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THE MORPHOLOGY OF FLIGHT VEHICLES
For someone who is fascinated with the phenomenon of flight and the aircraft that achieve it, there are several paths to their quantitative understanding. The traditional approach in schools of engineering proceeds from the mathematical, physical, and chemical fundamentals through a series of engineering sciences focused on solid and fluid mechanics to disciplinary studies with such names as structures, aerodynamics, propulsion, stability, control, and guidance. Only when all these foundations and pillars are erected for the "temple of aerospace engineering" is the student usually invited to synthesize his knowledge in a design project or in analyses of the overall configurations of flight vehicles.
Particularly for those who enter the field at an advanced or postgraduate level, we are concerned that this traditional path may call for unwarranted patience. It demands faith that, in the long run, all the pieces will fall together. Still building on the engineering-science fundamentals and trying to preserve much that is attractive about the disciplinary route, we offer in this book a partial alternative. We have found, in the vehicle's equations of motion, a unifying framework for studying many important questions that affect its design and arise during the extensive analytical investigation which contributes to that design. In Chapter 2 we shall present those equations and begin our discussion of their various terms. By way of introduction and motivation, however, we begin here with some qualitative descriptions of flight vehicles and the reasons for their general arrangement.
Much of what we say is concentrated on powered aircraft, that class of vehicles which is capable of sustained cruising flight within the lower atmosphere. Clearly, our point of view can be extended to vehicles which transport spacecraft to and from their extra-atmospheric environment. Even spacecraft themselves, when their structures are sufficiently rigid, fall within our scope, and some attention will be paid to all of these. The static and dynamic operation of submersible ships can also be analyzed, in common with balloons and airships, by rather standard aeronautical methods. Only when vehicles contact the interface between two terrestrial media, as in the case of surface ships and wheeled landcraft, do some of the force systems which act on them fall beyond the range which we hope to cover.
1.2 KEY FACTORS AFFECTING VEHICLE CONFIGURATION
Whole treatises have been written on why airplanes look the way they do. Stinton (1966) is a splendid example; the reader who would enjoy a discursive and less concentrated treatment of the subject is invited to browse in Stinton's book and others like it. Since our aim here is introductory, we must be brief and therefore skim over numerous important details. The several photographs and sketches accompanying this chapter will illustrate our remarks.
Every vehicle serves some transportation function—for people, cargo, armament, measuring instruments, or even just its own equipment and fuel. The enclosed volume which carries and shields this payload is the fuselage, although we observe that other vehicle elements usually assist in this function, an extreme case being the flying wing. The most efficient shape for simply containing material would be a sphere, whose minimum surface area for a given volume would need the least weight of structure for protection and support. This is a good shape for spacecraft and has often been adopted. When rapidly moved through an atmosphere, however, the sphere experiences too high an aerodynamic force (drag D) opposite to its direction of motion. Early designers, looking at birds and fish, found the remedy in streamlining—making the fuselage length in the flight direction quite large compared to its cross-sectional dimensions, and inclining most of its surface area at small angles to the relative wind.
Drag is unavoidable in practice. Hence, except for such transitory operations as gliding and atmospheric entry, a propulsive force is required, and it is furnished by one or more engines or rocket motors. Whether this system involves a reciprocating engine and propeller, turboprop, turbojet, turbofan, ramjet, liquid rocket, or solid rocket, it works by the principle of reaction propulsion. Backward momentum is imparted to the atmospheric gases (propeller), to material stored within the vehicle (rocket), or to a combination of these two, including products of combustion (most other devices). Thrust—which is the reactive force thereby applied to the vehicle—may simply counterbalance drag or may produce longitudinal acceleration, increased altitude, or, for a spacecraft, changes in the parameters of an inertial orbit.
The propulsion system is often housed in a distinct element of the vehicle, such as a nacelle or jet-engine pod. Alternatively it may be internal, with only an air inlet and exhaust nozzle visible from the outside. On some extreme configurations, such as the hypersonic ramjet, large portions of the aircraft's surface participate in slowing down the air (compression) before it enters the combustion chamber, and then reexpanding the combustion products into a high-speed exhaust slipstream.
Another force which dominates the performance on all but interstellar craft is the weight W exerted by gravitational fields. In level cruising flight, weight is counterbalanced by an aerodynamic force (lift L) normal to the flight direction. Some lift is usually contributed by the fuselage, but a more efficient device for its production is the wing—a flattened, often cambered or twisted surface which intersects the fuselage, but usually has its longest dimension (span) normal to the airspeed vector. In rectilinear cruising flight, the wing lies close to a plane parallel to the local horizontal. A well-designed wing is a marvelously effective device for lift generation, to the degree that the ratio L/D may approach 20 on powered aircraft flying below the speed of sound and exceed 40 on a high-performance sailplane.
