontal tails, HLFC provides a 15-percent reduction in block fuel from thatof a turbulent transport.87 Application ofHLFC to the engine nacelles has thepotential of at least an additional 1-percent block-fuel reduction with laminarflow to 40 percent of the nacelle length.88

Supersonic Laminar-Flow Control: F-16XLIn the late 1980s, the Laminar-FlowControl Project Office of the LangleyResearch Center reactivated a long-

dormant consideration of LFC for com-mercial supersonic transports as part of aNASA technology-development programfor high-speed civil transports. As is thecase for subsonic flight, potential benefitsof the application of LFC to supersonictransports include increased range,improved fuel economy, and reducedairplane weight. Reduced fuel consump-tion will not only improve economics butwill also reduce a potential adverseimpact of engine emissions on the earth’sozone layer from flight of supersonicairplanes at higher altitudes than those forsubsonic flight. Additional benefits ofreduced airplane weight at supersonicspeeds are a decrease in the magnitude ofsonic booms89 and a reduction in commu-nity noise during takeoff.90 Also, the lowerskin friction of laminar boundary layers ascompared with turbulent boundary layersis of even more importance at supersonicspeeds than at subsonic speeds becausethe associated aerodynamic heating of thesurface by the skin friction is an importantdesign consideration at supersonicspeeds.91 The Boeing Commercial Air-plane Company and the Douglas AircraftCompany of the McDonnell DouglasCorporation,92 both under contract to theLaRC LFC Project Office, first studiedneeded aerodynamic modifications andassociated structural and systems require-ments to arrive at a realistic assessment ofthe net performance benefits of super-

85 A generally-available technical report on the HLFC flight tests has not been published.

86 Richard H. Petersen and Dal V. Maddalon, NASA Research on Viscous Drag Reduction (Washington, DC: NASA TM84518, August 1982).

87 Block fuel is the fuel burned from airport gate to airport gate, excluding fuel burned due to any delays.

88 ALB files, LHA, P.K. Bhutiani, Donald F. Keck, Daniel J. Lahti, and Mike J. Stringas, “Investigating the Merits of aHybrid Laminar Flow Nacelle, The Leading Edge” (General Electric Company, GE Aircraft Engines, Spring 1993).

89 The magnitude of a sonic boom is proportional to the airplane lift which is proportional to the airplane weight at agiven cruise speed. If sonic-boom overpressures are reduced below a value of one pound per square foot, overlandsupersonic cruise may become allowable.

90 Takeoff noise is reduced by a reduction in takeoff thrust requirements resulting from lower weight.

91 Reduced aerodynamic heating increases material options, enhances the potential for unused fuel as a heat sink forairplane environmental control systems, and decreases the detectability of military aircraft.

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sonic-LFC implementation. Althoughpromising conclusions were reached, thestudies indicated the need for additionalresearch and development specific to thesupersonic application. Recommendationswere made for supersonic flight research onHLFC.93

After additional analyses, wind-tunneltesting and exploratory flight research at theDFRC on two prototype F-16XL airplanesdenoted as ship 1 and ship 2, DFRC alsoflight researched a laser-perforated titaniumglove installed on the left wing of ship 2(Figure 19).94 Under LFC Project Officemanagement, the Rockwell Corporation andthe Boeing Company manufactured andinstalled the glove and the Boeing andDouglas Companies supported DFRC with

the flight research and analysis. Specificobjectives were to determine the capabilityof active LFC to obtain a large chordwiseextent of laminar flow on a highly-sweptwing at supersonic speeds and to providevalidated computational codes, designmethodology, and initial suction-systemdesign criteria for application to supersonictransport aircraft. To make accurate mea-surements, the investigators installed anextensive array of hot-film, pressure, andtemperature instrumentation and providedreal-time displays of the measurements.They completed thirty-eight flights withactive boundary-layer suction and experi-enced very few problems with the suctionsystem.95 The laminar-flow data arecurrently restricted in distribution.

