aeronautics encyclopedia

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Aircraft AvionicsRobert G. LoewyGeorgia Institute of Technology

I. Definitions of Avionics Components (Glossary)II. Aircraft Avionics Systems, GeneralIII. Traditional Avionics, MEPIV. Avionics Applications Influencing Aircraft

Design; VMSV. Impact Of Smart Materials

VI. Summary

I. DEFINITIONS OF AVIONICSCOMPONENTS (GLOSSARY)

Actuator An element of a control system that will moveanother element, by providing a force, pressure or mo-ment (force acting through a lever arm) in response toa command signal.

Effector A control system element that will provide thedesired change in an aircrafts behavior; e.g., aerody-namic control surface such as a rudder, to changeheading, or a speed brake to reduce flight speed.

Linkage A control system component that carries use-ful signals, forces or moments from one location toanother location. These useful signals can be analogelectromagnetic or optical or digital, i.e., quantitative,and such transport can be within the aircraft or fromand to points external to the aircraft. Only when forcesand moments are transmitted are linkages mechanical.When digital signals are transmitted the linkages areoften called data buses. Data buses are the conduitsthrough which outputs are sent or inputs are receivedby a digital system or subsystem in order to performits function.

Power Source Most avionics system components requirepower sources independent of pilot/crew; i.e., avionicssystems are active systems. Power sources may beelectrical (e.g., batteries, generators, fuel-cells) or me-chanical (e.g., hydraulic pumps and reservoirs, pneu-matics, etc.)

Processor A system component which may analyze (i.e.,extract useful information from), combine or store sig-nals or may model aircraft behavior for comparativepurposes. Such operations may be analog or digital;when the latter, processors have much in common withcomputers, but usually having special, i.e., more lim-ited functions, rather than being general-purpose.

Sensors A device that responds to some physical quan-tity such as pressure or temperature (or conceivably achemical quantity such as acidity) by converting it to auseful signal.

Software The capability of digital processors and thecomplexity of their functions, defined above, are suchthat the (usually) specialized codes that command theiroperations are considered a separate avionics com-ponent. In written form such computer or processorcodes may require tens of thousands or millions of

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320 Aircraft Avionics

lines of instructions and their development may in-volve equal or greater expense than the hardwareelements of avionics systems whose components aredefined elsewhere in this glossary.

Transducer A device that takes a useful signal in oneform, say electrical, and converts it to another usefulform, perhaps optical. (Note that sensors and actu-ators are, in a more general sense transducers, butcommon usage restricts the meaning of the term as de-fined here.)

Transponder A component which, on receiving an Elec-troMagnetic (EM) signal, often coded, will respond bysending a similar signal, usually after a known, con-trolled delay time.

II. AIRCRAFT AVIONICSSYSTEMS, GENERAL

The term avionics results from combining aviationwith electronics, in recognition of the growing use andimportance of the application of devices making use ofelectronics in aircraft design, development and operation.Aircraft avionics systems, however, make use of compo-nents which may not all be electronic, and an understand-ing of their functions usually requires consideration ofthe whole system. Figure 1 illustrates a hypothetical sys-tem for control of an aircraft about its pitch axis (i.e.,pointing the nose of the aircraft up or down), whichwould boost the pilots force output in moving an aero-dynamic control surface by a variable and appropriateamount, depending on the aircrafts flight speed. In thiscase the pilots longitudinal sidearm controller motionis converted into an electrical signal by a motion1 sensor(Loewy, 2000). That electrical signal is converted to anoptical signal by an electro-optical transducer. Fiber opticlinkages carry the optical signal to a processor. After be-ing transduced back into an electric signal, it is amplifiedor attenuated there according to a second signal originat-ing from an airspeed sensor ( this may be simple gainchanges), so that the aircrafts pitch response will be thesame at all airspeeds (assuming this is a desirable char-acteristic). The signal from the processor then regulates avalve on a hydraulic actuator, which drives the aircraftselevator, i.e., pitch attitude control surface. Several com-ments may be pertinent for this illustrative example. Theelectromechanical input valve on the hydraulic actuatormight be considered a transducer, but for our purposesit is viewed as part of the actuator. Such an assembly isoften called an integrated servoactuator. Fiber optic link-

1 For illustrative purposes: side-arm controllers usually have force,rather than motion sensors.

ages (fly-by-optics, FBO) are used less often than elec-trical linkages (fly by wire, FBW) and in FBW systemsthere is no need for electro optical transducers. Wherefiber optics linkages are used it is usually because of theirsuperior capabilities for carrying large quantities of infor-mation (high bandwidth) and insensitivity to ElectroMag-netic Interference (EMI), including that associated withlightning.

Once aircraft were recognized as vehicles with realiz-able potential for transportation, the need for a number ofkinds of electronics-based equipment became apparent,based on the importance of increasing aircraft utility andsafety. These functions, roughly in the chronological or-der in which the related avionics equipments were firstadapted for use on aircraft in service use, are as follows:

1. Communicationa. Ground to airb. Air to groundc. Air to air

2. All weather, blind flying3. Navigation4. Limited visibility landing5. Bad weather avoidance6. Flight path stability augmentation7. Improved flight handling qualities8. Flight data recording9. Collision avoidance

10. Formation flying (military)11. Target acquisition (military)12. Secure identification (military)13. Crew/passengers comfort improvement14. Structural load alleviation15. Terrain avoidance16. Noise reduction

a. Internalb. External

17. Suppressing servo-aeroelastic instabilities18. Performance improvement

It will be noted that this list is long, some items involvefurther breakdown (Items 1 and 16), and since such adapta-tions continue apace, any attempt at completeness is likelyto soon be thwarted by new developments. For example,in-flight entertainment systems for commercial airlinersare not listed, but they could be considered avionics sys-tems. Their use is already commonplace and the servicesthey provide are growing by leaps and bounds.

As implied by the order of functions in the list,the application of electronic devices to aircraft can bethought of as beginning with radios for communications(Items 1(a) and (b)), between aircraft crew memberspilots, copilots,navigators, flight engineers, etc.and


Aircraft Avionics 321

FIGURE 1 Schematic of avionics components in a Fly-By-Optics (FBO) flight control system for the pitch axis. (FromLoewy, R. G. (2000). Avionics: A New senior partner in aeronautics. AIAA J. 37(11), 13371354.)

ground crew members; among dispersed crew membersof large aircraftalthough this is more likely to use tele-phone rather than wireless technologye.g., from thecockpit of military aircraft such as bombers, on the onehand, to the tail gunner via an intercom, for example, onthe other; and (Item 1(c)) between flight crews of differentaircraft.

Later, what can be considered radio technology, i.e.,transmitters and receivers of wireless EM signals, was ap-plied to navigation (i.e., helping the pilot know where togo) and landing aides (i.e., helping the pilot to land safely,particularly under reduced visibility conditions). In mili-tary applications of aircraft, such assistance by avionicsfor the pilot and/or other crew members was extendedto acquiring and identifying targets; pointing, firing orlaunching weapons; counteringi.e., thwarting throughso-called electronic countermeasuressimilar systemsused by the enemy; and identifying himself/herself asfriendly to members of the same forces. The last is knownas IFF, for Identification, Friend or Foe. All such func-tions can be performed more or less automatically to suchan extent that, taken together, such systems are often re-ferred to as the pilots associate. These kinds of avionicssystems are also referred to in the military as the Mis-sion Equipment Package (MEP). It is useful to think ofall avionics systems which assist pilots and crew in per-forming their mission, even if it is a civilian transportmoving passengers or cargo from one place to another,as MEP. Emphasizing the added-on nature of MEP, theaircraft in which it is installed is often referred to as thehost vehicle.

With the advent of devices (so-called control actua-tors) capable of moving aircraft control effectors (e.g.,aerodynamic surfaces) reliably and as quickly or morequickly than a human pilot, avionics systems could beused, not only to help the pilot and crew perform theirmissions but also to fly the aircraft safely. These automaticsystems are often referred to as Vehicle Management Sys-tems (VMS). To emphasize the highly integrated natureof VMS into the aircraft for which they are part of thecontrol systemon an equal, flight-safety footing alongwith airframe structure, aerodynamic shape and propul-sion systemsit is useful to think of VMS avionics aspart of the host vehicle. As might be expected, then,VMS avionics are not added on but are usually con-sidered during the design or developmental stages of theintroduction into service of a new or substantially modi-fied flight vehicle.

In a gray area between MEP and VMS are what, attheir introduction as early as 1917, were called auto-matic pilots or autopilots. These systems began by us-ing roll attitude sensors, heading sensors (e.g., magneticcompass), altitude sensors (e.g., barometric altimeters),airspeed sensors (e.g., pitot-static pressure tubes), etc toautomatically adjust (i.e., hold constant) wings in a levelposition, aircraft direction, flight altitude and speed, re-spectively, by sending appropriate corrective commands,for example, to rudder and ailerons (yaw and bank ef-fectors) and longitudinal control stick and engine throttle(speed and climb/rate of descent effectors). Such avionicssystems were initially thought of as relieving pilot fatigueon long flights (particularly in an era of low cruising



speeds). With increasing avionics component capabili-ties, as discussed in Section IV, autopilots have devel-oped into the much more sophisticated, Automatic FlightControl Systems (AFCS). Perhaps the most important ofthe enabling new capabilities is the greater responsivenessto commands on the part of actuators, i.e., their higherbandwidth or high frequency capabilities. Among theAFCS functions making use of these newer capabilitiesare included Items 4, 6, 7, and 13 through 18. The lastof these could include such measures as automatic pump-ing of fuel from one tank to another to keep the aircraftscenter of gravity in its most favorable position on longflights.

