Drag is the resistance to motion through a fluid. As applied to aircraft and spacecraft passing through the atmosphere, it is the component of the resultant force due to relative airflow measured parallel to the direction of motion. It is directly opposed to thrust. Lift is one of the four forces of flight acting on an airplane, the others being weight, thrust, and lift.
Theory of dragDrag is generated by nine conditions associated with the motion of air particles over an aircraft. There are several types of drag – form, pressure, skin friction, parasite, induced, and wave – which are described below.
Form and pressure dragForm drag and pressure drag are virtually the same type of drag. Form or pressure drag is caused by the air that is flowing over the aircraft or airfoil. The separation of air creates turbulence and results in pockets of low and high pressure that leave a wake behind the airplane or airfoil (thus the name pressure drag). This opposes forward motion and is a component of the total drag. Since this drag is due to the shape, or form of the aircraft, it is also called form drag. Streamlining the aircraft will reduce form drag, and parts of an aircraft that do not lend themselves to streamlining are enclosed in covers called fairings, or a cowling for an engine, that have a streamlined shape. Airplane components that produce form drag include (1) the wing and wing flaps, (2) the fuselage, (3) tail surfaces, (4) nacelles, (5) landing gear, (6) wing tanks and external stores, and (7) engines.
Skin friction drag
Parasite dragParasite drag is simply the mathematical sum of form drag and skin friction drag.
Total drag at subsonic speedsAll of these types of drag must be accounted for when determining drag for subsonic flight. The total drag is the sum of parasite and induced drag.
But the net (or total) drag of an aircraft is not simply the sum of the drag of its components. When the components are combined into a complete aircraft, one component can affect the air flowing around and over the airplane, and hence, the drag of one component can affect the drag associated with another component. These effects are called interference effects, and the change in the sum of the component drags is called interference drag. Thus,
Generally, interference drag will add to the component drags but in a few cases, for example, adding tip tanks to a wing, total drag will be less than the sum of the two component drags because of the reduction of induced drag.
Interference drag can be minimized by proper fairing and filleting, which induces smooth mixing of air past the components. No adequate theoretical method will predict interference drag; thus, wind tunnel or flight-test measurements are required. For rough computational purposes, a figure of 5 percent to 10 percent can be attributed to interference drag on a total aircraft.
Small items also add to the total aircraft drag and, although seemingly trivial, they can greatly reduce the aircraft's top speed.
Although prediction of drag and wind tunnel drag measurements of models yield good results, final drag evaluation must be obtained by flight tests.
Wave drag and supersonic flightWave drag occurs in supersonic flight, or flight above the speed of sound. Wave drag is a form of pressure drag. When an aircraft breaks the speed of sound, a shock wave is created. A shock wave is a strong pressure wave that creates a violent change in pressure. High pressure pushes on the front of the aircraft. This results in a large pressure drag toward the rear of the aircraft like that produced with form or pressure drag in subsonic flight.
The airplane's total drag determines the amount of thrust required at a given airspeed. Thrust must equal drag in steady flight.
Lift and drag vary directly with the density of the air. As air density increases, lift and drag increase and as air density decreases, lift and drag decrease. Thus, both lift and drag will decrease at higher altitudes.
The equation used to calculate drag is:
Where ρ is the density of the air, V is the velocity of the air (air speed), S is the surface area of the aircraft, and CD is the coefficient of drag.
Related entry drag coefficient
Related category AERODYNAMICS AND AERONAUTICS
Source: U.S. Centennial of Flight Commission
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