CP, locate by the cardboard.......

[rockhead]

.....cutout method. Has anyone here use it with a degree of success on LPR's. Seems to easy. Locate CP, build, and adjust weight as needed to locate CG 1 to 2 dia. ahead of CP.

 

[Zippy]

From what I've read about how to do the Cardboard Cutout Method I don't believe it finds the CP at all. I think it only finds the center of gravity of a piece of cardboard. Just my opinion though.

Zippy

 

[rockhead]

"I think it only finds the center of gravity of a piece of cardboard"
That's what I thought but then started to think that all components are the same material, 2 dimensional, and maybe a cut out like that would have the GC/CP at the same point (or close enough to use). I always get myself in a mess when I try to "think". Just to old to want to do that formula stuff. Heck forget how anyway! Lets see, mod one of my G scale train flatcars to a launch pad and recover with my RC off road truck. Built the flatcar and the truck so gota build the rocket.

 

[Zippy]

mod one of my G scale train flatcars to a launch pad and recover with my RC off road truck. Built the flatcar and the truck so gota build the rocket.

Now thats a project I'd like to see...

Zippy

 

[teflonrocketry1]

There are still some designs that are too complex to simulate using software (but the count gets smaller with every article I seem to write lately). For these designs the cutout method is the only option available. Micromister uses the cutout methods to simulate plastic model conversions of jet fighters loaded with ordinace under their wings and other odd rocs. Visit this thread: https://www.rocketryforum.com/showthread.php?s=&threadid=4251&highlight=cardboard+cutout

The cutout method indicates designs like spool rockets and the Saturn 1b kit from Apogee are unstable but we know otherwise.

Visit the Apogee website at: https://www.apogeerockets.com/rocksim.asp

And read the blue highlighted area titled: "Other Powerful Features of RockSim" to see what Tim Van Milligan has to say about the cardboard cut-out method.

Bruce S. Levison, NAR #69055

 

[rockhead]

OMG. Guess I won't be scratch building!!!!!!!!!!!! The price doesn't scare me but the program sure does. Way more than I would think I need just for a basic rocket.

 

[Zippy]

Just download the demo version. You can't save the design but you can take screen captures. I use the demo for all my scratch builts, unfortunatly the design has to be done all in one sitting but it's still a great tool. It's not that hard to use after playing with it a few times.

Zippy

 

[Fore Check]

Originally posted by ZippyOgiveHead
Just download the demo version. You can't save the design but you can take screen captures. I use the demo for all my scratch builts, unfortunatly the design has to be done all in one sitting but it's still a great tool. It's not that hard to use after playing with it a few times.

Zippy

I'll second that. I've laid out a couple rather ambitious projects on the demo version that I currently have on build table. I do, however, lament the fact that I have yet to invest in the full version for the use of such items as ring fins and radial tubes.

For basic modrocs, download the demo, play with it a bit, lay out your rocket, and go with it.

 

[teflonrocketry1]

rockhead,

Try out the Demo Version of RockSim to see if you like it. I can help you out by creating a few simulations of your designs that can be viewed and manipulated using the demo software. Just get me all the details in a post or PM.

There are other freewares you can use to simulate rocket stability and flight. Visit Mark Fisher's web site for descriptions, compairisons and links to them at:

https://fly.to/mrhq

I started out using WRASP (for flight simulations) and VCP (for stability simulations) both are classic programs (freeware) that have a lot to offer.

Bruce S. Levison, NAR #69055

 

[Zippy]

I forgot to mention RockSim is also great for trying out motor/delay combinations in allready existing designs. There are tons of Estes files available and lots and lots of others too. PML has the files available for all their kits and after actually flying a real one I can attest to RockSim's accuracy (as long as the files are well made) in the simulations. It's great to have a good idea of what that particuler model/motor/delay will do before really launching it.

No Apogee does not pay me a commission.
Zippy

 

[rockhead]

Thanks for all the informed replys. Guess after the holidays I'll have to get serious about designing a rocket to launch from a modded flatcar on my G scale train. I have the basic flatcar, short 4 wheel with no trucks for stability on the tracks. overall body is 9 inches so I'll need a rocket that will be in proportion and not look to weird. I did an Estes Gynomie and it's 10 inches but looks to long when suspended 5 inches off the car on the launch rod. Something in the 7-8 in. range and a little fater would look better. Will launch with the train stopped via remote. Why do I do these things??????????????????????

