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Thread: Prop diameter and pitch vs hp and fuel consumption

  1. #1

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    May 2020
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    Prop diameter and pitch vs hp and fuel consumption

    Forgive me for being an engineer and asking an engineer type question.

    For a given engine operated at a specific RPM at static conditions, how does the horsepower, static thrust, and fuel consumption vary vs changes in diameter of the propeller and pitch of the propeller?

    Lets say an C85-12F run at 2200 rpm. A 72.5 inch propeller at 44 inch pitch vs a 71.0 inch propeller at 44 inch pitch. Then say the same engine and rpm with 71 inch propeller and 42 inch pitch and at 46 inch pitch.

    Using the 71/42 prop, I would assume it would have the lowest fuel consumption. Any increase in diameter or pitch would require more throttle opening, a higher manifold pressure to keep the propeller spinning at the same 2200 rpm. Am I correct?

    And then the next part, would there also be an associated engine operating temperature change associated with the alternate propeller sizes and pitch, when the engine is run at the one specific rpm?



  2. #2

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    Jan 2007
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    I can address some of the last questions however, when flying I don't think you can say it is a static condition. The smart ones can answer the first part. If you change the pitch of the prop work of the engine increases/decreases so to keep the RPM at 2200 a flat prop will not have to work as hard and engine will not use as much fuel and temp might be lower depending on aircraft. DENNY
    Likes Pete Schoeninger liked this post

  3. #3

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    Mar 2015
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    I think your assumptions are right. Wondering why the interest in static numbers or is it just to understand the theory. As Denny says this static situation doesn't connect to actual flight. As manifold pressure increase so will operating temperature and fuel consumption in static but in flight increased cooling from higher speeds and may offset some. In general long wide cord props pull harder at takeoff speeds but are slower in the air.

  4. #4
    Aeronut's Avatar
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    Dec 2019
    Kittery, ME
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    Quote Originally Posted by Ab0ngcd View Post
    For a given engine operated at a specific RPM at static conditions, how does the horsepower, static thrust, and fuel consumption vary vs changes in diameter of the propeller and pitch of the propeller?
    You're correct in general that diameter and pitch would vary the amount work the propeller is doing and thereby affect horsepower and fuel consumption requirements. But that would only be minimally reliable intuition and not a reliable design basis. Where it gets tricky in my opinion would be quantifying those relationships for optimized design and performance. I would imagine that NASA or their precursor, NACA, has produced a research paper on this topic where they likely examined combinations of diameter, pitch, and number of blades as well as other variables like airfoil shape and finish of the prop blade.

    You should also consider that there may be a degradation in propeller performance after some diameter (tip speed becoming transonic even) and some pitch which would impact horsepower and fuel consumption requirements contrary to any mathematical relationship developed at some RPM. Think about the affect of these things on the flow of air around the airfoil. In fact, you may even want to model one blade in a CAD program to get a feel for how the air is behaving around that airfoil and deduce horsepower and thrust from the airspeed tested and lift generated at said airspeed. You should also be able to deduce the RPM required to generate this airspeed. Keep in mind that the airspeed on a prop is not uniform across the leading edge but a function of it's distance from the origin (the linear velocity of a point on a radius of a rotating body). I guess if you have CAD you could also just test the whole prop.

    If you have access to a physical test apparatus, you can also use a full scale model and test it at different RPMs and pitches to develop a profile to get a better idea. Perhaps I'm wrong, but I would imagine you can also create a scale model and use the Reynold's number to compare apples to apples.

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