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ThrustThrust is the forward-acting force which opposes drag and propels the airplane. In most general aviation airplanes, this force is provided when the engine turns the propeller. The same physical principles involved in the generation of lift also apply when describing the force of thrust. As explained in the lift section, Newton's second law states that an unbalanced force, F, acting on a mass, m, will accelerate, a, the mass in the direction of the force (F=ma) In the case of airplane thrust, the force is provided by the expansion of the burning gasses in the engine which turns the propeller. A mass of air moves through the propeller, a rotating airfoil, and is accelerated opposite to the direction of the flight path. The equal and opposite reaction illustrated by Newton's third law is thrust, a force on the airplane in the direction of flight. During straight-and-level, unaccelerated flight, the forces of thrust and drag are equal. You increase thrust by using throttle to increase power. When you increase power, thrust exceeds drag, causing the airplane to accelerate. This acceleration, however, is accompanied by a corresponding increase in drag. The airplane continues to accelerate only while the force of thrust exceeds the force of drag. When drag again equals thrust, the airplane ceases to accelerate and maintains a constant airspeed. However, the new airspeed is higher than the previous one. When you reduce thrust, the force of drag causes the airplane to decelerate. But as the airplane slows, drag diminishes. Once again, it maintains a constant airspeed. Now, however, the airspeed is slower than the one previously flown. |
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This page was last modified June 11, 2000 |
