Category Aircraft Flight

Supercharging and turbocharging

The power output of a piston engine can be considerably increased by using a supercharger to pressurise the air being fed into the cylinders, so that a larger mass of air is used in each working stroke. The use of a supercharger can, there­fore, improve the engine’s power-to-weight ratio.

An important advantage of a supercharger is that it enables an engine to operate at higher altitude than it could in normally aspirated (un-supercharged) form. As the altitude increases, the air density falls, and without supercharg­ing the mass of air taken in per working stroke would fall. Since there is less oxygen, less fuel can be burned, and there is a consequent loss of power.

The supercharger enables an aircraft to take off heavily laden from high altitude airfields on hot days. By cruising at high altitude, the aircraft may also sometimes be able to take advantage of strong tail winds.

A supercharger usually consists of a centrifugal compressor driven from the crankshaft. A turbocharger is similar to a supercharger, except that the com­pressor is driven by a turbine, which is powered by the residual energy in the exhaust gases. Unlike the supercharger, the speed of the turbocharger is, there­fore, not directly related to the engine speed. Because it makes use of otherwise wasted heat, the turbocharger is inherently more efficient than a plain super­charger, and has become the type normally used. Both devices can roughly double the power output for a given size and weight of engine.

For small aircraft, the disadvantage of turbocharging is that it adds to the cost and complication of the engine, and the boost pressure is yet another vari­able that the pilot has to monitor or control. There is little advantage in using a turbocharger, unless the pilot is able to take advantage of the benefits of high altitude operation. This in turn means that either the aircraft must be pres­surised, or an oxygen mask and supply system must be provided. Civil aviation regulations require that for high altitude operation, additional instruments, navigation and communication equipment must be installed, and the pilot must be suitably qualified to use them. In recent years, a number of pressurised turbocharged light aircraft have appeared, such as the Cessna Centurion. Garrison (1981) gives a good description of the pros and cons of turbocharged light aircraft.

Design for endurance

The purpose of an aircraft is not always to transport people or cargoes between two locations, sometimes the aircraft is used as a radar or visual observation platform, and in this case the main design consideration will be the length of time it can remain airborne, or its endurance.

In this case we require, not the minimum fuel flow over a given distance, but the minimum fuel flow in unit time. Here we will adopt the same approach as before and look at the airframe from an idealised point of view to get an initial idea of the way things behave. Following this we will look at the real engine behaviour to get a more accurate picture of the operational requirements of the complete aircraft.

If we take an initial guess, we would suppose that the best way to operate the airframe for maximum endurance would be to fly at the condition at which the smallest amount of work needs to be expended in unit time in order to overcome the drag force. The rate at which work is done is equal to power, so this operating point is equivalent to the flying speed and cor­responding aircraft attitude which results in minimum power, rather than minimum drag.

Because we are now concerned with power, rather than drag, we will con­sider the power required by the airframe and powerplant and plot them in a similar manner to the drag curves of Fig. 7.4. The power required curve is very easily derived from the drag curve. All we have to do is multiply each value of the drag by the speed at which it occurs and replot as in Fig. 7.10. Then we superimpose the power, rather than the thrust, curve for the particular power – plant we are using.

We find that the power reaches a minimum value at a speed slightly lower than the minimum drag speed. In constructing the power curves we must again remember that we are talking about an aircraft flying straight and level at a constant weight.

Now we have decided what the airframe is doing, we will take a simpli­fied look at the compromise which must be reached for the different power plant types, as we did when considering how to operate for best economy and range.

Fig. 7.10 Aircraft and engine power curves

Power curve is obtained by multiplying drag values (Fig. 7.5) by aircraft speed. Minimum power speed is about – minimum drag speed

Adding a fuselage

If a fuselage is now added to the wing we have basically the same problems which occurred on the isolated wing from the point of view of correcting the local load distribution, but we now also have to superimpose the flow pro­duced by the fuselage.

In isolation the fuselage will speed up the local air stream as it flows past, and that is precisely what happens to the local air stream at the wing centre section when the fuselage is added. This means that the local Mach number on the wing will be increased, thus adding to the possibility of locally strong shock waves being formed. The detailed flow in the junction between the wing and fuselage can be very complicated, and in general acute angles are best avoided. This leads to the conclusion that a centre-mounted wing is likely to be the best bet. However this solution is not desirable in such designs as transport aircraft, where a clear fuselage is essential. Indeed whether the wing is mounted low or high may be decided by such factors as ground engine clearance or under­carriage length rather than by pure aerodynamic considerations.

