Swing test help

[prowlerguy]

OK, I'm sure the answer to this is the rocketry equivilent to "PEBCAK", but I need help.

I have built an Estes Stormcaster. The only mods I made were to 1) change the engine hook to an E size, and 2) build a baffle on the forward end of the stuffer tube. The baffle is forward of the CG, so it should only enhance stability.

I really wasn't worried about stability on C/D engines, but I worried about the weight of an E9 perhaps moving the CG too much. So I went to EMRR, checked the CP entries, and made sure that I had at least one caliper of stability, even using the most pessimistic value found there.

If I were a brave soul, I would have stopped there and gone flying. However, I wanted to do a swing test as validation. So I put in my E9 and parachute, tied some twine around my rocket, balanced it, and taped the string in place. I then start swinging it, at the darn thing turned completely around and proceeded tail-first! I was concerned, but I originally figured this might happen, so I wasn't too upset. On a whim, I decided to swing test it with a D. Loaded engine, rebalanced on string, began swinging. The rocket began to wildly oscillate, never becoming stable. What!!??

So there I am. I can either go with my measurements, counting on a proven design, common sense, and the lack of negative statements about this rocket's stability. Or, I can add noseweight until the swing test works, killing the performance. Could I have botched the swing test somehow, or is the rocket too large to swing at the proper rate?

 

[davel]

The swing test is not a very reliable indication of stability. This method produces a 'flight ' path with a high angle of attack. If your CG is one caliper or more ahead of the CP, you are good to go.

 

[Mike]

Have you got the string exactly on the CG? You did move the string position after changing it to a D, didn't you?

Just a thought!

 

[cls]

The swing test is not a very reliable indication of stability. This method produces a 'flight ' path with a high angle of attack.

that's why everyone says use a really long string, as long as you can practically.

it's never failed me. in fact it's saved the rocket more than a few times.

 

[prowlerguy]

I could only let out about 6 feet or so and still keep the dang thing going around in a circle. And I did rebalance with each engine I tried (I also tried a C11, same results). I think I'll just load it up with a C11 this weekend and see what happens. Thanks for the feedback, guys.

 

[hokkyokusei]

It's a problem I've had. The rocket might still be stable. A swing test will generally prove a rocket is stable, but not that it is unstable.

I've had rockets fly back wards in a swing test that were subsequently proved to be stable.

So if it performs well in the swing test, you're good to go, and if it doesn't, well, you don't really know.

 

[Stones]

Prowler...
I just grabbed the stock RSim file for the stormcaster from EMRR and loaded it with an E9. Shows it as overstable. I would say that no nose weight should be added. Might want to fly it on a fairly calm day for it's 1st flight, just to see how it handles. My guess is, you'll be fine.

 

[powderburner]

prowlerguy, all the stuff already posted above is pretty much what I would have said. The swing test can indeed be squirrely, especially on models with minimal stability margins.

I usually try my best to get a smooth, wide swing by starting with about six or seven feet of string and holding my arm/hand overhead until the rocket is moving. Then I can slowly feed out another five or ten feet of string, but this is pushing it. With that much string, you have to be extra careful to be smooth on your speed all the way around or else you will start dipping your 'orbit' into the turf (a truly bad thing to have a prang BEFORE the first flight). And you have to be quick-fingered to take in the extra string when trying to stop. And even with the extra large circle, and a steady velocity, your rocket may still swing backwards.

Yes, it is frustrating. So go fly it once.

 

[Micromeister]

I also agree with much of what has already been posted with the exception of the mistaken notion that swing tests are not very reliable. If anything they err on the way overstable side. while marginally stable models just will not swing test not matter what you to. I've found that using a 5 to 10 degree nose down loaded CG helps in some models.
I'm with Powderburner, start out with about 5 or 6 feet of line to get the swing started then let out line to about 10 to 12 feet, maintaining your airspeed or increasing it a little. then bring the model back in. If you model doesn't stabiltize in the first swing or two at speed it isn't going to. Bring it in, add some nose weight or tape on some additional fin area and try again.
Try to keep your swing string as light and thin as possible. I've found 70 to 100 lb breaded kevlar makes a very light, strong 15 foot swing string.