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Thread: % Power and props.

  1. #1
    dhillier's Avatar
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    Default % Power and props.

    I should know the answer but..

    I have a O-235 with a Sensenich M76A 74" diameter by 52" pitch cruise prop on my PA-16.

    I am trying to establish approximate RPM values for 65 and 75% power for leaning and fuel consumption purposes.

    The O-235 operators manual just states RPM vs % power values but does not state which prop this is based upon.

    I have not found complete POH for the PA-16 but I do have a PA-22-108 POH (which, lets face it, is as similar as possible ignoring the gear) and that has figures based upon a standard prop which i believe is 74x48. The Sensenich installation guide shows both the PA-16 and PA-22 use 74" diameter props but differ in pitch - (standard / cruise / climb)

    PA-16 - 50/48/52 pitch.
    PA-22 - 48/-/50.

    My questions:

    Has anyone any experience between speed and fuel burn differentials for the different types of prop?

    For a given RPM I would expect the cruise prop aircraft to be flying at a greater IAS - but will it be doing so at a greater % power and thus greater fuel burn? Surely a greater pitch prop requires more power to spin at a given RPM?

    How would I expect the % power to vary between an engine with a climb, standard and cruise prop?

    How can I estimate what RPM gives a % power? In the PA-22 POH there is a formula of max cruise RPM - 10% = 75% power. Any comments on this?

    I ask all of this as the Clipper cruises at 100mph (TAS - the IAS is very optimistic at cruise speeds) at 2300 RPM with a cruise prop and burns 26L / 6.8 US gallons an hour.

    Both the PA-22-108 POH and O-235 manual have very different figures for speed vs fuel consumption.

    Obviously I do not expect a 1949 aircraft and an 800hr engine to perform exactly as per the book.

    Regards

    Dean

    N5240H

  2. #2
    Clayton Harper's Avatar
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    Default Re: % Power and props.

    You can get an average HP by looking at your average fuel burn.

  3. #3
    Harold Kroeker's Avatar
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    Default Re: % Power and props.

    On my clipper I have a Sensenich 76AM-2-50 prop (74" 50" pitch) and I normally cruise at 2350 rpm which is 75% according to the Lycoming Operators Manual https://www.lycoming.com/content/ope...-O-290-60297-9 . This burns fuel at 7.0 gph and gives me 105 to 110 mph.

  4. #4
    John Umbach's Avatar
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    Default Re: % Power and props.

    Some rules of thumb: full power static RPM will give you cruise 60% power. Add 25 RPM for every 1000' above the elevation you took the static RPM reading at. Example, if your airport elevation is 1500' and static RPM is 2300, and then you cruise at 4500', then your cruise RPM for 60% power is 2375. The beauty is that this works for Your airplane. Doesn't matter if the engine is getting tired, or if the RPM gauge is a little off.

    This works consistently across the general aviation fleet (fixed pitch, normally aspirated) to within about 3%

    Also, figure fuel flow at 0.48 pounds of fuel per horsepower per hour. 60% of 108 is 64.8, call it 65 HP. Multiply by 0.48 is 31.2 and then ÷ by 6 (6 pounds per gallon of fuel) is 5.2 gal per hour.

    Flight plan 5.5 GPH for a few trips to be conservative and be safe while you verify these numbers.

    I know you asked about 65% and 75% power, but this is usefull to know in my opinion.

    Sent from my LG-H812 using Tapatalk

  5. #5

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    Default Re: % Power and props.

    Quote Originally Posted by dhillier View Post
    For a given RPM I would expect the cruise prop aircraft to be flying at a greater IAS - but will it be doing so at a greater % power and thus greater fuel burn? Surely a greater pitch prop requires more power to spin at a given RPM?
    A greater pitch requires more power at a given RPM and a given airspeed. But if you increase airspeed the prop load is reduced. Even though flying at a higher airspeed requires more thrust, the cruise prop should be more efficient so not all of the added thrust has to come from added engine power.

    Quote Originally Posted by dhillier View Post
    In the PA-22 POH there is a formula of max cruise RPM - 10% = 75% power. Any comments on this?
    Propellers act on a cubic curve; Power = k x RPM^3. k is a loading factor that varies based mainly on prop design (including pitch) and airspeed. This formula also works with %power and %RPM. So if you take 100% - 10% = 90% and cube the 90%:

    0.9^3 = .729 or 72.9%

    So that says the prop would be at about 73% power. But when you reduce the RPM you'll also get a lower airspeed; that will increase the prop load some, which changes the k factor, and increases the % power some. As a result 73% is a bit lower than actual; 75% is probably close. So this formula from the POH makes sense as a rough estimate. Close enough for most purposes I'd think.

    Quote Originally Posted by dhillier View Post
    Both the PA-22-108 POH and O-235 manual have very different figures for speed vs fuel consumption.
    The reason being that the Lycoming manual has to apply to any prop that might be used on the engine while the Piper manual can be more specific since the prop is defined.

    As Clayton suggests, looking a fuel burn can be a good way to get your % power. If you are leaned to best power (peak rpm) you can multiply your fuel burn in lb/hr by 2 to get a good estimate of horsepower. Divide your horsepower by 108 and you've got the power setting.

    So if you're using 6.8 gal/hr; that's 40.8 lb/hr. Multiply by 2, and we've got 81.6 hp. Divide by 108, and you're at 75.6 % power.

    If you're also interested in actual horsepower when leaned to best economy (lean of peak) take that fuel burn rate and multiply by 2.4 (2.5 is closer for higher compression/fuel injected engines).

    Quote Originally Posted by dhillier View Post
    Obviously I do not expect a 1949 aircraft and an 800hr engine to perform exactly as per the book.
    The engine will perform really close to book values right up until wear starts to accelerate at end of life. So if everything is in good shape you should be getting very close to published performance. I don't have much experience on the aircraft side, but I'd assume that it's the same idea, if it's rigged well it should fly by the book.

    One other thing to consider when it comes to leaning is that the O-235-C1 is rated for Grade 80 fuel. If you're using 100 in it there is a lot of additional margin against detonation.

    Tim

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