I am currently learning about fins and the role they play in the stability of a rocket during flight. I came across a relatively minor problem.
Here is one of the resources I'm looking at: http://www.nakka-rocketry.net/fins.html
My question is, why does the lift force change direction when the center of pressure is located above the center of gravity? Apologies if it's a bad question but I haven't covered anything related to lift or drag in a very long time.
In Fig. 1A, the rocket is shown during the powered flight This is an ideal state, with all the forces acting through the CG and no external (perturbing) forces present. The rocket is stable and accelerating with exclusively linear motion along the line of thrust.
In Fig. 1B, a perturbing force is introduced, in this example, the force due to a gust of wind. The resultant of this pressure force acts through the CP, causing the rocket to rotate about its CG, changing slightly the angle of attack (alpha).
This change in angle of attack immediately generates a lift force, acting as shown (normal to the body) through the CP. This force balances the force due to wind, and the rocket remains stable, with its flight path only slightly altered.
Fig. 2A illustrates a rocket with the CP, CG locations reversed, that is, the CP is ahead of the CG. This is an undesirable scenario. In this figure, the rocket is initially stable, being in the same ideal situation as in Fig. 1A, with no perturbing forces present. Along comes a disturbing force, again a gust of wind, as illustrated in Fig. 2B. The wind force acts with its resultant through the CP, again generating a slight rotation, and consequential change in angle of attack. Again, a lift force is generated due to the change in angle of attack, but this time the lift force acts in the same direction as the wind force. The consequence of this is an unchecked rotation of the rocket about its CG, as shown. The rocket becomes unstable, that is, its flight path is no longer linear motion, but rotational motion is introduced. The rocket tries to turn around and fly backward. The thrust force from the motor does not allow this, of course, and so the rocket tumbles out of control.