Quantcast

Swing Test

The Rocketry Forum

Help Support The Rocketry Forum:

Mister Rogers

Active Member
Joined
Oct 24, 2004
Messages
43
Reaction score
0
Can anyone help me with interpreting the results of swing tests on scratchbuilt rockets? As a youngster, I always had trouble with this, and as a BAR, I am still having trouble with this. Oddly enough, even when the results of my swing tests tell me the rocket is horrible, I use my intuitive common sense regarding length, fin area, weight, motor size, etc., when I am designing them so I say "what the heck, I think it will fly okay despite the swing test" and it fly it anyway. They have always flown well and I have never had one become unstable.
What I do for the swing test is to prep the rocket just as if I am going to launch it with the engine (minus igniter) and wadding and chute. I then find the exact balance point (cg) and tie my string on with a slipknot and tape it in place. Then I go outside and swing my heart out. Sometimes it fly backwards, straight as an arrow, sometimes not. I would only worry if it "wobbled" in the swing test. I just tested one yesterday of my own design, sort of like a sidewinder missile but with a long ogive nose. It flew perfectly straight up and down, or vertical to the ground, just like it was sitting on a lauch pad, and perpendicular to the direction of "flight". It is a longish rocket, but with those huge fins I could't imagine it being unstable. I tried putting progressively more noseweight into it with same results. I've got almost as much noseweight as rocket! And it still sits straight up and down in the swing test, and I am swinging this thing as fast as I can. :confused: please help me....
 

tdn

Well-Known Member
Joined
Oct 4, 2004
Messages
47
Reaction score
0
Read this article. In short,

a) Your rocket's too long
b) Your swing line's too short

or c) You're not starting the test with the rocket flying straight in the first place.

Better to use RockSim, or do the cutout method, or work the barrowman equations.
 

Micromeister

Micro Craftman/ClusterNut
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 19, 2009
Messages
15,074
Reaction score
38
Location
Washington DC
I swing test everything I possibly can. I do NOT trust anything Roc-sim or ANY computer generated sim tells me.

there are of coarse limits to what can be successfully tested in this manor. tdn is right on about it could be to long, your sting to short, and/or your swing speed to low.

It helps to start out with the model with about a 10degree nose down from vertical position on the string. Try to start the model out in close increasing swing speed and letting out line to 10 or 12 feet, I use a 90lb bow string nylon braded nylon for my standard model swing test line I have 20 feet of line on the spent C6 motor caseing "handle/spool". I use every bit if it when I can. I use a 28lb kevlar thread for swinging micro-maxx models it's spool has 15 feet of line. all is used.
Hope this helps
 

tdn

Well-Known Member
Joined
Oct 4, 2004
Messages
47
Reaction score
0
Originally posted by Micromeister
I swing test everything I possibly can. I do NOT trust anything Roc-sim or ANY computer generated sim tells me.
I understand your reservations, but I think RockSim is pretty good, as long as you account for everthing in the rocket, and double check the CG by hand. If you want to verify the results, work the barrowman equations by hand (if you suck at math, get a math-geek friend to double check your work). If you're still in doubt, use the swing method, but realize that there might be reasons it could fail. Personally, and this is just me, I'd rather have a deep understanding of the situation than rely on one method.
 

Micromeister

Micro Craftman/ClusterNut
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 19, 2009
Messages
15,074
Reaction score
38
Location
Washington DC
Originally posted by tdn
I understand your reservations, but I think RockSim is pretty good, as long as you account for everthing in the rocket, and double check the CG by hand. If you want to verify the results, work the barrowman equations by hand (if you suck at math, get a math-geek friend to double check your work). If you're still in doubt, use the swing method, but realize that there might be reasons it could fail. Personally, and this is just me, I'd rather have a deep understanding of the situation than rely on one method.
Tdn:
I'm so glad you think you understand, and yes that is exactly what I do. Actually it's often quicker to do the Barrowman equations by hand then to get accurate results from Roc-sim. Hey but that's just me...I've been doing that stuff with a sliderule since the very early 60's. Barrowman, cardboard cutout and swing tests all have there place. Computers have their place as well, I have and use several computer design and simluation programs just like you ...but they are NOT the answer either. You couldn't be more correct when you said you'd rather have a deep understanding of the situation than rely on one method....why then was the first optional method recommended Roc-sim? ...The least accurate...Well; that's not fair, it's likely the most accurate if one takes the time to input the EXACT data on our model. But for most modelers; if it means we have to actually weight and measure things....it ain't gonna happen. At least not on models that are small enough to be "tested" in other ways. I am sick to death of folks quoting Roc-sim like its the Bible...It isn't, Check, Check, Check, It's ONLY a simulation, if we want it right we have to do the math ourselves. Check Check Check!!! Swing test in at least two fin positions relitive to the swing string. Do not rely on intuitive common sense on stabiltiy issues...SAFETY FIRST.
I'll Hop down now from my ol'e broken soapbox now.
 

GL-P

Well-Known Member
Joined
Jul 15, 2004
Messages
1,617
Reaction score
0
I've never swing tested anything in my life. I've never had a problem with VCP. I use VCP solely for the CP and then when the model is built I double check it. I always try for 2 caliber stability for safety.

The problem with swing tests is that you can't get the higher speeds a rocket will experience in flight. The CP moves back the faster a model goes. You may not be getting enough speed in your swing tests.
 

tdn

Well-Known Member
Joined
Oct 4, 2004
Messages
47
Reaction score
0
Originally posted by Micromeister
You couldn't be more correct when you said you'd rather have a deep understanding of the situation than rely on one method....why then was the first optional method recommended Roc-sim?
Quite honestly because it was the first one that came to me. I value it no more or less than the other methods, but I had to list something first.

