Category Aircraft Flight

Tandem wing

Tandem wing is a name given to any configuration where both the stabilising and lifting surfaces contribute significantly to the overall lift, and this term thus includes the canard arrangement. A number of experimental aircraft have been flown with two surfaces of almost equal area, but their handling and stability characteristics often gave problems, as in the case of the popular pre-war Flying Flea. In particular, such designs seemed to be prone to a pitching oscillation; as though neither wing could quite make up its mind which was supposed to be dominant. However the popular and successful homebuild Quickie shown in Fig. 11.9 has proved that the tandem configuration can work. Further information on tandem and canard aircraft can be found in Bottomley (1977).

Fig. 11.9 Tandem wing

The Quickie Q2 has two sets of wings of equal span. Burt Rutan was associated with this clever design, which uses composite materials, and is intended for the home builder. The use of the front wing to double as undercarriage legs, though ingenious, has disadvantages in terms of ground handling qualities, and shock loading to the wing structure on landing. A version with a conventional tricycle undercarriage is available

Structural influences

Although this book is primarily concerned with aerodynamics and flight mechanics, we must consider some of the important interactions between the structure and its aerodynamic characteristics.

The final shape of the aircraft often results from some form of compromise between conflicting aerodynamic and structural requirements. We have already cited the case of the elliptical planform. Another example is in design of aerofoil sections, where it is important to take consideration of the fact that a structure has to be fitted within the contour. Much recent research effort has concentrated on designing low-drag wing sections that are also relatively thick.

The materials and methods of construction can also affect the degree of aerodynamic optimisation that can reasonably be achieved.

Aeroelastics

Apart from the obvious fact that the structural strength imposes limits on the aerodynamic loads that can be tolerated, the flexibility of an aircraft can have a profound effect on its aerodynamic behaviour. The aerodynamic effects due to structural flexibility are grouped under the heading of aeroelastics. Aero – elastic problems can be subdivided into static cases, where the inertia of the structure has little effect, and dynamic cases, where the inertia is significant.

Static cases

Divergence

If an upward load is applied to the leading edge of a wing near the tip, then it will try to twist (leading edge up) as well as bend (Fig. 14.1). Similarly, if we

Fig. 14.1 Flexural centre

(a) Load applied at the leading edge twists the section nose-up (b) Load applied at the trailing edge twists the section nose-down (c) At one point, the flexural centre, the applied load will cause the wing to bend without twisting

apply a load near the trailing edge, it will twist in the other direction. There is, however, one intermediate position at which the applied load will produce just bending, with no twisting. This is known as the flexural centre.

If the line of action of the resultant aerodynamic lift is in front of the flexural centre, then the wing will twist, with the angle of attack increasing towards the tips, as shown in Fig. 14.1(a). As the angle of attack increases, the lift force will rise, causing the wing to twist even more. If the wing torsional (twisting) stiffness is too low, the twist can continue to increase or ‘diverge’, until either a stall occurs, or the wing breaks.

The solutions to this problem of torsional divergence include making the wing sufficiently stiff torsionally, and trying to ensure that the flexural centre is reasonably well forward. We shall describe the means of improving torsional stiffness later.

On forward swept wings, in addition to the torsional divergence described above, it is possible to encounter divergence due to bending, as illustrated in Fig. 14.2. As the wing tip bends upwards, the angle of attack increases, pro­ducing a greater lift, with more bending. If the bending stiffness is insufficient, the wing bending may diverge, until failure occurs. Bending divergence can also occur on unswept wings during violent sideslip.

Just as forward sweep increases the likelihood of divergence, rearward sweep decreases it, and on highly (rearward) swept wings, divergence is unlikely to occur. The problem of divergence, and other aeroelastic effects on forward-swept wings, is one reason why they were initially rejected in favour of the rearward-swept alternative.

Fig. 14.2 Bending on a forward-swept wing

On a forward-swept wing, as the wing bends upwards, the angle of attack will increase toward the tips. An increase in lift follows, and flexural divergence can occur

Because aerodynamic loading increases with speed, the risk of divergence increases as the aircraft flies faster. The minimum speed at which divergence occurs on any particular aircraft is known as its critical divergence speed, and the maximum operating speed must be less than this critical speed.

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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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.

The properly executed turn

Unlike a car, an aircraft cannot be turned satisfactorily by means of the yaw control alone. This is because there is no road to provide a reaction to produce the cornering forces. In an aircraft, the cornering (centripetal) force must be provided by aerodynamic means. When the rudder is deflected so as to yaw the aircraft, the force that it produces is actually outwards; the opposite direction to that required.

