TXFlyGuy
Well-Known Member
Found some interesting info...
Tip speeds at or near .65 Mach are the most efficient for high speed cruise flight.
90" Prop
1.9-1 ratio
3500 engine rpm / 1840 prop rpm
Tip speed = .642 Mach / 497 mph
90" Prop
2.21-1 Ratio
4100 engine rpm / 1850 prop rpm
Tip speed = .647 Mach / 497 mph
The following is taken from an article on propeller speed:
That phenomenon is very easy to spot in a high speed aircraft which has the capacity to run the propeller too fast for a given set of flight conditions. Here is an example. A few years ago, I was flying the factory Glasair-3 to an airshow. I was cruising at 13,000 feet, 2400 RPM, wide open throttle. I was running a bit behind schedule, so in pursuit of a few more knots, I decided to operate at max power (2700 RPM, WOT). It was something of a surprise when I lost about 15 knots of airspeed. I set the RPM back to 2400, and regained the lost 15 knots. Later I did the calculations to verify that the loss was due to the sudden loss of efficiency. It was.
So in the case of two identical airplanes, same engine and prop, both operating at max continuous rpm, which one is faster?
The one with the prop turning at 2300 rpm, or the one with the prop turning at 2000 rpm? Same engine, same power.
Tip speeds at or near .65 Mach are the most efficient for high speed cruise flight.
90" Prop
1.9-1 ratio
3500 engine rpm / 1840 prop rpm
Tip speed = .642 Mach / 497 mph
90" Prop
2.21-1 Ratio
4100 engine rpm / 1850 prop rpm
Tip speed = .647 Mach / 497 mph
The following is taken from an article on propeller speed:
That phenomenon is very easy to spot in a high speed aircraft which has the capacity to run the propeller too fast for a given set of flight conditions. Here is an example. A few years ago, I was flying the factory Glasair-3 to an airshow. I was cruising at 13,000 feet, 2400 RPM, wide open throttle. I was running a bit behind schedule, so in pursuit of a few more knots, I decided to operate at max power (2700 RPM, WOT). It was something of a surprise when I lost about 15 knots of airspeed. I set the RPM back to 2400, and regained the lost 15 knots. Later I did the calculations to verify that the loss was due to the sudden loss of efficiency. It was.
So in the case of two identical airplanes, same engine and prop, both operating at max continuous rpm, which one is faster?
The one with the prop turning at 2300 rpm, or the one with the prop turning at 2000 rpm? Same engine, same power.
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