# Propellor thoughts

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#### orion

##### Well-Known Member
When determining tip speed of a prop you must not only calculate the tip speed of the rotating prop but, you must also take into account the forward speed of the airplane. If you have the airplane's speed, and prop tip's speed, you then square both numbers, add the results together, and then take the square root of the sum. That will be the actual tip speed of the prop in flight.

#### Craig

##### Well-Known Member
Tip speed

Orion, et al -
So, if I figure it correctly, I have a prop 74" diameter, or 6.167 ft.
At 2,400 rpm (120 mph, say), I get 774.4 fps for the tip rotation speed, plus 176 fps for the forward airspeed. Squared, added, and then square root gives me a corrected tip speed of 794 fps.
Did my basic math add up right?

#### orion

##### Well-Known Member
Sounds right to me. And it falls right in the ballpark in that you really want the tip speed at cruise to be at about 80% of the speed of sound. Yes, that does vary with the application and prop design, but overall though, that's a common rule of thumb.