Category Aircraft Flight

Fan propulsion

A fan is essentially a propeller with a large number of blades, and therefore provides a means of producing a large amount of thrust for a given disc area. When there are many blades, so that they are close together, each blade strongly affects the flow around its adjacent neighbours. This interference can

have a beneficial effect if the relative flow speed is supersonic. The flow can be compressed gradually through a series of reflected shock waves, creating a smaller loss of energy than when the flow is compressed through a single shock.

Cruise climb

As the flight proceeds fuel is used and the aircraft weight changes. This change may be very significant; up to half the total weight on a long-range transport aircraft. Thus the lift will decrease. In order to operate at the best lift coefficient we therefore need to reduce the dynamic pressure. If the aircraft is powered by a gas turbine we do not wish to reduce speed, or the engine efficiency will suffer. The only alternative to reducing the speed is to reduce the air density by climbing as the flight proceeds. Fortunately this is possible even if we are operating near the limiting height described in the previous paragraph. As the weight of the aircraft reduces, so this limiting height increases (Fig. 7.8) and we can achieve the desired increase in altitude without being squeezed into ‘coffin corner’, the point where stall and high speed buffet occur at the same speed. This technique is known as ‘cruise climb’.

OTHER TYPES OF POWERPLANT 201

Some practical considerations

In practice, other considerations may influence the way in which the cruising height is selected and the way in which the cruise climb technique is operated.

In the first place, we must remember that the cruise is only part of the flight. The aircraft must both land and take off, and the flight plan must be optimised over all phases as a whole, not just during cruise. Thus for short journeys the cruising height is likely to be lower than for long-range flights.

As far as the cruise climb is concerned, factors such as Air Traffic Control requirements do not enable the technique to be followed as closely as the pilot would like. For example, in order to achieve safe separation of aircraft the height may be dictated by safety rather than economy, and airline pilots have to get permission from Air Traffic Control before changing height.

Swept wings in transonic flow

We have already had something to say about the swept wing in Chapters 2 and 8. In this section we will look at the advantages to be gained from using swept wings in the transonic speed range, emphasising how conflicting design requirements are resolved and a suitable compromise solution is reached for a particular aircraft.

We saw in Chapter 2 that sweeping the wing works because only the velocity component at right angles to the leading edge of the wing contributes to the aerodynamic performance, and so the free stream Mach number is effect­ively reduced (Fig. 2.17). For the supersonic wing this had two advantages. Firstly the strength of the bow shock wave was reduced, and secondly the char­acteristics of the wing could be made to be similar at both low and high speeds.

In transonic flow sweep works because of the same basic principle. In the case of a typical aircraft designed only for transonic cruise, however, the oncoming flow will be just below the speed of sound and sweep is used to maintain a high cruise Mach number while reducing the effective Mach num­ber seen by the wing section to a value just below the transonic drag rise.

Sweep is therefore an important weapon in the armoury of the transonic aerodynamicist, but it has its limitations. There is still a need to use relatively thin sections in order to delay the transonic drag rise as much as possible, and consequently wings tend to be quite flexible with resulting problems which are described later. Furthermore sweep introduces problems of stability (Chap­ters 11 and 12). The lift-to-drag ratio is also reduced for the reasons given in Chapter 2.

Because of such problems sweep angle is kept as low as possible and the transonic wing section is generally still a lot thinner than its subsonic counter­part. Thus the basic low speed performance of such wings is not very good and, as would be expected with a thin section, stall occurs at a comparatively low angle of attack; an effect which is made worse by the tendency for the local load at the tip of the wing to be high, as described in Chapter 2.

It is for these reasons that such aircraft, when flying at low speed, usually require to vary the section geometry by the use of leading and trailing-edge slats and flaps. Although these are expensive in terms of weight and mechanical complexity, they do permit a thin section configuration to be adapted to give reasonable performance at low speeds. The design of such devices is a complete story in itself, since there are different performance requirements at cruise, landing, take-off and during any low speed waiting (or stand off) which may be required by air traffic control.

It must be reiterated that the use of sweep simply allows us to use a higher cruise Mach number than the drag rise Mach number for the particular wing section employed. This is merely one technique which can be employed to give acceptable cruise performance and its use is coupled with ever improving detailed section design.

In our discussion above we considered the swept wing simply from the point of view of a wing of infinite span yawed to the main flow direction. In reality, as with the supersonic swept wing, there will be both a tip section and a centre section to complicate the issue.

Furthermore, the basic planform will modify the way in which the trailing vortex sheet forms (Chapter 2). The load distribution is consequently affected so that the load becomes concentrated near the tip, as we mentioned earlier. This is just what we would wish to avoid. Firstly the concentration towards the tip means that the bending moment at the root of the wing will be more severe, and secondly the increased loading peak in the tip region will make the stalling problems there even more severe.

To compound the problem, although we may have solved some of the exist­ing problems by the use of sweep, the flexural behaviour of a conventional wing structure causes the tip angle of attack to be reduced relative to the rest

Fig. 9.13 Reduction of tip incidence due to flexure of wing with sweep back

of the wing (Fig. 9.13). In some ways this is a good thing since load alleviation at the tip is what is required. However this has the effect of moving the centre of pressure of the whole wing forward and consequently altering the longitud­inal trim of the aircraft.

Looking at the above catalogue of woe, the reader might be forgiven for thinking that sweep should be avoided at all costs. This is not so. It is a vital technique in the design of aircraft of this type. However the problems discussed above will, at least, indicate that it needs to be used with due caution and that it is not such a complete answer to the proverbial maiden’s prayer as it might seem at first sight.

Control in transonic flight

When an aircraft is flying close to the speed of sound, the operation of a con­trol surface may cause the flow to change locally from subsonic to supersonic type, or vice versa. This means that the handling characteristics can change significantly, and in extreme cases, the controls may even reverse, making the aircraft almost unflyable. For example, application of right rudder will cause the left wing to travel a little faster than the right. If the aircraft is flying at the point where compressibility effects are causing a loss in lift and a rise in drag, then the faster moving wing may drop, so that the aircraft rolls away from the turn, instead of into it. This is a potentially dangerous characteristic, and great care has to be exercised when using the rudder, or indeed when making any control movements in the region of flight close to Mach 1.

Control reversal was sometimes encountered by the faster aircraft of the Second World War straying too far into the transonic region, but in most cases, this reversal was due to insufficient structural stiffness, leading to aeroelastic effects, as described in Chapter 14.

Take-off

The take-off is usually considered in a number of sections (Fig. 13.1). First there is the initial ground run, the sole purpose of which is to accelerate the aircraft as quickly as possible to the speed at which the wings can develop sufficient lift to permit take-off. This run must take place with the drag of the

(c) Climb out (b) Rotation (a) Ground run

Fig. 13.1 Take-off

Take-off may be divided into three phases:

(a) Ground run at low angle of attack giving low drag (b) Rotation where nose is raised to increase angle of attack (c) Climb out

aircraft at as low a value as possible to maximise the acceleration and therefore a low angle of attack is maintained. In the case of a tail wheel undercarriage, fitted commonly some years ago, the tail wheel must be raised as soon as the air speed permits adequate elevator control. This reduces the angle of attack to the required value. When sufficient speed has been reached the aircraft is ‘rotated’ until sufficient angle of attack is obtained for lift-off which is followed by climb out which should occur at the maximum angle of climb to allow optimum obstacle clearance.