Originally Posted by
mikehallam
a) Take prop Inches diameter divided by 12 for feet diameter.
b) Multiply by Pi (i.e. 3.142) for circle distance feet per rev.
c) Multiply by RPM (for tip's distance moved per minute)
d) divide by 60 to get tip speed in feet per second !
The speed of sound at sea level is 1100 ft/second -- Compare !
Hope that is O.K.
mike hallam.
That's not sufficient. You also need to incorporate that the aircraft is itself moving. You can have an RPM of zero and still have your prop tips approach the speed of sound - if the airframe itself is flying close to the speed of sound.
You can simply do a bit of pythagoras: Take the prop speed against the airframe (formula above), squared, and add the airspeed, squared. Take the square root of that to get the prop tip against airspeed. (Easiest if you do this all in metric numbers...)
Furthermore, I don't know the exact number but you will want your prop tip to remain below about 80% of the speed of sound. The prop itself is, after all, a wing (aerodynamically speaking). There will therefore be a local acceleration of the air. So even though the prop itself is subsonic, air may still be accelerated locally to supersonic speeds. You therefore need a buffer of about 15-20%.
Anyway, if your static prop speed is already at Mach 0.85, I can well imagine that your aircraft is LOUD.
I fly a GA8, which has a big prop and a 300 HP IO-540. Our POH, and noise certificate based on the POH, specifies a maximum RPM of 2500. 2700 is selectable but after passing a detent, and "in emergency only". It makes a huge difference.