Probably Dumb Question Regarding CP

Okay...this is probably a question where the correct answer is *obviously* one way or the other...but I will risk embarrassing myself anyway... :o)
Say, for example, I want to do an upscaled clone of "Rocket A", at 150%. If the CP of "Rocket A" is "x" inches from the nose...would the CP of the clone, be "1.5x" inches from the nose? Or is there some unforeseen factors I am ignoring; such as the increased diameter....

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Yes.
There are no stupid questions (before the launch button is pressed).
After that, I go to the launches to see the crashes!
Jerry
"Well maybe there is something to it then. I hate to say it, but a few in my section tell me Jerry has a point." - Greg Cisco

Jerry Irvine wrote:
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Jerry,
I would agree that the center of lateral area stays the same for a scaled change in dimension, but does the actual Cp remain remain in a scaled position as well? IIRC, there are some terms in the Barrowman and Galejs equations that are non-linear with respect to dimensions.

wrote:
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VERY close.
So yes.

Jerry Irvine wrote:
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Okay, thanks.
Greg, if you are reading this, I would certainly go with the advice of Jerry and Robert on this and I apologize for injecting any confusion here.

Greg Heilers wrote:
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Not quite that easy. Simply for discussion, consider that the aerodynamic forces on a rocket involve the moving air acting on the exposed surfaces of the rocket, the fins, NC, BT, etc. Surface area increases as the square of some linear dimemsion. If you double the size of a fin, say, it will have four times the surface area. This non-linear change of surface area with linear dimension is one thing which keeps the Center of Pressure from being a simply scaled parameter.
Center of Gravity is also apt to change as weight and mass also vary in a non-linear fashion. It is not safe to assume that doubling the size of a nosecone, say, will double it's weight.
That said, it IS almost always possible to upscale a model, particularly if the original is a more "traditional" looking rocket. You upscale the dimensions, determine where the Center of Pressure is (sim or design programs, cardboard cutouts, slide rules or abacus and geometry formulas, etc) then adjust the Center of Gravity by varying internal weight.
If you don't have one, grab a design/sim program from the net. Apogee has a free, demo version of Rocksim on their website. It will tell you where the Cp of a rocket is. Well, it will tell you where three Cp's are, pick the one you are comfortable with.
If nothing else, do a swing test on the finished model in flight trim (motor in, wadding and parachute packed) and see if it is stable. Add weight to the nose until it swings pointy end first. I've never had an unstable flight from a model which passed the swing test. (Not flame bait, guys, just a factual comment.)

Gary wrote:
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And I have never lost a model to unstable flight that I either ran a simulation on or did a swing test AND the recovery system deployed properly.
The caveat is this is since I have become a BAR, throw in engine sizes I didn't run sims on for altitude and drift prediction, spur of the moment on the field BAD decisions on motor size and I've lost models as much of anyone else.
When I plan carefully and downgrade motor size, if necessary, use creative rod angles to adjust for drift, use very heavy duty stuff in the recovery systems with nylon chutes and lastly use a beeper if it looks like my model will go into a cornfield, my recovery has been 100%.
OMG, I have just violated the tree gods and they WILL BE OUT TO GET ME!!! (So far haven't had a tree landing.) As Linus would say, "I'm doomed."
Best regards, Kurt Savegnago

Gary wrote:
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Yes it is that easy... The CP of a scale vehicle will be in the same relative position as the original.
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Yes, but if the CG of the scale model (With motor loaded) ends up in the same relative location as the original after adding nose weight, then you should have a stable flight on the scaled up model. Assuming of course that you had a stable original and a motor with suitably high thrust to get the scale-up up to speed.
- Robert Galejs

Robert Galejs wrote:
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Robert,
Thanks for the input. (I had just posted a response to Jerry about this and saw your reply). I would have sworn there were non-linear terms wrt dimension in your, and Barrowman's, equations.
What's really embarassing about this is that I had just read your report while doing a bash on a Fat Boy and was trying to figure out why stubbies seemed to be "more" inherently stable.
Its a fortuitous event for me, tho. I'm trying to figure out gliders and its apparent I have a misunderstanding of some of the Cp equations.
Maybe that's why I can't get one to glide worth a darn. ;)

The CP will tay in the same place, but the CG will probably move and therefore the stability may be affected. On scale-ups, the motor (at the back, obviously) tends to be a larger % of the weight. On the other hand, if you add an altimeter bay, that will add weight at the front.
-- David
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David wrote:
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Thanks to everyone for the input. I guess..."more-or-less", the CP will stay the approximately same (after accounting for the scaling factor). And the CG is easy enough to obtain. I just did not want to get 90% through the project...to discover that the CP and CG were so out-of-whack... that my project would be unflyable...lol.

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Yes, CP scales with the rocket. That's why some folks build MR versions of their larger rockets to work out the CG/CP, then put the CG at the same location.
When I do scale rockets, I calculate the CP for the original, then scale that dimension along with everything else. If you've got a spreadsheet for doing scales of different sizes, it just all works out.
    Bob Kaplow    NAR # 18L    TRA # "Impeach the TRA BoD"         >>> To reply, remove the TRABoD! <<< Kaplow Klips & Baffle:    http://nira-rocketry.org/LeadingEdge/Phantom4000.pdf www.encompasserve.org/~kaplow_r/ www.nira-rocketry.org www.nar.org
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