Number and shape of blades

The propeller must be able to absorb the power given to it by the engine; that is to say, it must have a resisting torque to balance the engine torque, other­wise it will race, and both propeller and engine will become inefficient.

The climbing conditions are particularly difficult to satisfy since high power is being used at low forward speeds; and if we do satisfy these conditions – by any of the methods suggested below – it will be difficult to get efficiency in high-speed flight. Thus, the propeller becomes a compromise like so many things in an aeroplane.

The ability of the propeller to absorb power may be increased by –

1. Increasing the blade angle and thus the angle of attack of the blades.

2. Increasing the length of the blades, and thus the diameter of the propeller.

3. Increasing the revolutions per minute of the propeller.

4. Increasing the camber of the aerofoil section of which the blade is made.

5. Increasing the chord (or width) of the blades.

6. Increasing the number of blades.

With so many possibilities one might think that this was an easy problem to solve, but in reality it is one that has caused considerable difficulty. First, the blade angle should be such that the angle of attack is that giving maximum efficiency; there is, therefore, little point in trying to absorb more power if, in so doing, we lose efficiency. The second possibility is to increase the diameter, in other words, to increase the blade aspect ratio, but quite apart from the bogey of tip speed, with large propellers there is the problem of providing enough ground clearance. The third would mean high tip speed and conse­quent loss of efficiency. The fourth, as with aerofoils, would simply mean a less efficient section; it would seem, too, that we must face even thinner aerofoil sections to avoid loss of efficiency at high speed. So we are left with the last two, and fortunately they provide some hope. Either will result in an increase in what is called the solidity of the propeller. This really means the ratio between that part of the propeller disc which, when viewed from the front, is solid and the part which is just air. The greater the solidity, the greater the power that can be absorbed.

Of the two methods of increasing solidity, increase of chord and increase of number of blades, the former is the easier, the latter the more efficient. The so – called paddle blades are examples of the former method. But there is a limit to this, first, because the poor aspect ratio makes the blades less efficient.

So, all in all, an increase in the number of blades is the most attractive prop­osition, and that is why we saw, first, the two-blader (yes, there has been a one-blader! – but only one); then, in turn, three, four, five, and six blades; and we might have gone to eight – and ten-bladers had not jet propulsion come along at the critical time.

After four or, at the most, five blades, it becomes inconvenient to fit all the blades into one hub, and it is, in effect, necessary to have two propellers for each engine. If we are going to have two propellers, we may as well rotate them in opposite directions (Fig. 4K, overleaf) and so gain other advantages which will become more apparent when we have considered the effects of the propeller on the aeroplane.