Originally posted by Aerobee300
how do I go about finding the CG CP relationship ?
You can find the c.g. by loading your rocket for flight, including motor and ejection wadding, and balancing it horizontally on a pivot. Measure and locate the c.g. the best you can (this technique can easily turn into a four-handed operation, if you know what I mean).
You can find the c.p., at least very approximately, by making a cardboard cutout of the profile of your model. If it has four fins, rotate them (mathematically) to the 45 degree position and make your cutout pattern based on the 'shortened' fin span. Balance your cutout horizontally on a pivot, and measure and locate the c.p.
The problem with this c.p. method is that it is conservative. It represents the aerodynamic effects of flying sideways. I have not seen too many rockets actually DO that, but the presumption is that if it is stable sideways, it is probably also stable at small angles of attack.
The actual flight behavior of objects, and their consequent stability limits, is just a bit more complicated. A fella named Barrowman did some R&D work for a NAR contest entry, and wrote out a bunch of equations by which stability limits can be calculated/estimated. This material is usually included as one of the back sections of the 'Handbook of Model Rocketry' (if you can get a copy).
Also, the balancing approach requires you to have a rocket built. This is not too handy if you just want to know whether a design is stable BEFORE you spend the time to build it. The Barrowman eqns will help you in this situation, and will give you a reasonable stability estimate even if there are some quirks and limitations to that approach.
To calculate the c.g. of a 'paper' rocket, try the following thread:
If you don't have Excel software on your computer to be able to see some of the attachments, let me know and I will dig them up and change them to some other format for you.