BOOK REVIEWS C f NOTICE$

SOUND BARRIER by Neville Duke & Edward Lanchbery

Published in 1955 by Philosophical Library, Incorporated

15 East 40th Street New York, New York

Price $4.50 Reviewed by Commander W. H. Cullin, USN

The Sound Barrier is a book written for laymen that meet its authors’ aim, namely, “a clear readable exposition of current aeronautical development.” The use of mathematics is negligible and the authors pre- suppose the readers have a minimum of technical background. The explanations of aeronautical phe- nomena are excellent and authoritative. The book is highlighted by good use of pictures and schematics to assist the explanations.

The book, one would suppose, would be exclusive- ly a recital of English efforts and successes in re- lation to the Sound Barrier. However, in the preface to the sixth edition, the authors apologize in these cryptic remarks, “and if, alas, the emphasis lies on American progress, it should be remembered that the US . releases news of its developments at an earlier stage than Britain, who will not admit the existence of a new aircraft until it has appeared in the sky.”

The book encompasses four parts, (1) Supersonic and High Altitude Flying, (2) Design for Speed (engines, aircraft, and pilot), (3) Design For Use, (4) Things to Come.

In the chapter on supersonic and high altitude fly- ing the authors first cover the fundamentals of the atmosphere and fluid mechanics of high speed flight, and it is done in an excellent manner. However, the English method or choice of words has a debilitating effect upon the explanations. An example is “Below the speed of sound, contraction is the dominating

factor; but the higher the speed the greater the fac- tor of expansion until at sonic speed itself there is a stage of fluctuating contraction and expansion that causes a pressure disturbance to travel in rises and falls like the ripples from a stone dropped into a lake; in other words, a sound wave.”

This chapter also discusses the effect of local Mach number, or compressibility effects on control of air- craft, in addition to comments on the cause and ef- fect of the phenomena of sonic bangs, and advances a number of current theories to explain them.

The authors further note an interesting report of Raymond Rice, the Chief Engineer of North Ameri- can Aviation, who estimates that the pressure of a shock wave generated by supersonic flight at 1,750 ft. is about 12 lb/sq. ft., and 50 lb/sq. ft., if the shock wave is set off at 250 ft. The latter is equivalent to the force of a 140 mph hurricane. Experts in England are quoted to consider that if sonic speeds were main- tained for a period of 30 sec., the resultant bang might approach the severity of bomb blast. In a lecture to the U. S. Institute of Aeronautical Sciences, Raymond Rice further suggested that as the pressures and di- rections of shock waves could be predicted, it should be possible to mobilize sonic bangs as a weapon of warfare. His idea was that an aircraft with a Mach Number of about 1.2 or 1.5. and as large a cross- section as possible, could be construct2 with the express purpose of generating aimed sonic bangs

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BOOK REVIEWS AND NOTICES

against an enemy as an alternative to bombs. (Under- scoring is the reviewer’s.)

In the engine portion of the chapter, Design for Speed, the authors have used the old chestnut of quoting horsepower potentialities that have no mean- ing when applied to ram and pulse jet engines. How- ever, the elementary descriptions of the various types of engines such as turbo jet, ram jet, pulse jet, and rockets, combined with the history of the develop- ment of these engines, is done in a most clear and interesting manner, and covers American, British, and French efforts.

The authors make some very candid statements, to wit: “Both Armstrong Siddeley and de Havillands are continuing work on the development of the rocket motors, and the Ministry of Supply has its own rocket research station about which practically no information is available. This, it is feared, is not only on account of security, but because Britain lags as far behind the United States in rocket propulsion as the United States was behind Britain in achieving jet- propelled flight in the first instance.”

It is considered that an error has been made by the authors in the following statement. “In this case the compressor is sprayed by water which evaporates, cooling the compressor and also the air that it is handling.” The normal practice is that the water, or water alcohol mixture, is sprayed directly into the air stream just prior to entering the compressor rather than on the compressor. The benefits claimed by the authors for thrust augmentation by water in- jection are correct. A very revealing statement made by the authors, especially when the recent commen- tary on the 540 engine of the U. S. Navy is consid- ered, is the following: “Although the development of a new engine may be initiated by Ministry re- quirements which are beyond the capacity of existing designs, or from a project planned jointly by aircraft constructor and engine manufacturer, jet engines have been generally kept slightly ahead of aircraft specifications.” This is a policy we could well emulate.

In the portion of this chapter, Design for Speed, devoted to aircraft, is an excellent explanation of induced drag, a discussion of boundary layer con- trol, and methods of reducing or forestalling critical Mach Number effects.

