To slow on the swing thing.

[Scotty Dog]

I know this has been a topic here before.But as usual,I cannt find the thread.So,heres the deal. I swing test all my scratch builds. Sunday I was trying to ballance my "Spear-Chuka.Well,let me tell ya.I had a heck of a time getting it so I could hang it on the string ballanced and then get it to fly.I would think I had the nose wieght correct and it would swing fine and then I try it again and it would go crazy. I was getting some frustrated and my fingers were about frost bit. Well, I got angry-ish and said "you will fly or bleed "!! And then I swang her for all I was worth .Ho-Ho- It straightened right up and flew.THEN it hit me. I was swinging it to slow at times. I needed to get the rocket up to speed for the fin tubes to work.So,I pulled out another NC (one with no weight) and started the whole thing over. It took 2-3 tries of adding small amounts of clay weight and BINGO.Took it to the field and it flew SWEET! Lesson learned!!!!!

 

[McKailas Dad]

And then I swang her for all I was worth .Ho-Ho- It straightened right up and flew.THEN it hit me.

I hope you are ok...:y:

At least you didn't stop swinging and have the string wrap around your head!

I can picture it now; Scotty Dog bound and gagged by his own rocket, laying in the snow...








Yea, I know. I'm kidding.

 

[Scotty Dog]

I hope you are ok...:y:

At least you didn't stop swinging and have the string wrap around your head!

I can picture it now; Scotty Dog bound and gagged by his own rocket, laying in the snow...








Yea, I know. I'm kidding.

You may be kidding,But your no that far off.It was me or the rocket that was going to cato!!!!!!!!!!!!!!!!!

 

[MarkII]

The lesson that I take from this is that it can be difficult to perform a swing test correctly. I don't use it myself; instead I use RockSim to obtain the CP location and the stability margin, and then I confirm the location of the CG on the actual rocket once it has been built. I'm not knocking the older method, though. The swing test is a good technique but it has to be done right, as your anecdote showed. You demonstrated a good point. Thank you for the insight.

 

[edwinshap1]

i went to a local club launch and the RSO was worried it wasn't stable, so another guy did a string test. i don't think it was travelling over 40 ft/s, and those fins were really big. i've never done the string test, i just do like mark and i *ahem*mark*ahem* down the CP and then check if my CG is at least 1.5-2 calibers ahead of the CP.

 

[Scotty Dog]

The lesson that I take from this is that it can be difficult to perform a swing test correctly. I don't use it myself; instead I use RockSim to obtain the CP location and the stability margin, and then I confirm the location of the CG on the actual rocket once it has been built. I'm not knocking the older method, though. The swing test is a good technique but it has to be done right, as your anecdote showed. You demonstrated a good point. Thank you for the insight.

I have to ask "What did people do BEFORE Rocksim and such programs"????? I dont have any of those type programs.Even if I did. Most scratch builds I do,well,I guess I would have a hard time trying to "build" it on sim. Some I think would be about impossible. I may have this wrong cuz of my lack of knowledge/use of these programs.Anywhoo-One thing I can say is that I swung tested all my scratchers and ALL have flown.Some not so good , and those were do to lack of proper motor size and delay.I also look at my experience of "Sundays" swing test fiasco as a day of education.(Priceless) As I look back,I had a similar issue with the Absolute Zero. In fact ,after I figured out that swing speed was a factor, I have re swung the AZ and actualy ended up removeing some of the nose weight. I want to see that AZ fly on a fat-C and then if the fins hold out,move up to a "D" .Now ,I also do relize that what I build-LPR and some MPR are small enough to do the swing thing. When I move up to the bigger rockets, Ill have to have the "Most Interesting Rocketeer" do the math.

 

[gpoehlein]

I have to ask "What did people do BEFORE Rocksim and such programs"?????

About all that was available was the "Cardboard Cutout, where you made a flat cardboard silhouette of your rocket and measured the CG of that - the idea was that the CG of the silhouette was the same as the CP of the 3D rocket. I don't know then the John Barrowman released his equations to calculate the CP mathematically.

My best swing test story is when I was designing my mini paper Saturn V (about 10-11 inches long and 13mm power). I tied the string around it and used a small piece of masking tape to hold the loop at the CG. Then I started to swing it over my head. The test was going pretty well until either the knot came loose or the tape let go and the rocket slipped out of the loop - next thing I hear is the WHAM of the model hitting my front door. I think that is when I broke down and bought my first copy of Rocsim!

 

[mjennings]

My swing test story is trying to swing test a Mean Machine. At the time I was of the opinion that everything should be swing tested. At that point in my education I should have known that a 6 foot rocket on a 15 ft cord wasn't going to be worth anything. In the end I think I had to reattach 3 of the 4 fins.

Like any test you need to understand the limitations of the tool you are using and that applies from every thing from the "That looks 'bout right" method to a full up CFD analysis.

 

[Micromeister]

Like you Scotty Dog I swing test just about everything. I don't trust ANY sim program at all. they've shown far two many models to be stable the have Proven to be anything but.

But to answer your question, before these programs were available we did the calculations long hand, With a Slide Rule, pencil and paper or in some strange configurations used the cardboard cutout method. but used the swing test to confirm the calculations or cardboard before flying the creation.
OH another BIG difference...we NEVER flew a new design or untested rocket with anyone else around. Which is still be best method of Testing new designs.
That's why full size rockets are "FLIGHT TESTED" in remote locations...NO simulation can predict exactly what a give vehicle will do in free flight all that accurately.
Those who rely on sim programs only are taking chances with their models and other peoples safety flying new designs at public launches.
Keep right on Swing testing it's the next best thing to flight tests.