The next most prominent feature of atmospheric aircraft is a group of lifting surfaces known as the tail or empennage. The most common arrangement has its location at the rear of the fuselage and consists of one portion (horizontal stabilizer) roughly parallel to the wing plane and a second (vertical stabilizer or fin) which is perpendicular, lying in the vehicle's central plane of symmetry. The immobile parts of these surfaces play a stabilizing role similar to a weathervane. Thus, when the vehicle sideslips, acquiring a component of relative wind normal to its symmetry plane, the vertical stabilizer gives rise to a yawing moment about a vertical axis through the center of mass (CM) and tries to rotate the fuselage axis toward the relative wind. As we shall discuss later, one historical feature of design evolution has been the tendency of the fin's area to grow progressively larger compared to the wing's.
The horizontal stabilizer applies pitching moments, which work to fix the inclination of the relative wind to the wing plane (angle of attack). It also assists with the "trimming" process of canceling pitching moments about the CM due to the wing lift, fuselage, etc.
The need for controls—movable portions of the various surfaces in the form of rotatable flaps along the trailing edges—is most clearly understood in connection with trimming. The wing lift depends on both angle of attack and airspeed, so that this angle must be readily adjustable to ensure that the weight can be supported in various flight conditions. Clearly, equilibrium demands that the resultant pitching moment about the CM be zero, but the contribution of wing lift to this moment often varies rapidly with angle of attack. The most efficient way to make the required pitching-moment adjustments has usually proved to be by controlling the tail lift with a trailing-edge "elevator." On vehicles which are to fly near or above the speed of sound, however, the tail must normally possess so much power that the whole horizontal stabilizer is rotated at the pilot's command. For instance, the Boeing 707 and many other transports are furnished with both a screw-jack arrangement that sets the stabilizer incidence for gradual trim changes and a rapidly actuated elevator for maneuvering, pulling up the nose during landing, and the like. Occasionally the horizontal stabilizer is placed far forward on the fuselage; in such cases it is called a canard (like a duck's bill).
For altering flight direction, banking the wings relative to the horizon plane, and performing other maneuvers, control surfaces must also be provided which can affect either the yawing moment or the moment about a longitudinal axis through the CM (rolling moment ). Yawing control is supplied by the rudder, a flap acting at the trailing edge of the vertical stabilizer. The rudder has a trimming function in such situations as a steady turn or multi-engine flight when one engine is inoperable. Rolling is accomplished by ailerons and/or spoilers, placed near each wing tip and deflected in an antisymmetrical manner. At high speeds, rolling moment may be exerted simply by differential rotation of two all-movable horizontal stabilizers. The wing flaps resemble control surfaces, but they are actuated slowly and only at low speeds, where they augment the lift to facilitate landing or takeoff.
Devices available to the pilot for moving the various control and trimming surfaces, cockpit layout, landing gear, and other internal details of the vehicle fall somewhat out of this chapter's scope. The book by Langewiesche (1944) has an excellent discussion of how the airplane flies and the proper use of controls. It is worth mentioning here that many large controls on high-performance vehicles are actuated by hydraulic or electric devices, since their direct mechanical operation by cables may require excessive effort from the pilot. Both power-boosted and manual controls may also be provided with automatic gadgetry which assists the pilot: autopilots are employed to help him maintain the direction, speed, and altitude of flight; elaborate stability augmentation systems (SAS's) modify the apparent dynamic behavior so as to improve controllability and render the "handling qualities" more acceptable to him.
Section 1.3 describes a historical series of flight vehicles, suggesting some of the reasons for their configurations in the light of the foregoing generalities. We must initially mention a few more observations which, we believe, apply quite broadly. The first is that any significant feature of a design can best be understood as the result of compromise among the recommendations of several engineering specialists who view it from different perspectives. Such compromises are known as "tradeoffs." An example might be determining the proportions of a low-speed wing. The aerodynamicist concerned with drag and cruising efficiency wants the wing area to be small (to minimize skin friction) and the span to be very large (to reduce the "induced-drag" penalty for producing lift, as we shall discuss further below). He is opposed by another specialist, seeking safe takeoffs and landings on short runways, who insists on high area to give plenty of lift at low speed. The structures man argues for reduced wingspan, so that the internal bending loads due to lift can be sustained with the least amount of material and the weight thereby reduced. Several others may simultaneously insert their requirements into the wing-design process. The role of skillful engineering leadership is then to arrive at an optimum configuration—a tradeoff which, in some sense, represents the best resolution of the conflict.
