What is MP?
Manifold pressure.
“@Airborne Again” mentions two a/c with fixed pitch propellers, e.g. no MP (manifold pressure) gauge.
So forget about MP and propeller pitch. Set power for 75% or less (typically 65% as previously mentioned) according to the POH; RPM to achieve the desired power output will depend on altitude. To lean for peak EGT you lean until you observe the peak RPM and then some until it drops again. The drop will happen relatively quickly, and if the engine gets rough you’ve leaned a little too far.
If you have an EGT gauge, you lean until you reach the peak, i.e until it drops again, and then enrich until you reach the peak.
At 65% there is very little chance you will harm your engine unless you let it run rough over time.
“@Airborne Again” mentions two a/c with fixed pitch propellers, e.g. no MP (manifold pressure) gauge. So forget about MP and propeller pitch. Set power for 75% or less (typically 65% as previously mentioned) according to the POH; RPM to achieve the desired power output will depend on altitude. To lean for peak EGT you lean until you observe the peak RPM and then some until it drops again. The drop will happen relatively quickly, and if the engine gets rough you’ve leaned a little too far.
One of the (only) good things about an electric constant speed propeller is that you can turn it off, use this fixed pitch procedure, and then turn it on again. I think that’s kind of fun, simple, and its the way I do it most of the time.
My question was not about how to actually manage the engine in practise. I know how to lean the aircraft and I know about the 75% limit etc. I also know that with a constant speed prop, MP and not rpm is the primary means of controling power. I am trying to improve my theoretical understanding.
To be very concrete. Take a PA28-181. Assume I am cruising at 7000 feet in standard conditions. For 65% best power, the POH tells me to set 2480 rpm which theoretically gives me 118 KTAS. It also tells me that at 65% best economy I would theoretically get 116 KTAS. However it doesn’t tell me the engine speed, which obviously must be less than 2480 rpm – otherwise the airspeed would be the same. And I understand that it will be less since the engine is producing less power at peak EGT.
Now of course I don’t care about the two knots in practise, but I’m trying to understand the theory behind this — how it is supposed to be done.
The engine makes most power just rich of peak, slightly richer than stoichiometric mixture.
65% power is 65% power regardless of engine speed, so neglecting propeller efficiency (which is higher at lower rpm) the plane should go the same speed at 7000 ft with any combination of engine control settings that produces 65% power. What will vary is the fuel flow required to produce 65% power.
Bear in mind that no airspeed indicator, tachometer, or propeller is accurate enough to replicate book figures to the level described. Tachometers are for example very frequently are 50 rpm low. The two knots is in the noise, and the manufacturer probably paid no attention to making the book match theory to that level. I suspect what they’re trying to communicate is that if you set 2480 rpm with the throttle, leaned for max rpm at that throttle setting, it’ll slow down a little if you subsequently lean to stoichiometric without touching the throttle.
I suspect what they’re trying to communicate is that if you set 2480 rpm with the throttle, leaned for max rpm at that throttle setting, it’ll slow down a little if you subsequently lean to stoichiometric without touching the throttle.
Thanks. That was my guess, too.
Best power is about 120F ROP for petrol engines.
Best economy is about 20F LOP for petrol engines. This is so close to peak EGT that there is no point in worrying whether you are peak or just slightly off it – especially as the curve is very flat around there.
I think most of the handbooks contain some dodgy data on speeds and power settings. The TB20 POH has figures which I can’t understand – very pessimistic relative to what I get, and what I get is absolutely verified with fuel flow and a GPS-calibrated ASI etc.
You can get second order but significant MPG improvements by flying at lower than usual RPM too, especially if this is combined with being slightly lean of peak.
Best Power has always been 50 or max 75 ROP for aircraft manufacturers since that will yield maximum cruise speed whilst still yielding an “acceptable” fuel flow (which in turn is important for their range numbers. Just forget about POHs as far as engine power settings are concerned.
Best Power has always been 50 or max 75 ROP for aircraft manufacturers
Well that might explain a few things 
since you need only 5 minutes in a plane to fnd that 50-75F ROP is definitely not “best power”.
It’s immediately obvious on speed at any altitude, and the difference between 50-75F ROP and 120F ROP is probably a 1000-2000ft difference in how high you can climb.
Deakin would argue that 50F ROP is a bad place to be because it is the CHT=MAX point.
