Cruise Settings P337
 

I was just wondering if anyone routinely uses lower RPM settings, 2200 or 2300 when at cruise? Is it quieter? Endurance? Etc. Just wondering if I could take advantage of others experiments.

Thanks

2350 RPM gives me about 150 - 154 true
I like this setting because my engines seem to just feel better and are smooth
If I'm in a hurry though it's 24-2500 RPM and about 158 - 160 true

Fuel burn, 18 vs 21

Its all in the curves
 

The weight of the airplane is 'constant' (at any instant)
That weight W requires an equal amount of lift L to stay aloft
To produce that lift induces a certain amount of drag D

The most efficient flight is in thin air when lift L is 'just enough' to hold the airplane up,
and drag is least (the 'coffin corner' for jets. Efficient but scary).

As throttle opens, airflow becomes more 'laminar'
So at max throttle engine breathes most air/fuel with least disturbance

As one climbs the engine as an air pump cant get enough air
So for fuel/air to remain stochiometric, reduce fuel to match less air

As throttle opens, airflow becomes more 'laminar'
So at max throttle, engine breathes most air/fuel with least disturbance

That's why most non-turbo cruise best around 7-9k feet
Throttle is full open but still getting enough air for available fuel flow
Adding more fuel wont add more power from burn, just richen mixture
Fly higher up and fuel must be reduced to available 02, so HP output drops off

Except for turbo.
But even turbo increases internal combustion 'drag' (energy to spin the turbo is not free).

Most efficient wing moves a lot of air just a little bit
A 'flat' blade at low RPM moves a lot of air a little distance per rotation
So prop efficiency better 'flat' as in takeoff
But delta V across blades (front to back) a function of cruise speed
The idea is to 'paddle' the air with least disturbance

And of course, internal engine friction a function of RPM
Turn the engine two times and it that produces two times the friction per unit of time.

If you can solve all those, you get an 'A.'

--

The only lower limit consideration I have read has to do with the internal harmonic balancing of the engine. Spin too slow and harmonics increase vibration in some manner. Which of course consumes energy as heat, not output.

E=MC^2
F=Mass x Acceleration
K=1/2 Mass *Velocity ^2

Those aren't just good ideas, they are the law.

Mine is a 1974 P337 with Riley intercoolers and Horton STOL kit.

I run mine at the book RPM and MP settings, and apply a MP decrease of between 0" and 2", due to the Riley intercoolers. In cruise, I reduce my manifold pressure to achieve book fuel flows for whatever cruise power setting I have chosen. If temperatures are hotter than I want (we have a lot of hotter-than-standard days here in Southern California), I use cowl flaps to manage temperature. If cowl flaps aren't enough (as sometimes happens), I reduce MP further. It's a dance between MP, fuel flow, and temperature. Usually not too difficult.

For me, at gross weight, I am about 8 knots below book cruise speed while in the teens. Given that my plane has Riley air conditioning which I presume is not factored into the cruise performance numbers (extra air scoops, etc.), and also the STOL kit, I'm pretty happy with this.

At 16,000 feet, I'm often running about 192 knots in the summer, but winter allows more like 204 knots. That's if I choose the high power cruise settings and 160pph (total).

If I try to manage temperatures by enriching the mixture, while keeping the cowl flaps closed, I can achieve 200 knots in the summer too, but fuel flow grows from 160pph (about 27gph total) to 216pph (36gph total)! Not a good trade off.

I've generally found my P337 to be quiet at 2400 and 2450 RPM (which are recommended for high speed cruise, depending on altitude). I have tried it at lower RPMS, and also lower power settings. I haven't found that the lower RPM makes much of a noise difference, nor much of an efficiency difference. Lower power settings, of course, are more efficient per mile, in terms of fuel.

Dan

Frank,
I flew my P337 at 2450 RPM, 30" MP and 24gph total, which was 50 LOP and roughly 65% power. I would see 185KTAS in the high teens with those settings with all CHTs less than 380 with the cowl flaps closed. My plane was air conditioned and had Riley intercoolers and GAMI injectors. FWIW, I fly my current P210 at 2300 RPM, 30" MP and 14.5 GPH 50 LOP which is also ~65% power and see 175-180KTAS at the same altitudes.