Category Aircraft Flight

The conventional wing

There are various methods of generating lift, as we shall describe, but we will start with the conventional wing.

In the conventional or classical aeroplane, each component serves one main function. The names and purposes of the principal components are shown in Fig. 1.3. In this classical configuration, nearly all of the lift is generated by the

The conventional wing The conventional wing The conventional wing
The conventional wing

The conventional wingprovide directional

stability and control

Tailplane (honzontal stabiliser) provides stability and control

Fig. 1.3 The classical aeroplane

Each component serves only one main purpose wing. The tail, which is intended only for stability and control, normally pro­vides a slight negative lift or downforce.

Early attempts at aviation were often based on bird flight, where the flapping wing provides both the lift and the propulsive thrust. The classical arrangement (often attributed to the English engineer Cayley), provided a simpler approach that was better suited to the available technology. Some unconventional arrangements do have theoretical advantages, however, and because of advances in technology, they are becoming more common. On some recent aircraft types, the tail, and even the fuselage may contribute significantly to the lift, but we will deal with such departures later.

The influence of aspect ratio

The amount of lift generated depends on the circulatory strength of the bound vortex, and on its length, which in turn depends on the span of the wing. A given amount of lift can be generated either by a short strong bound vortex, or a long weaker one. The longer weaker bound vortex will produce weaker trailing vortices, and as the downwash produced by the trailing vortices is responsible for the trailing vortex (induced) drag, the longer wing will produce less drag.

The longer the wing is, the weaker is the bound vortex required. For a given wing section and angle of attack, the strength of the bound vortex depends on the wing chord, so for a given amount of lift, the chord required reduces as the wing span is increased. Thus, wings designed to minimise trailing vortex (induced) drag, have a long span, with a small chord: in other words, the aspect ratio is high.

For a given wing section shape, any reduction in chord produces a corres­ponding reduction in depth. Therefore, as the aspect ratio is increased, it becomes more difficult to maintain adequate strength and stiffness.

Competition gliders or sailplanes often have wings with an extremely high aspect ratio, but for both structural and aerodynamic reasons, low aspect ratio wings are more suitable for very manoeuvrable aircraft such as the Hawk trainer shown in Fig. 9.2.

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Подпись: WINGS

Fig. 2.9 High aspect ratio and large wing area were used on the Lockheed TR-1, which was designed for long range and endurance

(Photo courtesy of Lockheed California Co.)

 

The influence of aspect ratio

Because high aspect ratio wings have a good ratio of lift to drag, they are used on aircraft intended for long range or endurance. The aircraft shown in Fig. 2.9 is a good example. It is noticeable that long-range, high-endurance sea-birds also have high aspect ratio wings. The albatross has an aspect ratio of around 18. However, very low aspect ratio wings, such as those of Concorde, produce less drag in supersonic flight, as will be explained in later chapters.

Boundary layer normal pressure (form) drag

Without the influence of viscosity, the streamlines or stream surfaces would close up neatly behind all parts of the aircraft, and there would be no wake. For a symmetrical shape such as that shown in Fig. 4.1, the streamline pattern and the pressure distribution would also be symmetrical, as in Fig. 4.1(a), and therefore, there would be no net resultant force. In fact, theoretical analysis shows, that if there were no viscosity, the pressure distribution would result in no net drag force on any shape. In the real case shown in Fig. 4.1(b), and Fig. 4.2, the streamline pattern and pressure distribution are not symmetrical, and a wake of slow-moving air is formed at the rear.

On shapes such as that shown in Fig. 4.1, the air pressure reaches its min­imum value at about the position of maximum section depth. Thus, over the tail portion, the air is flowing from a low pressure to a high one. As we have previously stated, this condition is known as an adverse pressure gradient, since the flow is likely to separate. Even if the flow does not separate, an adverse pressure gradient promotes a rapid degradation of available energy in the boundary layer, resulting in a reduction in pressure over the rear. Thus, on average, there is a lower pressure on the rear of the section than on the front, and therefore, there is now a net drag force, which is known as the boundary layer normal pressure (form) drag.

When the flow does separate, as illustrated in Fig. 4.1(b), the pressure down­stream of the separation positions is nearly uniform at a low value. Hence, the boundary layer normal pressure (form) drag will be high.

Boundary layer normal pressure (form) drag

Fig. 4.1 The effects of viscosity

(a) Theoretical flow pattern obtained when the effects of viscosity are ignored

(b) Typical actual patterns for a real air flow

In general, the further forward the separation positions are, the greater will be the area of low pressure, and the higher will be the drag.

Note, that since there is a loss of available energy in the boundary layer, Bernoulli’s relationship does not apply there, as it is based on the assumption that the amount of available energy remains constant. In the boundary layer and the wake, the speed and the pressure can be simultaneously lower than in the free-stream values.

