full throttle

[awash2002]

Would there be any harm in running the Comanche at full throttle and 2300 prop rpm 13.0 GPH at 7,500?

[n421nj]

What mp are you getting?

[awash2002]

I'm getting 22.3"MP

[AKar]

I'd expect no problems whatsoever from that setting. Only when getting above 24''...25'' or so, I'd remind you to pay attention to avoid very low cruise RPMs (which 2300 is not) together with near-peak-EGT mixtures. But that's just quick Stetson-Harrison interpolation of the graph, you'd always want to monitor the temps and feel the engine.

-Esa

[awash2002]

My CHT's was 359 EGT's was 1405

[ClipperLuna]

At that pressure setting you could also try pulling the prop back to 1900 or 1800 RPM and save some fuel. Someone correct me if I'm wrong, but I think the limits for 1800 or 1900 rpm operation are 25.0” and 25.5” respectfully.

[awash2002]

OK I'll do that but I really like going 160 knots

[EnDSchultz]

I recall seeing a performance graph in an old POH that indicated the max continuous MAP at 2400RPM was something like 28 or 29''. With that in mind, when I'm being lazy and just going for max speed, I leave the throttle at full all the way from takeoff to cruise, and only retard it for descent. When it comes time for power reduction after takeoff, I just pull the RPM back to 2400 and don't bother touching the throttle - ambient pressure is already below the max permissible by 1000-2000 MSL, and unless you're taking off right from sea level, you're going to be at least that high by the time you reconfigure for climb power.

Of course, if you want to baby the engine or if it's really hot out you can pull back to 24''/2400 as the checklist recommends, but based on the POH I've seen that's not strictly necessary unless higher power settings ends up resulting in excessively hot CHT.

[AKar]

For O-540-A, the full-throttle operation at standard sea level conditions is allowed for 2300 RPM and above. This is, of course, not to suggest to do anything stupid, such as immediately pulling back to 2300 RPM after takeoff and then leaning the mixture... with such double-insults, one could very well harm the engine. Nor would I suggest such a big reduction in RPM at all, I'd suggest leaving some margin there. While I don't think it is simulated, one should also note that the carburetor further enriches the mixture at wide-open throttle settings. Contrary, a small reduction in power actually leans the mixture some, and this combined with RPM reduction increase internal engine pressures and stresses. One certainly likes to think before acting, because being shy on the controls in wrong conditions can actually work against you.

I think the way of least hustle when flying the A2A Comanche is to leave the throttle full-in and slowly bring back the RPM while climbing higher. Eventually, the mixture needs to come back also.

When it comes to the cruise RPM, the propeller efficiency is the hard part. (The engine is simple: full throttle, mixture in practice as lean as you can smoothly get with carburetor gives you the most miles per gallon.) Now, the mathematically minded folks have nicely created some rules of thumbs for people of lesser ingenuity like me. I recommend the series of articles here for anyone interested. The absolute best efficiency of the propeller is typically achieved at relatively narrow range of advance ratios at around 1.0. The advance ratio as a number relates the forward true airspeed to the circumferential tip speed of the propeller. This means there would be a rough figure to optimize the RPM vs TAS. In his paper, Dr. David F. Rogers suggests that the RPM/KTAS ratio of about 14.5 gives a nice, rough rule of thumb of optimal RPM setting. If one cruises some 160 kts true, that would put the optimal RPM to about 2300 from propeller's efficiency point-of-view. This info needs to be combined with the engine's efficiency to get the actual power plant efficiency.

-Esa

[William Hughes]

That is pretty much how I fly it.

[awash2002]

Just after takeoff I pull it back to 24 squared 24"MP and 2400 rpm and lean to 14gph

[William Hughes]

OK I'll do that but I really like going 160 knots

That is my EXACT problem too!

[awash2002]

I prefer flying fast