Figure 19. Two-seat F-16XLSupersonic Lami-nar-Flow-Controlflight researchaircraft with asuction gloveinstalled on the leftwing. (NASA photoEC96-43831-5 byJim Ross).

92 Now part of Boeing.

93 A.G. Powell, S. Agrawal, and T.R. Lacey, Feasibility and Benefits of Laminar Flow Control on Supersonic CruiseAirplanes (Washington, DC: NASA Contractor Report 181817, July 1989); Boeing Commercial Airplane Company,Application of Laminar Flow Control to Supersonic Transport Configurations (Washington, DC: NASA ContractorReport 181917, July 1990).

94 NASA flight-test participants were: Dana Purifoy and Mark P. Stucky, research pilots; Marta R. Bohn-Meyer andCarol A. Reukauf, project managers; Michael P. Harlow, aircraft crew chief; Lisa J. Bjarke, DFRC principal investigator;and Michael C. Fischer, LaRC principal investigator.

95 See document number 6 at the end of this monograph for the flight log of the F-16XL number 2.

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Status of Laminar-Flow ControlTechnology in the Mid-1990s

The status of laminar-flow control tech-nology in the mid-1990s may be summa-rized as follows:• Design methodology and related

enabling technologies are far advancedbeyond the X-21 levels.

• Improved manufacturing capabilitiesnow permit the general aviation indus-try to incorporate natural laminar flowin some of its aircraft designs for chordlength Reynolds numbers less than 20million, but active laminar-flowcontrol, required for larger aircraft and/or aircraft with highly-swept wings, hasnot yet been applied to any operationalaircraft.

• Although some additional structuraland aerodynamic developments arerequired, the recent programs havebrought the promise of laminar flow formoderately large and very large sub-sonic transport aircraft much closer tofruition than ever before.

• Hybrid laminar-flow control simplifiesstructure and systems and offerspotential for 10- to 20-percent improve-ment in fuel consumption for moderate-size subsonic aircraft.

• Hybrid LFC may be the first applica-tion of suction-type laminar-flowcontrol technology to large high-subsonic-speed transports because of itsless risky nature.

• Although much of what has beenlearned about subsonic laminar-flowcontrol is applicable to supersonicspeeds, considerable additional work isrequired before supersonic laminar-flow control can be applied to opera-tional aircraft.

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GlossaryAACB

ACEE

AGARD

AIAA

Attachment line

Chord

DAG

DFRC

DOD

DOT

FAA

GASP

Glove

Hall

HLFC

Krueger flap

LaRC

LEFT

Length Reynolds number

Aeronautics and Astronautics Coordinating Board

Aircraft Energy Efficiency Program

Advisory Group for Aeronautical Research & Development, NorthAtlantic Treaty Organization

American Institute of Aeronautics and Astronautics

On a sweptback wing, the line at which the airflow divides to theupper and lower surfaces

The length of the surface from the leading to the trailing edge of anairfoil

Division Advisory Group

Dryden Flight Research Center

Department of Defense

Department of Transportation

Federal Aviation Administration

Global Atmospheric Sampling Program

A special section of an airplane’s lifting surface, usually overlayingthe basic wing structure, that is designed specifically for researchpurposes

Originator of a theory that indicates when laminar flow would be lostas a function of atmospheric particle size and concentration

Hybrid Laminar-Flow Control

A specific type of leading-edge high-lift device (flap) that retractsinto the wing lower surface. When used for an active laminar-flowcontrol application, the flap also shields the wing from insect impactsduring takeoff and landing and when retracted under the leading edgefor cruise, does not interfere with the upper-surface laminar flow.

Langley Research Center

Leading-Edge Flight Test

When the representative length in the formulation of the Reynoldsnumber is chosen as the distance from the body’s leading edge to theend of laminar flow, the resultant length Reynolds number can beused as a measure of the length of laminar flow attained.

