Effect of Fins on Bottle Rockets and Notes on Fins
The goal of the fins is to insure the stability of the rocket, by pulling the center of pressure (CP) at about 1 diameter of the rocket under the center of gravity (CG). To do that, we can act either on the area of the fins or on their position toward the center of gravity. The lower the fins will be, the less the area of the fins will be important.
Minimizing the area of the fins is important because, if they enhance the stability of the rocket, the fins have a negative influence on the performances. They increase the weight, the coefficient of drag and the frontal area towards the air penetration.
Drag is the sum of all the aerodynamic or hydrodynamic forces in the direction of the external fluid flow. It therefore acts to oppose the motion of the object, and in a powered vehicle it is overcome by thrust.
More of the rocket's mass should be concentrated at the front end of the rocket, while the fins are positioned as far aft as possible to maximize their effectiveness as a stabilizing device. Such an arrangement is stable because the fins are behind the center of rotation where their actions will force the rocket to stay pointed into the direction of air flow.
Any time turbulence begins to cause a stable rocket to rotate and veer off its direction of flight, the fins will develop lift forces -- because of the angle of attack they acquire relative to the airflow -- that will clock the aft end of the rocket back about the rocket's Center of Gravity until the nose and fins are realigned with the direction of air flow again.
The bigger the fins are , the...