Balance and stability

[Thruster]

With Airplanes the CG location is critical for controllability. If it's to far forward of the CP the plane is sluggish and hard to control, if it's behind the CP the plane is uncontrollable and will crash. Now I understand that the CG has to be in front of the CP for stability in a Rocket and I understand the math in figuring to CP is pretty involved. So it is safer to load the nose with a little weight to insure stability, what is the best way to locate the CG on a rocket. With RC planes you can lift the plane with your fingers at the wing tips and when it balances level that is where the CG is. Than you add or move weight so that the CG is where it should be.
So how do you balance these rockets do you hold the Tube with your finger or do you hang the rocket from a string ?

[barone]

With Airplanes the CG location is critical for controllability. If it's to far forward of the CP the plane is sluggish and hard to control, if it's behind the CP the plane is uncontrollable and will crash. Now I understand that the CG has to be in front of the CP for stability in a Rocket and I understand the math in figuring to CP is pretty involved. So it is safer to load the nose with a little weight to insure stability, what is the best way to locate the CG on a rocket. With RC planes you can lift the plane with your fingers at the wing tips and when it balances level that is where the CG is. Than you add or move weight so that the CG is where it should be.
So how do you balance these rockets do you hold the Tube with your finger or do you hang the rocket from a string ?
With a motor installed, I balance with a loop of string and a piece of tape (when practical). If the CG point seems questionable (further aft than one would suspect), I'll do a swing test (heck, it's already got the loop of string taped in place.....) :D On unproven designs, do a swing test.

[Shreadvector]

http://www.rocketryforum.com/showthread.php?t=41082

With Airplanes the CG location is critical for controllability. If it's to far forward of the CP the plane is sluggish and hard to control, if it's behind the CP the plane is uncontrollable and will crash. Now I understand that the CG has to be in front of the CP for stability in a Rocket and I understand the math in figuring to CP is pretty involved. So it is safer to load the nose with a little weight to insure stability, what is the best way to locate the CG on a rocket. With RC planes you can lift the plane with your fingers at the wing tips and when it balances level that is where the CG is. Than you add or move weight so that the CG is where it should be.
So how do you balance these rockets do you hold the Tube with your finger or do you hang the rocket from a string ?

[Thruster]

Thanks Fred, great link I have a lot of reading to do tonight. :)

[CPMcGraw]

...what is the best way to locate the CG on a rocket?...

The balance point of any rocket is...wherever it balances. I know, a cheesey answer, but remember that the CG will change location as the model flies. It is not static. The CG of an empty model is different than that of one loaded with a motor. As the fuel burns off, that CG shifts forward.

Big difference between balancing an airplane and balancing a rocket: You balance the airplane dry (fuel tank empty) as you want the airplane to still glide home after the engine dies. In a rocket, you have to balance with a motor installed (fuel tank full), because that represents the heaviest the model will be during the flight, and the farthest rearward the CG will be.

It's not as much the initial location of the CP or CG, as it is the distance between the CG and CP, which is important; and that the CG is always in front of the CP by at least one body tube diameter (usually the largest body tube along the centerline). This is what you see some of us talk about in the BARCLONE threads, the Stability Margin (or, Static Margin) of the model. As long as that margin is 1.00 or better, the model will typically fly well.

When you get margins over 2.00, you begin to get into an overstable condition. The model weathercocks severely, and may not reach a safe flying velocity off the end of the rod. It's the same as an airplane being nose-heavy.

Keep your models inside that CG/CP range of between 1.00 and 2.00, and they should all fly reasonably well.

[zixxerboy]

Iasked a similar question in "Ask the Docter".

I have just completed building my first two rockets. One is an Este “Hi-Flier” and the second is a scratch built. I have yet to launch them.

I too, am struggling with the swing test to confirm their stability. The amount of clay jammed into the nose cone to balance them seems excessive. The Este production rocket should be pretty close but looks like it will need a clump of clay about the size of two regular dice.

The Estes Tech Manual I downloaded says to install an engine before testing – but somewhere else I thought I had read that a spent motor casing should be used. That would cut down of the ballast I need.

Thanks

[rocketguy101]

Iasked a similar question in "Ask the Docter".
The Estes Tech Manual I downloaded says to install an engine before testing – but somewhere else I thought I had read that a spent motor casing should be used. That would cut down of the ballast I need.

Thanks
The motor should be fully loaded. Think about it: when do you want your model to be stable, at launch or at burnout?

Also your rocket should be "flight ready" i.e.: parachute installed, wadding, etc.

[Tom Swift]

I have just completed building my first two rockets. One is an Este “Hi-Flier” and the second is a scratch built. I have yet to launch them.

I too, am struggling with the swing test to confirm their stability. The amount of clay jammed into the nose cone to balance them seems excessive. The Este production rocket should be pretty close but looks like it will need a clump of clay about the size of two regular dice.


Many people have noted that the Hi Flier is NOT stable with a C (or even B) motor loaded. I seem to recall that a half ounce of clay in the nose is the recommended fix. About 2 dice-worth should do it, but please search the forums to confirm.

EDIT - Just found the other thread you mentioned. More info there.