Tip speed of a Prop
06-09-2005, 08:20 PM
#2
My Feedback: (1)
RE: Tip speed of a Prop
The square root of the sum of the squares
The simple answer is to calculate just the tip speed due to rotation, but is incorrect. You can find plenty of online calculators that do just that. That requires measurement of rpm, with prop diameter and using pi to calculate the distance traveled by the tip.
But the real answer requires knowing forward speed as well, since the prop tip is traveling a helical spiral through the air. Thus - the square root of the sum of the squares – take the rotational speed and the forward speed, square both terms, add, and take the square root. Make sure both numbers are the same units.
For model airplanes, the engines unload too, so the ground rpm is lower than the actual rpm in the air.
Then if you want to know the percentage of the speed of sound the tip is traveling, you need to factor in temperature too.
The simple answer is to calculate just the tip speed due to rotation, but is incorrect. You can find plenty of online calculators that do just that. That requires measurement of rpm, with prop diameter and using pi to calculate the distance traveled by the tip.
But the real answer requires knowing forward speed as well, since the prop tip is traveling a helical spiral through the air. Thus - the square root of the sum of the squares – take the rotational speed and the forward speed, square both terms, add, and take the square root. Make sure both numbers are the same units.
For model airplanes, the engines unload too, so the ground rpm is lower than the actual rpm in the air.
Then if you want to know the percentage of the speed of sound the tip is traveling, you need to factor in temperature too.