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LeRC

LFC

LTPT

NACA

NASA

NLF

OAST

RAE

Reynolds Number

Three-dimensional type surfacedisturbances

Two-dimensional type disturbances

TACT

Tollmien-Schlichting instabilities

USAF

WADD

Lewis Research Center (now Glenn Research Center)

Laminar-flow control

[Langley] Low-Turbulence Pressure Tunnel

National Advisory Committee for Aeronautics

National Aeronautics and Space Administration

Natural laminar flow

Office of Aeronautics and Space Technology of NASA

Royal Aircraft Establishment

A non-dimensional value equal to the product of the velocityof a body passing through a fluid, the density of the fluid, anda representative length divided by the fluid viscosity.

Three-dimensional type surface disturbances are those withwidth or diameter to height ratios near a value of one.

Examples of two-dimensional type disturbances are streamturbulence, noise, and surface irregularities having large ratiosof width (perpendicular to the stream flow direction) to height,like spanwise surface steps due to mismatches in structuralpanels.

Transonic Aircraft Technology

Very small two-dimensional type disturbances that may inducetransition to turbulent flow—named after German aerody-namicists Walter Tollmien and Hermann Schlichting.

United States Air Force

Wright Air Development Division

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Documents

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Document 1—Aeronautics Panel, AACB, R&D Review, Report of theSubpanel on Aeronautical Energy Conservation/Fuels.

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Document 2—Report of Review Group on X-21A Laminar Flow ControlProgram

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Document 3—Langley Research Center Announcement: Establishment ofLaminar Flow Control Working Group

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Document 4—Intercenter Agreement for Laminar Flow Control LeadingEdge Glove Flights, LaRC and DFRC

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Document 5—Flight Report, NLF-144, of AFTI/F-111 Aircraft with theTACT Wing Modified by a Natural Laminar Flow Glove

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Document 6—Flight Record, F-16XL Supersonic Laminar Flow ControlAircraft

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Abbott, Ira H., 4Adams, Earl, 23Antonatos, Philip P., 10Baer-Riedhart, Jenny, 17 nBaron, Robert S., 20, 23, 25 nBartlett, Dennis W., 17 nBjarke, Lisa J., 33 nBlizzard, Ralph G., 17 nBohn-Meyer, Marta R., 17 n, 33 nBoundary layer

laminar, 1, 2, 3-10, 12, 15, 16, 18, 20, 23, 26, 29- 30, 32, 33turbulent, 1, 6-7, 9-12, 20, 25, 31-32

Bower, Robert E., 15 nBraslow, Albert L., 5, 10-11, 13-14, 15 n, 20, 26 nBurrows, Dale L., 5Carlson, Gary, 23Caw, Lawrence J., 17Chiles, Harry, 21 nCollier, F.S., Jr., 26 nCortright, Edgar M., 14Davis, Richard E., 28-29DiAiutolo, Charles T., 15 nDisturbance inputs

insects, 11, 16, 20-21, 25, 26, 31-32noise, 4 n, 8, 10, 13, 18-20, 32stream turbulence, 4, 9-12, 26surface irregularities, 6, 7, 9-10, 11, 20, 25, 26-27, 31weather, 10, 12, 23-25, 26, 28-29, 31

Disturbance typesattachment line, 11, 29crossflow, 2, 8, 11, 17, 18-20, 22, 26,-27, 30three-dimensional, 6, 10, 26-27two-dimensional, 6, 7, 19-20, 26-27

Donlan, Charles J., 11 nDryden, Hugh L., 4Eaffy, A., 14Enevoldson, Einar K., 17 nFischer, Michael C., 23, 25 n, 33 nFisher, David F., 20, 23, 25 nFlight-test and flight-research aircraft

747, 28-29757, 18-19, 29-32Anson, 6AW52, 8B-18, 3-4Citation III, 29, 31F-111, 16-18, 66-70F-14, 17-18

F-16XL, 32-33, 71-75F-94, 8-9Hurricane, 4, 10JetStar, 20-29King Cobra, 4, 10Lancaster, 11-12Learjet, 29, 31Vampire, 6-7Victor, 20, 21WB-66D, 10X-21, 11-12, 16, 29, 34, 46-60

