Originally posted by rstaff3
I've always believed the CP of a pyramid should be 1/3 of its height above the base, and the CG can be tested against this. After all the discsussion on aerodynamics, I now question if that is true. I mean I still believe that that is a safe relationship, but am not sure if the CG can move behind that point.
One good think about draggy designs...they aren't going to far!
I concur with Dick. I have flown many variations of a 4 sided (4 sides and a square base) pyramid and a good rule of thumb is the motor should be not stick out below the bottom edge of the sides. This is assuming no nose weight.
If the pyramid is built from light weight materials such as cardstock, foamboard or corrigated cardboard, the CP of a pyramid is about 2/3 from the nose and the CG is behind that because of the weight of the base. Whenever possible the base should be recessed into the pyramid some as you did in you design. The motor's CG is about at it's middle, so to bring the overall CG of the rocket toward the nose, the motor should be as far forward as possible.
From the pictures, it looks like you must have put some sort of motor block in the motor mount. If the 6" motor could slide all the way up it should be far enough forward to fly safely.
I never use any form of internal motor block if I can help it because it always comes back to bite me in the a** when I want to fly a bigger motor, and I ALWAYS want to fly a bigger motor.
As the old saying goes, "I'll fly any rocket again, on a bigger motor, that don't crash the first time on a littler one."
The best way I have found to hold a motor in place is with a 1/4" - 1/2" masking tape thrust ring around the base of the motor and not a motor block up the tube. About 3 wraps of tape is usually all that is necessary.
One other point. Do not assume that a Tetrahedron (3 sides and a triangular base) will be stable just because a 4 sided pyramid of similiar dimensions is stable. For some reason the CG has to be more forward with a Tetrahedron than with a Pyramid. The base drag effect seems to be less.
The choice of motor for a draggy design is a whole nother subject.
BTW: What do the letters stand for on the rocket in the pictures?