C/S, best RPM for max speed

[Bob Axsom]

Most have read the performance report by the RV-10 pilot where he stated that the maximum speed was far below max RPM. Recently I was flying at 7500ft, max throttle, leaned for best power, trimmed for hands off level flight, Pictorial Pilot on with a specific course, Altrak holding altitude, and I increased the RPM in increments of 10 and recorded the TAS. I recognize some of the pitfalls of the test method, not retrimming after each change, etc. but still the results are food for thought:

RPM Kts
2450 171
2460 171
2470 170
2480 171
2490 171
2500 171
2510 171
2520 171
2510 171
2520 171
2530 170
2540 171
2550 171
2560 171
2570 172
2580 173
2590 174
2600 174
2610 174
2620 173
2630 172

Bob Axsom

[rv6ejguy]

This is the kind of data we need. Thanks for posting this. Can we trouble you for fuel flow at these rpms next time you are flying? This is very interesting.

[Bob Axsom]

The airplane has no fuel flow instrumentation. At 2450 RPM where I run it all the time the fuel flow is reliably 10gal/hr. Sorry I don't have it at the other RPM settings.

Bob Axsom

[cobra]

Bob-
Since your data indicates flat 171 kts speed between 2450 and ~2565 rpm, it would be interesting to see how much lower the rpms could fall and still maintain approx the same 171 kts speed (possibly improve your mileage a little)...

[Mike S]

Would also be interesting to know the manifold pressures at each RPM/speed index

Mike

[dan]

Did you re-lean at every RPM change? Shouldn't make a huge difference, but it does make a difference. You might find that the speed peaks at peak RPM if you do that.

Also, how exactly are you leaning? Leaning to peak indicated airspeed? Leaning to some specific value ROP? Just curious.

FWIW, when doing performance/speed testing, I lean to peak IAS after each RPM change.

)_( Dan
RV-7 N714D
http://www.rvproject.com

[Bob Axsom]

I have some digital EI gauges (tach for example) but the manifold pressure and the EGT are analog. For leaning my procedure is to run it up to peak and back off until I hit the first indication of smoothness (sweet spot) it is usually about 50 degrees rich of peak. So my answers to the questions are:

1 - I did not change the mixture at all during the test.
2 - Mixture 50 degrees rich of peak EGT.
3 - Manifold Pressure 22.7 in (It was constant through the full range of tests - I did record it but it was not a digital readout.)

Bob Axsom

[dan]

Try leaning to peak IAS, and readjust as necessary after each RPM or power change. You may see a few more MPH.

Keep an eye on your CHTs. This is an all-out performance test, not necessarily a representation of a "normal mode" of cruising.

)_( Dan
RV-7 N714D
http://www.rvproject.com

[hevansrv7a]

For a given Manifold Pressure MP, assuming equal proportions of fuel and air, more rpm will mean more power. There can be no doubt of this because rpm is exactly proportional to the volume of fuel-air mix pumped (with equal MP). The experiment needs to be repeated with equal mixture or, as suggested, leaning for speed. In effect, you can get this kind of result even with a fixed pitch prop by leaning to peak and then advancing the throttle a little.

[Bob Axsom]

Often in my academic days instructors would say something like "Ignoring the effects of ... this is true." I think that is a convenient way to make the math models work. There are many variables that need to be dealt with in conducting an experiment of this kind and I don't think the variables can be controlled adequately and objectively enough to produce the result you expect without biasing the experiment. In other words if you are convinced that something is true you tend to repeat the experiment and tweak the variables until the expected result is observed. My test equipment is certainly far to primitive to to give conclusive results but my test indicates to me that under some circumstances the maximum RPM achievable with a constant speed prop on a specific airplane does not provide the greatest aircraft system velocity. The question I guess is power to thrust conversion efficiency. If the power is greater for a given RPM then the pitch is greater and in a theoretical revolution of equal efficiency the higher pitched prop will travel farther. Is there an optimum prop blade angle of attack beyond which it starts loosing efficiency - especially in the case of the downward traveling blade. There is a node in my data centered on 2600 RPM beyond which there is a definite drop off in TAS - that is difficult for me to ignore.

Bob Axsom