Wiggy,
For an object to be in equilibrium, it has to verify two conditions:
1). Sum of forces = 0
Here we work in two dimensions so we consider only the forces along an X axis (longitudinal axis of the aircraft) and an Y axis (which is the lateral axis of the aircraft).
Sum of Fx = 0
Sum of Fy = 0
2). Sum of moments = 0
We consider here the moment around CG.
We can always apply the forces at the CG of an object if we consider the moment of each force.
We can perfectly well have an object with a total moment = 0 so it doesn't rotate. But the sum of the forces is not zero so it undergoes a translation motion.
By the way, if you cut the shape of a basic aircraft in a sheet of paper, put it on a table, then you push forwards with your finger its left wing (this simulates the thrust of the engine alive), and at the same time you prevent the rotation of the aircraft by pushing the tail to the right (this simulates the force of the rudder), the aircraft is drifting ! But it doesn't rotate. Try it and tell me your conclusions !
Last edited by manucordier; 22nd Jul 2011 at 17:04.