# Skin friction or Surface friction

These two between them form a large part of the total drag of an aeroplane – in the high subsonic range, the major part. The sum of the two is sometimes called profile drag but this term will be avoided since it is apt to give an impression of being another name for form drag, whereas it really includes skin friction.

The total drag of an aeroplane is sometimes divided in another way in which the drag of the wings or lifting surfaces, wing drag, is separated from the drag of those parts which do not contribute towards the lift, the drag of the latter being called parasite drag. Figure 2A illustrates an old type of aero­plane in which parasite drag formed a large part of the total. Figure 2B (overleaf) tells another story.

Lines which show the direction of the flow of the fluid at any particular moment are called streamlines. A body so shaped as to produce the least poss­ible resistance is said to be of streamline shape.

We may divide the resistance of a body passing through a fluid into two parts –

[5] Form drag or Pressure drag

[6] To allow the tail to touch the ground before the main wheels (Fig. 6.7a). This is hardly a practical proposition.

[7] To have a much higher undercarriage (Fig. 6.7b). This will cause extra drag and generally do more harm than good.

[8] To provide the main planes with a variable incidence gear similar to that which is sometimes used for tail planes (Fig. 6.7c). This involves consider­able mechanical difficulties.

[9] To set the wings at a much greater angle to the fuselage (Fig. 6.7d). This means that in normal flight the rear portion of the fuselage sticks up into the air at an angle which not only looks ridiculous, but which is inefficient from the point of view of drag. It gives the appearance of a ‘broken back’, but has sometimes been used for aircraft designed for deck landing since it not only gives a low landing speed, but a quick pull-up after landing.

The only real answer lies in the design of flaps and slots which must be such that the effective camber of the wing can be altered so as to give

We assumed at an earlier stage, that the area of the wings was bound to remain constant, but inventors have from time to time investigated the problem of providing wings with variable area.

Since W = CL. іpV2 . S

W/S = CL. ipV2

It is easy to work out simple problems on minimum or landing speeds by using the now familiar formula –

Weight = Tift = CL. ip У2 . S

If we denote the maximum value of the lift coefficient by CL max, and the landing speed by VL, then our formula becomes

W = CL . max ip У2 . S

[10]The quotation from The Stars in their Courses by Sir James Jeans is given by courtesy of the Cambridge University Press.

[11] A car is travelling along a road at 50 km/h. If it accelerates uniformly at 1.5 m/s2 –

(a) What speed will it reach in 12 s?

(b) How long will it take to reach 150 km/h?

[12] A train starts from rest with a uniform acceleration and attains a speed of 110 km/h in 2 min. Find –

(a) the acceleration;

(b) the distance travelled in the first minute;

(c) the distance travelled in the two minutes.

[13] If a motorcycle increases its speed by 5 km/h every second, find –

(a) the acceleration in m/s2;

(b) the time taken to cover 0.5 km from rest.

[14] During its take-off run, a light aircraft accelerates at 1.5 m/s2. If it starts from rest and takes 20 s to become airborne, what is its take-off speed and what length of ground run is required?