Category Aircraft Flight

Aircraft size

Any observer of commercial aircraft over the past few decades cannot fail to have noticed that the size of airliners has dramatically increased, particularly for the longer ranges, hence the A380, Fig. 13.12. The reason for this is quite simple. Apart from the wings, contributions to the overall drag of the aircraft come from a variety of other sources, including the fuselage. For similar fusel­ages the capacity increases as the cube of the diameter while the surface area only increases as the square. Since the drag is dependent on the surface area this means a reduction in the drag contribution per passenger, and a consequent improvement in the economy of operation.

Of course other considerations work to restrict the size. In order to provide an attractive and useful service, the airline must operate a reasonably frequent service over a given route. If this results in the aircraft having to operate with a substantial number of seats empty this clearly undoes any improvement due to the increase in size.

Other factors include the provision of passenger handling facilities at air­ports which are able to deal with a very large number of passengers joining or leaving a large aircraft. Any reader who has had the unfortunate experience of being a passenger on a diverted jumbo jet will have first-hand experience of the chaos which can result at the unsuspecting passenger terminal of the receiving airport.

Getting the load distribution right

The loading distribution for a swept wing of constant section and geometric incidence along the span (Fig. 9.14) shows that, as well as an increase in local load towards the tips, there is a decrease in the centre section. The region of high load means low pressures on the top of the wing surface. This in turn means that the local velocity, and hence Mach number, will also be high in this region. Thus the tip region will be the first to encounter the transonic drag rise and stall, while the rest of the wing, particularly the centre section, is com­paratively lightly loaded.

If this state of affairs is not corrected, the wing will not be particularly efficient. It may be thought that this would not create too much difficulty because the Mach number could be pushed up slightly and the reduction in performance near the tip tolerated. However increase of Mach number in the tip region would lead to unacceptable shock-induced flow separation resulting in buffet and even stall.

We saw in Chapter 2 that the load variation across the span could be altered by modifying the wing planform. This is also true with swept wings. Unfortunately, in order to remove the tip load peaks and boost the load in the centre section an inverse taper would be required. This is clearly not a good idea from the structural point of view. The alternative solution of using twist is

Fig. 9.14 Pressure distribution on a simple swept wing

There are strong low-pressure peaks near the leading edge at the tips

Fig. 9.15 The Republic XF-91 had inverse-tapered wings

the one nearly always adopted. Figure 9.15 shows a rare example of an aircraft with inversely tapered wings.

Typically the wing of a transonic transport will have 5% washout (reduced geometric incidence towards the tip) over the span and employ the more struc­turally acceptable conventional taper.

Unfortunately the use of twist to correct the load distribution can only pro­duce the desired result at a particular design angle of attack. As the speed reduces so the load distribution will tend to revert to the previous undesirable form and leading and trailing-edge flaps must be used to correct further.

Automatic control systems and autopilots

The earliest form of automatic control or autopilot consisted of a device to keep the aircraft flying on a steady heading at constant height. They normally employed a number of mechanical gyroscopes which were used to sense the motion of the aircraft and apply suitable corrective control inputs. These so-called ‘inertial’ systems have been developed to a high level of precision and sophistication, particularly in military aircraft and missiles, and can provide highly accurate guidance and control. Laser-based electronic inertial and GPS sensors have now largely replaced the mechanical gyroscopes.

During the Second World War, guidance and navigation systems using ground-based radio transmissions were developed, primarily for bombing missions, and these were subsequently adapted for civilian applications. Nowadays, an autopilot may be linked to a complex set of navigation systems and instruments, and can be programmed so that the aircraft follows a pre­determined flight pattern, including variations in speed, altitude and direction.

Satellite-based navigation systems can now provide pinpoint positional indica­tion and may eventually replace the older ground-based systems altogether.

As we shall describe in later chapters, automatic control systems can be used to help enhance stability, to improve performance and manoeuvrability, and to help provide safe landing, particularly in poor visibility. A good introduction to avionic systems is given in D. H. Middleton’s book.

Take-off configuration

Since it is clearly an advantage for an aircraft to be able to operate from the minimum possible length of runway there is a strong pressure on the designer to have as low a stalling speed as possible. This requirement is not compatible with the demand for low drag at cruise and the section is modified for take-off by means of leading-edge slats, trailing-edge flaps and other devices already mentioned. For light aircraft the runway requirements are generally not too great and a simple trailing-edge flap deflection, or indeed no modification at all, may be sufficient. For high performance aircraft or transonic transport aircraft, however, more sophisticated high lift devices are required (Chapter 3). In par­ticular some sort of leading-edge slat or droop will normally be employed. Figure 13.2 shows a typical take-off configuration.

