Swing Test and Marginal Stability

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lcorinth

lcorinth

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I'm prepping this little scratch build for launch tomorrow.

DSCN0758.jpg

According to my OR design, it should have been 1.29 caliber stability. However, I didn't bother to weigh the parts.

When I checked the CG, it was further back than on the sim. So I did some measurements, and did a swing test.

The swing test was tricky - it was my first time doing it. First couple of swings, it got going backwards (it's windy outside today). When I tried it inside, it was fine. Then, I took it back outside, and instead of just swinging it around, I tossed it in the right direction, and it passed the swing test.

BUT, according to my measurements, the CG with wadding, chute, and an E9-6 motor has the CP at 24.3cm. The CG is at 21.9cm. That's a 2.4cm difference, and the airframe is 2.48cm - so it's just shy of one caliber stable (unless I mismeasured).

Whaddya think? Did I mismeasure? I know the swing test isn't perfect, but I've always read that a rocket which passes the swing test is stable. And a second after launch, the CG will shift forward (launching on a 1500mm micro rail). Do you think this rocket should be stable, or do I need to add a gram or two of nose weight?

Thanks!
 
lcorinth said:
I'm prepping this little scratch build for launch tomorrow.

View attachment 263108

According to my OR design, it should have been 1.29 caliber stability. However, I didn't bother to weigh the parts.

When I checked the CG, it was further back than on the sim. So I did some measurements, and did a swing test.

The swing test was tricky - it was my first time doing it. First couple of swings, it got going backwards (it's windy outside today). When I tried it inside, it was fine. Then, I took it back outside, and instead of just swinging it around, I tossed it in the right direction, and it passed the swing test.

BUT, according to my measurements, the CG with wadding, chute, and an E9-6 motor has the CP at 24.3cm. The CG is at 21.9cm. That's a 2.4cm difference, and the airframe is 2.48cm - so it's just shy of one caliber stable (unless I mismeasured).

Whaddya think? Did I mismeasure? I know the swing test isn't perfect, but I've always read that a rocket which passes the swing test is stable. And a second after launch, the CG will shift forward (launching on a 1500mm micro rail). Do you think this rocket should be stable, or do I need to add a gram or two of nose weight?

Thanks!
Click to expand...

Swing test are a really good why at find weather or not a model is flight worthy. The fact you had trouble getting the model to swing nose first outdoors IS concerning. Wind or no wind your model should be able to swing test outside nose first (the ONLY Stable condition). Swinging Backwards is not in any way showing the model as stable. If your model is that close to neutral stability it will like not give you a safe and stable flight.

If your measurements show the Static margin is LESS then one caliber, adding a bit of nose weight is what should be done. Add enough to get the CG forward of the LCP by at least one cailber.
 
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I'd be inclined to add a little more nose weight. It is true that as the propellant burns, the CG will shift forward, making the rocket more stable. The problem is that if it's marginal right at lift-off then it may clear the rod, tip over, then become stable and head off on its new random near horizontal trajectory.

One of my rockets did exactly this. It was fine in a swing test, but turned out to be marginal. On its first flight it did indeed tip over, fly on a low trajectory, and crash. It didn't damage either itself or anything else, and after I'd added a bit more nose weight it flew perfectly well. It's been flying well ever since, in all sorts of wind conditions.
 
Counteracting the effect of burning propellant in shifting the CG forward, if the wadding and recovery system is packed loosely in the airframe, the sudden accceleration kick of ignition and liftoff may cause the wadding and chute to slide backwards (toward the motor) as the rocket accelerates, which will cause the CG to move rearward.
 
Swing test checks the stability of a rocket in a tight turn (high angle of attack). I have had models that don't swing well that have flown purfectly well on all sorts of situation. Make your line as long as possible helps. If you did actually get it to fly straight once then I'd say it is fine.
 
First of all; great paint job.

As far as the “Swing Test” goes; as others have mentioned many a rocket has been successfully flown that will not pass a swing test. From somewhere I once read that the Estes “Alpha” will not swing test successfully, I know mine won’t, but Alpha’s have been successfully flown just short of a Gazillion times, even the ones with those heavy plastic fin assemblies.

Your rocket is short and you are using an “E” sized/weighted motor and the fins aren’t particularly large and possess very little sweep beyond the end of the body tube. That fin characteristic is what I believe allows the “Alpha” to be stable in flight when otherwise it wouldn’t be. (Base Drag?)

Personally I’d put some additional weight in the nose cone. How best to do that would depend on whether the NC is balsa or hollow plastic. The further forward you can place the weight the less you will need.
 
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