Expert: Paul Soderman - 10/10/2008

Question
Dear Mr. Soderman,
    I'm Matt Orloff and am working on a science fair project on how twist angles affect efficiencies of propellers. I just have a few questions about my project.
1. To conduct my experiement, I was going to hook up a DC motor to a small propeller keeping chord length constant and change the twist angles, by using a household fan to make the propeller spin, I would then use the voltage from the motor to measure effeciency. Does this sound like an accurate experiment?
2. What is the purpose for a twist angle?
3. How could a twist angle possibly affect the performance or efficiency of a propeller and the vehicle?
Thank you for your time and it is gladly appreciated.
                 Sincerely,
                        Matt Orloff

Answer
Matt
Propeller blade twist is used to optimize the angle of attack of each blade section.  Because the blade section velocity varies with radius from the hub, each section sees a different velocity vector which is the vector sum of free stream and blade velocities.  That way each section can be set for a given flight condition and rpm to have the same angle of attack or same loading.  You certainly don't want any section to be stalled.  By optimizing the loading you optimize the efficiency and performance.

I don't understand your experiment enough to comment.  How can you change propeller twist angle ?  How can driving a propeller backward simulate a thrusting propeller ?  Which motor voltage are you measuring ?
Paul