BLUEPRINTS: Acting Forces
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Four forces that are constantly acting on a moving aircraft.

Lift

When the wing is tilted at a certain positive angle, called the angle of attack, and moves rapidly, the air is deflected over the upper surface of the wing. In accordance with Bernoulli's principle, the airflow increases and the pressure exerted on the wing decreases over this surface.

This reduced pressure on the wing means that there in a reduction of the force the wing pushes downward on the air beneath it, and the molecules of air. Because the air below the wing has slowed down, the pressure on the underside of the wing increases. Now the pressure on the upper side of the wing is now lower than the pressure on the bottom. Thus, the wing is lifted up.

Weight

Weight is the natural weight of an airplane. This is useful when an airplane is trying to land. The pilot attempts to minimize lift and maximize the weight. Reversing everything that happens during lift can achieve this.

The wing is tilted at a negative angle and thus the air moves rapidly under the bottom surface of the wing. Again, in accordance with Bernoulli's principle, the pressure will decrease along this surface. The pressure will then increase along the upper surface of the wing. This will cause a downward push on the airplane.

Drag

Drag is the natural resistance to airflow. The entirety of the airplane produces some amount of drag. The easiest way to feel drag is to place your hand outside a moving car.

Thrust

Thrust is the force that must be generated by the engine(s) on an airplane in order to overcome the drag of an aircraft. Thrust is to drag, what lift if to weight.
 

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