A. The aircraft is able to rise into the air and to keep in the air because of the forces working on it. The motion itself maintains those forces.
B. When moving in the air, the aircraft produces an upward force which is called lift and acts at right angle to the direction of the air stream. When moving the leading edge of the wing pushes the air out of the way. Part of this air flows rapidly over the wing and part of it flows under the wing, both parts joining behind the trailing edge. The important thing is that due to the curved upper surface the air flowing over the wing travels faster than the air flowing under the more or less flat bottom surface. The air traveling across the top of the wing creates a reduced pressure on the upper surface. The air traveling along the bottom of the airfoil is slightly compressed and develops increased pressure. The difference in pressure between the air on the upper and lower surfaces of the wing produces lift.
C. To produce lift, the airplane wing must move through the air at high speed. This high speed is produced by a force of thrust which is acting in the direction of the airplane’s motion. Both a propeller and a jet engine produce thrust.
D. Drag is the resistance an airplane meets in moving through the air. The faster the airplane moves, the greater will be the drag.
E. In any position of flight the airplane is acted upon by four forces, the last being weight, or gravity, the downward acting force.
F. Lift opposes weight and thrust opposes drag. Drag and weight are forces inherent in anything lifted from the earth and moved through the air. Thrust and lift are artificially created forces used to overcome the forces of nature and enable an airplane to fly. The engine-propeller combination is designed to produce thrust to overcome drag. The wing is designed to produce lift to overcome weight.
G. In straight-and-level unaccelerated flight, lift equals weight and thrust equals drag. Any inequality between lift and weight will result in the airplane entering a climb or descent. Any inequality between thrust and drag while maintaining straight-and- level flight will result in acceleration or deceleration until the two forces become balanced.
H. The lifting power and the drag of a wing depend on the angle of attack, the shape and the size of the wing, density of the air and the speed of the flight.
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