That's where my thinking was leading me. You put it into words pretty good Keeping a fixed pitch prop turning at say 2500 rpm doesn't do a whole lot for speed, though if in level flight - prop @ 2500 rpm, engine at 2500 rpm - might make for better miles per gallon. For take off/climb - prop 2500 rpm, engine @ 4500 rpm no real benefit. But varying the pitch of the blades... that's a whole 'nother story.
On 7/13/2012 10:00 PM, krnet-requ...@mylist.net wrote: > CVT - continuously variable transmission > > I have designed 2 of them for small vehicle applications at my last > job, and worked on 2 others that someone else designed. I will tell > you that though in theory they are simple, in practice they are full > of gremlins. I wouldn't go as far as to say you can't do it, but I > don't think would be worth the development effort. It won't be > lighter than a standard gear reduction for sure. > > That said - > I guess I'm missing the benefit. With a "constant speed" prop, you > (well, the prop gov.) vary the prop pitch to control engine and prop > rpm, and thus load on the engine dependent on a throttle setting. > Driving a fixed pitch prop at one rpm while varying the engine rpm > doesn't get you anything except a prop that spins 2400 rpm from idle > (or tries before killing the motor) to somewhere in the power-band. > Even if you made the output (prop) rpm adjustable, there's no gain > because FP props are most effecient at a small range of rpm while > (hopefully the powerband of the engine) in a specified airspeed > range. The only thing you might get out of it is the ability to tune > the engine to a specific prop/airframe, which we do already (but in > reverse) by playing with prop diameter, pitch, blade profile, etc... > > It works great in a ground vehicle because you can tune the engine to > max efficiency, or max power, and the cvt can change gear ratios as > wheel rpm (speed) increases to keep the engine at the same rpm. It > doesn't work like that in an airplane.
