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How to Calculate Prop Tip Speed

The speed at which the tip of an airplane's propeller slices through the air is a good measure of the propeller's efficiency. The longer a propeller's diameter, the faster the tip moves. Up to a certain point, the faster it moves the more thrust it produces. When it approaches the speed of sound, however, the gain in thrust drops off and the faster speed only produces more noise. The recommended range for peak prop efficiency is a tip speed between 0.88 and 0.92 mach, i.e. between 88% and 92% of the speed of sound.

Things You'll Need

Ambient temperature (Fahrenheit)
Propeller diameter (inches)
Propeller RPM

Instructions

- 1
  Calculate propeller tip speed in miles per hour by multiplying prop diameter, RPM, and the constant 0.002975. For example, 80 inches multiplied by 2,600 RPM and 0.002975 equals 618.8 miles per hour.
- 2
  Calculate the speed of sound for the given ambient temperature by adding 460 to it, taking the square root, then multiplying by 33.4. For 59 degrees Fahrenheit, add 59 and 460 to obtain 519. The square root of 519 is 22.78. Lastly, 22.78 multiplied by 33.4 yields 760.9 miles per hour.
- 3
  Divide the tip speed in miles per hour by the speed of sound. In our example, 618.8 divided by 760.9 gives you a mach speed of 0.813. This propeller isn't producing the maximum thrust it could. You can either increase RPM or design a longer propeller in order to obtain a faster, more efficient tip speed.

Aviation