I've seen referances on the forum about a card board cut out method. Can someone please explain what it is, how do you use it and how do you make it?
I would add to the excellent information provided here by others, that the cardboard cutout method provides the CENTER OF LATERAL AREA of the rocket. When you cut out the rocket's silouette, the balance point is the center of lateral area. As someone pointed out above, this is where the CP would be if the rocket were pointed 90 degrees to the airstream, in other words, pointed completely sideways to the direction of flight. That's about as much of a 'worst case scenario' as you can get! Therefore, the CP derived from the cardboard cutout method is about as conservative and 'worst case' as you can get.
Once you have the balance point of your rocket from the cardboard cutout method (center of lateral area) the actual rocket can be loaded with the desired rocket motor, wadding, chute, etc. so it's flight ready (less ignitor) and balanced using a loop of string slid along the body until the rocket balances horizontally, which determines the Center of Gravity, or CG. This is the ACTUAL CG of the rocket. Measure from the end of the body tube to this balance point, and compare to a similar measurement made of the cardboard cutout-- if the CG is at least one body diameter ahead of the center of lateral area, you should have a good stable rocket.
Center of lateral area DOES give you an over-conservative "CP" location usually, which is why rockets designed by cardboard cutout methods tend to be overstable. Still, that's better than understable, (neutrally stable, or completely unstable!) That's where Rocksim or some of the other programs using their methods or the Barrowman Equations come in handy... they get you actually closer the real CP point.
CP varies in flight, it moves around based on the outside conditions of the rocket in it's flight, and the air it's moving through. One important thing to remember is that, generally speaking, the CP tends to move FORWARD the slower the rocket is travelling, or the greater the angle of attack of the airstream it's moving through. That's why rockets tend to become more stable as they pick up speed during flight, and also why high angles of attack like rod-tipoff (rod whip) and gusts of wind and other things can make rockets go unstable or do other unwanted things at slower airspeeds (like just after liftoff). That's why you need at least one caliber (body diameter) stability, so account for the CP moving forward.
Fortunately with solid-propellant rockets, the CG also moves forward as the propellant burns, so this helps offset the CP movement effect. (Incidentally, liquid fuelled rockets are the opposite-- the CG moves REARWARD as the propellant is expended and is getting to lower levels in the tanks-- helpful to know if you're flying Cold Power rockets, as some folks on here are!)
Good luck and hope this helps! OL JR