Tip Speed

The prop tip speed can be found, in feet per second, given the RPM and prop diameter from the following formula:

Prop tip speed = 2nRN

Where: 2 = a constant n = 3.14…

R = prop radius in feet N = prop revs per second

Given the following figures the prop RPM in FPS can be found:

Prop radius = 74 inch/2 = 37 inch = 3.08 feet Revs per second = 2700/60 = 45 revs per second

Prop tip speed = 2nRN

= 2 x 3.14. x 3.08 x 45 = 870 FPS

An alternative and simpler formula to the one above is as follows:

RPM x diameter

229.3

2700 x 74 inches

229 3 = 870 FPS

In the example above, the propeller rotating at 870 FPS on take-off at full RPM would be operating very close to Mach 0.8, where efficiency begins to deteriorate and the prop’s noise level is about to exceed the maximum allowable limits. An aircraft designer may consider a propeller reduction gear to reduce the tip speed to a value of 0.7 to 0.8 of the engine speed. One example of using prop reduction gear was demonstrated on the Australian CAC Wirraway, a licensed built version of the North American Harvard. The Wirraway’s geared engine driving a three-blade prop at a slower RPM was much quieter than the Harvard’s distinctive growl on take-off.

So far, we have only considered the effects of the rotational velocity on the prop tip speed (vector A-B on Diagram 2, Propeller Terminology). To this vector we must add the vector representing the props advance per rev (B-C). We now have the third vector in the triangle (vector A-C) corresponding to the propeller’s helical flight path. Because the vector A-C is of greater length than vector A-B, it follows the prop tip speed will be higher when the aircraft’s forward speed is increased from zero up to cruising speed. Using the above formula again, and the 74 inch propeller turning this time at 2400 RPM, we find the prop tip speed to be approximately 775 FPS (236 m/ sec) when the aircraft is stationary Increasing the plane’s forward speed up to 142 knots, the prop tip speed increases to around 810 FPS (247 m/sec). The noise level would be fairly high but acceptable at this speed.

Generally at tips speed of around 600 FPS (183 m/sec) the prop will be relatively quiet but, the noise level will start to increase around 700 FPS (213 m/sec). At 880 FPS (268 m/sec) the prop could be unacceptably noisy, as mentioned above. When the prop tip speed approaches the speed of sound, compressibility problems and tip vortex losses increase, which in turn reduces thrust, efficiency and increases prop torque and noise. Mach 0.8 or 880 FPS (268 m/sec) is about the maximum tip speed a normal prop can safely operate to,

but there are a few exceptions with some props designed to run at transonic tip speeds (Mach 0.8 to Mach 1.20).