Another key concept is that of the design point. For many aircraft, considerable flying time is spent near a certain combination of standard atmospheric altitude and Mach number, and there is a desire to ensure especially efficient operation at this point in the envelope of possible flight conditions. The various parts are therefore optimized with this condition in view, although constraints such as a limit on takeoff run cannot be completely overlooked. Some vehicles give rise to extreme difficulties because of having two distinct and incompatible design points, one example being the penetration bomber which must cruise supersonically at high altitude and also fly down over the "nap of the earth" just below M = 1. There are also air-superiority fighters and multipurpose aircraft whose design is dominated by considerations of dynamic maneuverability or by the need to do several different things with equal efficiency.
As you know, flight vehicles come in a bewildering multiplicity of shapes, sizes, and arrangements of components. As we attempt to clarify the rationale for these differences, it is useful to ask questions such as "What was the design point?" and "What were the technological resources available at the time of development?" These are not always easy to answer, but one is surprised, after having reached some depth of understanding, at how often the actual configuration seems to lie pretty close to an optimum under the given circumstances. Aerospace engineers tend to set for themselves very demanding objectives and requirements. Someone has remarked that, given the technology when they were first built, most successful aircraft are just barely able to do what is asked of them. Whether one is studying a Wright biplane, a supersonic transport, or a man-rated booster, he discovers much truth in this "theorem of the barely possible."
1.3 SOME REPRESENTATIVE FLIGHT VEHICLES
Since pictures are a poor substitute for the real thing when the latter has the dimensions of, say, the B-70, we urge you to seek opportunities for first-hand inspection of machines like those reviewed in this section. The Smithsonian Institution, in Washington, D.C., exhibits an outstanding collection of historical aircraft, engines, and spacecraft, as does England's National Science Museum in the Kensington area of London. At the Wright-Patterson Air Force Base near Dayton, Ohio, there is free admission to the Air Force Museum, whose unique series of transport, military, and experimental aircraft covers the period from 1912 through to the present.
Limited both by space and the aims of this book, we shall point to only a few highlights of a dozen or so designs. Much more extensive, for instance, is the treatment of Stinton (1966); the forthcoming treatise by Hoff (still in preparation) promises to be a compendium of engineering information and critical judgments on the world's aircraft. Each year, McGraw-Hill publishes Jane's All The World's Aircraft, an illustrated summary of performance and other features of current types (see Taylor, Ed.).
1. Strut and Wire-Braced Biplane of World War I Era (Curtiss R-4)
During the first decade of powered flight, 1903-1913, configuration was going through rapid, somewhat haphazard evolution. There were therefore many deviations from the prototypic arrangement outlined in Section 1.2, e.g., the Wright Flyers had canard stabilizers and employed antisymmetric warping of the wing structure for roll control. By the time of the R-4 (Fig. 1.1), however, morphology had evolved until it was close to that of more recent aircraft.
A modern designer would naturally reject the biplane wing because the numerous compression struts and bracing wires required for strength and stiffness cost a huge penalty in drag. Several factors forced the choice of biplanes and even occasional triplanes. The tradition persisted of adopting very thin airfoil sections, with maximum thickness only a few percent of wing chord, and thus providing little depth for the structural beam which sustained the lift loads. A certain minimum span of wing was essential to ensure that thrust exceeded drag just after takeoff and in slow flight generally, as can be seen from the formula relating induced drag (due to lift) Di wing span b, weight W, and dynamic pressure q:
Di = W2/πqb2 (1.1)
Di constitutes the bulk of drag at low vc, with the contribution of skin friction, which is roughly proportional to v2c predominating near top speed. Finally, the best available materials were high-strength woods, coated fabric for skins, and steel. The steel was so heavy that its application had to be limited to engine parts, wires, and certain truss structures. With all these constraints, the designer had to go to a biplane in order to obtain a structural "box" adequate to the bending moments, shears, and torques which it had to withstand.
Excerpted from ENGINEERING ANALYSIS OF FLIGHT VEHICLES by Holt Ashley. Copyright © 1974 Holt Ashley. Excerpted by permission of DOVER PUBLICATIONS, INC.
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