The ideal aerofoil

But what characteristics do we want in the ideal aerofoil section? We cannot answer that question fully until a later stage, but briefly we need –

1. A High Maximum Lift Coefficient. In other words, the top part of the lift

curve should be as high as possible. In our imaginary aerofoil it is only about 1.18, but we would like a maximum of 1.6 or even more. Why? Because we shall find that the higher the maximum CL, the lower will be the landing speed of the aeroplane, and nothing will contribute more towards the safety of an aircraft than that it shall land at a low speed.

2. A Good Lift/Drag Ratio. If we look again at Fig. 3.16, we can see that at a particular angle of attack, the lift/drag ratio of the aerofoil has a maximum value. This ratio does not occur at the angle of attack for minimum drag (Fig. 3.15) or at that for maximum lift coefficient (Fig. 3.13), but somewhere in between. Why is this ratio important? Because to get the smallest possible resistance to motion for a given weight we must operate at this angle of attack, and the higher the maximum lift/drag ratio, the smaller the air resistance that will be experienced.

The real importance of both high lift/drag ratio and high CLm/CD, dis­cussed below, will become clearer when we talk about aircraft performance (Chapter 7). Tet us just note here that both are important from the point of view of aerofoil design.

3. A High Maximum Value of CL3n/CD. The power required to propel an aeroplane is proportional to drag X velocity, i. e. to DV. For an aero­plane of given weight, the lift for level flight must be constant (being equal to the weight). If L is constant, D must vary inversely as LID (or CL/CD). From the formula L = CL. jpV2S it can be seen that if L, p and S are constant (a reasonable assumption), then V is inversely pro­portional to VCL (or CLm). Thus power required is proportional to DV, which is inversely proportional to (CL/CD) X CLi/2, i. e. to CL3n/CD. In other words, the greater the value of CLm/CD, the less the power required, and this is especially important from the point of view of climbing and staying in the air as long as possible on a given quantity of fuel and as we have seen, getting the best economy from a piston-engined aircraft. If the reader likes to work out the value of this fraction for dif­ferent aerofoils at different angles, and then compares the best value of each aerofoil, it will be possible to decide the best aerofoil from this point of view.

4. A Low Minimum Drag Coefficient. If high top speed rather than econom­ical cruise is important for an aircraft, then we will need low drag at small lift coefficient, and hence small angles of attack. The drag coefficient at these small angles of attack will be related to the minimum drag coefficient (Fig. 3.15).

5. A Small and Stable Movement of Centre of Pressure. The centre of pressure of our aerofoil moves between 0.75 and 0.30 of the chord during ordinary flight; we would like to restrict this movement because if we can rely upon the greatest pressures on the wing remaining in one fixed pos­ition we can reduce the weight of the structure required to carry these pressures. We would also like the movement to be in the stable rather than in the unstable direction.

Looking at this another way: as we have explained, the moment coeffi­cient at zero lift is slightly negative on most aerofoils, and about the leading edge becomes more nose-down as the angle of attack is increased, and this tends towards stability. Yes, but our real reference point should be about the centre of gravity and, as we have also explained, this is usually not only behind the leading edge but also behind the aerodynamic centre, and may even be behind the trailing edge. So, in fact, this is not what we want for stability about the centre of gravity. On the contrary, we would prefer the exact opposite, i. e. a slight positive (nose-up) moment coefficient at zero lift, and this decreasing to negative as the angle of attack is increased. Most aerofoil sections do not give this; but later we shall find that there are means of achieving it.

6. Sufficient Depth to enable Good Spars to be Used. Here we are up against an altogether different problem. Inside the wing must run the spars, or other internal members, which provide the strength of the structure. Now the greater the depth of a spar, the less will be its weight for a given strength. We must therefore try to find aerofoils which are deep and which at the same time have good characteristics from the flight point of view.

Compromises

So much for the ideal aerofoil. Unfortunately, as with most ideals, we find that no practical aerofoil will meet all the requirements. In fact, attempts to improve an aerofoil from one point of view usually make it worse from other points of view, until we are forced either to go all out for one characteristic, such as maximum speed, or to take a happy mean of all the good qualities – in other words, to make a compromise, and all compromises are bad! It is perhaps well that we have introduced the word ‘compromise’ at this stage, because the more one understands about aeroplanes the more one realises that an aeroplane is from beginning to end a compromise. We want an aeroplane which will do this, we want an aeroplane which will do that; we cannot get an aeroplane which will do both this and that, therefore we make an aeroplane which will half do this and half do that – a ‘half and half affair’. And of all the compromises which go to make up that final great compromise, the finished aeroplane, the shape of the aerofoil is the first, and perhaps the greatest, com­promise.