I think the main application for PSRUs in aircraft, is if you're running an engine with an absurdly-high operating RPM/power-curve, that you need to bring down to make using a prop practical...
For example, if you're powering your plane with a Subie motor that red-lines at 6k+, you're obviously going to want to reduce that down a bit, as there really aren't that many prop-combos that will work in that power-band unreduced... The other reason, would be to isolate engines with less-durable crankshafts, from the forces associated with a prop hub... That said, Continental DOES or DID make a version of the O-300 with a geared PSRU, for certified aircraft - and it seemed that they did so entirely for powerband reasons (to turn the engine at over 3K, while turning the prop at the normal 2400-2600 seen in most certified direct-drive designs... On 7/13/2012 1:51 PM, Matt Elder 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. You need to be able to vary the > pitch of the prop most importantly to get efficiency at different > airspeed while keeping the engine at a speed it likes to make power in > (or run more efficient in). The prop is the actual transmission. So > in effect, the hyd/electric prop govs. are akin to CVT's. In reality, > I'd love to be able to vary diameter too! And telescope the wings > > If I'm off on my thinking here, please elaborate. > It sounds like you want/need a gearbox with an oil system so that you > can run a const. speed prop > > Matt E. > >> On 7/12/2012 6:03 PM, John wrote: >>> I was wondering if anybody has heard of anybody doing a CVT type setup >>> instead >>> of the PSRU? (Constant Velocity Transmission) search youtube for CVT. >>> seems to me that it could weigh about the same and keep the prop at say >>> 2400 rpm >>> while the engine rpm varies. >>> From what I've seen they're all set up to let drive wheel rpm vary >>> while engine stays >>> in "best power" - >>> > > _______________________________________ > Search the KRnet Archives at http://tugantek.com/archmailv2-kr/search. > To UNsubscribe from KRnet, send a message to [email protected] > please see other KRnet info at http://www.krnet.org/info.html
