The drag force acting on a propeller?

  • Thread starter jeebs
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Hi,
I am a university student of physics and currently have a project in which a "car" is propelled using only the energy stored in a balloon. Two possible ways of storing energy in the balloon are to inflate it and allow it to blow air out of the neck, or to twist it around and allow it to drive a propeller. I am trying to work out which might be the best option from an aerodynamics perspective, because I obviously want to minimise the drag on my car as much as possible.

My question is, for the propeller, is there a difference in the drag that the blades cause when it is stationary compared to when it is rotating? I have a suspicion the answer is going to be that the faster the rotation rate, the more drag, but I really need a way to quantify this and so far I haven't found much thats useful so far.

I ask this because, as the balloon untwists, the angular velocity of the propellers is going to peak then decrease to zero. I need to compare the drag force from the decelerating propeller to that of the shrinking balloon.

If its too complicated to explain on here (which I suspect it could be), a link to a website that explains/derives an expression for the drag as a function of rotation rate would be really helpful to me, or anything else you think might be useful, I want to hear it, reccomended textbooks, anything.

Thanks.
 
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