Efficient propellers

High efficiency does not depend on just getting a good ratio of thrust to resist­ance, since that would imply using very small pitch and helix angles. A small helix angle means that the blades would be whirling round at high speed doing a great deal of work against the resistance, without actually doing much useful work in moving the aircraft forwards.

The efficiency with which a propeller is working is the ratio

rate that useful work is done,
amount of power required to overcome the resistance

The useful work rate is the product of the thrust produced, and the forward or axial speed of the propeller.

The power required to overcome the resistance is the product of the resist­ance torque and the blade angular rotational speed.

thrust x axial speed

Propeller efficiency is.

resistance torque x rotational speed

From the above, it is seen that efficiency depends not only on (thrust/resistance) but also on (axial speed/rotational speed). By looking at Fig. 6.4, it may be seen that reducing the helix angle would improve the first ratio (thrust/resistance torque), but decrease the second ratio (axial speed/rotational speed) by an almost equal amount.

Theoretical analysis shows that for high efficiency, the blades will need to be operating like a wing producing a high ratio of lift to drag, and that under these conditions, the best helix angle approaches 45 degrees.

Since the helix angle increases towards the centre of the propeller, it follows that only part of the blade span can be operating at the most efficient angle at any time. The outer portion of the blades produces the majority of the thrust, so the blades are normally operated at a pitch angle which gives maximum efficiency on their outer portion.

The theoretical analysis also shows that as the blade lift-to-drag ratio is increased, the propeller efficiency becomes less sensitive to helix angle.

If the outer portion is running at its optimum helix angle, then a large por­tion of the inner section will be running at a high and inefficient angle. The central portion of a propeller often does little more than increase the resistance torque. It is, therefore, normal to terminate the inboard ends of the blades at a streamlined spinner, which thus serves a more than merely aesthetic function (see Fig. 6.6). On advanced prop-fan designs, such as that illustrated in Fig. 6.9, the engine itself may take the place of the spinner.

For very high propeller efficiency, we need to use the same kind of low-drag aerofoil section shapes for the blades that we use for wings. As with low-drag wings, however, these high-efficiency blades are intolerant of running off their design angle of attack. High-efficiency propellers, therefore, depend on the accurate matching of pitch to flight speed, and engine speed to power. Advances in control systems, and the development of better blade section profiles, have enabled considerable improvements in propeller propulsion to take place.