Calculating Rotor Moment of Inertia and Time to Slow Down

[Alwyn Hartman]

Ok, here we go.

In a rotating mechanism (helicopter rotor), at a state of equilibrium, the rotor consumes a certain amount of energy from the shaft to maintain a constant angular velocity (since there is a measure of resistance present over the span of the rotor).

Lets suppose that the moment of inertia from all sources of drag acting on the rotor, transmitted to the shaft at a speed of 600RPM is measured to be 200Nm. If the rotor is made of two blades then each blade will have a moment of 100Nm?

Each rotor blade measures 3m from the shaft centre point and has a mass of 80N.

Second, assuming that the moment of inertia remains 200Nm for all angular velocities, how long will it take to slow the rotor to 0 RPM?


Sorry if its vague, I am engaged in a purely academic design of a helicopter but have become lost in the rotating physics!

 

[Lsos]

"Moment of inertia" is essentially how hard it is to accelerate or decelerate the rotor. When you say "moment of inertia from drag is 200Nm" you're thinking of the torque from drag. That said if the total torque on the shaft is 200 Nm then indeed each blade will contribute 100 Nm.

In order to answer how long it will take to slow the rotor to 0 rpm, THIS is where the "moment of inertia" comes in. This depends on the shape of the rotor, but your rotor seems simple enough that you could estimate a reasonable value by some calculations (you need to google the formulas for these). Once you have torque and moment of inertia, how fast the rotational speed changes is a matter of another simple formula (again a google search away).

 

[Alwyn Hartman]

Wow, that's probly the most useless piece of advice anyone gives everyone nowadays.

The whole point of coming on this forum is NOT to just be redirected away to another search query. Perhaps you may not have thought of this but, maybe I would like to interact with a real person, not just some static page I can't ask questions to. Why don't we just all get our university degrees from google, the google university... preposterous.

 

[HallsofIvy]

What's preposterous is you insulting some one who is trying to help you.

 

[russ_watters]

Thread locked. Try again, with some humility, appreciation and respect for someone who is only trying to help -- and doing a good job of it.