The term boundary layer drag (profile drag) is used to describe the combined effects of boundary layer normal pressure drag and surface friction drag. It is often convenient to combine these two forms of drag, as they both depend on the wing area and the dynamic pressure (l/2pV2). At constant altitude, both of these contributions to drag rise roughly with the square of the speed.

Thrust and momentum

The propeller, the jet, and indeed all conventional aircraft propulsion systems involve changes in momentum of the air. When a change of momentum occurs, there must be a corresponding force, but it should not be thought that thrust is caused directly by the change of momentum, with no other mechanism being involved. As we have seen in the above examples, the force is produced and transmitted to the structure by pressure differences acting across the various sur­faces of the device. It is perhaps best not to think of rate of momentum change and force as cause and effect, but as two consequences of one process. In mak­ing practical measurements, or even theoretical estimates, we normally have to consider a combination of pressure-related forces and momentum changes.

Comparison between jet and propeller for thrust production

Figure 6.3 shows a jet aircraft and a propeller-driven one producing equal amounts of thrust at zero forward speed. In the case illustrated, the jet engine is transferring energy to the slipstream or jet five times as fast as the propeller. Since this energy must ultimately have come from the fuel, it indicates that the propeller-driven aircraft is producing the thrust more economically.

When the aircraft are in motion, the jet engine will still transfer energy to the air at a faster rate than the propeller at any given thrust and forward speed, but the difference in energy transfer rate becomes less marked as the speed increases.

Pure rocket propulsion

The pure rocket will work at very high altitude and in the vacuum of space. The high speed of the exhaust gases and the added weight of the oxidant that must be carried, however, mean that it is extremely inefficient in comparison with air-breathing engines at low altitude.

The thrust of a rocket motor comes from the high pressure on the walls of the combustion chamber and exhaust nozzle. The same high pressure produces the acceleration and momentum change of the exhaust gases.

Rockets have been used to assist the take-off, and for experimental high altitude high speed research aircraft, but one production rocket aircraft was the Second World War swept tailless Messerschmitt Me 163. The motor used two chemicals, one of which was highly reactive and, if it did not explode during a

Fig. 6.40 Turbo-ramjet propulsion for very high speed flight

The Lockheed SR-71 was capable of flight at Mach 3+

Note the central shock-generating movable spike in the axi-symmetrical engine intakes, and the exhaust nozzles fully open for operation with reheat The photograph was taken as the aircraft was manoeuvring at a high angle of attack. The strong conical vortices generated by the fuselage strakes and the wing have been made visible by the clouds of water vapour produced (not smoke). The engines have flamed-out leaving spectacular fireballs. The engine has a very complex internal variable geometry, and any mismatch is liable to produce a failure of the combustion process, leading to flame-out (Photo from Duncan Cubitt, Key Publishing)

heavy landing, was liable to dissolve the occupant. It was reportedly unpopular with pilots!

The swing-wing

One of the most obvious ways in which to satisfy the conflicting requirements imposed by a large speed range is to provide some mechanism to vary the sweep angle of the wing. Although this seems an attractive solution the mechanical problems faced in such a design are considerable. The hinge mechanism must clearly be at the root of the wing and this is the very position at which bending moment and structural demands will be greatest. Other important mechanical problems may be encountered such as the requirement to keep underwing stores, such as missiles or fuel tanks, aligned with the free stream direction as the sweep angle is changed on a military aircraft. It will also place restrictions on the positioning of the engines since wing mounting will clearly lead to severe complications.

In spite of these difficulties this solution has been employed on a number of aircraft, including the Tornado (Fig. 11.12), which was designed to fulfil a variety of roles from strike aircraft to high speed interceptor, and on the F-14 (Fig. 8.2). Both these aircraft are required to operate at high speed at low alti­tude. If the wing is operating at a relatively high loading then the increase in angle of attack due to an upwards gust will be less than that for a wing with a lower loading per unit area. This is because the more highly loaded wing will be operating at a greater angle of attack. A gust at a given flight speed will thus produce a smaller percentage change in angle of attack than it would for a wing operating at a reduced loading. This is a particularly important consideration for high speed low altitude operation and a swing-wing produces a suitable compromise.

Another method of sweep variation which has been proposed is to simply yaw the whole wing in flight as on the experimental NASA AD-1 shown in Fig. 8.16. This solution is not without its own complications, though, and some mechanical hinges may still be required (e. g. for any wing-mounted com­ponents, such as vertical stabilisers or at the wing fuselage junction). More­over the configuration is inherently asymmetrical in the swept configuration, and this is likely to lead to drag penalties because of the need for aerodynamic trim.