Although avionics systems in the MEP category can, inmany instances be added on well after the fundamentalaircraft design is completed, those responsible for their in-tegration into the host aircraft still must provide (a) spacewithin the airframe, (b) stress-free mounting points whichlimit the shock and vibration transmitted to this equip-ment, and (c) an environment of limited maximum temper-atures and EMI and acoustic fields. Such must, of course,also be provided for VMS avionics. Further, when trans-mitting/receiving antennae (the internal/external linkages)are involved, their locations should minimize interferencewith the signals to be sent or received and cross talkto/from other EM sources. This is particularly challeng-ing for aircraft designed to have low radar cross sections,so-called stealth configurations.

Another way to categorize avionics systems in broadterms is to note that there are those for which the con-sequences of failures are such that a pilot can correct orcompensate for them with reasonable effort, or those forwhich it is not reasonable to expect a pilot to do so. For ourpurposes, in this consideration, it does not matter whetherthe failure is of the type in which the system simply stopsworking or if it causes the system to drive to a full authorityposition unbiddena so-called hard-over failure.

Systems for communication, navigation, or bad weatheravoidance may well be important for safety of flight andhence be duplicated or provided in multiple installationsof higher redundancy; but the consequences of their fail-ure can reasonably be expected to be compensated forby a pilot if means exist to identify their improper oper-ation. As a consequence, the duplicated systems do notusually have to operate simultaneously, but can be left ina stand-by mode until needed. Other systems, havingto have frequency response characteristics well beyondwhat a pilot can do simply to operate effectively, includethose for maintaining proper aircraft attitude in all weatherflying, performing limited visibility landings, augmentingflight path stability, and suppressing aeroelastic instabili-ties. The consequences of such systems failing, therefore,will unfold much too quickly for a pilot to respond effec-

tively. The architectures of such systems should then besuch as to include at least triple redundancy and a con-tinuous comparison of performance among them in realtime, so that a failure can be identified, in what is oftencalled voting, and an automatic shut off of the systemwhich has failedor has even been subject to degradedperformancewill take place.

III. TRADITIONAL AVIONICS, MEP

A. Communication and NavigationSystems in General

Radio technology is based, fundamentally, on the fact thatan alternating electrical current (ac) in a wire will radi-ate EM energy into space. If the relationship between thelength of the wire and the frequency of the ac, f , is suchthat the wire length is half a wavelength, , almost allthe power not turned into heat in the wire will be radi-ated. This behavior of half wavelength wires is the basisof EM transmitting antenna design. A half wavelengthwire which intercepts the EM radiation will also convertits energy into ac current most efficiently and is the basisfor receiving antenna design. Note that the relationshipbetween frequency and wavelength is given by

f (inHz) = c

(in distance per second),

where c is the velocity of propagation of EM radiation,which is that of light in a vacuum (about 300 106 m/s).

Sending and receiving antennae can be based (1) on theground (terrestrial), (2) in aircraft or (3) in spacecraft. Ingeneral, the larger the antenna in terms of wavelengths ofthe radiation transmitted, the narrower will be the patternof radiation. This can lead to some large airborne antennae(see Fig. 2, for example). Some antennae are designed tobe omni-directional or nondirectional, i.e., they transmitEM radiation in a spherical pattern. In such a case the ratioof received to transmitted energy is equal to

Receiver Antenna Area4R2

,

where R is the distance, or range, between transmittingand receiving antennas.

Although specific portions of the frequency spectrum(i.e., all the values of f to be used) must be allocated toprevent different systems from interfering with each other,for many years and by general agreement, radio transmis-sion frequencies have been designated in the followingbands (Skolnik, 1962) (Table I).

There is a marked tendency to use higher and higherfrequencies, and some of the categories in Table II havealso been widely used for about 50 years, but with some



FIGURE 2 Early warning E-2C aircraft. (From Skolnik, M. I. (1962). Introduction to Radar Systems, McGraw Hill, New York.)

different applications overseas (Reference Data for Engi-neers (1985) (Table II).

Many factors affect the transmission of EM radiation.Some are a function of radiation frequency, others a func-

TABLE I Designated Frequency Bands for EM Radiation

Name Abbreviation Frequency Wavelength

Very low VLF 3 to 30 kHz 100 to 10 kmfrequency

Low frequency LF 30 to 200 kHz 10 to 1 kmMedium MF 300 to 3000 kHz 1 km to 100 m

frequencyHigh frequency HF 3 to 30 MHz 100 to 10 mVery high VHF 30 to 300 MHz 10 to 1 m

frequencyUltrahigh UHF 300 to 3000 MHz 1 m to 10 cm

frequencySuperhigh SHF 3 to 30 GHz 10 to 1 cm

frequencyExtremely high EHF 30 to 300 GHz 10 to 1 mm

frequency

tion of the electrical characteristics of the earth (whichinfluence ground waves, those propagating along theEarths surface), atmospheric noise (such as caused bylightning), ionospheric properties (which influence skywaves, those reflected by characteristics of the Earthsatmosphere). The influence of the Earth is, as might beexpected, important for transmissions from ground sta-tions, and is sometimes referred to as causing site sen-sitivity. Very high frequencies (i.e., above 30 MHz) aremostly line-of-sight waves, and above 3 GHz, atmosphericand precipitation scattering and absorption becomesignificant.

B. Terrestrial Based Navigation SystemsThe term avionics usually implies equipment carriedand/or functions carried out aboard aircraft. To understandsome of their complexities, however, it is useful to knowsomething of the ground-based systems with which the air-borne avionics components interact. There are, in general,two kinds of ground-based navigation systems; so-called



TABLE II Letter Designation of High-Frequency EMRadiation

Letter Frequency Letter Frequencydesignation range designation range

L 0.39 to 1.55 GHz Xb 6.25 to 6.90 GHzLs 0.90 to 0.95 GHz K a 10.90 to 36.00 GHzS 1.55 to 5.20 GHz Ku 15.35 to 17.25 GHzC 3.90 to 6.20 GHz Ka 33.00 to 36.00 GHzX 5.20 to 10.90 GHz Q 36.00 to 46.00 GHz

a Includes Ke band, which is centered at 13.3 GHz.

point sources and those that establish an EM radiationgrid in space. Among point source ground systems are om-nidirectional (nondirectional) beacons, which allow air-borne direction finders to establish a heading direction orbearing to the known position of the beacon. Directionfinders consist essentially of a rectangular loop antennawired so as to send the difference in signals in the op-posite, vertical sides of the loop to a receiver. This dif-ference is zero when the sides of the loop are the samedistance from the beacon, at which point the plane of theloop is perpendicular to the line joining the aircraft andthe beacon. The antenna loop is, therefore, rotated aboutan axis parallel to and equidistant from the two verticalsides sensing the beacons signals, and its orientation mustbe noted when the receiver indicates a null reading. Tominimize the aerodynamic drag on an aircraft in which adirection-finding antenna is to be mounted, two fixed an-tenna loops can be mounted so that their planes are at 90to each other and the phase of their signals are comparedelectrically from one to the other to achieve the same effectas mechanical rotation. Direction finding systems can, al-ternatively, have the beacon placed in the vehicle and therotating loop antenna and receiver at the ground station.

The nearly constant speed of EM radiation has led to itsuse to measure distance. Although other means of radioranging (i.e., means to measure distance) exist, perhapsthe simplest in concept is known as DME, for DistanceMeasuring Equipment. This system is internationally stan-dardized. Its operation is depicted in Fig. 3 (from Kaytonand Fried, 1997). A transmitter-receiver on board the air-craft, known as an interrogator, sends a pair of very shortEM pulses (3.5 s long and 12 s apart), repeated from 5to 150 times per second. A transponder at a fixed, knownground station, on receiving these pulses, retransmits themafter a 50 s delay. The avionics component on the aircraftautomatically determines the difference between sendingand receiving times (very short compared to the period ofthe highest repetitive rate) subtracts the transponder de-lay and shows the distance from the ground station on acontrol panel display.

Bearing, as provided by direction finders, and distance,as provided by a DME, from the same ground station, al-lows the calculation of position relative to that ground sta-tion. In geometric terms, by establishing range and bear-ing, ground point sources allow an aircraft to place itselfon the space curve intersection of a sphere (from range in-formation) and a semi-infinite vertical plane which hasone edge at the fixed ground station (from bearing infor-mation). If barometrically determined altitude informationis added, the position of the aircraft will be known.

Another kind of ground-based point source is intendedto provide aircraft occasional, positive and absolute loca-tion information, often known as a fix. These EM radia-tion transmitters are known as marker beacons and theysend a narrow, fan-like pattern vertically at fixed pointsalong the nations airways, with the patterns maximumwidth aligned with the center-line of the airway on whichthey are located. Receivers in the aircraft provide the pilotwith information as to which beacon has been or is beingtraversed.

