# Homework Help: Moment of inertia formula for a propeller

1. May 13, 2012

### localrob

Is there a moment of inertia formula for a 2 bladed propeller?
If you only have the mass and the length of the propeller, I would think a good estimate would be using a rod rotating about the middle. 1/12ml2

2. May 13, 2012

### Staff: Mentor

"Good" depends upon what you want to use the estimate for. Does a simple rod adequately describe the geometry and mass distribution of the propeller you have in mind?

3. May 13, 2012

### localrob

I think that for a high school physics problem, it can be assumed to act like a rod. So when finding the rotational kinetic energy of a spinning propeller, I could use the rod formula.
But I was curious if there were another generally accepted formula for a propeller that I could use. I'm not sure what the distribution of mass is for an actual propeller, and nothing other than the mass and length are provided.

4. May 13, 2012

### Staff: Mentor

No doubt manufacturers of propellers will have detailed models of their propellers, including all the geometry and mass distribution. But for a problem where the only information you have is mass and length you're rather constrained to choose the rod as a model.

5. May 13, 2012

### localrob

That's how I feel too. I did see a post on these forums where someone found the formula to be 2/3mr^2, but I don't know where that came from.