Discussions of the aero-isoclinic wing, a supersonic straight wing biplane, reverse thrust, the flexible deck, V.T.O. aircraft and flight power control sys- tems are also included.

A very interesting and extensive discussion is made of the pilot’s limitations in relation to new aircraft design (i.e., G forces, prone position for pilots, pres- sure suits, pressurized cabins). The authors state “There is little difficulty or discomfort, provided the body is properly supported, in sustaining a force of 10 G for periods of from 20 to 40 sec. In direct ac- celeration, this is the equivalent of from zero to 1,098 mph in 5 sec.” The fu-st manned rockets will probably take off with an acceleration of 6 G lasting

about a minute, to reach speeds in the neighborhood of 25,000 mph. A further point is made that gives one considerable thought. “If two aircraft with speeds a little faster than those already flown by Bill Bridge- man in the Douglas Skyrocket were to come out of the clouds a mile away travelling head on, neither pilot would have time to see the other machine before the crash occurred.” Quite apart from the question of illumination, speed itself sets its own problem as it begins to outpace the nerve impulses of the human system.

The third chapter, “Design For Use,” discusses the question of sweep back, new military features, and questions of reliability in the design of commercial aircraft.

The authors take you step by step through the de- sign considerations and the compromises required, in general terms, to produce the trio of British “V” bombers, Vickers Valiant, Glosters Javelin, and the Aero 698, delta wing Vulcan. A similar treatment is made in this portion of the book of commercial jet airliners’ design, with cost per passenger mile, ex- tensive testing done, etc. It should be noted, however, the cost per passenger mile and freight cost per ton mile is given in English money abbreviations and therefore subject to further interpretation.

In the final chapter, Things to Come, the authors point out that so far, designers have sought to save weight by dispensing with the fuselage in the experi- mental flying wing, and have succeeded in dispensing with parts of the tail assembly in delta wing aircraft. They note the only unproductive part of the aircraft, in the aerodynamic sense, that still remains constant, is the landing gear. In view of these comments, a dis- sertation on the values of the new seaplane configura- tions is conspicuous by its absence.

The authors bring out a salient point in the fact that Aviation Medical research, in the area of the effects of high speed flight environment on the crew and passengers, should be completed prior to the time the aeronautical engineer first sits down to his drawing board, which is usually 5 to 10 years before the projected airplane is first flown.

The book, “Sound Barrier” is readable and invig- orating in its coverage of the material concerning aeronautical development to date and should be of interest to anyone desiring the broad picture of cur- rent aviation. The authors’ remarks in the final chap- ter are, I believe, revealing and worthy of mention here again:

“The scientist and the designer have much in com- mon with any artist, and their approach, interpreta- tion, and vision, are just as individual. What one favors, another rejects; and it was through the in- dividual’s unwavering faith in sometimes seemin~lv conflicting convictions that Britain fu-st gained su- premacy in jet and high speed flight. Many ways lead to the same end; there is no simple, direct route -that is the crux of the matter, the vital point which must be borne in mind when politicians talk glibly of nationalizing the aircraft industry.”

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THE THEORY OF AEROELASTICITY By Y. C. Fung

Published by John Wiley and Sons., Inc., May, 1955 490 pages

Reviewed by Lieut. (jg) I. H. Stockel, U.S.N.R.

Dr. Fung states that his book is an outcome of a course in areoelasticity which has been taught at the California Institute of Technology since 1948. It treats a complex subject at the graduate level; consequent- ly, all who have need for a reference to aeroelasticity will gain by his effort.

Aeroelasticity concerns the mechanical behavior of elastic structures subjected to aerodynamic forces, Prerequisite to its study is an understanding of strength of materials, dynamics of mechanical sys- tems, and e!ementary and advanced aerodynamics. These are summarized in Chapters 1,12, 13,14 and 15.

There are two kinds of aeroelastic problems: the stability problem for which only the conditions of instability are sought; the response problem for which the displacement amplitudes as well as fre- quencies are desired. These are presented in C h a p ters 2 through 11.

Dr. Fung’s work contains a healthy quantity of physical reasoning. The fundamental principles are emphasized and those assumptions which he finds expedient are honestly admitted and clearly stated. An extensive bibliography of available literature is included at the end of each chapter and frequent and specific reference is made to these listings in the text material.

Chapter 1 reviews strength of materials, elemen- tary aerodynamics and dynamics of mechanical sys- tems. The elementary theories of bending and torsion are reviewed. The reader is reminded to judge the applicability of these elementary theories only by the accuracy with which they describe the influence of load on deformation. The formulation of aeroelastic problems requires that the deformed shape of an elastic structure be determined for its loading and the solution does not include a stress analysis. Shear cen-

n. ter is defined, with particular emphasis being given to the number of possible definitions of this term. Elas- tic axis, flexural center, center of twist and flexual line are defined. The concept of “influence” between load and deformation is developed. The uniqueness of the elastic problem and the symmetry of the “in- fluence” matrices are established.