The VHF (Very High Frequency) band listed in Table Iis used for voice communications to, from, and amongaircraft. By combining communications and navigationfunctions in the VHF band, some avionics componentscan be made to do double duty. The success of this schemehas resulted in what is known as VOR (for VHF Omni-directional Range) and its adoption as an internationalstandard. In this system, the ground station radiates twosignals: one is omni-directional radiation whose carrierVHF frequency is modulated at 30 Hz; the second is acardioid (heart-shaped) pattern in the horizontal plane thatrotates at 30 rps. The airborne receiver experiences bothtransmitted waves as 30 Hz signals and the phase anglebetween them, as related to the rotation angle of the car-dioid pattern, determines the bearing of the VOR beacon

FIGURE 3 DME operation. (From Kayton, M., and Fried, W. R.(1997). Avionics Navigation Systems, Wiley, 2nd ed., New York.)



from the aircraft. There is also a Doppler version of VOR(see Section III.E) in which a 9960-Hz carrier frequency isfrequency modulated by the (simulated) rotation of a largediameter (480 wavelengths or 44-ft-diameter) antenna2 soas to be varied by 480 Hz at 30 Hz. The same airborneequipment can be used to sense phase, hence bearing to theDoppler VOR beacon, as with ordinary VOR, but with lesssite sensitivity and greater accuracy. Maximum bearing er-rors at 20-mile distance with standard VOR are about 3and with Doppler VOR, about 0.5.

The military uses point source ground stations whichcombine systems for determining both bearing and dis-tance measurement. These systems are known as Tacan(Tactical Air Navigation). The distance measuring func-tion, i.e., range determination, is accomplished usingthe same pulse and frequency configurations as stan-dard DME. The Tacan omni-bearing operation, however,(a) uses frequencies from 960 to 1215 MHz (almost 10higher than VOR) so that smaller antennas can be used;(b) employs a multi-lobe radiation principle which im-proves bearing accuracy; and (c) enjoys equipment eco-nomics as a result of using the same radio frequencies forrange and bearing determination.

The so-called hyperbolic systems, such as Loran,Omega and Decca, provide an alternative means of po-sition determination. These systems, rather than usingpoint sources, consist of groups of transmitting stationsthought of as forming chains. A chain consists of atleast three stations, of which one is a master transmitterand the other two are secondary transmitters. Each sta-tion in a chain transmits EM pulses which are groupedclosely in time and repeated at a certain rate. The inter-val between the repeated transmissions of these groups ofpulses is known as the Group Repetition Interval (GRI),and it identifies a particular chain. The number of pulsesin a group, the interval between them, the envelope whichdefines pulse shape, as well as GRI, establish the transmit-ted signal format, and it identifies each station in a chain.Since the positions of the stations are known, as are thetiming of signals transmitted from master and secondarystations, the difference of the Time Of their signals Ar-rival (TOA) at an aircraft informs the aircraft that it mustbe somewhere on a space curve, which happens to be ahyperbola, in a horizontal plane determined by baromet-ric altitude. A series of these TOAs, then, establishes aseries of hyperbolas, as shown in Fig. 4 (from Kayton andFried, 1997). If follows that TOAs from that master andanother secondary station establishes a second series ofhyperbola. The two specific TOAs informs the aircraftas to which two hyperbolas it must be on; their intersec-tions (plus barometric altitude) establish the aircrafts po-

2 Actually a ring of individual EM transmitting elements.

FIGURE 4 Hyperbolic lines of constant TD for a typical master-secondary pair. (From Kayton, M., and Fried, W. R. (1997).Avionics Navigation Systems, 2nd ed., Wiley, New York.)

sition in space, as shown in Fig. 5 (Kayton and Fried,1997).

Since LORAN-C is affected by sky waves and usesground waves, sophisticated corrections must be made toachieve maximum accuracy, and the usual ranges, a func-tion of transmitter power, are measured in hundreds ofmiles. Accuracy of about 1/2 km is achieved roughly 95%of the time, when differential techniques (see, for example,Section III.C) are used, adding redundant station pairs.

In the Omega system (which has been shut down for sev-eral years), transmitters emitted continuous waves, ratherthan pulses, and hyperbolic lines of position were estab-lished by phase differences in signals received from a mas-ter/secondary station pair. Because one phase differencebetween two continuous wave signals defined a series ofhyperbolas, multiple frequencies were used to eliminatethe ambiguity of which hyperbola was the pertinent one.

FIGURE 5 Hyperbolic lines of constant TD for a typical triad.(From Kayton, and Fried, W. R. (1997). Avionics Navigation Sys-tems, 2nd ed., Wiley, New York.)



Five frequencies were used at each station; four commonand one station-unique. The frequency bands being VLF(see Table I), these signals were propagated with low atten-uation between the Earths surface and a particular layerof the ionosphere, to great ranges. In fact, only 8 transmit-ting stations worldwide constituted an Omega system withaccuracy within about 4 nm, 95% of the time. When 30such stations were employed, using differential techniquesamong redundant pairs, position errors were diminishedto about 2 km, 95% of the time, within 1000 km of themonitor station.

Decca is also a system based on phase differences, butone that uses low frequency carrier waves between 70 and130 kHz. Two station pairs are typically 110 km apart andthe range of coverage is typically 320 km.

With the advent of digital technology, both the accuracyand flexibility of the earlier systems information process-ing have increased. Further, the components, particularlythe airborne components, size and weight have been re-duced. Since many aircraft radio communication and nav-igation systems have shared both the same parts of theEM frequency spectrum and a common technology, bothground-based and airborne components of terrestrial sys-tems which provide digital communication and navigationfunctions have been developed as integrated systems. Thatis, the same EM carrier waveforms are used to carry bothfunctions.

Two basic types of terrestrial integrated communi-cation-navigation systems, centralized and decentralized,are in widespread use by the military. Operation of theformer is dependent on a central site, from which all usersdetermine their positions on an absolute basis. Such anarrangement facilitates the control of many users, moreor less simultaneously, although users ordinarily receiveinformation on their positions automatically on the basisof periodic requests from the ground-based central siteor node. Users in these Position Location and ReportingSystems (PLRS) are cooperating users; i.e., they areequipped with Radio Sets (RS) equipped with accurateclocks and send a signal, individually identifiable, tothree or more ground stations (MSs for Master Stations).The MSs also have very accurate clocks and compar-isons of their timing signals are made with those of theRS. Knowing clock signal differences and signal TOAinformation at two MSs, places the user (the RS) on thespace curve intersection of two (imaginary) spheres. Alti-tude information, based on barometer data, establishes theposition of the RS at one of two points on this space curve,and TOA data from a third MS eliminates this uncertainty.

Operation of the second, decentralized type of systemis such that each user determines its own coordinates rel-ative to other users positions. Since it is independent ofcentral sites, decentralized systems are often called node-

less. Decentralized systems clearly require the airbornecomponents to be both receivers and transmitters.

C. Satellite-Based Navigation SystemsRelatively soon after the successful orbiting of man-madesatellites about the Earth, attempts began with the objec-tive of replacing or supplementing terrestrial radio naviga-tion systems through the use of earth satellites. The majoradvantages provided are those of (1) coverage, since thelines of sight, with enough satellites in the proper orbitscan be made to reach all points on earth; and (2) verystable operation. Their transmission frequencies (L bandnavigation signals from the satellites and S band telemetrydownlink to and up-link from the ground station) are alsosuch as to make them all-weather systems. Two majorsatellite radio navigation systems are included in an in-ternationally recognized Global Navigation Satellite Sys-tem (GNSS); they are the U.S. Department of DefensesNAVSTAR Global Positioning System (GPS) and theRussian Federations Global Orbiting Navigation Satel-lite System (GLONASS). Both systems have three ele-ments: (1) a constellation of earth-orbiting satellites (eachhas 24, as of this writing), (2) ground stations; and (3)receiver/processor units in the user aircraft. The satellitestransmit EM signals which the ground station uses to trackthem and from which the user aircraft determines its posi-tion relative to the satellites. Very accurate atomic clocksaboard the satellites are the heart of GPS. TOA process-ing in the ground stations allow simultaneous ranging frommultiple locations, and since the ground stations positionsare knownalso allows determination of the satellites lo-cation, velocity, and predicted orbital positions. The satel-lites orbital position information is sent from the groundstations to the satellites, which transmit it to the user air-crafts receiver processor, together with timing signals.The user aircrafts processor uses TOA data to establishits position relative to (at least three) of the GPS satelliteswhich, combined with the transmitted data on their posi-tions, allows the user aircrafts position to be known. Anobvious advantage from the avionics viewpoint of GPStype navigation systems is that the equipment in the useraircraft can be passive to the extent that user aircraftneed not transmit EM signals.

A variety of corrections are required in GPS orGLONASS to achieve the position accuracy desired; suchinclude those compensating for clock errors, the rotationof the earth, ionospheric and tropospheric refraction, etc.Differential principles can be used to eliminate errorscommon both to the user and a reference ground station.In Differential Global Positioning Systems (DGPS), areference ground station receives the same navigationsignals as the user aircraft, but since its position is known,



all the errors in its calculated position can be determined.These become error corrections when the reference groundstation transmits them to the user aircraft. Through the useof DGPS, position errors can be reduced to within 1 to10 m, depending on the users distance from the referencestation.

