High speed propellers

The sudden increase in fuel costs that occurred in the mid-1970s, caused man­ufacturers to look again at the possibility of designing propellers suitable for flight at high subsonic Mach numbers.

The primary approach to solving the problem of supersonic tips, is the same as that used for transonic wing design, as described in Chapter 12. Essentially, it is necessary to keep the maximum relative velocity on the blade surface as small as possible. Thin ‘transonic’ blade sections may be used, and the blades may be swept back producing the characteristic scimitar shape shown in Fig. 6.9.

Accurate control of pitch, enables high lift/drag ratio sections to be employed, and this in turn allows the use of large helix angles, so that the resultant relative flow speed past the blade is minimised.

A large number of blades is used, in order to reduce the thrust force per blade. As with wing lift, blade thrust is related to the circulation strength. By reducing the thrust per blade, the circulation is reduced, and hence the maximum relative speed on the upper surface is lowered.

Figure 6.9 shows a typical design for a contra-rotating configuration. Such propellers are variously described as prop-fans or unducted fans, and though

they may look very different from older designs, they are, in principle, still propellers. Although not providing such efficient propulsion as low speed pro­peller designs, unducted fan propulsion is more efficient than the turbo-fan system that it is intended to replace.

The relative airflow speed at the tips of such propellers is designed to be supersonic in high speed flight, and they are therefore very noisy. This is the major obstacle to their use in civil transport aircraft.

It should be noted, that reducing the propeller diameter will not reduce the tip Mach number, because in order to produce the same amount of thrust, the smaller diameter propeller will have to rotate at a higher speed.