Fullerton, C. Gordon, 17 nFulton, Fitzhugh L., 23, 25 nGoldsmith, John, 6Hall, G.R., 12, 29Harlow, Michael P., 33 nIndustry

Boeing, 18-19Douglas, 22, 32-33Handley Page, 7, 20Lockheed-Georgia, 21-22, 23 nNorthrop,5-6, 8, 10-12Rockwell, 33

Instrumentationhot-film, 18-19, 33Knollenberg probe, 25

Jacobs, Eastman N., 4, 5 nJohnson, J. Blair, 23Kayten, Gerald, 13Klebanoff, P.S., 4Laminar-flow control

active (LFC), 1-2, 5-6, 7, 8-9, 10-12, 3, 15-16, 22- 29, 32-34, 46-60, 61-65, 71-75combined active & passive, 2, 18, 29-32, 34passive, , 1, 4, 8, 16-17, 19, 29-32, 66-70

Leonard, Robert W., 14 nLinn, Kenneth, 20Mach number, 9, 11, 19, 32Maddalon, Dal V. , 26 nMallick, Donald L., 20, 23, 25 nMcCarty, Bill, 17 nMcMurtry, Thomas C., 20Mersereau, William D., 20Meyer, Robert R., Jr., 17 nMontoya, M.C., 25 nMuraca, Ralph J., 14 n, 22 nNagel, Adelbert L., 15 nNatural Laminar Flow (NLF), 1, 4, 8, 16-17, 19, 29-32, 66-70Nice, Edward, 23Organizations

Aeronautics and Astronautics Coordinating Board, 14

Index

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American Institute of Aeronautics and Astronautics, 13Department of Defense, 14Department of Transportation, 14Federal Aviation Administration, 10, 14National Advisory Committee for Aeronautics,1, 3, 10, 15National Aeronautics and Space Administration, 1, 10-11, 14, 28Ames Research Center, 18Dryden Flight Research Center, 8, 17, 20-29, 33Langley Research Center, 3, 4, 10, 13, 15, 20-21, 31, 32-33Lewis Research Center, 28Office of Aeronautics and Space Technology, 14National Bureau of Standards, 4Royal Aircraft Establishment (Great Britain), 6, 8U.S. Air Force, 5-6, 8, 10-12, 31Wright Air Development Division, 10

Perforation methodelectron beam, 24-25laser, 25

Peterson, John B. Jr., 20Pfenninger, Werner, 5, 8, 22Prandtl, Ludwig, 1Programs

Aircraft Energy Efficiency (ACEE), 14-16, 21-22, 26Advanced Technology Transport, 13 nGlobal Atmospheric Sampling Program, 28Leading Edge Flight Test, 21-26, 29-31Simulated airline flights, 21Supersonic Laminar Flow, 32-33, 71-75

Pumping system, 8Purifoy, Dana, 33 nReukauf, Carol A., 33 nReynolds number, 2, 3-4, 5, 6, 7-10, 18, 27, 30-31, 34Reynolds, Osborne, 2Schlichting, Hermann, 4, 29, 30Schneider, Edward T., 17 nSchubauer, G.B., 4Skramstad, H.K., 4Stack, John P., 23Steers, Louis L., 17 nStucky, Mark P., 33 nSuction type

continuous, 5-6, 8, 10, 24-26, 32-33slotted, 5-6, 8-9, 22-23, 24-26strips, 7

Swann, Michael R., 17 nTetervin, Neal, 5Tidstrom, K.P., 4Toll, Thomas A., 13 n

Tollmien, Walter, 4, 29, 30Trujillo, Bianca M., 17 nVendolski, Walter, 20Visconti, Fioravante, 4, 5von Doenhoff, Albert E., 4Wagner, Richard D., 23, 26 nWilson, James A., 20, 23Wind tunnels