GPS receivers can be quite small; one military versionis known as the MAGR (for Miniature Airborne GPS Re-ceiver) and the entire component, except for its antenna,is contained inside another avionics assembly; e.g., an in-ertial navigation system (See Section III.D, below). Theantenna itself, in a U.S. Navy system, is contained in acircular housing with a diameter of less than 125 mm, hasa height (thickness) of about 40 mm, and weighs about 2 n(mass of about 0.1 kg).

D. Inertial Navigation SystemsThe basis of inertial navigation is dead reckoning (seeSection III.E, below), using accelerometers mounted onthe aircraft to measure accelerations and integrating theirsignal outputs over time, first to obtain velocities and thena second time to determine position. Inertial NavigationSystems (INS) are self-contained, requiring no cooperat-ing ground stations or satellites sending EM signals tothe user aircraft; thus, they are not subject to interfer-ences by an enemy or the weather. Since the processing ofthe fundamental sensor output is an integration over time,however, errors grow with time, and, if the orientation ofthe accelerometers is not accurately known, aircraft at-titude changesas a result of atmospheric disturbancesor deliberate maneuverswill contaminate accelerationsignals with changing gravity components. Correctionsfor these effects are accomplished in so-called strap-down inertial systems in which gyroscopes are added tosense angular motions. These strap-down inertial sys-tems have become practical with the advent of Ring LaserGyros (RLGs) and Fiber Optic Gyros (FOGs). Thesedevices correct for changes in acceleration direction elec-trically, so that the linear, horizontal velocity and posi-tion predictions are as they should be for the purposes ofnavigation.

When inertial systems are activated, they must bealigned, to set the aircrafts initial position and velocityproperly and to orient its axes relative to the Earth; this pro-cess is known as gyrocompassing. The Earths rotation,of course, imposes a centripedal acceleration whose mag-nitude and direction (with respect to the vertical, i.e., thelocal normal to the earths mean surface) varies with geo-graphical position. This and other such small errors growsufficiently with time as to make hybrid systems, suchas those which update inertial systems periodically us-ing GPS data, for example, in a process known as aiding

worth their additional complexity. INS is widely used inthe military and on large civil passenger aircraft.

As principal components of inertial systems, ac-celerometers and gyroscopes have been subject to intensedevelopment efforts to improve their accuracy and elimi-nate responsiveness to influences which contaminate theiroutputs. Design of accelerometers for inertial navigationsystems are most often based on one of three concepts.The first is that of a pendulum on flexuresbeams withvery low stiffness in one direction, but stiff in the othertwo, perpendicular directionsand electrically restrainedto a zero deflection at zero or reference acceleration. Thisprovides for rebalancing to ensure that response to oneacceleration will not change the direction of sensing forthe next. The second makes use of very small, microma-chined silicon masses mounted on springs that are softin one direction, stiff in the other two, also electrostaticallynulled; and the third employs vibrating beams whose stiff-ness is so low in the direction of vibration that tensile forcevariations along the beam length cause changes in the fre-quency of vibration, thus indicating acceleration along thebeam length.

Many types of gyroscopes are used in aircraft applica-tions for either indicating or providing signals in automaticsystems which provide control of aircraft attitude angle orangular rates. The earlier forms used a spinning wheelmounted in a gimbal (so as to be free to rotate about anaxis perpendicular to its spin axis) and floated at neutralbuoyancy. Angular motion of the gimbal axis in a planecontaining the spin axis would then cause precession aboutthe gimbal axis, which would indicate the gimbal axisangular rate. If this response were to be available againfor later motions, this precession angle would have to bereset, and such would be done by magnetic torquers,according to a rebalance algorithm. These and othergyroscopes were developed further, including such re-finements as two perpendicular gimbals, electrostatic sus-pension, etc. The less expensive, less maintenance proneversions with drift rates of about 0.1 deg/h are still useful,for example, in tactical missiles, but are too inaccurate forlong-range navigation.

Although modern optical angular motion sensors arestill called gyroscopes, they function on other than New-tonian mechanical, i.e., inertial, principles. Because oftheir accuracy, dynamic range, linearity, maintenance-freenature, and reliability, RLGs and FOGs are now used inINSs for almost all commercial and military aircraft. Oneof two optical principles are used in these devices, but ineither application, two laser beams propagate in what isessentially the same closed, planar path; one clockwise,the other counterclockwise. If the device containing thepaths rotates about an axis perpendicular to the plane ofthose paths, the Sagnac Effect (1967), which results from



the fact that light waves are unmoved by motion along thelight path of the medium in which theyre transmittedmakes the lights path in the direction of device rotationappear to be longer and the lights path in the oppositedirection appear to be shorter. The RLG makes use of aresonator principle, the FOG can use that principle orinterferometry.

Because the front part of a laser light beam is coherent(i.e., all components are in phase) the interference be-tween two beams propagating in opposite directions in anoptical resonator forces a standing wave within the opti-cal cavity. When this type of RLGs housing rotates aboutthe circular paths centerline, then, the nodes and/or antin-odes of the standing wave, which are fixed in space, can becounted and interpreted as angles in the azimuthal direc-tion around the circular path. The light sensor of an FOGusing the interferometry principle experiences phase dif-ferences where the two, counter-rotating laser light beamsemitted simultaneously are recombined, since ones pathis longer and the others shorter, depending on the senseand magnitude of the angular rotation of the device aboutthe paths centerline. The positions of the lines of inter-ference can then be interpreted as a measure of rotationangle. Most optical gyros used in INSs, as of this writ-ing, are of the interferometer type; employing light pathsof between 10 to 40 cm in length; weighing between 5and 20 n (mass between 0.52.0 Kg) per axis; and havingroot-mean-square accuracies of about 0.05.

The typical INS, using these components, then (Kaytonand Fried, 1997), requires about 800016,000 cc in vol-ume, 30150 W of power, weighs approximately 85130 n(mass between 914 Kg) and has a velocity accuracyof about 0.75 m/s (rms) and navigational accuracy of1.5 km/h. These modern airborne systems are relativelyexpensive ($50,000 to $120,000).

E. Doppler Radar and DeadReckoning Systems

Dead reckoning is an old maritime term used to describenavigating (itself a maritime term) by using known initialposition, the vehicles velocity vector (speed and direc-tion), and how long that velocity has been maintained, todetermine the vehicles new position. If velocity is mea-sured, say, relative to the surface of a body of water, itis clear that positions determined by dead reckoning willbe in error by the existence of currents in that water. Forships whose speed is not great relative to currents, this isimportant; for fast flying aircraft it is much less so. Useof Doppler radars to measure relative speed in moderndead reckoning systems, however, has some significantadvantages; for example, like INS, they are self contained,needing no terrestrial or satellite cooperative station; their

transmitter power requirements are small; and they workvery well for low vehicle velocities.

As to the principle on which Doppler radars are based,consider that wave motion emanating from a source mov-ing with respect to the receiver is sensed as having achanged frequency; the magnitude of the change depend-ing on the relative velocity, higher if the source and re-ceiver are moving closer, lower if they are moving fartherapart. This so-called Doppler effect is experienced al-most every day acoustically, for example, if a fast-movingauto or train passes with its horn or whistle blowing. Ina directly analogous way, the frequency of a radar signalreturn shifts if the transmitter and reflecting surface haverelative velocity along the line of EM transmission. Thisprovides a means, using reflection returns, to determinethe speed of an aircraft relative to the ground or waterover which it is flying. Doppler radars, mounted on anaircraft, use microwave frequencies in an internationallyauthorized band, between 13.25 and 13.4 GHz. This pro-vides narrow beams of EM radiation, which can be pointedat the ground at relatively steep angles. The last has theadditional benefit of reducing the probability of detectionin military applications.

For Doppler navigation, at least three radar beams areneeded to determine three components of velocity relativeto the earths surface, and three aircraft attitude measure-ments in three perpendicular planes are needed to resolvethe Doppler radar measurements into components in anearth-related, geodetic coordinate system, as needed fordead reckoning navigation (Fig. 6). If the three Dopplerradar beams are arranged as shown in Fig. 7, and a differ-ence taken of the returns from signals A and B, the Dopplershifts of the lateral components will cancel, whereas thelongitudinal components, being of opposite sign, will beadded. This arrangement, known as a Janus system(after the Roman God who could see both backward andforward), increases system accuracy. For the usual beamangles to the horizontal of about 70, a Janus system will

FIGURE 6 Resolution of aircraft velocity into navigablecomponents.



FIGURE 7 Lamda arrangement of three Doppler radar beams.

have an error in horizontal velocity of only about 0.015%per degree of error in the aircrafts pitch attitude. Withoutthe Janus arrangement the same metric would be about 5%.

F. Instrument Landing SystemsWhat is known as the Instrument Landing System (ILS)consists of (1) localizer transmitters, located at the cen-terline of and off the ends of runways, which provide lat-eral guidance to aircraft approaching to land; (2) glideslope transmitters located beside runways near the endof the runway over which the aircraft first passes in landing(the threshold), which provide vertical guidance; and (3)marker beacons reporting progress along the glide-path tothe pilot of the landing aircraft.

