Method of finding minimum landing speed

Consider this problem –

Find the wing area required for an aeroplane of mass 1500 kg, if the minimum landing speed is to be 35 knots (65 km/h) and the maximum value of the lift coefficient for the aerofoil used is 1.2 (assume the air density to be 1.225 kg/m3).

Data: W = 14 715 N

p = 1.22= kg/m3 VL = 65 km/h = 18m/s CL max = 1.2 S = ?

So 14 715 = 1.2 X – X 1.225 X 18 X 18 X S 2

c. S = 62 m2 approx

This is rather a large wing area for an aeroplane of this weight, and it is doubtful whether the structure involved would not make the total weight greater than 14 715 N, in which case, of course, the landing speed would be above 35 knots.

Suppose we could use a flapped wing with a maximum lift coefficient of 1.8 instead of 1.2, then, neglecting any small increase in weight, the necessary wing area to produce the same landing speed would be –

(1.2/1.8) X 62 = 41 m2 approx

It would certainly be much easier to design a wing structure of this size so as to conform to a total weight of 14 715 N and, further, the reduced wing area would enable a much greater maximum speed to be obtained.

As another problem, let us compare the minimum landing speeds of the fol­lowing –

(a) A sailplane of wing loading 100 N/m2.

(b) A training machine of wing loading 400 N/m2.

(c) A fighter of wing loading 1500 N/m2.

(d) The S.6b of wing loading about 2000 N/m2.

(e) An airliner of wing loading 3000 N/m2.

Supposing other things to be equal, e. g. taking p as 1.225 kg/m3, and assuming each machine is fitted with an aerofoil section having a maximum lift coeffi­cient of 1.12, then –

(a) Wing loading = W/S = 100 = CL max fpVL2

= 1.12 X I X 1.225 X VL2

У 2 = (100 X 2)/(1.12 X 1.225) = 146 .•. VL = 12 m/s, i. e. 23 knots or 43 km/h

Similarly for

(b) Landing speed = 47 knots (87km/h)

(c) 91 knots (169 km/h)

(d) 106 knots (195 km/h)

(e) ? knots (? km/h). Reader, work it out.

Such is the type of problem which confronts the designer of an aeroplane in the very early stages, when, by a process of calculations, he has to decide such important items as the wing area, the type of aerofoil, and the landing speed. It will now be obvious that in order to settle these he must know the weight – the weight of an aeroplane which he has not yet commenced to design! Here comes the first great guess; but it is a guess based on experience, and often proves remarkably accurate. A decision as to landing speed and as to the type of aerofoil will then decide the wing area, on which the whole lay-out of the aeroplane depends, so it will be seen how important is this question of landing speed and its influence on the whole design of the finished aeroplane.