Airframe dynamics of a model rocket roll control system?

[leright]

Hello,

I am hoping to build a model rocket with a roll control system for my senior project. Since the idea of building or obtaining a wind tunnel seems infeasible I will need to theoretically calculate how this rocket will behave with various canard fin angles and at various air speeds. I assume first I will need to measure the moment of inertia of the rocket airframe. Can someone give me an idea of how to do this? I assume somehow I would mount the rocket to some type of rod (which itself doesn't significantly affect the moment of inertia) and measure the angular acceleration as a function of torque, right? So I would need some type of gyro or accelerometer and a torque transdicer of some kind, right? Can someone help me out here?

Also, I would need to be able to determine how much torque is applied to the rocket airframe at different airspeeds and different fin angles. Can someone help me with this?

 

[Cyrus]

You might want to see if you can find the MOI by using a simple pendulum. The period is related to the MOI about the axis of rotation. Just a simple first though. I tried it once but you need very very low friction. Id probably not try it though.

 

[leright]

You might want to see if you can find the MOI by using a simple pendulum. The period is related to the MOI about the axis of rotation. Just a simple first though. I tried it once but you need very very low friction. Id probably not try it though.

I don't quite follow you. Sure, with a bob swinging on a string, the period of the pendulum will depend on the MOI (which is a function of the weight of the bob and the length of the string.) However, I am trying to measure the moment of inertia of the rocket about its roll axis.

 

[AlephZero]

I don't quite follow you. Sure, with a bob swinging on a string, the period of the pendulum will depend on the MOI (which is a function of the weight of the bob and the length of the string.) However, I am trying to measure the moment of inertia of the rocket about its roll axis.

The inertia for the pendulum is I + mr^2 where I is the MOI of the bob about its CG, m is the mass and r is the length of the string. If the pendulum has a small bob and a long string I is negligible compared with mr^2 but that isn't always the case.

To use this method to find I, you want to make r small. Putting the pivot of the pendulum directly on the outside of the case would be one way. Then I would be the same order of magnitude as mr^2.

Another way would be mount the rocket horizontally on a rotating spindle, and use a pulley and a weight on a string to accelerate it with a known torque. With a small weight, the accleration will be small, so you can measure the time for the string to unwind a known length and calculate the acceleration. Do it with a few different size weights, and you should be able to allow for any friction in the pivot.