Airplane Drag Flying

Posted by Admin on

The myriad forces acting upon an airplane in flight may be simply depicted as the basic four: lift, gravity, thrust, and drag.

In order for a plane to get off the ground, the lift it is generating must be greater than the weight of the plane, or the pull of gravity. Once in the air, gravity is still trying to pull the plane back to the ground, so lift must be maintained for it to stay at a given altitude. When lift is reduced, such as in a descent, the plane loses altitude as a result of the force of gravity. When the plane is not accelerating up or down, the forces of lift and weight are equal.

Thrust moves the plane forward. Whether thrust is generated through a propeller, jet engine, rocket, or even the gentle glide of powerless flight, the ever-present need for it in sustaining flight is the same. Thrust opposes drag and the two forces balance when the airplane sustains a constant speed. Acceleration or deceleration depend upon a variance of thrust with respect to drag.

Drag is the resistance of the air to a body moving through it, and it comes in several types. The two most basic types of drag are parasite drag and induced drag. Parasite drag is the combined effects of form drag and skin friction. Form drag is the result of the air displacement due to the physical shape of the airplane. It can be minimized through careful design and streamlining, giving most airplanes, especially gliders, their characteristically beautiful shape. Skin friction drag is caused by the resistance of airflow across the surface of a body and can be reduced by making the skin surfaces as smooth as possible, and especially by making the airplane as small as possible, which reduces total surface, or wetted area. Parasite drag increases with the square of the increase in airspeed. For example, if you double your airspeed, the parasite drag of the plane will increase to four times the amount that was present at the original airspeed.

Induced drag is the other half of the pie. Induced drag is present whenever the airplane is generating lift, indeed, the drag is lift induced. This drag is due to a characteristic rearward inclination of the lift vector, a natural by-product of the air displacement created by the wing as it flies.

Unlike parasite drag, which increases as airspeed increases, induced drag varies inversely as the square of the plane’s airspeed. In this case, the slower an airplane is flying, the greater the induced drag it generates. This means that induced drag is highest when the plane is flying slowest, such as during landing, and much less at normal cruise speeds.

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