What is the equation for calculating the moment of inertia of a propeller?

[Moolan]

Hi all,
I am currently working on a project where i need to find the work out from a propeller. But to do so, i need to know the moment of inertia of the propeller. I did a search online and i found that the moment of inertia is

2/3MR^2

I was thinking of assuming the propeller to be a flat rectangular piece then i can use

1/12M(L^2 + B^2)

I need help in deciding which equation is right... as both equation differs quite a lot. Or is the moment of inertia of a propeller something else?

 

[Galileo]

Hmm, a more accurate model would be to approximate the moment of intertia of one propellor blade and then multiply that by the number of blades to get the total moment of inertia. Depending on the shape of such a blade you might decide whether you want to use a rectangular board, a cilinder or something else to model it. Think about the shape and mass distribution of the blade.

 

[Moolan]

Is there an experiment which i could carry out to calculate that inertia?

 

[FredGarvin]

If you actually have a propeller it is rather easy to determine the moment of inertia experimentally. If you can count the oscillations of a pendulum you can calculate the moment of inertia. We do it quite often as a sanity check against models on rotors and such.

 

[Moolan]

I do have a tachometer. Could you outline the way how the experiment can be performed?

Edit: Do you actually mean using the pedulum experiement and do some algebraic manipulation to get the Inertia? But I tought that the period equation varies from object to objects due to difference in inertia.

 

[FredGarvin]

Instead of typing it all out, I found this: http://www.eng.fsu.edu/dynamics1/inertia/inertia.doc

One note is that the initial displacement of the pendulum has to be relatively small (<15° or so) so that sinq = q.

 

[Moolan]

Thanks for the link, everything is now making more sense