All three, localizer, glide slope, and marker beacontransmitters, radiate continuous wave EM energy at ra-dio frequencies. Their radiation patterns in space and inspecific frequency channels provide signals to an aircraftreceiver indicating deviations from the desired height asa function of range from the end of, and lateral displace-ment from the centerline of, a particular runway. The re-ceiver, then, displays information to the pilot that only isnulled when the aircraft is on course to landing, andthese information signals grow with the level of deviationto either side or above/below the proper course. In autoland systems, such deviation signals are hard-wired tothe AFCS (see Section IV). In the ILS airborne equipment,a Morse-Code identification signal is received audibly inthe cockpit on the localizer band, and a voice transmissionfrom the airports control tower may also be provided. Sig-nal standards for ILS are established internationally, andabout 1500 ILSs are operational at airports throughoutthe world.

G. Collision Avoidance SystemsAlthough ground control of aircraft flight plans and flightpaths in real time have as a prime objective eliminating

the possibility of mid-air collisions, controlled flight intoterrain, or collisions on airport taxiways or runways, ve-hicle borne avionics equipment plays important roles inthese functions. Installation of airborne, mid-air collisionsystems provides protection against such calamities in-dependent of ground control and in addition to the na-tions Air Traffic Control (ATC) system. The Traffic Alertand Collision Avoidance System (TCAS) uses a scanningradar transmitter in one aircraft to trigger the response ofa transponder in any aircraft so equipped within its range.In the version of TCAS most used, TCAS II, the distancebetween two aircraft and their altitude separation are cal-culated, based on the transponder signal returns, and thecrew is alerted about 40 sec before the closest Point ofApproach (CPA) is to be reached, if the separation is pre-dicted to be small. This alert is known as a traffic advi-sory and displays the range, bearing and altitude of theaircraft posing collision danger. If the danger continues,about 25 sec before CPA, a Resolution Advisory (RA)appears showing the climb or descent maneuver recom-mended to increase the miss distance. Since both TCAS-equipped aircraft must be properly advised as to how tochange their flight paths, TCAS II has an air-to-air datalink communicating between the two aircraft, to coordi-nate RAs. A version known as TCAS I does all of thisexcept displaying RAs. All air carrier aircraft operatingin U.S. airspace with 10 to 30 passenger seats must haveTCAS I, all with more than 30 seats must have TCAS II.

As air travel departure and arrivals increase and, withthem, airport congestion, the danger of collision betweenaircraft on the ground also increases. Ground-based sys-tems to aid the regulation of ground movement of aircraftinclude surface movement radars and taxiway lights mod-ulated to indicate specific taxi routes. These dont, at thiswriting, require avionics equipment on the aircraft. Thereare, however, aircraft systems that use transponders to al-low ground-based interrogators, located with taxiwaylights, to derive identification and location informationand relay it to tower controllers.

Commercial, in fact all civil, aviation must keep a safealtitude above terrain in all flight modes other than take-off and landing. Military aircraft, however, must oftenapproach the ground for weapon delivery, precise recon-naissance, orfor extended flight timesto avoid detec-tion or defensive weapons. Those with the last of theserequirements are usually equipped with terrain follow-ing equipment. These are automatic systems having For-ward Looking radars and Infra-Red (FLIR) sensors andradar altimeters (see Section III.I, below). The first twoof these measure the range and angle from the horizontalof the terrain before the aircraft. Flight path control com-mands, based on a computed terrain profile in a verticalplane, based on the forward-looking radar returns with



pilot monitoring, directly control the pitch attitude, henceangle of attack and lift of the aircraft, so as to main-tain a desired height above the terrain. The radar altime-ter checks the altitude prediction, and the FLIR providesback-up data to ensure that the radar commands flight oversuch obstructions as power lines. In civil aviation, Con-trolled Flight Into Terrain (CFIT) has become a safetyissue of increasing concern, particularly in mountainousregions and under conditions of reduced visibility. Appro-priate avionics therefore, may soon be appearing on all air-craft above a certain size in terrain avoidance applications.

H. Weather Radar SystemsAirborne weather radar systems make use of the reflectiv-ity of clouds, precipitation, dust particles at low altitudesand ice crystals at high altitude. Their intent is to deter-mine the position of air having the kinds of motion that canmake flight dangerous or uncomfortable. The magnitudeof reflections, compared with stored models, allows pre-cipitation rates and whether it is rain, snow or hail, to bedetermined. Positions are determined by the elevation andscanning (heading) angles of return signals and range bygating, i.e., enabling the receiver only at the specifictimes when a signal reflected from that range would be re-ceived. Further, the wind speeds and turbulence intensitiesare measured by Doppler effects, usually by processing thereturn timing of pairs of pulses.

Weather radars and multiple use radars with weatherdefining functions are usually mounted in the nose ofthe aircraft (see Fig. 8, from Kayton and Fried (1997))carrier wavelengths in either C or X band are used, and theweather displays usually show weather formations withinabout 100 km ahead of the aircraft. Warnings can be givento the pilot if stored levels regarded as the maximum al-

FIGURE 8 Avionics placement on multi-purpose transport (From Kayton, M., and Fried, W. R. (1997). AvionicsNavigation Systems, 2nd ed., Wiley, New York.)

lowable for safe operations are exceeded for wind speedsor turbulence; 5 m/s, for example, is often considered sucha limit for turbulence. The atmospheric phenomena asso-ciated with thunderstorms and microbursts and knownas wind shear is susceptible to detection using Dopplerprocessing avionics. This extremely dangerous conditionis associated with a column of air with high downwardvertical velocity flows which, on contact with the groundmust flow horizontally (i.e., radially) outward in all di-rections. An unsuspecting aircraft flying directly towardthe vertical column, near the ground (as in a landing ap-proach), is flying into a head wind until it passes the centerof the column, at which time it is abruptly subject to a tail-wind. The associated loss of lift may not be restored beforedisastrous contact with the ground. Weather radar to sensethese potentially calamitous conditions are installed at afew airports. Plans are underway to install weather radarsspecifically capable of detecting wind shears on all com-mercial aircraft with detection range capability sufficientto allow this atmospheric phenomenon to be evaded.

I. Radar AltimetersFor certain specialized functions, down-looking radars areused to measure tape-line altitude; i.e., distance abovethe ground immediately below the aircraft. Low flying mil-itary aircraft may use radar altimeters for correlating ter-rain contours with stored map-matching navigation infor-mation. In civil aircraft operation, radar altimeters can beused to assist automatic landing systems. Although othertechniques have been used, in one currently used radar al-timeter, a continuous wave is generated whose frequencyis modulated with a period much longer than the time re-quired for the ground-reflected signal to return to the air-craft. Comparison of generated frequency with returned



frequency for the known modulation schedule, then, isa measure of elapsed time, hence the distance above theterrain. Antennae beam width, for this function, must bewide enough to provide a return from a flat earth when theaircraft pitches and rolls over the normal range. Radar al-timeters have the 4.2 to 4.4 GHz frequency band assignedto them, allowing use of two small, microstrip antennas,one to transmit, one to receive, producing beam widths ofbetween 40 and 50. If the separation between this pair isat least about 0.8 m, the transmitted/received signals mu-tual interference will be reduced enough that, even afterthe loss of signal intensity experienced in ground reflec-tion, useable information will still remain in the receivedsignal. Radar altimeters are usually only used for tape-linealtitudes of less than 1500 m; current continuous wave ver-sions require less than 1 W of power and have accuraciesof about 0.5 m below 30 m and about 2% above 30 m.

IV. AVIONICS APPLICATIONSINFLUENCING AIRCRAFTDESIGN; VMS

A. Flight Path Stability; AFCSMeans to provide equilibrium, stability, and control foraircraft are obviously essential. Yet devices and arrange-ments of aerodynamic surfaces to carry out these func-tions are usually detrimental to such aircraft performanceas how far the aircraft can carry a given payload, its max-imum speed, etc . Tail surfaces, for example, add weightand aerodynamic drag, and having the aircraft CG forwardof the wings center of lift as required for positive natural,i.e., unaugmented, stability in pitch, requires a downloadon the tail for equilibrium. The present-day capabilitiesof avionics, with actuators capable of high-frequency re-sponse to control inputs, allow designers to deliberatelytake advantage of the often superior range-payload, high-speed, and high-maneuverability performance of config-urations whose unaugmented flying qualities are grosslyunsatisfactory, by incorporating an avionics-based Stabil-ity Augmentation System (SAS) to compensate for un-stable flight path characteristics. Figure 9 (from Loewy(2000)) illustrates one of the advantages of Reduced StaticStability (RSS) in pitch that was exploited in the basic de-sign of the F-16 Falcon jet fighter. By integrating so-calledRSS into the basic aircraft design, equilibrium of pitchingincrements was achieved with the horizontal tail generat-ing lift rather than download, so that the lift that the wingmust produce to carry the weight of the aircraft is reduced.This means a smaller wing with less structural weight anddrag can be incorporated into the overall design. Further,for equal offsets of the wings lift from the aircraft CG, ei-ther fore or aft, the tail lift (with RSS) can be less than the

tail download (with positive static margin) so that the tailsdrag, i.e., trim drag, will be less. The cumulative effectof these beneficial design changes, resulting from allow-ing an avionics SAS to compensate for RSS, is to reducethe fuel required for the same range-payload combinationand a higher net thrust-to-weight ratio. The latter is par-ticularly important for fighter aircraft because it improvesmaneuverability.

