First post / Prop sizing questions

[mmarien]

Interesting question. I read somewhere that prop efficiency was best at about 1800 RPM and as mentioned here the one blade prop is the most efficient. On the other hand there are modern turbine engines that are mostly pushing air that bypasses the jet engine. The turbines have multiple vanes turning at ~30,000 RPM. The vanes could be considered multiple blade props. So there is probably room somewhere between slow moving single blade props and very fast multi blade props that would be efficient at the peak hp range of a piston engine.

There is the heat problem of running any engine at higher RPM's. My O320 run cooler at 2300 RPM as opposed to 2700 RPM. I needed a nice cool day to open it up and keep the CHT within reason. I'm pretty sure that if you put your automobile in a lower gear and got the RPM up to 4000 or 5000 RPM at highway speeds you'll probably have a heat problem.

And there is that cowling problem behind the prop on most planes, except maybe the VariEze and derivatives. Maybe a smaller prop would be more efficient beside or behind the fuselage rather than in front of it. Just thinking out loud here.

Having said all that, if they put the amount of engineering that goes into an automobile engines into airplane engines, we'd probably have better solutions. Rotax comes to mind. Nearly 90% of LSA's use the Rotax engine. Maybe there is a reason for that. Same hp at twice the RPM and half the weight.

One final thought. The PT6 turboprop uses a unique solution of running the exhaust gases of the engine through a separate turbine that runs the prop at a much slower speed. Maybe what is needed is a torque converter to separate the prop from the engine.

Any solution needs to be done quickly before fossil fuel powered engines become obsolete.

[Eric Page]

On the other hand there are modern turbine engines that are mostly pushing air that bypasses the jet engine. The turbines have multiple vanes turning at ~30,000 RPM.

I think you're confusing two parts of a jet engine: the fan that pushes bypass air and the turbine that drives it. Rotational speed varies somewhat from model to model, but roughly speaking the N1 fan on a typical airliner engine spins at somewhere between 2,200 and 3,000 RPM. The N2 compressor spool spins in the neighborhood of 10,000 RPM and the turbine spool at ~25,000 RPM. The fundamental difference is that the N1 fan is pushing air -- that is, functioning like a propeller -- while the turbine is being driven by hot gasses.

[Jeffrey Meyer]

Propeller vs Impeller, incompressible vs compressible flow, extremely low rotor solidity vs extremely high rotor solidity. Completely different engineering design criteria.
Propellers are rotating wings. the free-air wing tip contributes zero lift to the wing - not some lift, not a little lift, zero lift. On the other hand, the tip of an impeller blade contributes a major portion of the lift. This is because the shroud (or duct, or whatever you want to call it) around the impeller prevents radial flow of the air that then tries to flow tangentially around the blade. To prevent that tangential flow we put another blade very close to our blade - so close that the air has no choice but to flow in the axial direction of the rotor. This "no choice" is called "pressure". We are now working in the compressible regime rather than the incompressible regime of a free-air propeller, and the closeness of the blades is called the "high solidity" of the impeller.
And last but not least, on a bypass turbofan, the diameter of the first stage bypass rotor is bigger than the inside diameter of the intake duct. This helps to smooth the transition from incompressible flow to compressible flow.