And I think it can be fairly accurate assuming 2 things -- One, that you choose from the database of parts (as opposed to guessing about a custom made part) and two, that you give yourself a comfortable margin of error. I always give myself a little overstable for insurance.

And three, I don't discount common sense. If it's roughly the size and shape of an Alpha III, and made of more or less the same materials, it'll probably fly more or less the same way. If RockSim gives it "over stable" or very close to it, I feel pretty comfortable flying it, assuming I can do so the first time in relative isolation.

Now, I'll grant you that you have far more experience that I, and my designs so far have been "safe" (nothing "weird" like forward fins), so I trust your judgement. But I have yet to prang, cartwheel, or landshark.
 

Micromeister

Micro Craftman/ClusterNut
TRF Lifetime Supporter
TRF Supporter
Joined
Jan 19, 2009
Messages
15,074
Reaction score
38
Location
Washington DC
[Now, I'll grant you that you have far more experience that I, and my designs so far have been "safe" (nothing "weird" like forward fins), so I trust your judgement. But I have yet to prang, cartwheel, or landshark. [/B][/QUOTE]

I hadn't either until banking on some roc-sim Nike-Smoke micro scale model, 3 different sizes, and a couple other scales as well. not one was stable. luckly they were all micro models so no harm done, but a TARC two stage was a much bigger problem.. and the team Swore the model had a 2.5 cal stabilty, even had a diagram of sorts. the model was completely unstable. BAD DATA entry, but the kids Believed the thing would fly! Check, check, check.
 

tdn

Well-Known Member
Joined
Oct 4, 2004
Messages
47
Reaction score
0
Originally posted by Micromeister
BAD DATA entry
Well, that's just the thing. A program is only as good as the data you put into it. Plug the same wrong values into barrowman and you'll end up with an answer just as wrong.
 

Mister Rogers

Active Member
Joined
Oct 24, 2004
Messages
43
Reaction score
0
Originally posted by tdn
Read this article. In short,

tdn, thanks, I read the article on the apogee website and it was very informative. I feel a lot better about why I was seeing the results I was. I will look into a computer based simulation but common sense has worked so far;)
 

GL-P

Well-Known Member
Joined
Jul 15, 2004
Messages
1,617
Reaction score
0
and the team Swore the model had a 2.5 cal stabilty, even had a diagram of sorts. the model was completely unstable. BAD DATA entry, but the kids Believed the thing would fly! Check, check, check.
Could be bad data entry, could be some trim weight came loose under boost, could be overstable. could have had fin flutter, could've...

Isn't this why we are rocketeers? We love the challenge!:D
 

powderburner

Well-Known Member
Joined
Jan 18, 2009
Messages
7,356
Reaction score
4
Mister Rogers,

Have you scanned through all the old tips and advice that is available on old TRF threads on these subjects? Swing testing, stability calculations, and computer simulations have been covered pretty extensively here on TRF. You might want to read:
http://www.rocketryforum.com/showthread.php?threadid=4251&highlight=swing+test
or
http://www.rocketryforum.com/showthread.php?threadid=5384&highlight=swing+test
or
http://www.rocketryforum.com/showthread.php?threadid=10948&highlight=swing+test
or
http://www.rocketryforum.com/showthread.php?threadid=8970&highlight=swing+test
or
http://www.rocketryforum.com/showthread.php?threadid=7612&highlight=swing+test
 

Stymye

Well-Known Member
Joined
Jan 21, 2009
Messages
7,568
Reaction score
4
first there were swing tests ..... than the computer age came along
how many people do you see still riding a horse to work?

I swing test sometimes if it's a scary design , and if I can't figure out how to get an accurate simulation
 

teflonrocketry1

Well-Known Member
Joined
Jul 31, 2011
Messages
1,625
Reaction score
1
I vote for the computer simulations. Nothing like building a nice model rocket and swinging it overhead and either having the string break or the model fall out of the loop or crack a fin when you try and stop the motion. I busted a few this way before I discovered the software programs like RockSim, now that's all I use.

I got started using VCP freeware for stability calculations by using tips from Mark Fisher's web site (watch out for the pop-ups) at:

http://mcfisher.0catch.com/software/vcptips.htm

Mark's work inspired me to stretch the RockSim program into being able to simulate things like tube fin, ring fins and side pods (visit the Apogee web site for these articles).


Bruce S. Levison, NAR #69055
 

powderburner

Well-Known Member
Joined
Jan 18, 2009
Messages
7,356
Reaction score
4
Let's see here . . .

Swing test: about three cents' worth of string, about 30 seconds, reasonably good assurance of stability

Rocksim: about $100 worth of software (not counting the computer to run it on), about 30 minutes, reasonably good assurance of stability

If it is so easy to do a swing test, why wouldn't you want to include it?
 

DynaSoar

Well-Known Member
Joined
Mar 15, 2004
Messages
3,007
Reaction score
0
Originally posted by stymye
first there were swing tests ..... than the computer age came along
how many people do you see still riding a horse to work?

Darn fewer than there would be if people weren't forced to live without horses by zoning laws, and ridiculed overtly and covertly for not owning at least 1 vehicle per driving person in the family by the people who have to keep themselves convinced this is the only proper way to live.
 

Stymye

Well-Known Member
Joined
Jan 21, 2009
Messages
7,568
Reaction score
4
easy there bulldog..... I am just using it as an analogy, nothing more.
 

JStarStar

Well-Known Member
Joined
Jun 6, 2009
Messages
2,528
Reaction score
56
But is the horse stable by the RockSim method??

;)
 
Top