If the unaugmented flying qualities of an aircraft aresufficiently poor to constitute a risk to flight safety, how-ever, the reliability of the SAS must be extremely high,e.g., approaching that of primary structure. Further, pro-visions must be made to assure the kind of controllabil-ity that avoids hazardous aircraft attitudes and/or angularrates even in the unlikely event of hard-over, i.e., abrupt,full control travel, system failures. In the case of the F-16,this resulted in a quadruply redundant SAS. Implicationsfor the aircrafts design went beyond the fundamental per-formance and maneuverability characteristics mentionedbefore. As stated in (Droste and Walker (1990), Sincethe pilot would not be able to control the aircraft in thepitch axis without the electrical system, there was no jus-tification to retain a mechanical pitch system. The pilotscommand could now be electrically combined with thestability system with no penalty. Removal of mechanicalconnection between pilot and the control surfaces was thelogical result. That is, making the SAS highly reliable andfail-safe allowed the use of an FBW flight control systemwith no mechanical control system to back-up the elec-trical system. FBW control systems also facilitate meansto enhance the way an aircraft responds to a pilots com-mand. These aircraft avionics systems are referred to asControl Augmentation Systems (CAS).

It should be clear that avionics systems capable of SASand CAS functions can also perform autopilot functionsas mentioned in Section II, above. When all these functionsare integrated into one system, it is commonly referred toas an Automatic Flight Control System (AFCS).

B. Load AlleviationAnother example of avionics systems usage is in maneu-ver load-limiting. The prototype of the F-16, the YF-16,was designed for a limit normal load factor (n) of 9 timesgravity, or 9g, throughout most of its flight envelope,but as low as 6.5g in some critical areas. To prevent pilotsfrom exceeding these limits, pilot commands were atten-uated by avionics-based controls as a function of Machnumber (forward speed divided by the speed of sound),altitude, and Angle Of Attack (AOA). This system alsolimited AOA in an absolute sense, i.e., independent ofMach number and altitude. The associated automatic AOA(or n)limiting schedule is shown in Fig. 10 (from Loewy



FIGURE 9 Relaxed static stability. (From Loewy, R. G. (2000). Avionics: A New senior partner in aeronautics.AIAA J. 37(11), 13371354.)

(2000)). The production version of the F-16 was designedwith a 9g limit normal load factor throughout the oper-ational envelope, eliminating the need for this particu-lar avionics system, but roll rate (angular velocity aboutthe aircrafts longitudinal axis) limiting was retained withvariable limits as a function of AOA as a means of en-suring good handling qualities at the extreme maneuverattitudes at which strong dynamic coupling between an-gular motion about two inertial axes exists.

A second example of a load alleviation avionics systemis provided by the arrangement used to extend the wingstructures fatigue life as an interim measure during thedevelopment of the Lockheed C-5A military transport air-plane. The operational concept on which this automaticsystem was based followed from two physical laws, oneaerodynamic, the other in mechanics of materials. First,a wing will have minimum drag induced by lift (induceddrag) if the spanwise lift distribution approaches an ellipti-cal distribution. This will maximize range-payload perfor-

mance, assuming that proper account is taken of the struc-tural, i.e., weight consequences, attendant on the bendingand shear loading associated with such a spanwise distri-bution of lift. The second physical law is often expressedin so-called Goodman Diagram form: (McClintock andArgon (1966), for example), which shows that the lowerthe steady stresses in a material, the higher are the allow-able alternating stresses for which the material will haveindefinite life; the so-called endurance limit. Because amajor source of alternating stresses on airplane wings isgusts (i.e., atmospheric turbulence) in cruise flight, anautomatic system was installed on the C-5A during itsdevelopment phase that sensed the onset of turbulence(using, as sensors, accelerometers or wing bending straingauges) and called for both ailerons, i.e., those on portand starboard wings, to retrim slightly upward when gustloads exceeded some level. This reduced the lift on theoutboard wing sections. To preserve total lift, of course,the aircraft pitch attitude would be increased, but the total



FIGURE 10 Original YF-16 AOAG Limiter Concept. (From Loewy, R. G. (2000). Avionics: A New senior partner inaeronautics. AIAA J. 37(11), 13371354.)

effect would be a spanwise lift distribution deviating fromthe approximately elliptical by having less load outboard,more inboard. This load distribution reduces steady wingroot bending moments and associated steady stresses be-cause of bending, relative to that produced by ellipticaldistributions.

With this avionics system operative, the aircraft wouldhave optimum cruise efficiency in smooth air, i.e., turbu-lence below some predetermined level, and lower aero-dynamic efficiency when cruising in turbulent air. In tur-bulence, however, its wing root structural material wouldenjoy a more favorable position on the Goodman Diagram,so as to improve the wing structures fatigue life. If such asystem were found to be advantageous for an operationalaircraft in the part of preliminary design concerned withwing structural weight, its designers could rely on lowercombinations of steady and alternating stresses than wouldhave existed if the more nearly elliptic spanwise lift distri-bution had been carried under all atmospheric conditions.Note that no quick-acting automatic actuation would berequired for this spanwise lift distribution modification inrough air. However, because flight path control effectors(ailerons) are involved, only limited authority would begiven the actuators. Thus, if the system failed so as not tooperate, the pilot could compensate by reducing airspeed,

hence loads caused by turbulence, to levels of acceptablealternating stresses. If it failed with the ailerons in theup position, the pilot could take some range-preservingactions which might, for example, adversely affect flighttime, but preserve flight safety. In either case, however,appropriate pilot action would depend on having displaysthat reveal the existence and nature of the failure in thissystem. Redundancy in this kind of automatic control sys-tem and a requirement for full or partial authority opera-tion after a single failure, however, would not be necessarycharacteristics.

Load limiting as described in the F-16 example clearlyrequires VMS response as quick or quicker than a pilotcan command. Thus, as a consequence of the level of re-liability needed, all possible failures in systems compo-nents must be accounted for in design stages and adverseconsequences minimized. This requires that multiple sen-sors and processors be provided for redundancy. It is notunusual for large commercial jet transports to have fivefull authority digital processors to control pitch, roll, andyaw; and to have each such computer divided into twophysically separated channels. The first one, the controlchannel, is permanently monitored by the second one, themonitor channel. In the case of disagreement between con-trol and monitor, the computer affected by the failure is



FIGURE 11 Gust frequency spectrum. (From Loewy, R. G.(2000). Avionics: A New senior partner in aeronautics. AIAA J.37(11), 13371354.)

turned off automatically, while the computer with the nexthighest priority takes control. In addition, to prevent com-mon mode failures, designers of such systems will oftenaccept the cost penalties associated with dissimilarity toprovide two types of computers.

C. Crew/Passenger Comfort ImprovementIn addition to the emphasis on safety associated with theresponsibility of carrying passengers, competition amongairlines to win the business of these passengers motivatescommercial transport airplane designers to make flightas comfortable as possible for their customers despiteatmospheric turbulence. As seen in Fig. 11 (from Loewy(2000)) there are significant frequency components in at-mospheric turbulence which place systems designed toameliorate those effects above 2 Hz; and this frequency isbeyond a human pilots control input capabilities.

It is possible to take advantage of avionics systemshigh frequency capabilities, to carry out AFCS, load lim-

FIGURE 12 Turbulence damping: Principle of one lane (from Loewy, R. G. (2000). Avionics: A New senior partnerin aeronautics. AIAA J. 37(11), 13371354.)

iting and gust alleviation for comfort purposes using manyof the same system components. Figure 12 (from Loewy(2000)) is a control diagram for processors in a systemto provide the last of these three functions. In that fig-ure the words Fuselage Acceleration indicate the per-tinent sensor, and the words Control Surface indicatethe aerodynamic effector (ailerons or elevators, or both).As an indication of the kind of improvement possible us-ing such systems, Fig. 13 (from Loewy (2000)) shows themagnitude of Gust Load Alleviation (GLA) achieved ontests. Here wing bending moment is a direct indicator ofdynamic lift variations caused by continuous turbulence,which will result in passenger/crew vertical accelerationsas measured by wing accelerometers and bending straingauges and using the wings ailerons as corrective controleffectors.

D. Suppressing Servo-Aeroelastic InstabilitiesStructural problems of a dynamic and/or aeroelastic natureare usually thought of as falling into either self-excited orforced vibratory classes. In aircraft, forced structural dy-namic response of airframe components has many sourcesof excitation. Some of these are (1) gusts, i.e., atmosphericturbulence, as mentioned in Sections IV.B and C; (2) air-craft wake induced turbulence (examples include tail buf-fet as caused by both boundary-layer separation and bytrailing vortices from rotor/propellers where such exist;(3) engine and rotor/propeller vibratory hub forces andmoments; (4) rotor/propeller blade tip passage in closeproximity to a fuselage; (5) transmission of gear box vi-brations at tooth contact frequencies; and (6) rapid fireweapon recoil and/or muzzle pressures in military aircraft.The phenomenon known as flutter of wings and tailsurfaces, the latter usually coupled with aft fuselage mo-tion, and both sometimes coupled with control surface



FIGURE 13 Power spectral densities of bending moment due tocontinuous turbulence with GLA System On & Off. (From Loewy,R. G. (2000). Avionics: A New senior partner in aeronautics.AIAA J. 37(11), 13371354.)

deflections, is in the self-excited, i.e., aeroelastic stabilityclass. So are the whirl-flutter and coupled rotor-wing phe-nomena which can be thought of as including mechanicalinstabilities, such as helicopter so-called ground reso-nance (a misnomer), as a subcase. Pilot-Induced or Pilot-Assisted Oscillations (PIO or PAO) can also be a specialcategory of these phenomena.