The generalized aerodynamic force is shown to be a function of angle of attack, Reynolds’ Number, Strouhal Number, Mach Number, dynamic pressure

and an area. Lift, drag, pitching moment and their associated coefficients are defined. Vortex theory for wings of infinite and finite span is summarized and a wing having an elliptic lift distribution is discussed. The “strip theory” for spanwise lift distribution is described.

The fundamental principles of generalized coordi- nates and Lagrange’s equations are reviewed and the applications of these are demonstrated with five ex- ample problems. Normal coordinates are defined and complex representation of harmonic functions is de- scribed. Lastly, the torsional oscillations and coupled torsion-flexure oscillations of a cantilever beam are treated.

Chapter 2 introduces the main body of the work with some problems in civil and mechanical engineer- ing. Because the structural shapes are “unstream- lined,” these problems are not given a theoretical treatment. Experimental results are reviewed to ex- plain the natures of the aerodynamic forces and sev- eral representative examples of aeroelastic oscillation are given.

Chapters 3 and 4 treat steady-state aeroelastic stability problems. Wing divergence is employed to illustrate fundamental concepts and methods of solu- tion. This problem is formulated for an idealized three-dimensional wing. The “strip” assumption is introduced and an “exact” solution is obtained for a rectangular wing. The results of two approximate methods based on an assumed deformed shape (the second. of which employs Lagrange’s equations) and one method employing successive approximations are compared to the “exact” results. Dr. Fung takes this opportunity to introduce matrices, matrix calcu- lus and the reduction of differential and integral equations into matrix equations with finite differ- ences. An application of the latter is outlined for the divergence of a wing having varying section.

Chapter 4 considers steady-state aeroelastic prob- lems in general. The analysis of wing divergence is extended for steady-state problems to include more accurate aerodynamic theories. Loss of aileron effi- ciency, reversal of control, aerodynamic lift distribu- tion over a rigid wing, the effect of elastic deforma- tion on lift distribution, swept wings, tail efficiency, static longitudinal stability of an aircraft, and twist-

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ing of propeller blades are discussed with varying degrees of analytical development.

The phenomenon of flutter is treated in Chapters 5, 6 and 7. Chapter 5 employes dimensional analyses to give a general discussion. Elastic stiffness and mass balancing are related to flutter prevention and it is shown that flutter occurs when energy is added to the airfoil by the airstream. Experimentation in flut- ter analysis and similitude are discussed. Chapter 5 concludes with a resum6 of the early history of flutter.

Chapter 6 presents two approaches to the problem of flutter of a cantilever wing. To facilitate the intro- duction of analytical solutions, the first assumes quasi-steady aerodynamic derivatives and employs Galerkin’s method. The second, based on linearized two-dimensional thin airfoil theory, is less restrictive: the method of generalized coordinates is used.

Chapter 7 extends the analysis of flutter to the three dimensional wing. Generalized coordinates are employed and the choice of these coordinates and the limitations of this theory are discussed. The subject is closed with some remarks on the control and pre- vention of flutter.

Chapter 8 introduces the “response” type aeroelas- tic problem. The response of an airplane to gust load- ing is employed as a vehicle for explanation. Some mathematical concepts are reviewed and the response to a gust of known character is determined. The sta- tistical aspects of this problem are considered where-

in the concepts of probability and distribution func- tions are included. Lastly, the response of an airplane to a measured gust is calculated.

Chapter 9 continues the considerations of response with discussions of “buffeting” and “stall flutter.”

Chapter 10 demonstrates the application of the Laplace transform to the solution of both types of aeroelastic problem, response and stability. These ap- plications are illustrated with examples in “gust” re- sponse and flutter. The Laplace transformation and the theorems which govern its use are reviewed briefly.

Chapter 11 concludes the subject of aeroelasticity with a general formulation of aeroelastic problems. The “closed loop” nature of these problems is recog- nized. They are described with block diagrams in which the functional operation of each block is deter- mined and the net behavior of the total system is sought.

Chapters 12 thru 15 conclude the book with a con- densed study of advanced aerodynamics. Much of the aerodynamic theory required by the preceding chap- ters is included here. The subjects treated are the fundamentals of nonstationary airfoil theory, oscillat- ing airfoils in two-dimensional imcompressible flow, oscillating airfoils in two-dimensional compressible flow, and unsteady motion with an abbreviated des- cription of experiments and experimental results.

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