Power Settings & Efficiency

[uwochris]

Hey guys,

According to my Seminole POH, you can obtain 65% power through the following various MP and RPM settings (at a pressure altitude= sea level):
- 2100 & 24.5
- 2200 & 24
- 2300 & 23.5
- 2200 & 22.9

The question is- is there a specific combination that is best for engine efficiency? ie) is it better to choose a high RPM and low MP combo as opposed to a low RPM and high MP combo? In all cases, fuel flow and TAS will be the same- I guess the only differences will be in operating temps (EGT, CHT, etc)??

Thanks for all comments,

Chris.

 

[coonass one]

The prop is an airfoil, so the faster it turns through the air, the less efficient it becomes (greater parasitic drag). Yes, you can change the rpm and manifold pressure and keep relatively the same fuel flow, but the efficiency of the prop is the key. I believe the best efficiency you can get from your lycoming engine with a constant speed prop (non turbo-charged) is at an altitude where you have full throttle and about 22" mp, and about 2200 rpm, or about 8000'.

The engine is an air pump. The prop is directly connected to the engine, and therefore controls engine speed. The slower the engine, the less air moves through, therefore lowering your fuel flow as well. This combined with a slower moving/more efficient prop = more overall efficiency. Hope this helps.

 

[PropsForward]

Fuel vs. Time

Just don't forget to figure the cost of the entire flight also. If you are renting the airplane for a 3 hour flight to xyz at $90/hr dry, and you debating over saving 5gph fuel to be more effecient and taking 30 minutes longer in the flight, you will spend more money for the flight. (about $25-$50 in this example).

 

[DirtyBeech]

what coonass is trying to say is push the power levers all the way forward. then push some more. you're doing it right when they appear to bend a bit. then monitor your engine gauges. if any one of them exceeds a redline you have two options: 1. cover the gauge and continue partial panel -or- 2. placard it inoperative. don't worry about the props or the mixtures. play with them from time to time so that it appears to your student/passenger/copilot/examiner that you are doing some sort of pilot stuff, but it really doesn't matter. lastly, ignore your fuel burn. dinosaurs->meteor->fossils->oil->exxon-> pa44...tough sh*t for them. your student/passenger/copilot/examiner will pay for more. they will be impressed enough that you got somewhere quickly.


last time i saw a coonass involved with something powered by a lycoming he was hanging out the door, tow bar in hand, trying to manually extend the gear.

 

[mattpilot]

I believe there is no difference if your the pilot. Just set it however you like it to obtain desirable temps & stay within the green arcs.

I think the main difference is to the operator/owner. The slower the prop turns, the slower the Tach turns, the more flying time you can get out of it. So if you don't actually pay for the 100hr inspections, i wouldn't worry about it.

 

[EagleRJ]

All those settings probably have nearly identical specific fuel consumptions. The low RPM/ high MP settings are probably a little more efficient, as well as quieter for the occupants.

If maximum efficiency is your goal, load for an aft CG and don't tanker a lot of unnecessary fuel. Remembering the cowl flaps is key, too. When I flew the PA44, I would taxi with the cowl flaps open, climb with them in Trail, and cruise with them closed. IIRC, leaving them open during cruise shaved about 10 knots off the airspeed.