String test flys backward

[Mailman]

Hey y’all,
Last night I threw together a little Estes star trooper I love 13mm minimum diameter. Built it stock but with a Kevlar shock cord. As soon as I popped in a motor I could tell the CG was going to be questionable. So I string tested it and it flew stable but backwards. I’m thinking nose weight but then I read that it’s ok for small rockets to string test flying backwards. Makes me think CG is behind CP and I could have a problem...

 

[Steve Shannon]

Hey y’all,
Last night I threw together a little Estes star trooper I love 13mm minimum diameter. Built it stock but with a Kevlar shock cord. As soon as I popped in a motor I could tell the CG was going to be questionable. So I string tested it and it flew stable but backwards. I’m thinking nose weight but then I read that it’s ok for small rockets to string test flying backwards. Makes me think CG is behind CP and I could have a problem...

I’ve done string tests before on rockets I knew were stable and had them fly backwards during the string test.

 

[GlenP]

I wonder if this is kind of like the Deep Stall phenomenon on the Rutan designed Long EZ canard-pusher airplane, it can be trimmed for normal attitude flight, but if you pitch too high or have a little too much aft c.g. you can get stuck in a stable trimmed high attitude stall as well, in which the control surfaces are no longer effective.

So, the rocket may very well have two stable configurations, normal flight and tail-first flight. Getting the string test to show stability in the tail-first configuration does not necessarily exclude the possibility of a stable nose first configuration as well.

 

[jqavins]

The two previous replies come as new information to me, and I assume they know what they're talking about. But I'd add a little nose weight anyway, if it were my build. What's certain is that this is the behavior you would get if the GC were a decent amount behind the CP, so even if there is another possible explanation, I'd want to be sure. Which doesn't mean you have to do what I would do.

 

[OverTheTop]

I have never heard of a rocket flying backwards during this test.

Try a longer string maybe. It needs to simulate a straight flight to a reasonable approximation.

 

[SpaceManMat]

Had this happen before. Along the same train of thought of OTT; swing test is a stability test of a rocket in a tight turn (high AOA). Does not necessarily mean it’s unstable in a real flight.

 

[OverTheTop]

If it fails the swing test with a long string I would be worried

 

[Mailman]

Thank you all for your concern. I ended up adding a few grams of nose weight and she flew like a dream

 

[OverTheTop]

Looks great!

 

[jqavins]

Sounds like a good decision, and I love the silver paint.

 

[mjennings]

Too short of a string is a common swing test issue, and is apparent if you dig into the math of what it is doing. Also if you don't nail the CG as your attach point that's an issue. I put a lot of weight into the tip of my Gemini DC as a result of backward flying swing test as a kid, that probably doesn't need to be in there.

 

[GlenP]

String tests can be difficult to perform. You could also try the cardboard cutout balance point to find C.P. and compare that to the actual rocket balance point C.G., that should be conservative, since you are using a 90-deg angle of attack to estimate C.P. rather than a near zero angle of attack like RockSim or Open Rocket uses (Barrowman eqs).