Propellers

[gravey]

Does anyone know a formula to calculate at which engine RPM the propeller will go the speed of sound? Thanks in advance...

 

[pilotyip]

Radius

(the sq root of (the sq of the (diameter divided two times the RPM) plus the (sq of the of the fwd speed of the airplane in ft per sec))/speed of sound in ft per second at the specific temp at which you a trying calculate.

 

[Andy Neill]

ALMOST....you forgot pi in the first part.

I believe

(the sq root of (the sq of the (pi*diameter/2 RPS in ) plus the (sq of the of the fwd speed of the airplane in ft per sec))/speed of sound in ft per second at the specific temp at which you a trying calculate.

or

((((pi*D/2)*RPS)^2 + V^2)^0.5)/S

where D is the prop diameter in ft
RPS = RPM/60
V = airspeed in fps
S = speed of sound in fps

So for a 6' prop turning at 2400 RPM on an aircraft flying 150 kts on a day when the speed of sound is 600 kts, the prop tip would be just under 0.8 Mach.

 

[BeechSlapped]

We're getting warmer here........

The equation that Andy posted is correct, except for the "pi(D/2)" term. The distence travelled by the propeller tip is equal to the circumferance of a circle of radius "D/2". But the equation for circumferance is (2*pi*r). Since r=D/2, the "2" in the term cancels out. Since the original poster wanted the RPM for the tip reaching the speed of sound:

RPM = (60/(Pi*D)) * (a^2 - V^2)^0.5


Where:

RPM = The RPM that will yield a tip speed of Mach 1.0 (Rev/min)
a = speed of sound at current temperature (ft/s)
D= diameter of propeller (ft)
V = speed of aircraft (ft/s)

Beech Slapped

 

[pilotyip]

don't forget fwd speed

Yea you are right I left some stuff out, but don't forget the fwd speed and prop tip speed combine to give a net velocity. The TAS of the prop tip which goes up with altitude, and the speed of sound goes down with temp at altitude. There is a net speed that combines the rotational TAS velocity of the prop plus the fwd TAS throught the air. In the P-3 and the Electra our Vne was controlled by the tip speed of the prop, as it approched .8 M, I believe, you could not exceed that speed even though the airframe limit was well above that limit dicated by the prop. If I remember Vne at SL was 405 KTS?, and at 310, it was like 205 kts.

 

[MetroSheriff]

Props are for boats.

 

[Typhoon1244]

So how fast would a Maglev train have to go before its wheels went supersonic?

 

[pilotyip]

Anyone can fly jets

Takes a real pilot to work all those levers at once, throw in turbo waste gate controller and intercoolers and you really have a hand full. But the Electra only had one lever per engine like a jet, made it easy to fly like a jet.