 

[Micromeister]

Back to your original question, the cardboard cutout method finds the LCP "Literial Center of Pressure" based on the outline of the object being modeled. Like Teflon mentioned there are some designs that are just to complicated to model accurately with our current MR software. PMC's of most aircraft is a good example, but simple 3f&NC rockets can be tested this way also. If is important to do a cutout of a side and top view of the subject if the model is asymmetrical, average the results to find the LCP. One view is fine for symmetric models.
Once you know the LCP of your cutout simple add a point a least one caliber ahead fo the LCP. that is were your finished models CG will be. add nose weight or larger fin area to adjust the models CG to that point.
Yes it is easy. If done with care, you will have a model with will be on the overstable side but very flight worthy.

 

[Micromeister]

Rockhead:
Heres a pic of the cardboard cutouts used to make this 144th scale F104 micro-mixx pmc stable

 

[Fore Check]

Originally posted by Micromister
Back to your original question, the cardboard cutout method finds the LCP "Literial Center of Pressure" based on the outline of the object being modeled. Like Teflon mentioned there are some designs that are just to complicated to model accurately with our current MR software. PMC's of most aircraft is a good example, but simple 3f&NC rockets can be tested this way also. If is important to do a cutout of a side and top view of the subject if the model is asymmetrical, average the results to find the LCP. One view is fine for symmetric models.
Once you know the LCP of your cutout simple add a point a least one caliber ahead fo the LCP. that is were your finished models CG will be. add nose weight or larger fin area to adjust the models CG to that point.
Yes it is easy. If done with care, you will have a model with will be on the overstable side but very flight worthy.

"Overstable" by this method may be a bit of an understatement.

Most of the rockets I've laid out on RSim show to be stable by either Barrowman or the RSim method but rather unstable by the cardboard cutout method. Making them stable by the cardboard method would increase their RSim/Barrowman stability to calibers of 5 or 6+

As you say, you will surely have a flightworthy model, but performance is probably negatively affected by the overbuild on the fins or the nose weight.

 

[Micromeister]

Forecheck:
don't get me started on Rocksim..It has as many flaws as it is helpful.
We are talking about LMR models. I'd rather have folks building and flying model that are safe and overstable, than worry to much about a few extra feet of total altitude. especially new users.
I'm not knocking automated tools or computer programs, they have there place. but ya have to answer the original question asked before we go off on a tangent with alternate ways.

I think the question was has anyone used this method with any degree of success? the answer is a resounding YES.

Speaking of other options... there is also a couple pretty good mod roc programs out here Win roc 4.5 is now freeware. (very good stuff) and RocketCAD-4 has a shareware version, with the full version around 20 bucks.

 

[Fore Check]

Fair enough -

Enlighten me a bit on the flaws of RSim if you will. I've been relying on it for relatively simple designs (one nose cone, perhaps a transition or two, non tubular or ring fin sets, various motor configurations but no outboards.) Is it trustworthy in your opinion?

 

[Micromeister]

