Aerodynamics - Four Forces

..	Weight - Lift - Thrust - Drag

Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight. In steady flight, the sum of these opposing forces is always zero. .. .. ..

	Thrust
		The forward force produced by the powerplant/propeller. It opposes or overcomes the force of drag. As a general rule, it acts parallel to the longitudinal axis. For an aircraft to start moving, thrust must be exerted and be greater than drag. The aircraft continues to move and gain speed until thrust and drag are equal. In order to maintain a constant airspeed, thrust and drag must remain equal, just as lift and weight must be equal to maintain a constant altitude. If in level flight, the engine power is reduced, the thrust is lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate. To a point, as the aircraft slows down, the drag force will also decrease. The aircraft will continue to slow down until thrust again equals drag at which point the airspeed will stabilize. If the engine power is increased, thrust becomes greater than drag and the airspeed increases. As long as the thrust continues to be greater than the drag, the aircraft continues to accelerate. When drag equals thrust, the aircraft flies at a constant airspeed.
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You can read more about the concepts above in the Pilot’s Handbook of Aeronautical Knowledge. Chapter 4: Principles of Flight Chapter 5: Aerodynamics of Flight