Category Aircraft Flight

It is tempting to ignore biplanes and multiplanes as being of purely historical interest, but old ideas have a habit of returning, and small biplanes have once again become popular for aerobatic and sport flying.

The wings of early aircraft had little or no bending stiffness, and had to be supported by external wires and struts. The biplane configuration provided a simple convenient and light structural arrangement, which was originally its main attraction.

A biplane produces virtually the same amount of lift as a monoplane with the same total wing area and aspect ratio. The biplane, however, has the smaller overall span, which makes it more manoeuvrable. The highly aerobatic Pitts Special, shown in Fig. 2.26, is an example of a modern biplane which is not merely a gimmick. The manoeuvrability of biplanes was one factor that led to their retention even when improvements in structural design had removed the necessity for external bracing.

Reductions in drag can also be obtained by careful attention to the shape of the wing tip. This is particularly true in the case of aircraft with untapered wings. Although untapered wings are not the best shape in terms of minimising drag, they are often used on light aircraft because of their relative simplicity of con­struction, and their docile handling characteristics. (The inboard section tends to stall first.)

Two simple approaches; the bent and the straight-cut tip are illustrated in Fig. 4.13. Both of these tip designs are said to reduce drag by producing

separation of the spanwise flow at the tip, resulting in a beneficial modification of the tip flow-field. It should be noted, however, that unusual tip shapes are often intended primarily to inhibit tip stall, rather than reduce drag. Upward bent tips are evident on the Aerospatiale Robin shown in Fig. 4.14.

For aerodynamic efficiency large slowly rotating propellers are preferable, but unfortunately, small piston engines develop their best power-to-weight ratio at relatively high rotational speeds. For light aircraft, therefore, the added cost, complexity, weight and mechanical losses of gearing sometimes make it pre­ferable to use direct drive, and accept a slight degradation in engine efficiency due to running at low rotational speed. When small automotive engines are adapted for home-built aircraft, some form of gearing is often used. In the case of turbo-prop propulsion, the rotational speed of the primary engine shaft is so high that gearing is almost essential.

Once gearing is accepted, then the propeller diameter is limited only by prac­tical considerations such as ground clearance, so highly efficient propellers can be used. The propellers of the Lockheed Super Hercules shown in Fig. 6.6 are driven by geared gas-turbines.

The effect of altitude on the drag curve is very similar. As the altitude increases the density is reduced and this can be compensated by an increase in cruising speed to keep the dynamic pressure constant. If the aircraft attitude is kept con­stant both lift and drag coefficients will remain constant as before, and the drag curve will be shifted to the right in exactly the same way as before (Fig. 7.5).

Maximum speed

The maximum speed in level flight that can be attained by the aircraft can be deduced very simply from Fig. 7.4. In order to achieve the maximum speed we need the intersection between the drag and engine thrust curves to be as far to the right as possible. This is clearly obtained when the engine is at the maximum throttle setting.

This seems to be a very simple situation, but a word of caution is necessary. We have assumed a comparatively simple form for the drag curve in our dis­cussion. Compressibility effects may have an important influence on this for a particular aircraft. Such factors as the buffet boundary (Chapter 9) may then limit the maximum speed. High speed aircraft may also be limited by the maximum permissible structural temperature, which may be approached due to kinetic heating effects (Chapter 8). These factors may restrict the permitted maximum to a value below that which would be suggested by the simple ‘avail­able thrust’ criterion. Additional limitations may be imposed by the constraints on the engine operating conditions (Chapter 6).

Increasing the wing loading has the primary effect of shifting the whole of the drag curve to a higher speed (Fig. 7.5) without increasing the drag itself. Therefore a high wing loading, and consequently small wings, is desirable from the point of view of obtaining high speed.

A similar argument might lead the reader to suppose that high altitude is also desirable for high speed. To some extent this is true but it must be remem­bered that increase in altitude implies a reduction in temperature, and thus a lower speed of sound. This means that the flight Mach number will be increased for a given air speed at high altitude. Compressibility effects will therefore be apparent at a lower air speed and this will impose an important restriction, par­ticularly for aircraft designed for operation at subsonic or transonic speeds.

The conventional aerofoil revisited

In Chapter 5 we saw how the flow characteristics over a conventional aerofoil changed with increasing free-stream Mach number from a shock-free low speed flow (Fig. 5.18(a)) through the developing shock wave system at tran­sonic speeds (Fig. 5.18(b)) until the fully developed shock system is obtained at higher Mach numbers (Fig. 5.18(c)). In transonic aircraft we are particularly concerned with the intermediate type of flow shown in Fig. 5.18(b) in which the oncoming flow is still subsonic.

First let us take another look at the pressure distribution on a conventional aerofoil section (this is shown again in Fig. 9.4) and how this relates to the flow is shown in Fig. 5.18(b). We see at once that there are two potential problems. First there is a very high suction peak which occurs locally near the leading edge of the aerofoil. This means very high velocities in this region, and con­sequently high Mach numbers. The second problem occurs because of the very high adverse pressure gradient on the downstream side of this suction peak. This is liable to coalesce into a relatively strong shock wave (the shock wave which terminates the supersonic patch in Fig. 5.18(b)) and this may also induce boundary layer separation, with all the problems that entails!

Thin sections

The increase in the surface velocity over the aerofoil section is caused by two factors – the thickness of the section and its angle of attack. Thus one way in which the local Mach number over the top can be limited is to use a thin sec­tion. This has certain aerodynamic penalties associated with it, however, as we have already seen in Chapter 2. Firstly the range of angle of attack over which the wing will operate without stalling will be reduced, and secondly it is obvi­ous that the problems of fitting in a satisfactory wing structure get more and more severe as the section thickness is reduced (Chapter 14).

Supercritical sections

So far we have attacked the problem of developing a wing section suitable for transonic flight simply by using as thin a section as we can in order to limit the velocity increase due to thickness. However, as we get near to the speed of sound, the achievable wing loading is limited unless the flow becomes locally supersonic. We therefore have to design supercritical aerofoils in which this supersonic flow is adequately catered for.