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Best Cruise RPM

todehnal

Well Known Member
I thought that I would ask what cruise RPM most of you are running. I have right at 100 hr. flight time, and I find that 5200 seems to be a real sweet spot. According to a power chart that I recall seeing somewhere, 5200 RPM provides the peak torque, even though it is not quite max HP. Using this setting I am seeing a fuel burn of less that 4 1/2 GPH, and about 112 kts true at 3,000 to 5,000 feet. So, how does that compare with the rest of the flying RV-12s??

Tom
 
guessing

I am no sure but from the charts the 912 has about 128 + 0r - NM of Torque at about 4900 Rpm and burns about 5 GPH I would think that the best cruse for fuel burn the RPM should be about 5000 RPM

Just a WAG

My View

Joe Dallas


I thought that I would ask what cruise RPM most of you are running. I have right at 100 hr. flight time, and I find that 5200 seems to be a real sweet spot. According to a power chart that I recall seeing somewhere, 5200 RPM provides the peak torque, even though it is not quite max HP. Using this setting I am seeing a fuel burn of less that 4 1/2 GPH, and about 112 kts true at 3,000 to 5,000 feet. So, how does that compare with the rest of the flying RV-12s??

Tom
 
I am no sure but from the charts the 912 has about 128 + 0r - NM of Torque at about 4900 Rpm and burns about 5 GPH I would think that the best cruse for fuel burn the RPM should be about 5000 RPM

Just a WAG

My View

Joe Dallas

Wondering if prop pitch has a lot to do with these number....I haven't got to this point yet.
 
Wondering if prop pitch has a lot to do with these number....I haven't got to this point yet.

I'm sure prop pitch does Ric. I have no way of actually measuring that, however, my full power climb out RPM is about 5250 to 5300. I recall some posts of guys cruising at around the 5400 RPM range. I have done that, and I do get a higher cruise speed, but the fuel burn sure seems to go up, as does the noise level. At 5200, the engine just seems to be loafing along. Just my layman's observation............ Tom
 
I plan on 5300 rpm getting 4.5 gph and 118KTAS.

The wheel pants do seem to add about 3 or 4 its. I get about 3.5 hours with reserve of 4 gals.
 
as does the noise level.

many pilots of all RV's use noise canx headsets....worth the money!
 
I cruise as close to 5500 as I can get without the ups and down of the autopilot sending the revs over 5500. Fuel burn is just over 5 and CTAS is between 115 and 119 Kts depending.
 
Why not use manifold pressure to set power instead of RPM. ;) When flying through thermals or in continuous light chop RPM varies +-100 RPM or more while MP stays constant. Unlike the old Lyc and Conts where we think about 65% or 55% power I like to think of what speed I want to fly when running a rotax in a RV12 similar to driving your car.:pMost of my flying is close to home so Unleaded Fuel is usually not a factor at $2.80 a gallon. Because we have all kinds of info on board our RV12s like real time winds aloft, GS, Fuel Flow, etc we can make more precise adjustments when flying a cross country. Big tails winds throttle back 100kts, big head winds throttle up 120kts.:D Sunday afternoons enjoy the ride 90kts. IMHO setting my MP when in a rush so that I bump up above 5500RPM on occasion won't hurt the rotax at all so long as CHT and Oil temp is in the green.:rolleyes: Water cooled cylinders make for a more uniform engine temps so no worry about over heating or super cooling.:)
 
As part of my Phase 1, I recently did some straight and level runs between 2,500' DA and 5,000' DA in very calm conditions, which made it possible to accurately maintain altitude. My TAS figures were 11-12kts faster than the POH figures, which I assume are for a basic RV-12 without wheel pants. I have wheel pants, main gear leg fairings (ex RV-8), and transition fairings on main gear and steps, which all make a difference I guess. Not that speed is the RV-12's strong point, but I'm very happy with the results. My prop is pitched to 5680-5720 WOT, which will get me to 128-130kts TAS. Incidentally, we don't have the 120kt limit on LSA's in Australia so I haven't broken any rules. In calm conditions I normally cruise at about 5100-5200 RPM. At that setting, fuel consumption seems to be about 4.5gph, and I get around 115-119 kts TAS, although I'm still fine-tuning the fuel burn figures.
 
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Why not use manifold pressure to set power instead of RPM.
I've always wondered why the Van's performance charts aren't based on manifold pressure - it seems more predictable than RPM in a world of ground-adjustable props. The only constant is that a built-to-spec RV-12 has a manifold pressure sensor and display.
 
I've always wondered why the Van's performance charts aren't based on manifold pressure - it seems more predictable than RPM in a world of ground-adjustable props. The only constant is that a built-to-spec RV-12 has a manifold pressure sensor and display.

Actually, neither is entirely predictable because power is a function of both, and with the prop pitch being adjustable neither one is a good indicator.
RPM was chosen as a reference since that is what most pilots that fly airplanes on the lower performance end of the spectrum are accustomed to.
 
I have been using @ 5,250 rpm for the bulk of my RV-12 cruise aviating - my engine seems happy there. In any kind of turbulence or rising air / descending air, the rpm tends to deviate from the cruise setting value a fair amount due to prop load, etc. - as is typical for a fixed-pitch prop - particularly if the AP is working to hold altitude. The precise digital readout on our tachometer tends to be all over the place - just get used to it and don't bother chasing it! I sometimes use Manifold Pressure to initially establish a power setting since it tends to be less influenced by the changing forces influencing the prop / engine rpm.
 
My Best cruise RPM 2550

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This is the last pic of a trip to AC house :)
 
Rich -

I usually cruise @ 5,250 RPM - my engine and I like that setting. I typically figure around 4.0 GPH (HOBBs) and/or 4.9 GPH (Actual flight time) - my HOBBs time is usually @ 20% higher than actual flight time due to taxiing, warm-up, etc. Those consumption numbers reflect some 230 hours of operation.

At 5,500 feet MSL, I typically see 118-120 knots TAS. My RV-12 has Van's standard wheel pants and I consider my prop to be pitched for general use (climb-out performance is good as is cruise).

- David
 
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Thanks for the info, Dave. My engine has a happy face at about 5000-5100 and 21" MAP WITH 4.5 GPH based on flight time. I see about 113 KTAS. I guess we all have our pet numbers!

BTW, the posted MFD pic from another blogger seems really high on GPH at over 9 GPH! Might need calibration!

Rich
 
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