Variable pitch prop - VPP - for motor efficiency optimization

[quax]

Is someone out there, who has experience with that topic?

The idea: It should be possible, to run a motor in its best RPM (efficiency) range, adjusted accordingly to the input power. Averaged it should be possible to increase the usable power.

I would like to know something more about “max efficiency“ related to “input power“. I can imagine, that it results in a constant RPM over powerrange or something like that. Before I start with any realization, I would like to have more base knowledge.

Such a control should result in a very good acceleration over a great speed range, full speed-up at take off and also in the air, at higher speed. I expect more an acceleration control than a speed control.

Any ideas ?

Quax

[Bruce Abbott]

For a fixed input voltage, the maximum efficiency of a motor occurs at a single rpm and current (and therefore a single input power level). However, if the motor has a high peak efficiency, it will also have a fairly broad range of currents at high efficiency, with rpm only reducing slowly as load increases. When input voltage is reduced the rpm goes down proportionately, and efficiency drops somewhat. Also the peak efficiency point moves towards a slightly lower current.

To get the greatest acceleration you need to achieve maximum thrust. A fixed pitch prop gives maximum thrust only at one particular airspeed, where the blade AOA is optimum (only slightly affected by rpm). Using a variable pitch prop allows you to optimise for a range of airspeeds - low pitch for maximum static thrust and high pitch for best thrust at maximum speed.

Adjusting the pitch to maintain constant motor rpm (or current) gives constant power loading, which can be matched to the motor's best operating point. This should also give close to optimal thrust at any airspeed (within limits - infinite pitch is obviously not optimal!).

Overall efficiency gain is much more a function of the prop than the motor. The ultimate solution would be to have a prop with variable diameter and blade shape as well as pitch.

[quax]

If I understand correctly, efficiency optimization is more complicated as I thought and should not be the aim of implementing a simple VPP.
Thanks for your explanations, Bruce!

On the other hand side a RPM-control via pitch comes near to a constant thrust over a wide range of airspeed and may be worth to be added as an integral part of the motorcontrol.

Probably an unfamiliar feeling, this kind of control :cool:

I'm quiet sure to give that version a try - even if it's only for curiosity.

Are there additional applications imaginable :confused:

Quax

[MatC]

It's a nice idea...
And you can make it "feel" nice by making the throttle act as a combined speed and acceleration input: ie at part throttle you get the most efficient acceleration and a medium top-speed, at full throttle you aim for max power and sod the efficiency.

For a given prop and model, the input voltage to the motor and the ideal pitch are just functions of model speed and throttle position, although some reasonably careful measurements of the motor parameters (easy) and prop parameters (tricky) would be needed.

To make it easier, auto prop pitch will make a fantastic difference to speed/accel/efficiency, and is a doddle to calculate (at least roughly) from windspeed. Nice project, show us pictures when you do it :)

[Blackaces]

Sorry to bring an old thread alive but it seems relevant to place my discussion in this thread as oppose to a new one.


I have been playing with the idea of a mechanical and electrical variable pitch system for a little while now.
I don't mean anything like the 4D systems that you can find these days though thrust reversing would rock, im talking about the idea of coupling prop angle to Airspeed.

so far almost all of the designs i have looked at have used either a servo system mixing prop angle with throttle (or manual control) or a mechanical system using counter weights all directly or indirectly linking prop angle to RPM.

what i want, is a system that has a very fine angle for static thrust but a course angle for higher airspeed.

currently my design is:
Using a swash plate like mechanism mounted behind the prop, control the props AoA by moving the swash plate forward(finer) or backwards (coarser).

Id then attach the swash plate to a light spring holding it in the full back position with the prop at a high AoA.

What my idea was that as i applied throttle, the high drag created by the high AoA of the prop would over come the spring and reduce the angle.

As the aircraft speeds up, the air would naturally supply less force to the propeller allowing it to re-increase the angle of attack.

Theoretically this should give me a high static thrust, while providing a good top speed.

some of the issues im having though, and the reason i brought it up, is because i A) im not sure of the right prop length(advance is arbitary as its changed by the system anyway) B) the kind of motor i would need to run it (i currently use a 2200KV out-runner) and C) because im forcing the propeller to be under constant load trying to over come the spring( load should remain fairly constant across all airspeed i believe) how will that effect my running time and motor/electronics wear?

My last question is, i got my idea from looking at full scale CSU props, so i guess from experience, would putting this kind of system onto my RC actually give me any kind of good performance increase? or will it just be a cool novelty?

thanks

Aces