 

[n3tjm]

I could never get a swing test done right. How do you do it before the days of rocksim? Simple. Fly it

 

[cjl]

Like you Scotty Dog I swing test just about everything. I don't trust ANY sim program at all. they've shown far two many models to be stable the have Proven to be anything but.

But to answer your question, before these programs were available we did the calculations long hand, With a Slide Rule, pencil and paper or in some strange configurations used the cardboard cutout method. but used the swing test to confirm the calculations or cardboard before flying the creation.
OH another BIG difference...we NEVER flew a new design or untested rocket with anyone else around. Which is still be best method of Testing new designs.
That's why full size rockets are "FLIGHT TESTED" in remote locations...NO simulation can predict exactly what a give vehicle will do in free flight all that accurately.
Those who rely on sim programs only are taking chances with their models and other peoples safety flying new designs at public launches.
Keep right on Swing testing it's the next best thing to flight tests.

The problem is that a swing test does not always accurately show a rocket's stability. If a rocket is stable in the swing test, you can be pretty sure that it'll be stable in flight. However, the reverse is not really true - a rocket which fails the swing test can fly just fine (and I've seen this happen many times). This can be for a number of reasons.

First, as was hinted at with the mean machine story above, a long rocket cannot easily be swing tested. This is because the airflow will effectively appear curved to the rocket unless the string is much longer than the rocket (many times its length). A long rocket can be swung by a relatively short string, yes, but it may or may not appear stable even if it is quite stable in reality. Also, stability is highly dependent on angle of attack. A rocket can be unstable at all angles of attack above 30 degrees, and it will fly completely fine. In flight, all that matters is low-AoA stability, since at all times in flight, it is at a low angle of attack. However, you would have a heck of a time getting a swing test to show it. Swing tests can start at nearly any angle of attack, so a rocket which is unstable at high alpha can easily appear unstable all the time when swing tested.

Also, simulations aren't nearly as unreliable as you make them out to be. Properly done, simulations are much better than a swing test at predicting in-flight stability, and even simple simulation programs (such as rocksim) can be very useful tools when used correctly.

 

[KennB]

I could never get a swing test done right. How do you do it before the days of rocksim? Simple. Fly it

That explains some old flights but what about those after Rocksim? :tongue:
Mostly just kidding.

Penns Creek. Sounds nice.

 

[MarkII]

I have to ask "What did people do BEFORE Rocksim and such programs"????? I dont have any of those type programs.Even if I did. Most scratch builds I do,well,I guess I would have a hard time trying to "build" it on sim. Some I think would be about impossible. I may have this wrong cuz of my lack of knowledge/use of these programs.Anywhoo-One thing I can say is that I swung tested all my scratchers and ALL have flown.Some not so good , and those were do to lack of proper motor size and delay.I also look at my experience of "Sundays" swing test fiasco as a day of education.(Priceless) As I look back,I had a similar issue with the Absolute Zero. In fact ,after I figured out that swing speed was a factor, I have re swung the AZ and actualy ended up removeing some of the nose weight. I want to see that AZ fly on a fat-C and then if the fins hold out,move up to a "D" .Now ,I also do relize that what I build-LPR and some MPR are small enough to do the swing thing. When I move up to the bigger rockets, Ill have to have the "Most Interesting Rocketeer" do the math.

Actually, I was trying to give you props. You did some good field testing of the swing test and identified an important but seldom-mentioned condition that had to be met to produce useful results. You then discussed how increasing the speed for some previously ballasted rockets of yours gave you better information about their stability. This allowed you to reduce the ballast mass that you had added, a move which would make them more efficient and perform better. This excellent additional information demonstrated the real-world value of conducting the swing test correctly, using sufficient speed to provide a fair test of the rocket's restoring surfaces. It was a good and informative post. In my response, I mentioned that I used a different method to determine the stability of my rockets, but I also stated that the results of the swing test could be viewed as valid as long as it was performed correctly, and your post was evidence of that fact. You demonstrated an important factor that improves the swing test's predictive ability. All good.

To answer your question, in the old days, people who wanted to determine the location of the center of pressure in their rockets worked their way through the Barrowman equations. The swing test doesn't provide a direct way to identify the CP location; it takes a different approach, producing a simple simulation of the rocket's flight in order to provide an indication of stability. But the test does allow one to deduce a location for the CP in a gross sense, because if the rocket demonstrates stability during the test, than one can reasonably conclude that the actual CP is likely 1-2 body diameters aft of the string's attachment point. The swing test is a simulation, and as in all simulations it has its share of limitations. cjl mentioned the biggest issue with it, which causes the swing test to be excessively conservative and to potentially yield spurious failures. As long as one recognizes those issues, one can obtain reasonably valid results from the swing test for smaller rockets when more complex and more thorough methods are not available.

Short of flight testing, the best method of determining a rocket's stability probably involves using a wind tunnel. These aren't always available, of course, but even with them, the tests must still be set up correctly to produce valid results.

 

[Scotty Dog]

Actually, I was trying to give you props.{quote} HMMM. Somewhere I think you and I started off on the wrong track. I read your post and felt good about your response. I was pleased that you mentioned that the issues I discussed were important and useful insites to others.My reply was only touching on the fact that some of us dont have the programs. And I still wounder WHAT can be designed on these programs. I know I have read posts mentioning that some programs have thier limitations on what can be designed. One was "complicated designs of fins." Ya know, back in the day, you were supposed to swing test your finished rocket before flying it. I do have, somewhere ,a card that was included in the kits with the instructions with illustrations on how to perform a swing test. Now, after reading other replies I have added another factoid into doing a swing test -the length of the string can play into it. Anywhoo- I plan on moving up and building some bigger rockets and in the future , I do plan on getting a program. I did do a trial of RS from Apogee and had some fun playing with it.Untill then- Schwing



https://www.jh4c.com/schwing.gif