Swing Test Question

[Rocket_Man]

I am trying to swing test my airplane but I dont know which axis to do the swing test. Does it matter which axis? Should it be stable over any axis?

If I take my model and go to balance it on a piece of string, doesnt the nose cone take on more weight as the model is swung in a circle due to centrifugal force?

Cause if I attach the string to my model and it wont balance, then I add BB's
to the nose so it will balance, when I swing the model the BB's become "heavier" due to centrifugal force thus causing the nose to swing out?

Or are the fins supposed to counter balance the centrifugal force of the
weights?

I am trying to understand this "center of pressure" concept which I believe to be a fulcrum of balance where the airflow over the fins will counter balance any centrifugal force of the nose weight if the model is swung in a circle.

So the more weight I add to the nose then the less "sensitive" or more stable the model will become?

What if I cant get the model to swing fast enough to get airflow over the model and the minute I start swinging it it goes nose heavy.

Any help doing this swing test with an airplane would be greatly appreciated!

 

[FastCargo]

I'm probably going to mix up the physics here...when you balance the mass of the model (on a fulcrum, hanging from a string, whatever), that point is the center of mass.

Any aerodynamic forces that cause the model to rotate should cause it to rotate around it's center of mass.

Also, the center of mass is exactly that. The idea is that for a rigid body (like a rocket), any force that acts on that center of mass acts on all the parts of the rocket equally. In other words, the nose might be getting heavy, but so is the rest of the model. Your model won't go 'nose heavy' unless it A) flexes a lot or B) the weight shifts while you are swinging it.

But you have to start with balancing the model first.

FC

 

[mjennings]

Another thing to keep in mind is that the length of our string must be much greater than the length of the rocket being swing tested. For example a 2 ft rocket should be at the end of a 10 to 15 ft string. Other wise teh angle of attack (angle the airflow makes with the rocket along the long axis) varies greatly over the length.

I may not be following your description the best but it appears that you are attaching the string to the rocket and then trying to drive the center of gravity to the attachment point. You should find the CG first by balancing the rocket on a straight edge make the point and attach the string there. As you add nose weight of course this position will change.

If your rocket is a "standard rocket shape" then one axis is fine. If it is airplane or "non-standard" configuration you should do at lest 2 swing test.

Check the Handbook of Model Rocketry by G.H. Stine for info most library systems should have a copy.

FC your Physics was pretty decent.

Hope this helps

 

[Rocket_Man]

Well like you both have said the critical point is to balance it on the string, which I cannot do. It has a delta wing and I just cant get it to balance.

My center of gravity seems decent and I am going to add a couple of pennies for nose weight and am just going to launch it. I weighed it today and it came in at 4.2 ounces with a D12 in it so a couple of pennies wont hurt. Max lift-off weight for a C11-3 is 6 ounces.

So with nothing around to damage and launching on a C engine over some tall grass, I am just going to go for it.

If it goes then fine, if it crashes O well. I am not going to build something and then be afraid to fly it. If it fails to exist then so be it. :y:

 

[edwinshap1]

it has to be bilaterally symetrical, or it's one weird plane indeed.

if you lay the tail end on your left index finger, and the nose end on the other, and bring them together, you should end up with a fairly good representation of the center of gravity (cg). then try to tie a string so that it won't completly tilt either back or forth. once that's done, swing like hell, but it won't matter, the swing test is terrible and has never once passed any of my rockets, even though all of my rockets are stable...