FAA Aviation Mechanic Powerplant (AMP)PropellersHard
A mechanic is calculating the rotational tip speed of a propeller with a diameter of 6 feet turning at 2,500 RPM to evaluate proximity to sonic speeds. What is the approximate rotational tip speed in miles per hour?
- AAbout 535 mph
- BAbout 700 mph
- CAbout 450 mph
- DAbout 300 mph
Show answer & explanationAnswer & explanation
Correct answer: A. About 535 mph
Tip speed = pi x diameter x RPM = 3.1416 x 6 ft x 2,500 = 47,124 ft/min. Converting to mph: 47,124 ft/min x 60 min/hr = 2,827,440 ft/hr; divided by 5,280 ft/mile = approximately 535.5 mph.
Why the other options are wrong
- B. Too high; would require a larger diameter or higher RPM.
- C. Close but does not match the precise calculation.
- D. Too low; results from underestimating the circumference or RPM.
Propeller Tip Speed Calculation
Rotational tip speed = pi x diameter x RPM, used to evaluate how close propeller tips are approaching the speed of sound.
- Formula: tip speed (ft/min) = pi x D x RPM
- Convert ft/min to mph by multiplying by 60 and dividing by 5,280
- Forward airspeed must be added for total resultant tip speed
Memory trick: Pi times D times RPM gives you the spin speed king