Number and shape of blades

As with wings, increasing the aspect ratio of the propeller blades reduces the drag or resistance. However, the amount of thrust that can be produced, depends on the total blade area, so the use of high aspect ratio blades may result in an unacceptably large propeller diameter. Large high-powered propeller-driven aircraft often have low-aspect-ratio ‘paddle’ blades.

A small number of blades is preferable as it reduces the mutual interfer­ence effect between blades. However, maintaining sufficient total blade area to transmit the required power through a given diameter may necessitate a compromise between aspect ratio and number of blades. The Spitfire, which started life with only two blades and 1000 bhp, ended up as the Seafire 47 with six blades (on two co-axial contra-rotating propellers) and 2350 bhp. The diameter was limited by ground clearance. The Lockheed Super Hercules, shown in Fig. 6.6 uses high-aspect-ratio six-bladed 4.11 m diameter propellers.

Increasing the number of blades also reduces the amount of thrust that has to be produced by each blade, which is an advantage in high speed operation, as it lowers the maximum local Mach number on the blades. The importance of limiting the Mach number is described later.