Equilibrium

Newton’s First Law of Motion states that an object at rest or moving in a straight line will remain at rest or continue to move in a straight line until acted on by some other force. When an aircraft is flying straight and level at a constant speed, it is in a state of equilibrium, should thrust increase, it will begin to accelerate until drag equals thrust again, at which point the aircraft will have reached a new equilibrium at a higher speed. Similarly, if thrust is decreased while maintaining straight and level flight, the aircraft will begin to decelerate until the total drag is once again equal to the total thrust. It may be tempting then to conclude that an aircraft is in a state of equilibrium when thrust is equal to drag, and similarly when lift is equal to weight. While true in some instances, it is more correct to say that an aircraft is in equilibrium when the sum of all horizontal forces is zero and the sum of all vertical forces is zero. This will become more evident in the coming lessons.