OH man I don't want to start this again.. Rocksim IS a wonderful, tho WAY overpriced MR tool. I use it, Ver.7.01 or higher is getting better..BUT it is as much as 20% over estimating every single sim., I can't begin to tell you how they came up with their "cardboard cutout" portion. it seems extremely high. and I just found out we should be using 2 caliber static margins using the rocksim cp/cg findings.
Last weekend I flew 4 micro scale models which the program gave me a solution static margins of 1.35 to 1.8 cal. stability. Every one of the 4 were totally unstable. Was this the programs fault? I'm not sure, probably not..but "I was led to believe" these models were stable by the program. I had to make some "assumptions" in the "Modeling" on screen of some custom parts not available in the programs library.. which may have altered the outcome. I'm reasonably sure after the fact, that it's something I did..however I didn't bother to swing test or double check the work..why?...because we have all sort of assumed that if it checked out in the program.. it is fine. That is were we're all making a big mistake. This particular problem raised is ugly head fairly often.
I can't be sure because I do not use the program on a daily basis. It looks like we must be extremely careful while entering cp cg data for our models and the individual parts. It may be necessary to including balancing the real part if it's already made to get a very accurate CG. Very precise measurments need to be used if using the override mass and locations option from the real model and/or its parts.
If the program tells you the design is stable, even thou you entered some of the parts incorrectly, while under or over guestimated the mass and/or cg's your output solution is going to be wrong. Our false sense of security in the program needs to be tempered back to reality. it's a tool, a good tool but it's only as good as the info it gets. GI/GO
Bottom line: Don't rely solely on rocksim for your stabiltiy solutions. double check it by hand if necessary or with another cp/cg calculator.
I hope pointing out stuff like this isn't construded as trashing the program. It's not ment that way, but I'm really getting very tired of hearing folks quote "rocksim" as if it were absoulte. There is a winning TARC team licking thier collective wounded pride this week after just such an oversight in their staged egglofter that was swarn to have had a 1.8 cal stablity with large forward fins, which on closer examination was more like .2xx something. Just be careful.. It's only a tool.

 

[Zippy]

OH man I don't want to start this again.. Rocksim IS a wonderful, tho WAY overpriced MR tool.

I can't argue with that. If the price were half as much I might actually buy a copy instead of just playing with the demo.

Zippy

 

[Fore Check]

Micromister -

What I've noticed is that the mass data compiled for the components in the database, while being close, is a tad off.

In the case of micro rockets, I imagine that this problem is exarcebated because the weights are so low and the surface areas of fins, parts, etc. are so small that the slightest variation will have a much greater impact on the model in actual practice.

I guess I mean to ask this: can it be relied upon for fairly accurate Cp determinations? Then, once the model is complete, balance it to see if the Cg matches the simulation and adjust a bit of weight as necessary to achieve the stability caliber you put together in the sim (by moving the Cg as necessary)? Again, you could only do this with certainty if the Cp determination is pretty accurate.

 

[rstaff3]

The example TARC problem that micromister referenced was a big error in the data entry in the design. If the components you enter aren't shaped like the ones on your model, the CP will be wrong. The rocket was way underpowered also.

My opinions:

1. I have found the Rsim CP estimates to be reliable and can be trusted. (my experience, not a guarantee) However, the Rsim must accurately reflect the design. I've accidently added an inch to the length of a BT I probably would have used the same number in a hand calc of the CP.
2. I think the best way to set the CG is to design your rocket without being too anal about the component weights, and set the CG/mass over rides in the final sim.
3. I have also found the altitude estimates to be inflated. However, I run mine in zero wind conditions which never meets with reality. Start playing with the environmental settings and these will come down closer to reality.
4. The sims will show a stable flight for underpowered/unstable designs. Check thrust to weight ratios. Look at the sim details to see the speed as the model leaves the rod.
5. Especially on the wilder designs, a bit of extra insurance in the nose is a good thing.
6. Static margin is not a 100% guarantee of stability.

But this is off-topic since I never have used the Rsim cutout CP estimate.

 

[Micromeister]

Dick is right, this is sort of off topic, I do agree with much of his observations. I know he is far more familiar with the program than I, Which is also part of the solution. becoming more familiar with the finer points of the program.
The short answer to your question is; Sure, it's as accurate as you make it. just don't take a computer programs word for it, Check, Check, Check..on most lprs a swing test isn't that bad an idea either.
I also agree that underpowering is something we all need to look at more carefully.
Hope this helps

 

[rockhead]

WOW!! Again thanks to all you great folks. I'm bookmarking this one for all the info and links. Should be required reading For newbee builders. Won't be around for a bit as my mother passed away at age 89. So Merry Xmas to all and hope to continue this soon.
Ed

 

[Micromeister]

Ed:
My deepest regrets hearing of your mothers passing. A lose of a loved one at any age is tough, knowing she's in a better place always helps.
God bless and keep you in your hour of need.

 

[rstaff3]

rockehead, I echo micro's sentiments. I'm sorry to hear about your loss. We look forward to hearing from you down the road!