Control at high angles of attack

At low speeds, where the angle of the wing is high, and the aircraft is flying in a nose-up attitude, the tail surfaces may be partially immersed in the wake of the wing. To prevent this, the horizontal tail surface may either be mounted low so that it is out of the wake, as on the Tornado (Fig. 3.14), or high, as on the C-17 (Fig. 10.20). When the high option is chosen, the tail may need to be

WING TIP ROLL REACTION CONTROL VALVE

Fig. 10.21 Reaction controls are necessary on the VTOL Harrier (Diagrams courtesy of Rolls-Royce pic)

mounted very high indeed, as on the C-17, otherwise it may come under the influence of wing wash just before the stall. This creates a very dangerous con­dition known as superstall, where not only is the aircraft stalled, but the lack of control means that the pilot can do nothing about it. The stall is likely to deepen, and recovery at low altitude is impossible; a situation that has caused a number of fatal accidents.

Because of the dangers of stalling, particularly at low altitude, a number of automatic devices may be fitted to warn, or otherwise help the pilot to avoid a stall. These include a flashing light, and an audible warning that is triggered off if the angle of attack is sensed to be too high.

On small aircraft, the onset of a stall can often be sensed by the shaking of the control stick, caused by buffeting of the control surfaces by the turbulent separated flow. On aircraft with powered controls, this buffeting may not be transmitted, and a stick shaker mechanism can be fitted to shake the stick artificially, and thus provoke the required conditioned response from the pilot.

In some cases, particularly on large airliners, a stick pusher may be incor­porated. This is a device that automatically pushes the stick forwards to reduce

the angle of attack when it is sensed to be too high. A good description of such devices is again given by Davies (1971).

The use of reliable fly-by-wire systems enables aircraft to be flown much closer to the stall than was previously considered either possible or advisable. Some control systems may even allow the aircraft to be flown in a partially stalled condition.

Take-off and landing

Take-off and landing present a particularly hazardous part of the flight of an aircraft. During take-off the aircraft will be heavily loaded with fuel for the journey and the engines will be working at high rating in order to take off in as short a distance as possible. The take-off for a commercial airliner is further complicated by the need to adhere to appropriate noise abatement procedures. This may typically involve an initial climb at high angle in order to put the maximum distance between the aircraft and the ground at the boundary of the airfield. This may then be followed by the need to reduce throttle setting as a populated area is reached.

Landing, too, has its difficulties. The pilot has to manoeuvre the aircraft to a precise touch-down point in three-dimensional space. Furthermore when touch-down is achieved the aircraft must be flying in the correct direction, aligned with the runway, and must be at a low air speed to facilitate bringing it to a halt in a reasonable distance while retaining a safe margin over the stalling speed.

Take-off and landing have common features; for instance the object of the exercise is to transfer the weight from wheels to wings (or vice versa) in as short a distance as possible. However the conditions of weight are considerably different because most of the fuel will generally have been used on landing. Also the engine power output used will differ considerably in the two cases. For these reasons we consider them in separate sections.

The wing-bound vortex

A major breakthrough in the development of theoretical aerodynamics came when it was realised that a wing or lifting surface thus behaves rather like a rotating vortex placed in an air stream. This apparently odd conceptual jump was important, because it was relatively easy to mathematically analyse the effect of a simple vortex placed in a uniform flow of air.

In the simplest version of the theory, the wing is represented by a single vortex, which is known as the wing-bound vortex. In later developments, the wing is considered to be replaced by a set of vortices, as described further in Chapter 2. In Chapter 2 we also show how this vortex concept is very helpful in understanding the flow around a wing, and in analysing the influences of wing planform and general geometry.

The boundary layer and its control

One of the most important advances in the study of aerodynamics was the discovery of the influence of the ‘boundary layer’. This is a very thin layer of air adjacent to the surface of the aircraft. Despite its thinness, it holds the key to understanding how air flows behave, and in particular how lift is generated.

In this chapter we shall show how a knowledge of the behaviour of the boundary layer can enable us to improve the lift, drag and general handling characteristics of aircraft.

A major breakthrough

At the time of the Wright brothers’ first flight, a significant part of the basis of modern aerodynamic theory already existed in the form of the so-called clas­sical theory of fluid mechanics. In the early stages of its development, this theory took no account of the effects of viscosity (the stickiness of the air), with the unfortunate result that lift and drag forces were not predicted. For some time, therefore, it was little more than a toy for mathematicians. Equations which correctly took account of viscosity had been derived, but these ‘Navier-Stokes’ equations are extremely complex, and in their complete form they were of little practical use, until invention of the digital computer.

A big breakthrough occurred a few years after the Wright Brothers’ historic flight, when Prandtl found that the effects of viscosity were only important and apparent in a very thin layer adjacent to the surface. He called this the boundary layer. For an aircraft wing in cruising flight, it is, at most, only a few centimetres thick.

Although an exact analysis of the flow in the boundary layer was not possible, approximate methods based on experimental observation were developed. Outside the boundary layer, the effects of viscosity are negligible, and develop­ments of the classical theory can be used. By combining boundary layer theory

with the classical theory, it eventually became possible to produce results that were of real practical use.