The design of systems to suppress aeroelastic oraeromechanical instabilities must consider such systemsas SAS, active, if such exist, because of their possibleeffect on the unaugmented aircrafts structural dynamicand/or aeroelastic behavior. When a stable aeroelasticmode is destabilized by a flight path control system such asSAS, this is often called spillover. Aeroelastic stability(and flight path stability, for that matter) must, therefore,be assured with all systems functioning in all possiblemodes of operation, including partial failures. Active mod-ification of structural and/or aeroelastic phenomena bymeans of avionics systems may, in any event, be thoughtof as acting by virtue of either reducing the (usually aero-dynamic) forcing functions, generating directly opposingforces, or by introducing stiffness changes and/or addi-tional damping into the motions crucial to the instability.

The U.S. Air Force investigated the use of avionics toreduce airframe structural design criteria to ensure aeroe-lastic, i.e., flutter, stability more than 20 years ago. Inthat research, a modified B-52 jet bomber aircraft wasflown 18 km/h faster than its flutter speed. The FlutterMode Control (FMC) system employed in that programhad vertical accelerometers in pairs at four locations onthe wing, which produced signals which, processed byshaping filters, drove outboard ailerons in one indepen-dent loop, sensors to surfaces, and outboard flaperons, ina second. The system was predicted to increase flutterplacard speed by more than 30% by increasing damping

in and improving coupling between the structural modesactive in the aeroelastic instability.

V. IMPACT OF SMART MATERIALS

Smart materials are thought of as those which produceelectric voltages when strained (the direct piezoelec-tric effect, for example), and/or become strained whensubjected to electric or magnetic fields or temperaturechanges. Examples of the latter behavior include piezo-electric materials (the converse piezoelectric effect),magnetostrictive materials, and shape memory alloys(which change states, and thereby dimensions or shapes,depending on temperature), respectively. The strain sens-ing characteristic gives piezoelectrics potential for use assensors, the strain-inducing behavior of the last three, po-tential as actuators. Among the unique properties of suchmaterials for use in avionics systems are included the pos-sibility of distributing them throughout a structure, ratherthan concentrated at specific points as are such sensors asaccelerometers or strain gages, or as actuators are, whichdrive the rotation of a control surface, for example. Insome applications, very high frequency response is avail-able, as well. Where distributed sensing has advantages,smart materials are considered for embedding as just an-other fiber in advanced filamentary composite materials,and embedding may be considered for distributed actua-tion too, but usually at a surface of the structure.

Using smart materials and structures techniques alsoholds promise for more spatially continuous variations ofshapes and other properties, with the elimination of dis-continuities in slope angles at the surfaceoften impor-tant from aerodynamic considerations.

Piezoelectric materials are particularly promising foravionics functions requiring high frequency sensingand/or actuation. For example, techniques are being devel-oped which could be integrated into turbine engine con-trols, to allow turbo-machinery performance increases andreductions of chemical and/or particulate elements in theirexhaust harmful to the environment. Turbine engine per-formance generally improves the closer their operationsare taken to compressor stall. Yet stall in service opera-tion is unacceptable. Feedback control has been shownto be capable of extending the effective stable flow rangeof axial compressors and rejecting persistent disturbanceswhich, otherwise, would cause the system to incur rotatingstall.

As regards achieving more efficient and cleaner com-bustion, a key consideration is in avoiding combustioninstabilities. Such instabilities can involve large ampli-tude acoustic oscillations sufficient to cause mechanicalor thermal damage, and passive approaches to avoiding



them have generally not been satisfactory. On the otherhand, tests using active control systems have been shownto be promising. A pressure sensor at an upstream loca-tion in the combustor where all axial acoustic modes areexpected to be significant, can provide a signal processedso as to modulate the flow of a secondary gaseous fuelstream into the combustor, with gain and phase changingwith stability characteristics in real time. The secondaryfuel injector actuator requires a modulation rate variablefrom 0 to 1500 Hz and the processor must rely on an ob-server to identify the amplitudes, frequencies and phasesof several combustor modes in real time (Loewy, 2000).At this writing, it appears that a fuel injector suitable forsuch an automatic combustion instability controller, usingsmart materials, could be integrated within existing enginefuel-feed systems. Both the above applications of avion-ics components and techniques, and presumably others, tothe design and operation of intelligent turbine enginesseem highly likely, at some point in the future.

Although no applications of smart materials in avion-ics systems of any kind have appeared in aircraft presentlyin service use, they continue to have considerable promise:for example, to reduce structural vibration resulting fromtail buffet; to increase the aircraft speed at which panelsexposed to the airstream will flutter; to change the twistand thereby improve the performance of rotor blades; andto produce lifting surfaces (wings and tails) particularlythose constructed with tailored filamentary compositematerials, which will have superior aeroelastic stabilitycharacteristics.

VI. SUMMARY

It is useful, particularly in view of the integration of avion-ics components into more than one functional system, tothink of these systems as being of the kind that (1) changethe behavior of the uncommanded airframe, (2) change thebehavior of the uncommanded engines, (3) modify the pi-lots command signals, (4) provide mission-related com-mand signals; and (5) provide mission-enabling informa-tion to the pilot. The last of these is a function that allowsthe pilot to fly where he/she wants to go and to use theroutes he/she chooses to get therein the case of gen-eral aviation, commercial and military transport aircraft.In the case of combat aircraft, equipment providing thisfunction can be considered part of the payload, because

finding the enemy and/or neutralizing the enemy may bethe fundamental reason the mission is undertaken in thefirst place. These broad functional categories can be listedas follows:

1. Changing behavior of uncommanded airframesa. Flight path stability augmentations (SAS)b. Aeroelastic instability suppression (FMC)c. Load reduction

2. Changing behavior of uncommanded enginesa. Compressor stall avoidanceb. Emission ameliorationc. Noise reductiond. Improving combustion efficiency

3. Acting as pilots associatea. Threat avoidance maneuversb. Target tracking/weapon pointingc. Formation flying/station keepingd. Automated landings

4. Providing mission enabling pilot informationa. Communicationb. Navigationc. Collision/terrain avoidanced. Target/threat location

SEE ALSO THE FOLLOWING ARTICLES

AIRCRAFT INSTRUMENTS AIRCRAFT PERFORMANCEAND DESIGN AIRCRAFT SPEED AND ALTITUDE AIR-PLANES, LIGHT FLIGHT (AERODYNAMICS)

BIBLIOGRAPHY

Droste, C. S., and Walker, J. E., (1990). A Case Study on the F-16Fly-by-Wire Flight Control System, AIAA, Inc., Reston, VA.

Kayton, M., and Fried, W. R. (1997). Avionics Navigation Systems,2nd ed., Wiley, New York.

Loewy, R. G. (2000). Avionics: A New senior partner in aeronautics,AIAA J. 37(11), 13371354.

McClintock, F. A., and Argon, A. S., (1966). Mechanical Behavior ofMaterials, Addison-Wesley, Reading, MA.

Reference Data for Engineers: Radio, Electronics, Computer and Com-munications (1985). 7th ed., Sams, Howard W. & Co., Indianapolis,IN.

Sagnac Effect, Review of Modern Physics (April 1967). Vol. 39, No. 2.Skolnik, M. I. (1962). Introduction to Radar Systems, McGraw Hill,

New York.

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Aircraft AerodynamicBoundary Layers

Jean CousteixONERA and SUPAERO

I. IntroductionII. Body in Motion in a FluidIII. Stresses and Heat Fluxes in a FluidIV. Laminar and Turbulent FlowsV. High Reynolds Number Flows

VI. An Example of Boundary Layers:Falkner-Skan Solutions

VII. Laminar-Turbulent TransitionVIII. Turbulent Boundary Layers

IX. Drag ReductionX. Concluding Remarks

GLOSSARY

Boundary layer Thin layer of viscous, possibly turbu-lent, flow near a wall where the velocity exhibits veryfast variations normal to the wall. Boundary layers de-velop at high Reynolds numbers.

Diffusion of momentum (or heat) Transport of momen-tum (or heat) by viscosity (or thermal conductivity).

Dissipation Transformation of kinetic energy into heatdue to the deformation work of viscous stresses.

Drag Component of aerodynamic forces aligned with therelative velocity between the body and the free stream.

Inviscid flow Approximation in which viscous effects arenegligible.

Navier-Stokes equations Equations governing fluid flow.Reynolds number Dimensionless number which gives

the magnitude of the ratio of inertial forces to viscousforces in a flow.

Skin friction or wall shear stress Stress at the wall due toviscosity. The corresponding force applied to the bodyis parallel to the wall.

Stress tensor Second-order tensor defined at any pointin a flow and determining a force applied to a surfaceelement bounding a volume of fluid. The dimension ofstress tensor components is a force per unit surface.

Viscosity Property of a fluid by which momentum dif-fuses in a flow. Viscosity smooths inhomogeneities ofmomentum.

