The drag force acting on a propeller?

[jeebs]

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 that's 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.

 

[Achy47]

Hi there! Your project sounds really interesting. In terms of the aerodynamics of the propeller, you are correct that the drag will increase as the rotation rate increases. This is due to the increase in the relative velocity between the air and the propeller blades, which leads to a larger force acting on the blades. However, there are other factors that can also affect the drag, such as the shape and angle of the blades, and the design of the propeller itself.

One useful resource for understanding the drag force on rotating objects is the Kutta-Joukowski theorem, which relates the lift force on an airfoil (such as a propeller blade) to the circulation around it. This theorem can be used to derive expressions for the drag force on the blades as a function of their rotation rate.

In terms of recommended textbooks, "Fundamentals of Aerodynamics" by John D. Anderson Jr. is a great resource for understanding the basic principles of aerodynamics, including those related to rotating objects. Additionally, "Aerodynamics for Engineers" by John J. Bertin and Russell M. Cummings also covers the fundamentals of aerodynamics and includes a section on rotating objects.

I hope this information is helpful to you. Good luck with your project!