I fly model aircraft. Recently I changed from a "1300kv" motor to a "1500kv" motor. The difference is in speed; the 1300kv can go about 16,400rpm full-throttle and with a fully charged battery and the 1500kv can go nearly 19,000rpm. (This is no load speed, but the prop adds very little load when the aircraft is not flying.)
I noticed that when I increased the throttle beyond about 75% the motor and propeller became incredibly loud. Below that they were loud, but no where near as much.
So I did some ballpark calculations. I'm using a 7×3.5" propeller – which means it has a 7" diameter and 3.5" pitch. In this case we are only interested in diameter. 7" diameter means 3.5" radius, or 8.86 cm. Doing 19,000 rpm means than I'm doing about 317 rps. According to 2 * pi * r the circumfrence of the prop's tip is 55.64 cm. Plug this into 317 rps and we get 17,638 cm/s or 176.4 m/s. Which is about half the speed of sound.
Although this is not actually the speed of sound it's very close to it, and much higher than I expected. So what does that mean? I figure that although the propeller itself isn't traveling faster than the speed of sound, the air it pushes out of the way is. My physics teacher showed that if you take a metre stick and swing it through the air (avoiding hitting anyone) it makes a wooshing sound – the air is moving faster than the ruler was. So the prop is pushing the air away faster than this and causing this very loud noise.
Is my reasoning correct? Is my prop breaking the sound barrier?!
Best Answer
In general yes, you may have supersonic flow in some areas.
But prop is subsonic, and should work fine till 0.9mach (~300 m/s) if it have optimal shape for that speed (some planes even had variable prop's angle of attack to get this for all speeds).
So, in your case you must have excessive noise due to turbulent air flow due to non-optimal prop shape. When flow goes from laminar to turbulent - you should hear significant increase in noise.