High speed helicopters and convertiplanes

One way to get around the problems of high speed flight in helicopters is to equip them with wings and a conventional propulsion system in addition to the rotor. In high speed flight, the wings take over the provision of lift, and the rotor blades can rotate slowly, or even be folded back.

High speed helicopters and convertiplanesThe co-axial contra-rotating helicopter rotor mentioned above also has an advantage in high speed flight, as it can be arranged that the lift is provided only by the advancing pairs of blades (one on each side of the aircraft) which, therefore, do not need to move much faster than the aircraft.

High speed helicopters and convertiplanes

Fig. 1.29 High speed helicopter

The BERP tips helped to take this modified Lynx to a record-breaking 249.1 mph. By the standards of conventional aircraft this is quite slow

(Photo courtesy of Westland Helicopters Ltd)






High speed helicopters and convertiplanes

Fig. 1.30 The Boeing-Bell V-22 Osprey convertiplane

The convertiplane combines the VTOL capability of the helicopter, with the economy and speed of a turbo-prop, but the cost and complexity is considerable (Photo courtesy of Bell Helicopter Textron Inc.)

Another solution is a convertiplane, such as the Bell V-22 Osprey illustrated in Fig. 1.30. In this design, the rotor blades are tilted forwards in high speed flight to become large-diameter propellers. As with the other solutions described above, this apparently simple arrangement is not without its problems, and it took decades of development and several different experimental aircraft before a production type was finally built.

Leading-edge devices

In addition to trailing-edge flaps, a variety of leading-edge devices may be used. The simplest of these is the leading-edge flap, which is used to increase the cam­ber of the leading edge. Leading-edge flaps can take the form of a simple hinged section, as in Fig. 3.13(f), but a more effective arrangement is to slide or fold the flap forward to increase the wing area, as shown in Fig. 3.13(g). Leading – edge flaps may be seen deployed in Fig. 3.15.

We can utilise the principle of boundary layer control by introducing a gap or slot in the leading edge, as in Fig. 3.13(g). Like the slotted flap, the air gap allows a fresh boundary layer to develop behind the slot, which helps to prevent leading-edge separation. The slot may be formed by moving the lead­ing edge forward, in which case, the articulated portion is known as a slat. Leading-edge devices are particularly useful for thin aerofoil sections where leading-edge separation is likely to occur.

Simple unflapped aerofoils normally generate a maximum lift coefficient value of less than 2, but as long ago as 1921, Sir Frederick Handley Page and

Leading-edge devices

Fig. 3.15 Leading-edge flap deployed on a Tornado

G. V. Lachmann managed to achieve lift coefficient values as high as 3.9 using multiple element aerofoils. The patented Handley Page leading-edge slat was a feature of several aircraft built by the Handley Page company.

There are many variations in the mechanisms used for leading-edge devices. In some cases, the slat is held in by spring tension, and extends automatically at high angle of attack by the action of leading-edge suction.

Turning the flow the other way – the expansion

We have now seen how the flow can be turned by a shock wave resulting in an increase in pressure, density and temperature of the air as the flow is almost instantaneously slowed by the shock. If we turn the air in the opposite direction

Turning the flow the other way - the expansion

Fig. 5.16 Expansion

The flow accelerates around the corner through an expansion fan. Pressure decreases so pressure gradient is favourable for the boundary layer, which remains attached

(Fig. 5.16) we find that the pressure decreases as do both density and temper­ature, while the speed increases. If we look at the process in greater detail we see that the process of expansion is not sudden as in the case of the shock wave compression, but takes place over a well defined area.

It is interesting to observe that at supersonic speed the flow is much better able to negotiate this type of corner than it is at subsonic speed where bound­ary layer separation would almost certainly result (Chapter 3). In fact the degree of turn that can be achieved by a supersonic stream is quite surprising.

At first sight it seems strange that the faster flow is better adapted to making sudden changes in direction, but the clue to why this should be so has already been given in Chapter 3. The problem in subsonic flow is that the boundary layer separates and one of the primary causes of this is an increasing pressure in the direction of the flow; an adverse pressure gradient. If we now look at the change in pressure around the corner for the subsonic case we see that there is, indeed, an adverse pressure present. In supersonic flow, however, the pressure gradient around the corner is in the favourable sense and acts to prevent boundary layer separation (Fig. 5.16).

