ABSTRACT
Associated with an equilibrium (or trim) state is the notion of its stability. At the equilibrium state, of course, all the forces and moments balance each other. So, an object in equilibrium can, in principle, remain in a state of equilibrium forever. However, in practice-and this is true for all natural systems-there are disturbances. For an airplane in flight, this may, for example, be due to wind or due to pilot input. Disturbances displace a system from its equilibrium state. The question then arises, what happens to the system after it is disturbed? Does it regain its equilibrium state or not? This leads us to the notion of stability.
