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2.4.  FORCES ACTING ON AN AIRCRAFT

Figure 2.2 shows the four forces that act on an aircraft in flight; they are weight, lift, thrust, and drag. The weight of the aircraft and its occupants, fuel, and cargo must be lifted against the force of gravity. In designing aircraft the lightest and strongest materials possible are used.

Figure 2.2. Four Forces Acting on an Aircraft.

Lift is the force that overcomes gravity. Lift is obtained through the action of air moving past the wings or rotor blades of an aircraft. How to get maximum lift is a major problem in wing and rotor-blade design.

Thrust is the force that puts the aircraft into motion relative to the ground and brings the force of lift into existence. Conventional aircraft are pushed or pulled forward by one or more reciprocating or turbine engine-driven propellers or jet engines.

Drag, the resistance to forward motion, is created by the flow of air over the surface of the aircraft. Figure 2.3 shows how different shaped objects are affected by airflow. The two kinds of drag are induced and moving through the air. While the aircraft is flying, high-pressure air below the wing tends to flow into the low-pressure area above the wing. The two pressures mix at the wing tip and create a vortex (whirlpool). The vortex creates a suction effect at the ends of the wing and causes induced drag that varies directly with the angle of attack. Parasite drag is created by the entire aircraft, excluding induced drag. It is caused by protrusions such as landing gear, rough surfaces, and air striking on the aircraft's frontal surfaces.

Figure 2.3. How Different Objects are Affected by Airflow.

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