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RPM and MPG

We’ve had various discussions on this one in various – mostly unrelated – threads.

Lower RPM results in reduced mechanical losses so should deliver a better specific fuel consumption (HP per fuel flow) and thus better MPG.

In the past I found a definite gain of a few % at 2200 versus 2400, but later tests failed to reproduce it.

Today I flew in very calm air and was able to do some measurements. Between 2400 and 2200, 4.5% to 5.5% improvement.

Conditions: 4700ft, +3C, Q1031, marginally LOP, around 138kt IAS. Result reproduced on reciprocal headings.

The gotcha is that you can’t do this at high altitude because you can’t get enough power. It will work at say FL120-140.

Administrator
Shoreham EGKA, United Kingdom

Large bore Continental have a SB recommending not to go below 2300RPM.

The slower RPM in addition to some improvement on propeller efficiency, may also provide a more complete burn and therefore a slightly better SFC?

Oxford (EGTK)

I’ve always flown my TR182 (Lycoming O540) at 2200/25", because it’s quieter that way. Of course then you run into the “over square” old wives’ tale.

KPAO, United States

Peter wrote:

Between 2400 and 2200, 4.5% to 5.5% improvement.

It’s like driving a car 100km/h on 4th gear 3000RPM or 5th 2500RPM the same speed.
I also try to choose one of the lowest RPM setting for a given power percentage according to my POH.
Usually:
75% – 2400RPM
65 – 2300
55 – 2200
45 – 2100

Works well and of course all of those settings are “over-square” but there is nothing wrong with it.

Poland

Some analysis was attempted here.

The slower RPM in addition to some improvement on propeller efficiency, may also provide a more complete burn and therefore a slightly better SFC?

I think that one big reason why low rpm works is in conjunction with LOP. The LOP mixture burns slower so this works better with lower rpm, because more time is needed to align the bulk of the pressure pulse with the optimal conrod angle, and the lower rpm delivers this extra time.

If you fly at peak EGT or ROP I don’t think you get much of an MPG improvement, and this may explain why I was often unable to simulate it – I fly mostly at peak EGT.

Administrator
Shoreham EGKA, United Kingdom

The gotcha is that you can’t do this at high altitude because you can’t get enough power.

One of the main advantages of a Turbo Arrow is that I can maintain sea pressure up to a good height.
I regularly cruise at 12,000’, LOP, at 2,250 RPM @ 31”.
I burn 34 l/h, getting a cruise of 150 k/h, which is very economical and low-wear effective.
I would like to think that this a very efficient money-saver; although I know some would claim the cost/maintenance of the Turbo itself might qualify this claim.

Rochester, UK, United Kingdom

As I find this topic quite interesting, it finally lead my way to register here in the forums to contribute a bit rather than just reading.

I’m recently testing on every flight the implications of RPM on cruise efficiency (which is: Miles per Gallon for a given speed, because by reducing MP you can always lower fuel flow sacrifying speed). Actually my testing includes low-level and mid-level flying, below oxygen levels which I will add starting in summer this year.

With my Turbo Comanche I’d like to see around 150 KTAS, which is around 130 KIAS in 10k feet. This speed gives reasonable time consumption but staying short of “exhaustive” fuel burn rates, which I see when flying 160 KIAS. Additionally, I can stay Lean Of Peak.

Keeping the indicated fuel burn rate (digital monitor) constant, I note an increase of about 2 Knots of Airspeed for each 100 RPM reduction. So instead of flying 2500 RPM going down to 2000 RPM increases indicated speed by about 8 to 9 knots at constant fuel flow (122 indicated at 2500 rpm vs. 130 indicated at 2000, constant FF).

LOP at slightly below peak EGT – having the GAMI injectors installed – I see fuel burn rates of about 30 l/h. Or in other words: On 15 Gallons of fuel from one of the outboard tanks I fly 1:45 hours at this speed and setting, and the engine does not quit :-) You can imagine the resulting range hauling 90 Gallons of fuel on board, some 1700 nm.

Interestingly, with lower RPMs of 2200 and below, I can’t go far below peak EGT on the lean side. At 2100 and below upon leaning the temperature stays constant at peak EGT over quite a portion of the lever movement range, but does not fall below peak until engine slightly starts to shake. Above about 2200 RPM – for example at 2400 – I can lean farther down with smooth engine run practically until the engine starves. Up to now I haven’t fully understood why this behaves different with lower RPMs. So at and below 2000 RPM, for example in order to keep CHT down at the Lean Side I can’t reduce mixture with throttle wide open, but have to reduce MP instead. I’ll investigate further on this.

On the other side, for example, I can’t fly 130 KIAS at FL100 with higher RPMs on the Lean Of Peak side due to a rising cylinder head temperature. I can only stay lean and at the same time obtain 130 KIAS when reducing the engine speed to at least 2100, or better 2000, reducing CHT at the same time. With higher RPM my CHT rises to 380°F and above, which I do not want. So with higher RPM I would have to fly on the rich side – thus burning more fuel for cooling.

So far I stopped testing at 2000 RPM, however maybe I’ll continue down to 1800 RPM (which is indicated as maximum range cruise in my POH). Another major advantage is a very comfortable noise reduction at RPMs of 2000 and below, where I start to fly without headset – something I never thought to be possible without pressurization.

One drawback, however, is that at these low RPMs the Turbo provides only a few inches of MP. It seems as if low RPMs together with reasonable indicated speeds doesn’t match for higher altitudes due to low manifold pressures, but I’ll see that. High up it seems as if I will have to use at least higher RPMs in order to have further benefit from the Turbo.

By the way, I think that the opinions of Mike Busch, Savvy Aviation and John Deakin are well-known here..they state more or less that you should fly with the lowest allowable RPM .

Cheers, Udo

Germany

@UdoR good to see a Comanche pilot on the forum, I think there are a couple more forumites who fly this great type.

30 lph at 150 KTAS on a turbocharged Lycoming 540 is seriously impressive. 8 usgph is around 40 % power on a normally aspirated variant, the turbocharger heating means it is below 40% on a turbocharged engine.

Oxford (EGTK)

What do you mean? Turbo Twin Comanches have IO-320s.

Mainz (EDFZ), Germany

I was about specifying Turbo Single Comanche, knowing how rare they are. But indeed it is one of the few singles. However I don’t want to take over this topic talking about Comanches in particular (although you can bet that I’m loving talking about that beautiful peace of metal work). I’d rather like to share and learn here how to perfectly adjust and set a piston engine in order to even make use of this hobby in a more efficient (and at least a bit more environmentally friendly) way.

Udo

Germany
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