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  1. #1
    Captain Thor's Avatar
    Join Date
    Mar 2013
    An undisclosed Southeast Asian nation...

    Propeller L/D Max

    OK big brains, here's a puzzler.

    Some years ago I spent a lot of time studying ways to get maximum range out of a piston powered aircraft and one thing I worked on unsuccessfully was mathematically determining the required prop RPM to achieve maximum propeller efficiency at a given airspeed.

    We all know that flying the wing at L/Dmax (roughly Vy + or - some small corrections) will give us the greatest efficiency from the wing.

    I reasoned at the time that one could improve efficiency and therefore cruising range by ensuring that the other important airfoil on the aircraft, namely the propeller, is also "flying" at L/Dmax.

    My logic is like this: At a given airspeed (aircraft), any given propeller blade angle will produce a given propeller blade angle of attack. If we can adjust the RPM to achieve a blade angle of attack of LDmax, then we should be getting maximum efficiency out of the propeller.

    So the end state I'm trying to achieve is Wing at L/Dmax and Propeller Blades at LDmax at the same time and a formula for calculating required prop RPM.

    At the time, I got far enough to understand that there is some "mean" spot in the propeller blade that one would have to use as the place to measure blade angle but that's about as far as I got.

    I've asked a lot of people this question, including an aeronautical engineer and have either gotten blank looks or some story about how it wouldn't really effect efficiency that much or that I'd have to go out and test it in the aircraft.

    Can't do that. I'm still at the "Postulate a Theory" stage of the process. I need help with the geometry of calculating the RPM required at a given indicated airspeed to achieve L/Dmax with the propeller blades.

    Any takers?

    (How's that for the first real post on an aviation discussion forum?)

    Thor Odinson... you have betrayed the express command of your king. Through your arrogance and stupidity, you've opened these peaceful realms and innocent lives to the horror and desolation of war! You are unworthy of these realms, you're unworthy of your title, you're unworthy... of the loved ones you have betrayed! I now take from you your power! In the name of my father and his father before, I, Odin Allfather, cast you out!

  2. #2
    The first official SkyViking.Net T-Shirt will be given to the first member to submit a useable solution to this puzzle.
    I intend to fly until my beard gets caught in the propeller. And since I don't plan on growing a beard, that may be a while. Ernest K. Gann, age 69

    Daily Diatribe - (Carl's Blog)

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