Swing test

[STRMan]

I've always understood that during a swing test, if the rocket points into the direction you are swinging, it is stable. If a rocket points away from the direction you are swinging, it is unstable, and if a rocket just sort of flips back and forth into and away from the direction of rotation, it has neutral stability.

Does a swing test work for really long rockets? I've been building a 6 foot long rocket based on a BT-55 sized tube. No matter how I swing it, I can't get it to act stable, even when I remove the engines. I'm going to try some more nose weight next.

Could the swing test not be a viable way to check a rocket's stability when swinging a really long rocket. It seems like there is a lot of rocket length trying to be turned into the radius of the swing circle. It seems like most of the air moving over it's fins and body are coming in at an angle. I was just wondering if there is a point when the swing test becomes an invalid way of testing stability due to a rockets length and the practical radius it must be swung.

Any thoughts?

[snaquin]

I've always understood that during a swing test, if the rocket points into the direction you are swinging, it is stable. If a rocket points away from the direction you are swinging, it is unstable, and if a rocket just sort of flips back and forth into and away from the direction of rotation, it has neutral stability.

Does a swing test work for really long rockets? I've been building a 6 foot long rocket based on a BT-55 sized tube. No matter how I swing it, I can't get it to act stable, even when I remove the engines. I'm going to try some more nose weight next.

Could the swing test not be a viable way to check a rocket's stability when swinging a really long rocket. It seems like there is a lot of rocket length trying to be turned into the radius of the swing circle. It seems like most of the air moving over it's fins and body are coming in at an angle. I was just wondering if there is a point when the swing test becomes an invalid way of testing stability due to a rockets length and the practical radius it must be swung.

Any thoughts?

Unless your fins are really tiny, it should be easy to design it stable if it's 6 feet long. Because of the long length it's quite possibly over-stable {RockSim estimating the distance between the CG and the CP as greater than two body tube diameters}. Compare your design to the Mean Machine or Cobra 1500. Because of the long lengths much smaller fins can be used to stabilize these two rockets.

http://www.ninfinger.org/~sven/rockets/catalogs/estes79/79est34.html

I wouldn't think a swing test would be practical for a really long rocket. If you have some basic fin dimensions and parts list with a motor choice it could be mocked up in RockSim to give you an estimation.

.

[Zeus-cat]

I have found that the swing test is difficult to perform on large or heavy rockets. As a result, I no longer try to do it on rockets over about 18 inches in length. I think you are correct on why the test isn't accurate on a 6 foot rocket. To get the angle of attack small enough you would need a long string or rope. The potential of damaging the rocket while starting or stopping the swinging would be considerable. I suggest you use one of the computer programs made specifically for model rockets to determine the stability.

[tbzep]

Some rockets will be unstable with swing tests and still be good and stable in flight. I've read about it many times, but since I've never done a swing test, I've never had that happen. ;)

[Mark II]

The problem is exactly as Zeus-Cat described. In order to get an effective swing test for a 6 foot long rocket, you would need to have a VERY long string (something on the order of 100 feet or more), so that you could swing it in a circle that had a large enough radius. To swing a rocket in that large of a radius, you would probably need to be at least 20 feet tall, too. :eek:

Here is a fuller explanation of the issue:

http://www.apogeerockets.com/education/downloads/Newsletter53.pdf

Mark

[PaulK]

The problem is exactly as Zeus-Cat described. In order to get an effective swing test for a 6 foot long rocket, you would need to have a VERY long string (something on the order of 100 feet or more), so that you could swing it in a circle that had a large enough radius. To swing a rocket in that large of a radius, you would probably need to be at least 20 feet tall, too. :eek:

Here is a fuller explanation of the issue:

http://www.apogeerockets.com/education/downloads/Newsletter53.pdf

MarkGood info, thanks for the reference.

[STRMan]

Good info indeed! I can only seem to be able to read the first page of the document, but I get the idea.

So, a good rocket may in fact fail the swing test, but it seems if a rocket passes the swing test, it is almost certainly going to be stable. (Unless there is something in the last two pages of this article to contradict this.)