Atmospheric Flight

Drag

Aerodynamic drag is the force acting on an aircraft in the direction of the wind, or opposite to the flight direction. There are several causes for drag on an aircraft. The most basic source of drag is the friction of the air flowing on the surface of the vehicle. Air, and all fluids, have a resistance to sliding on a surface. This resistance comes from viscosity, a tendency for the fluid to resist the shearing or sliding of one layer of fluid past another. Because of the viscosity, the layer of fluid closest to the surface acts as if it sticks to the surface. Each layer of fluid on top of that slides a little bit on the layer below it. It tries to stick, so it takes a force to keep it sliding. Outside the thin layer, the fluid is moving at the full speed of the flow away from the body, and there is no more friction effect. The thin layer of fluid where the friction forces act is called the boundary layer. The flow of an imaginary fluid with no viscosity is called and inviscid flow. The flow of a real fuild with viscosity is called a viscous flow.

We can measure the speed of flow at different points inside the boundary layer.

The accumulation of this viscous friction force over the whole aircraft is called skin friction drag, or viscous drag.

Another source of drag is called pressure drag. The section on lift discusses what happens if the angle of attack is increased too much, and the airflow can no longer smoothly follow the wing's surface. In this case, the airflow separates from the surface and forms a large pocket of recirculating fluid. The flow around the outside of the pocket eventually matches speed with the free stream flow, but near the pocket, the flow speed may be rather fast to go around the pocket, so the pressure may be fairly low. There is some lift produced by the low pressure in this pocket, but mostly, the low pressure acting on the inclined surface creates drag. Whenever there is a region where the surface of the aircraft is directed backwards and it has low pressure acting on it, then there is pressure drag. Normally, well designed aircraft have very little pressure drag.

A third source of drag is related to the process of making lift on the wing. This is called induced drag. Energy is lost in the process of making lift because of the airflow around the wingtips. The pressure difference between the upper and lower surface of a wing cannot be maintained near the tip, because the flow leaks around the edge of the wingtip. This leakage creates a tip vortex that trails off behind the wing.

You can see this vortex when the plane flies through smoke.

There is a drag force associated with the energy lost in the tip vortices, called induced drag. If the lift is spread out over a longer wingspan, the effects of the wingtip are reduced so there is less energy lost in the tip vortices. Long slender wings like those on a seagull or a sailplane are called "high aspect ratio" wings, and are much more efficient at making lift without very much induced drag. This is expressed as lift-to-drag- ratio (L/D). Low aspect ratio wings, like on a crow or a fighter airplane, have much more induced drag.

There is one more source of drag that has to do with the compressible nature of air in the atmosphere. When flying near the speed of sound, the air becomes compressed near the nose of the airplane. The pressure is higher in that region than it would be for slow flow. Also, when the flow accelerates to high speed and low pressure over the top of the wing, it may actually get moving faster than the speed of sound. This pocket of supersonic fluid eventually must slow down, and when it does, it forms a shock wave. The energy lost in the process of compressing the flow through these shock waves is called wave drag. When flying near the speed of sound, or faster, there is also a pressure drag from the energy lost in compressing the air as it flows around the aircraft.

As the aircraft moves through the air it makes pressure waves. These pressure waves stream out away from the aircraft at the speed of sound. This wave acts just like the ripples through water after a stone is dropped in the middle of a still pond. At Mach 1 or during transonic speed (Mach 0.7 - 0.9), the aircraft actually catches up with its own pressure waves. These pressure waves turn into one big shock wave. It is this shock wave that buffets the airplane. The shock wave also creates high drag on the airplane and slows the airplane's speed. As the airplane passes through the shock wave it is moving faster than the sound it makes. The shock wave forms an invisible cone of sound that stretches out toward the ground. When the shock wave hits the ground it causes a sonic boom that sounds like a loud thunderclap.

The energy lost in the process of compressing the airflow through these shock waves is called wave drag. This reduces lift on the airplane.

The amount of drag depends on:

1. the size of the aircraft
2. details of the shape and smoothness of the aircraft
3. the lifting efficiency of the wing, related to the wing "aspect ratio"
4. the dynamic pressure (density and speed)

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