Quote: carusoam
Reality strikes...
Crash, I think what you have been told about prop speed and speed of sound (although correct) has been simplified to explain why the RPM range is chosen for a particular prop. It sounds more mysterious than the more mundane actual reasons.
So my guess is.... somebody at the airplane factory noted prop speed vs. effectiveness (angle of attack too). Engineers like to test this kind of stuff and write long reports that go with that.
They (engineers) need to answer stuff like: How much pull can I get out of this prop from take-off through cruise? and How is this best accomplished?
At take-off, with no airspeed, through rotation and climb, 2,700 rpm must be the most pull available to get the plane off the ground quickly and safely (including safely for the engine).
Realisticly was it 2705 rpm or 2690.32 rpm that gave the best performance? Again somebody simplified the system to help the pilot not need to be a flight engineer. Another factor of safety is added to the system because of the accuracy of the rpm gauge in the aircraft may be off 10s of rpm or worse.
During cruise, the same prop selects an RPM that is comfortable(lower vibration), an efficient match to the airframe (speed vs. mpg), economical (to make, sell, operate).
Ordinary propellers are clearly not efficient at anywhere near supersonic speeds (prop or airplane). Compressibility of air makes it far from ideal, and gets further away from bernoulli's principles as the speed of the system increases.
We want air molecules to flow smoothly over the airfoil in an ideal manner. compressibility, mass of air molecules, momentum, and friction all work against efficiency.
It would be my guess that aircraft engineers stay a fair percentage (25% ???) away from the speed of sound (at any condition) The parts of the propeller that reach this plateau would not be worth the added weight, stress, or other costs.
Clearly the point is well taken, the faster the tip speed, the more noise is generated. You don't need to go to supersonic speeds to generate a lot of noise. A whistle or horn intentionally makes a lot of noise with very little air speed. Mooneys are not quiet beasts on take-off and neither are Bonanza's.
Our European brothers are seeing new propeller designs that achieve quiet operation and still strive for good pull through the flight regimes.
Question for N601X, On the Jabiru is that direct drive? Often, a high rpm engine, rotax and auto engines included, a gear box (or belt) will be used to bring the prop speed back down into the 2,700 rpm range.
a 64" prop spinning at 3,300 rpm gives a 514 mph tip speed (using maropers method above)
prop circumference = Pi X D = tip distance traveled per revolution.
Still 30+% short of the speed of sound at any altitude or temperature.
Respectfully submitted
-a-