The tendency of a body to remain in its condition of rest or motion is called inertia. Equilibrium is the absence of acceleration, either linear or angular. Equilibrium flight exists when the sum of all forces and the sum of all moments around the center of gravity are equal to zero. An airplane in straight and level flight at a constant velocity is acted upon by four forces: thrust, drag, lift and weight. When these forces exactly cancel each other out, the airplane is in equilibrium.
Trimmed flight exists when the sum of all moments around the center of gravity is equal to zero. In trimmed flight, the sum of the forces may not be equal to zero. For example, an airplane in a constant rate, constant angle of bank turn is in trimmed, but not equilibrium, flight. An airplane in equilibrium flight, however, is always in trimmed flight.
An airplane does not have to be in straight and level flight to be in equilibrium. Figure 1-1-4 shows an airplane that is climbing, but not accelerating or decelerating, i.e., there are no un- balanced forces. It is another example of equilibrium flight. Thrust must overcome drag plus the parallel component of weight. Lift must overcome the perpendicular component of weight.
An airplane with sufficient thrust to climb vertically at a constant true airspeed can achieve an equilibrium vertical flight condition. Thrust must equal weight plus total drag, and lift must be zero (Figure 1-1-5).
An airplane does not have to be in straight and level flight to be in equilibrium. Figure 1-1-4 shows an airplane that is climbing, but not accelerating or decelerating, i.e., there are no un- balanced forces. It is another example of equilibrium flight. Thrust must overcome drag plus the parallel component of weight. Lift must overcome the perpendicular component of weight.
An airplane with sufficient thrust to climb vertically at a constant true airspeed can achieve an equilibrium vertical flight condition. Thrust must equal weight plus total drag, and lift must be zero (Figure 1-1-5).