As illustrated in Fig. 10.12, to execute a level turn properly, the aircraft must be banked, and the lift increased so that the horizontal component of lift is exactly the right size to provide the centripetal force required for the turn, and the vertical component exactly balances the weight. Normally a certain amount of rudder control is necessary in order to keep the aircraft pointing in the intended direction. Excessive use of the rudder, however, produces a skidding turn, with an uncomfortable sideways acceleration, and a potentially danger­ous sideslip.

vertical component of lift

Horizontal component of lift

Weight

Fig. 10.12 Turning flight

For a correctly banked turn, the lift force must be increased so that its vertical component exactly balances the weight. The horizontal component can then provide the required centripetal acceleration

The precise coupling between roll and yaw varies from one aircraft design to another. In general, a combination of aileron and rudder movement is required, but most aircraft can be turned smoothly using ailerons alone. Some early Farman aircraft had no rudder at all. The balance between rudder and aileron control also depends on whether the aircraft is climbing, descending, or flying level. A more detailed description will be found in practical flying manuals such as Birch and Bramson (1981).

Note, that once a properly executed turn has been initiated, the control stick or handlebars are returned to somewhere near the neutral or mid-position, and the aircraft keeps turning. Holding the stick over would cause the aircraft to continue rolling. This is quite different from steering a car, where the steering wheel must be held in the turned position.

One very special case where flat turns were necessary was in the man – powered Gossamer Albatross shown in Fig. 10.13. Because of its exceptionally low power, this aircraft required a high-aspect-ratio wing with a span similar to that of a large airliner, and could only fly close to the ground. In a banked turn the wing tip would be likely to hit the ground. The aircraft was therefore turned by means of the canard foreplane, which could be canted over so as to

produce a sideforce component to pull the nose round. Note that no fin or rud­der was provided.

Speed stability

As we explained in Chapter 4, in level flight, the contributions to drag from surface friction and normal pressure rise roughly as the square of the speed. The trailing vortex drag, however, decreases with speed, because the circula­tion, and lift coefficient required, decrease. In Fig. 4.21 we showed how the contributions to drag vary with speed. It was shown that the resulting total drag has a minimum value. The curve of resulting drag is repeated in Fig. 11.18. If we try to fly at a speed less than the minimum drag speed whilst trying to maintain a steady flight path then a decrease in speed will cause increased drag. The thrust of turbo-jet engines is not very sensitive to speed changes, so on jet – propelled aircraft the increase in drag will slow the aircraft down further. Similarly, a small increase in speed will result in less drag, so the aircraft will tend to fly even faster. Therefore, at speeds less than that for minimum drag, a turbo-jet aircraft suffers an instability of speed.

On piston-engined aircraft where the power is not greatly affected by the speed, a reduction in speed is usually accompanied by an increase in thrust, since power = thrust x speed. Up to a point, therefore, the increase in thrust

Fig. 11.18 Speed instability and the effect of air brakes, etc

When an aircraft is flying slower than the minimum drag speed, as at A, then any increase in speed results in a reduction in drag if the pilot maintains a steady flight path. The aircraft will therefore accelerate until point B is reached where the thrust and drag are once again in balance

Conversely, if the speed falls, then the drag will rise, and the aircraft will slow producing more drag. The vicious circle continues until the aircraft stalls. In the landing configuration, the deployment of flaps, landing gear and if necessary, air-brakes increases the boundary layer (profile) drag. This lowers the minimum drag speed, and consequently reduces the speed at which the onset of speed instability occurs

SPEED STABILITY 317

Fig. 11.19 Air brakes not only slow the aircraft down, but may be useful in

preventing speed-instability

(Photo courtesy of Alistair Copeland)

tends to compensate for the increase in drag, so piston-engined aircraft are less prone to speed instability.

There are also other reasons why turbo-jet aircraft are more prone to speed instability. When we looked at aircraft performance, we saw that the most eco­nomical flying speed is above the minimum-drag speed. For piston-engined air­craft, where the equivalent air speed (EAS) at cruise is only about two or three times as fast as the landing speed, the landing speed is normally fairly close to this minimum point. Any tendency to speed instability is, therefore, slight, and can be easily controlled by the pilot. For high speed turbo-jet aircraft, the cruis­ing (EAS) speed may be many times greater than the landing speed. Thus if the cruise is to be efficient, the landing speed will be well below the minimum drag speed, and speed instability becomes a more serious problem.

The problem of speed instability on turbo-jet aircraft is made worse by the fact that the response to throttle changes is much slower than for a piston – engined type. If the pilot of a turbo-jet propelled aircraft tried to flatten out and float down to a three-point landing, as was the custom in the piston-engine era, he might find himself taking-off again instead.

To solve the speed-instability problem, air brakes may be fitted as shown in Fig. 11.19. These devices increase the drag, and have the effect of pulling the minimum drag position point further to the left on the curve, as shown in Fig. 11.18. Flaps also help to increase the drag, and are normally deployed more fully for landing than for take-off. On Concorde an automatic throttle control system was used to help iron out the inherent speed instability at low speeds.