define Center of Force , Neutral Point and Centre of Gravity , with reference to Aircraft?

Answer 1

Hi. Start with this site... http://www.rc-airplane-world.com/how-airplanes-fly.html
Center of Force is easy to understand on a single engine propeller plane. It is the engine center line. Center of gravity is the point in the plane where weight in front equals weight in back. (In a small plane it is about at the pilots hips.) The neutral point is "A point where two fields are equal in magnitude and opposite in direction so that the net field is zero."

Edit. Also from the web... "All airplanes become more stable as the CG is moved forward and less stable as it is moved aft. There is a limit, though, to how far aft the CG can move before the airplane becomes unstable. We call this point the "aft CG limit," or "neutral point." It is called the neutral point because, with the CG at this point, the airplane is neutrally stable in pitch. If the CG is kept forward of the neutral point, the airplane will be stable. If it moves aft of the neutral point, it will be unstable."

Answer 2

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The CG is a variable point in the longitudinal axis. Theres a large methodology to calculate the forward and backward position of the CG. This is a safe range near the center of pressure that obviously is different for each aircraft. In the majority of cases, the CG is a little behind the Center of Pressure (where the lift acts). Remember that the lift generates an aerodynamic moment that tends to pitch the aircraft. So the CG behind the CP create a moment that offsets the aerodynamic moment. But if we increase the lift, the aerodynamic moment also increases, but the aircraft weight cannot increase to get the equilibrium!!! Then, what happen??? Thats when the tail acts (horizontal surface) this is used to control the pitch moment in the aircraft to reach the equilibrium. Thanks!