Ames 12-Foot Tunnel, 18Langley Low-Turbulence Pressure Tunnel, 4, 18

Wright, Howard T., 14 nYoung, Ronald, 23, 26 n

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About the Author

Albert L. Braslow graduated from the Guggenheim School of Aeronautics, New York University, in 1942(with a Bachelor of Aeronautical Engineering degree) and continued with post-graduate studies at the Univer-sity of Virginia, UCLA, George Washington University. and Langley Research Center. During over 50 yearsof research in fundamental and applied aircraft and space-vehicle aerodynamics at and for the LangleyResearch Center of the National Aeronautics and Space Administration (previously the National AdvisoryCommittee for Aeronautics), he has published over 60 technical reports, book chapters, and encyclopediasections. Included in his research were activities at subsonic, transonic, supersonic, and hypersonic speeds inthe areas of wing design, boundary layers, stability and control, aircraft performance, propulsion aerodynam-ics, aerodynamic loads and heating, wind-tunnel techniques, advanced technology evaluation and integration,and development and documentation of space-vehicle design criteria for structures. As an internationally-known pioneering researcher on laminar-flow control and transition from laminar to turbulent flow, he haslectured on these disciplines for the American Institute of Aeronautics and Astronautics, the [North AtlanticTreaty Organization’s] Advisory Group for Aeronautical Research and Development, the University of Texas(Austin), George Washington University, University of Tennessee Space Institute, University of Virginia,University of California (Davis), the Continuing Education Program of the University of Kansas for eightyears, and the Lewis and Ames Research Centers of NASA.

Monographs in Aerospace History

This is the thirteenth publication in a new series of special studies prepared under the auspices of the NASAHistory Program. The Monographs in Aerospace History series is designed to provide a wide variety ofinvestigations relative to the history of aeronautics and space. These publications are intended to be tightlyfocused in terms of subject, relatively short in length, and reproduced in inexpensive format to allow timelyand broad dissemination to researchers in aerospace history. Suggestions for additional publications in theMonographs in Aerospace History series are welcome and should be sent to Roger D. Launius, ChiefHistorian, Code ZH, National Aeronautics and Space Administration, Washington, DC, 20546. Previouspublications in this series are:

Launius, Roger D. and Gillette, Aaron K. Compilers. Toward a History of the Space Shuttle: An AnnotatedBibliography. (Monographs in Aerospace History, Number 1, 1992)

Launius, Roger D. and Hunley, J. D. Compilers. An Annotated Bibliography of the Apollo Program. (Mono-graphs in Aerospace History, Number 2, 1994)

Launius, Roger D. Apollo: A Retrospective Analysis. (Monographs in Aerospace History, Number 3, 1994)

Hansen, James R. Enchanted Rendezvous: John C. Houbolt and the Genesis of the Lunar-Orbit RendezvousConcept. (Monographs in Aerospace History, Number 4, 1995)

Gorn, Michael H. Hugh L. Dryden’s Career in Aviation and Space. (Monographs in Aerospace History, No. 5,1996).

Powers, Sheryll Goecke. Women in Aeronautical Engineering at the Dryden Flight Research Center, 1946-1994.(Monographs in Aerospace History, No. 6, 1997).

Portree, David S.F. and Trevino, Robert C. Compilers. Walking to Olympus: A Chronology of ExtravehicularActivity (EVA). (Monographs in Aerospace History, No. 7, 1997).

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Logsdon, John M. Moderator. The Legislative Origins of the NationalAeronautics and Space Act of 1958: Proceedings of an Oral History Workshop.(Monographs in Aerospace History, No. 8, 1998).

Rumerman, Judy A. Compiler. U.S. Human Spaceflights: A Record ofAchievement, 1961-1998. (Monographs in Aerospace History, No. 9, 1998).

Portree, David S.F. NASA’s Origins and the Dawn of the Space Age.(Monographs in Aerospace History, No. 10, 1998).

Logsdon, John M. Together in Orbit: The Origins of InternationalCooperation in the Space Station Program. (Monographs in Aerospace History,No. 11, 1998).

Phillips, W. Hewitt. Journey in Aeronautical Research: A Career at NASALangley Research Center. (Monographs in Aerospace History, No. 12, 1998).