This difference in the ability of subsonic and supersonic flows to turn cor­ners is not just of academic interest. The supersonic aerofoil section shown in Fig. 5.17 is perfectly adequate for use at its design speed, but will have a very

Turning the flow the other way - the expansion

Fig. 5.17 Supersonic aerofoil section (double wedge)

This section has good supersonic but poor subsonic performance

poor subsonic performance. As we shall see in Chapter 8, this makes the designer’s life much more difficult since, with the exception of some missiles, most aircraft have to land and take off and must therefore be capable of satis­factory operation at both subsonic and supersonic speed.

By-pass or turbo-fan engines

For a given amount of thrust, the Froude efficiency can be improved by increas­ing the mass flow rate of air, while reducing the jet speed. This can be achieved by increasing the size of the low pressure compressor stage, and by-passing some of the compressed air around the outside of the combustion chamber and turbine, as illustrated in Fig. 6.23. As in this illustration, separate spools are normally used for the low pressure by-pass and high pressure core stages. The hot and cold jets are arranged to have about the same velocity at exit.

The low pressure by-pass stages are effectively ducted fans, and by-pass jet engines are described as turbo-fans. In Britain, the term was originally only applied to high by-pass ratio engines (described below).

Supersonic aircraft

The order of the book may appear a little puzzling to the reader at this point. In Chapter 5 we saw how the speed range went from subsonic to supersonic and hypersonic via the intermediate transonic stage. Thus it would appear that transonic aircraft design should be considered before we come to the higher speeds. However, as we also saw in Chapter 5, the transonic flow regime is in many ways much more complex than that of fully developed supersonic flow and it is for this reason that we consider supersonic aircraft first. We need to be aware, though, that supersonic aircraft will also need to fly transonically and so the considerations outlined in the next chapter will also influence their design.

One of the most striking aspects of aircraft is the vast variety of shapes that are used. A few of these are shown in Figs 8.1— 8.4, 8.8, 8.16 and 8.18. We see that some wings are straight, some are swept, some are small and some are large. All have provided successful solutions to the problem of flight at super­sonic speeds.

As in most engineering design problems, the answer is to be found in the fact that the design process is one of compromise. Although an aircraft may be designed for high speed, unless it is an air launched missile it still needs to land and take off and so has to fly at low as well as high speed.

As well as the speed range required of the aircraft, other considerations such as the degree of manoeuvrability required may have an important influence on the overall configuration.

In this chapter we look at the ways in which the wing and the complete aircraft can be designed to achieve a satisfactory compromise. The particular solution chosen depends acutely on the precise role the aircraft is designed to fulfil.

The Lightning (Fig. 8.1) was designed to serve as a relatively lightly loaded high altitude interceptor. The more recently designed Grumman F-14

Fig. 8.1 Highly swept wings

Thin-section highly swept wings were used on the Lightning which was designed as an interceptor, with high speed and climb rate as its major objectives (Photo by N. Cogger)

(Fig. 8.2) has to perform in a variety of roles. The Eurofighter Typhoon (Fig. 8.3) is designed as a highly manoeuvrable transonic and supersonic ‘air superiority’ fighter. The Concorde (Fig. 8.4) is a passenger carrying transport intended to fly economically at supersonic speeds, yet is required to have a reasonably efficient subsonic cruise as well as a good airfield performance.

Indicating instruments

The pilot is effectively part of the aircraft’s control system, and he needs to have a good indication of the results of his action. Figure 10.2 shows the primary instruments available on a typical light aircraft. In some aircraft, many of the individual instruments are now replaced by a display on a form of com­puter screen, as seen in Fig. 10.3.

Ailerons (roll control)

Rudder (yaw control)

Elevator (pitch control)

Fig. 10.4 The primary conventional control surfaces

Yaw control

On conventional aircraft, the yaw-control pedals are connected to a mov­able rudder, which is attached to the vertical stabiliser or fin, as illustrated in Fig. 10.4. Operation of the rudder effectively produces a camber of the vertical stabiliser surface, and this hence generates a sideways force. Less commonly, the whole fin surface is turned so that it is inclined to the flow. Since the side – force is applied well behind the centre of gravity, it produces a yawing moment.

For reasons that we shall give later, yaw control is not used as the primary means of changing direction, except when manoeuvring on or very close to the ground.

On large aircraft, several independently driven rudders may be provided (on a single fin), mainly for reasons of safety. The use of multiple rudders can also enable the balance between yaw and roll action to be controlled, according to whether an upper, or a lower rudder is used. On multi-finned aircraft, two or more rudders may be used, operating in parallel.