301

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302 Aircraft Aerodynamic Boundary Layers

PHYSICAL and theoretical aspects of boundary layerflow are presented, stressing important effects such as fric-tion drag and separation. Laminar-turbulent transition andturbulence effects are also described. These phenomenahave a strong influence on the development of bound-ary layers but their understanding and modeling are stilllargely open questions. Finally, drag-reduction techniquesby means of riblets and laminarization are presented.

I. INTRODUCTION

The aerodynamic flow around the wing of a civil air-craft in cruise conditions is characterized by a very largeReynolds number on the order of several tens of millions.The Reynolds number can be interpreted as the order ofmagnitude of the ratio of inertial forces to viscous forces.At high Reynolds numbers, viscous effects are negligible.This assumption is valid almost everywhere in the flow ex-cept close to the walls where viscosity is effective. Viscouseffects are also important in wakes and jets.

With appropriate hypotheses, the Navier-Stokes equa-tions are drastically simplified and the boundary layer the-ory makes flow analysis much easier. Viscosity is respon-sible for friction drag or separation, which are among themost important characteristics of a wing. These remarksexplain the key role played by boundary layers in aerody-namics.

As a general rule, the boundary layer flow is not laminarall along its development on a wing. Laminar-turbulenttransition and turbulence take place more or less early ac-cording to the conditions of boundary layer development.These phenomena have a strong influence on the behaviorof boundary layers. At the present time, the understand-ing of these phenomena is not complete and no definitivemodel exists.

Basic concepts of aerodynamics and fluid mechanicsare reviewed in the first sections of this paper. The rest ofthe presentation is devoted to a description of the physicaland theoretical aspects of the following topics: boundarylayer concept, laminar-turbulent transition, turbulence ef-fects, and drag reduction techniques.

II. BODY IN MOTION IN A FLUID

In standard aerodynamics, air is a fluid considered as acontinuum. The concept of fluid particle is defined asa volume of fluid large enough to contain a great num-ber of gas molecules and small enough to be character-ized by averaged quantities which are uniform over thefluid particle. A fluid flow is characterized by thermody-namic properties such as pressure, temperature, density,

FIGURE 1 Fluid entrainment by a flat plate moving parallel toitself.

etc. A fluid particle is entrained at the local velocity of theflow.

In an inviscid flow, excluding the possibility of modi-fications of the molecules structure at high temperaturedue to chemical reactions, for example, the spatial or tem-poral variations of the thermodynamic properties of thefluid are associated with velocity variations or with inho-mogeneities of the initial or boundary conditions.

In a real flow, other phenomena modify the flow prop-erties. Fluids, like solids, are heat conductors. Thermalconductivity is a property of the fluid that tends to evenout the temperature differences in the fluid. If a volume ofhot fluid is in contact with a volume of colder fluid, ther-mal conductivity creates a heat flux which transfers heatfrom the hot fluid to the cold fluid. Fluids have anotherpropertyviscositywhich tends to smooth the momen-tum differences in the fluid, which implies a smoothingof velocity differences. For example, a body with a flatsurface moving parallel to its surface entrains the fluid inits neighborhood (Fig. 1). Momentum diffuses in the fluid.Momentum diffusion is similar to the heat diffusion whichwould take place if fluid at rest were in contact with a hot-ter (or colder) plate. As shown in Fig. 1, the fluid velocityis equal to the velocity of the body along the fluid-body in-terface. In standard aerodynamics, the temperature of thefluid and of the solid are also identical along the fluid-solidinterface.

A. Forces in the FluidLet us consider a fluid volume V surrounded by fluid andlimited by a surface (Fig. 2). Let d be an element ofthis surface. First, let us assume that the flow is inviscid.Along d the volume V is submitted to a force normalto d which is called the pressure force. The pressureforce points towards the inside of volume V . In a viscousflow, the pressure force is also present but another force isexerted on volume V . This additional force due to viscosityis not generally normal to d . In a viscous flow, the fluidin V can also exchange heat with the outer fluid throughd .


Aircraft Aerodynamic Boundary Layers 303

FIGURE 2 Local forces on the surface of a volume of fluid. Thevector n is a unit vector normal to d and pointing outwards fromV .

B. Forces on the BodyNow, let us consider the surface of a solid in contact with afluid (Fig. 3). In an inviscid flow, along a surface elementd the fluid exerts a pressure force on the solid; this pres-sure force is normal to d . In a viscous flow, an additionalforce due to viscosity is tangential to d ; heat transfer canalso occur between the fluid and the solid through d .

In aerodynamics, it is often convenient to consider thatthe fluid is moving around the body at rest. Indeed, itis shown that if the motion is stationary (independent oftime), the flow around the body moving through the fluid isequivalent to the flow around the body at rest immersed ina moving fluid. Only the relative motion of the body withrespect to the fluid is significant. In particular, in a windtunnel, the body is at rest and the fluid moves around it.

An airfoil is a two-dimensional body designed to pro-duce a lift. The relative velocity of the fluid with respectto the airfoil measured far ahead of the airfoil is the free-stream velocity V (Fig. 4). The chord c of the airfoilis the distance between the leading edge and the trail-ing edge. The chord line connecting the leading edge andthe trailing edge forms an angle the angle of attackor the incidencewith the free-stream velocity. The totalforce exerted on the airfoil is obtained by integrating thepressure forces and the viscous forces over the bodys sur-face. The aerodynamic forces are resolved into the lift Land the drag D acting perpendicular and parallel to V,respectively.

FIGURE 3 Forces applied to a surface element of a body in rel-ative motion in a fluid.

FIGURE 4 Aerodynamic forces on an airfoil.

III. STRESSES AND HEAT FLUXESIN A FLUID

A. Modeling of Stresses and Heat FluxesFrequently, the flow is described by the Euler variables,i.e., by the velocity field at any point in space and at anytime. The description is completed by the density, pres-sure, and temperature fields.

The velocity gradient is a second-order tensor whosecomponents in an orthonormal axis system are ui

x j. The

velocity gradient is broken down into a symmetric ten-sor si j (si j = s ji ) and a skew-symmetric tensor ri j (ri j =r ji ):

ui

x j= si j + ri j

with

si j = 12(

ui

x j+ u j

xi

)and ri j = 12

(ui

x j u j

xi

)

The rate of strain tensor si j defines the deformation ofa small volume of fluid, whereas tensor ri j defines therotation of this volume. Indices i and j can take the values1, 2, or 3; x1, x2, x3 are x , y, z, respectively; and thecorresponding velocity components are u, v, w.

Let V be a control volume of fluid (Fig. 2). The forceexerted on the control volume over its surface element dis

d f = d f pressure + d f viscositywhere the pressure force and the viscous force are givenby:

d f pressure = pi j n j d ei (1)d f viscosity = i j n j d ei (2)

The Einstein notation is used in the above formula.The repetition of an index in a term implies a summa-tion. For example, i j n j is equivalent to

j=1,3 i j n j (i.e.,

i1n1 + i2n2 + i3n3).


304 Aircraft Aerodynamic Boundary Layers

The quantities p and i j are called stresses. They havethe dimension of a force per unit area. The stresses p andi j are defined at any point in space, whereas d f pressureand d f viscosity depend on the surface element via the unitnormal n.

In relations 1 and 2, p is the pressure, i j is the viscousstress tensor, i j is the second-order unit tensor (i j = 1if i = j , i j = 0 otherwise), n j are the components of theunit vector orthogonal to d and pointing outwards fromvolume V , and ei are the unit vectors of the referenceorthonormal axis system. Formulas (1) and (2) express thatthe pressure force is normal to d , whereas the viscousforce can have a tangential component. It is shown that i jis a symmetric tensor (i.e., i j = j i ).

The amount of heat received by the control volume Vthrough d per unit time is

d Q = j n j dwhere j are the components of the heat flux vector.

As a good approximation, air is considered as aNewtonian fluid, i.e., the viscous stress tensor is expressedby a linear function of the rate of strain tensor:

i j = 2si j + ulxl

i j

where and are viscosity coefficients. This relationshipassumes that ri j does not create any viscous stress.

For most practical purposes in standard aerodynamics,Stokes hypothesis, 2 + 3 = 0, is valid. Then, the vis-cous stress tensor is given as:

i j = 2si j 23ul

xli j

Stokes hypothesis implies that an isotropic compressionor dilatation does not create any viscous stress.

The dynamic viscosity coefficient can be obtainedfrom the kinetic theory of gases or experimentally. For air,a good representation of is obtained from Sutherlandsformula:

= 0

TT0

1 + S/T01 + S/T

Where 0 = 1.711 105 Pl (the unit is 1 Poiseuille =1 kg m1 s1), T0 = 273 K, S = 110.4 K. Sometimes, thekinematic viscosity coefficient is used, =

.

The Fourier law is used to express the heat flux vector:

j = Tx j

where the thermal conductivity is related to the viscositythrough the Prandtl number P:

P = cp

For air, the Prandtl number is considered as a constantP = 0.725 for temperatures below 1500 K, and the spe-cific heat at constant pressure cp is also a constant, cp =1005 J kg1 K1.

Viscosity and thermal conductivity are thermodynamicproperties of the fluid. Even when the fluid is at rest,these properties are defined. The behavior of the flowis influenced by the effe

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