Question about CG and CP

[redleder]

I have what I think is a simple question but will need to read more. I have the Madcow mini Cowabunga, which according to the spec sheet says it weighs 4oz and has a CP of 11.6" from the nose tip and a CG of 10" from the nose tip. On the official weigh in, without motor and paint yet, it is 5.66oz and my CG is 7.9" and OpenRocket is showing a CG of 9.75". See pictures below. My question is this, What do I really need to know about CG and CP calculations and why are they so important? Is it just that the CG needs to be in front of the CP by at least 1x the diameter of the main body tube? Any tutorials would be helpful. I adjusted the weight and CG in OpenRocket and this is what it is showing me for CG/CP with a D and E motor. As well as flight sims.

Thank you,

Jarod

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Flight sims with different motors






No motor

 

[AfterBurners]

I never worked with OR but used RS. I have a MC Patriot and according to Mike @ MC he said to use the Barrowman's 2D and the RS simulation when calculating the CG and CP. Like I said not sure what OR settings are, but yeah basically you want the CG ahead of the CP by at least 1x the width of the body diameter. I added 27oz of nose weight to my Patriot and on a J500 it hits about 3000 plus feet and deploys the chute at 0.00

Also when I balance mine on a chair and adding tape rolls and such to compensate the weight of the motor and case it comes in about what it says it should be at.

 

[T-Rex]

I am assuming that you made the adjustments to the CG & weight with no motors in the sim. (the last of your screen shots)
I would push the chute back toward the bottom, in both your static tests and the simulation, and check again, that will be closer to reality.

Yes CP & CG are that simple in basic terms. When flights go super sonic, the CP will move forward, but for your simulation that is not a concern. CG will also move forward slightly as fuel burns away, again not a concern in your simulation. CG forward of CP, good. CP forward of CG, BAD. Real missiles have a CP forward of CG so that they are highly maneuverable, however they also have active guidance to make corrections. Our rockets rely on a static guidance, so we have work within those rules.
In simplest terms, CP is the point at which the forces exerted on the rocket by the fins come together. The CP can be changed by fin size, shape , and location.

If you have a cluster rocket with the motors canted so that the thrust vectors come to the same point, and one of them fails to ignite, the rocket will still fly more or less straight, as long as the CG is forward.
I am not an expert, so someone may chime in with a bunch of math, but simple works for me so far.

BTW, looking at the flight data in the first attachment, I would change to a D12-5......

 

[redleder]

I am assuming that you made the adjustments to the CG & weight with no motors in the sim. (the last of your screen shots)
I would push the chute back toward the bottom, in both your static tests and the simulation, and check again, that will be closer to reality.

Yes CP & CG are that simple in basic terms. When flights go super sonic, the CP will move forward, but for your simulation that is not a concern. CG will also move forward slightly as fuel burns away, again not a concern in your simulation. CG forward of CP, good. CP forward of CG, BAD. Real missiles have a CP forward of CG so that they are highly maneuverable, however they also have active guidance to make corrections. Our rockets rely on a static guidance, so we have work within those rules.
In simplest terms, CP is the point at which the forces exerted on the rocket by the fins come together. The CP can be changed by fin size, shape , and location.

If you have a cluster rocket with the motors canted so that the thrust vectors come to the same point, and one of them fails to ignite, the rocket will still fly more or less straight, as long as the CG is forward.
I am not an expert, so someone may chime in with a bunch of math, but simple works for me so far.

BTW, looking at the flight data in the first attachment, I would change to a D12-5......

Yes, I modified the main CG and mass in the simulation. Switching to a D12-5 also removed the warning. Thank you. I get the gist now which makes perfect sense. My last question is how is CP calculated. If the sim software says one thing, and my the spec sheet says another and both the OR file and spec sheet came from MadCow, which one is right. They are almost 2" different. which makes a big difference with the motor in it.

 

[1tree]

Answering the "What do I really need to know about CP and CG and why are they so important?"

Not to offend if you already know, but let's start with definition:

CP or Center of Pressure is the point at which if you suspend the rocket in moving air it doesn't turn either direction.

CG air Center of Gravity is that point at which the rocket will balance.

Now for the interaction - generally you want the CG at least one body diameter in front of the CP. thus if you suspend the rocket there it will turn much like a weather vane. So as the rocket clears the rod (or rail) it will want to fall turning about the CG. But by this time there is enough forward movement that it is like wind keeping it pointing in the direction of travel. And this keeps the rocket traveling in a straight direction (up).

Generally we measure stability in terms of "caliber" which is the same as the diameter. Having said that, the more "stable" (or further forward the CG is from CP), the more likely the rocket is to weathercock into the wind. This can be compensated for by ensuring the rocket is going faster when it clears the rod.

 

[Coop]

When I'm running a sim, I configure the internals (shock cord, chutes, etc.) as far aft as centering rings and bulkheads permit --give me a "worst case" (also read: realistic) scenario by which to measure the CG. I'll also try different motors to get the heaviest motor which negatively affects the CG (usually a smokey). This way, if nose weight needs to be added, I can do so with confidence that it'll be adequate for anything I'm likely to run.

I would not sweat too much your built weight vs the manufacturer's listed weight. Techniques and materials vary from builder to builder and we generally learn to build lighter (when we need to) as we gain experience. You may require more nose weight to drag your CG forward, but... eh. No real worries --even if it's real heavy, there's high thrust motors in every diameter nowadays...


Later!

--Coop

 

[markkoelsch]

This rocket would qualify as a short and stumpy rocket. There is a tweak to RS, and also OR that covers this. Basically, both sims underestimate the base drag. If you can add that back in you stability will improve.

Do a search for Rocksim short and fat rockets.

 

[Nytrunner]

To try and she'd some light on why all your CP results are different; the actual resulting Center of Pressure on a rocket in flight is calculated by integrating the pressure distribution and shear forces acting on the Rocket's surface. That is not a trivial thing.

To make it easier for hobbyists, the Barrowman Equations are super simplified reductions of the CP calcs that are best suited for "General" rocket geometry (ie, 3 or 4 fnc, roughly 10:1 Length/Width). Openrocket used the Barrowman simplifications. Rocksim makes fewer assumptions and goves a more accurate (less conservative CP position. Yours is shorter than 10:1, and as mark stated, Base-drag isn't accounted for accurately.

 

[redleder]

Thanks everyone for all the great information. Very helpful.