If, however, we are faced with a situation in which there is some choice over the fuselage geometry and we are not simply restricted to using a straight tube, we find that we have another design parameter at our disposal. As well as modifying the local flow at the wing centre section by changes in the shape of the wing itself, we can also change the flow by modifying the local cross­sectional shape of the fuselage in order to make the local streamlines follow the shape they would adopt on the infinite wing. Alternatively, if the basic form of the fuselage must remain unaltered, a suitable fillet can be used at the wing/fuselage junction.

Longitudinal and lateral stability

In the previous chapter, in Fig. 10.1, we defined the three turning motions; pitch, yaw and roll. Pitching stability (nose-up/nose-down motion) is known as longitudinal stability.

Lateral stability is a term used rather loosely to refer to both rolling and yawing. These two motions are very closely interconnected, as we noted when describing control surfaces.

Fortunately, the coupling between longitudinal and lateral static stability is normally weak, and for the purposes of our simple introduction, it is conveni­ent to treat them separately. This again, was part of the traditional approach. It should be noted, however, that in highly manoeuvrable aircraft, the cross­coupling can be significant.

Longitudinal static stability

Aerofoil centre of pressure and aerodynamic centre

For an aerofoil, the point along the chordline through which the resultant lift force is acting, is known as the centre of lift, or centre of pressure. On a cam­bered aerofoil, the centre of pressure moves forward with increasing angle of attack, as shown in Fig. 11.2(a).

When a cambered aerofoil is set at an angle of attack where it produces no lift, we find that it still gives a nose-down pitching moment. Since there is no force, this moment must be a pure couple. Figure 11.3 shows how this arises physically. The downforce on the front of the aerofoil is balanced by an upward force at the rear, so there is no net force, but a couple is produced.

It is a surprising feature of aerofoils that there is one position on the chord line where the magnitude of this pitching moment does not change significantly with varying angle of attack. Therefore, as illustrated in Fig. 11.2(b), we can represent the forces on an aerofoil as being a combination of a couple and a lift force (L) acting through that position. The position is known as the aero­dynamic centre. It is useful to have such a fixed reference point, because, as the angle of attack reduces towards zero, the centre of pressure moves further and further aft, eventually disappearing off towards infinity.

Flying down the glide path

The above description perhaps gives a deceptively simple view of the landing procedure. Flying an accurate approach is a very demanding exercise and there is more than one way of going about it, the choice being determined by the aircraft type and pilot preference. The term ‘glide path’ for this part of the landing is somewhat misleading. It is perfectly possible to fly this part of the approach with the engine idling and this was a popular method some years ago.

With a gas-turbine engine in particular, the safer method is to fly down the glide path using a significant amount of power with the aircraft flaps being

used to provide a high drag setting. This procedure gives better control. The throttle setting can be decreased as well as increased, the latter being the only option available in the true gliding approach. Even more important is the fact that a gas turbine engine is very slow to pick up from idling speed when the throttle is suddenly opened. It is therefore a safer procedure to fly the approach under power to facilitate recovery from an aborted landing. The improved control afforded by this procedure has, however, led to its wide adoption even for light piston-engined aircraft.

Assuming that the pilot has broadly got the aircraft set up at the correct angle of attack and throttle setting to follow the required glide path, there will inevitably be small corrections needed from time to time. Here again the pilot has some choice in the matter. Provided the aircraft is not dangerously near the stall, such corrections can be made by controlling the aircraft angle of attack by elevator movement. This will result in some change in speed as well as glide angle. The alternative is to change the throttle setting and for piston-engined aircraft this method is frequently preferred because of the smaller change in speed. For jet aircraft and especially large ones, the former method is frequently used. This is because of the slow response of the engine, which makes accurate correction difficult. Further, if the aircraft is heavy, it will take a long time for the speed to change, which minimises the main dis­advantage of the method.

When flying down the glide path the pilot must have some means of check­ing that he is flying to the correct glide slope. Nowadays a variety of aids are available, and some of these are discussed below. In the absence of more complex aids he will need some reference markers, which may be simple radio beacons, at known distances from the runway threshold. He can check the height on the altimeter on passing these markers and estimate the required descent rate appropriate to the speed of the aircraft. In order to help to the correct descent rate the aircraft is fitted with a Vertical Speed Indicator (VSI) which works by sensing the rate of change of atmospheric pressure as the aircraft descends.