Aerodynamics question

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

jammer

Member
Joined
Aug 20, 2003
Messages
18
Reaction score
0
I just built an Alpha 1 w/my son's Boy Scout Troop as part of their space exploration merit badge. I tried to do something a little different from the boys by reversing the fins so they swept up. I ran a stability test on it with fishing line and it went well. Except when I went to launch it with my son, it went nose over tail to about 20 feet up and then popped the streamer at about 2 feet up.

Has anybody seen this happen before?

The rocket is officially retired to hanging from my son's ceiling, which is kind of a shame, because it is one of my better paint jobs, Blue Angels Blue with a yellow nose cone. I guess the next one will run the same paints with a different fin configuration.

Thanks in advance!

Doug
 
Jammer,

Short answer, in reversing the fins, you moved the rocket's center of pressure forward and disturbed the stability. in any rocket, the balance point or Center of Gravity (CG) should be at least one body diameter AHEAD of the point on the rocket about which all aerodynamic forces acting upon that rocket are centered (Center of Pressure or CG).

The Alpha is a relatively short rocket and needs the swept back fins to maintain this balance. Each external component on the rocket has it's own center of pressure and these add up in a somewhat complex relationship to determine the CG for the rocket as a whole. Making a radical change suce as reversing swept-back fins will indeed dramatically change the performance of the rocket.

I hope I don't sound condescending as that's not my intent. I'm just not sure where your experience level in aerodynamics is and want to ensure that you understand.

Did this explanation help any?

Jeff
 
When you did the fishing line (swing) test was a motor installed and was the rocket fully prep'd for flight with recovery wadding and a parachute? If you did not put the motor in the rocket when you did the swing test, or did not include the parachute and wadding, the swing test may have indicated a stable design. With the motor, parachute and wadding in place, the balance point would shift and your test might have indicated the ustable condition you observed.

I don't like the swing test much. I prefer to use simulation programs like RockSim and VCP. You can get a demo version of RockSim at https://www.apogeerockets.com The best part about RockSim is that in addition to flight stability the program gives you an idea where a given rocket under a particular set of conditions will land.

Bruce S. Levison, NAR #69055
 
This can be done even with a short rocket. You have to put nose weight in the nose cone. It take quite a bit. I know this from expeirnce. One of my designs have forward swept fins. I did the swing test on it and it was un stable so iadded nose weight and thought it was stable weoll when I launched it it was not stable. So I brought it home and put more nose weight in it. It is stable I guess that it took about 2 1/2 oz of clay to get it stable. It has nad some great flights on some B6 motors. If it is not to windy this weekend I might lose it when I put it up on a C6. So if the rocket is still in flyable shape add some nose weight and do a swing test (if you need to know how to do this use a search on here or send me a PM and I will walk you through it) I have put a picture of my rocket it is painted bright red know.
 
Oh by the way Welcome to TRF. You will find alot of help on here. I hope this does not keep you from enjoying the model rocket hobby.
 
rocket trike,

Everything you said was true but you left out one item that will also increase stability.

Length.

By increasing the length you increase the "moment arm" of the nose weight thus moving the center of gravity even further forward with a much less increase in overall mass.

In other words...add less nose weight and more tube! It'll really change the looks of a model too!

BTW...no offence...but compared to all your medival rocket designs... this one looks like a tulip!

LOL!

I like it.

sandman
 
Well, just assuming that the only change was the reversal of the fins, I simmed it out and here's what Rocksim says...

Here's the normal config (note CP-CG relationship)

edit (oops wrong file :) )
 
And here's a reverse-finned Alpha (otherwise stock, not CP-CG).

As a side note, I managed to get this one to 100 feet on a C-6 and it spins like a freakin pinwheel all the way :)
 
Sand

The tulip started as a Battle Axe. I had a problem with w chute and the bottom part of the rocket got damage. So I cut it off and put a new motor mount in it. I put the fins on so that is why it is short. So I guess you could call it a Battle Axed tulip.:D :p :D :D :p :D :p
 
Tons of great advice...Jeff, none condescending and very informative. I'll probably keep it retired and take one of the leftover kits and rebuild it.

Thanks!

Doug
 
I did a quick sim too. It looks like you can fly it stable if you add 3/4 oz. to the nose.
 
jammer,

I forgot...WELCOME aboard!

So...what's your next project?

sandman
 
I'm working on a Quest Courier for my son and also have a Gemini DC for me. With a little luck, I should have them both done by Monday. I just need to decide on the colors to paint them. I figure the Courier will be white with black fins, and I think I'll paint the Gemini blue or red.

And, I've stil got to do another Alpha

Thanks for the welcome!

Doug
 
welcome jammer!

on that Alpha modification, I'd be willing to bet that with a tube coupler and about 8-10 more inches of body tube that it would be stable. She'd also look pretty darned cool too!

jim
 
Originally posted by jflis
welcome jammer!

on that Alpha modification, I'd be willing to bet that with a tube coupler and about 8-10 more inches of body tube that it would be stable. She'd also look pretty darned cool too!

jim
It's marginally stable with the 8" extension on a C6-7 but 10" would work.
 
Originally posted by KermieD
You forgot to add a motor to the regular Alpha for the CG.

Yep. I was having technical difficulties with the image exports and had to retry exporting the standard Alpha 3 or 4 times. I guess I did forget the engine on the one that finally worked :) We do know that the Alpha is a stable design though, so no harm no foul I guess.
 
Back
Top