What causes drag?

Answer 1

Answer: turbulated or swirling air behind the vehicle

Answer:

Drag is the generic term for friction caused by the interaction of a fluid or gas on a fluid or solid. Drag is somewhat related to the materials interacting (as regular solid on solid friction is), a great portion related to shape, and slightly related to by temperature. Surface area of the substances directly correlates to the amount of drag experienced.

A flat thin surface placed so largest face is towards the flow of fluid, it has all it's area in contact with the fluid and if the force is perpendicular to the surface, and drag is maximized. Rotate the surface and you change the angle the fluid hits the surface and how much is imparted to it. Turn it sideways (90' from flat) and you have the least amount of drag: initial surface area is dropped by 2 to 1000's of times. The thicker the second surface is, the less drag is reduced by rotating the surface. This is called "angle of attack" when it comes to wings and air. Angle of attack changes both friction and lift.

A wing (on a plane) is in an offset teardrop shape because (what I was taught in school but read somewhere is not quite right ..) the flow speeds over the wing need to be different to create the force of lift (vertical pressure.) The distance the fluid has to move in the same time increases with more offset. The bigger the difference in flow speeds, the greater the lift (to a limit of course.) BUT. Since an increase in surface area in contact equals an increase in friction, the wing produces lift but "fights" against forward movement: induces drag.

Design of the wing, the body, the control surfaces, even the engine all change flow characteristics of air over a plane, and therefore change the drag generated. If there were no drag, much less powerful engines would be needed to over come drag.

Gas and liquid flows come in two generic types: laminar and turbulent. Laminar is seen in the middle of a river - with no obstructions the fluid all travels in orderly layers and in straight lines. Remember an object in motion tends to stay in motion in the same direction unless acted on by an outside force. Turbulent flow is what you see when the laminar flow hits and object. The friction between the object and the fluid causes the fluid to "stick" around the object and clashes with the laminar flow from the rest of the fluids. These cause eddys and whorls (spinning vortices) in the fluid and increase the drag felt by the fluid.

Subsonic (less than the speed of sound, ~700mph at sea level) planes use the old offset teardrop shape to generate large amounts of lift and because the speed forward is not near as important with high speed planes (generally military) they want slower laminar flow over most surfaces to increase mileage and control by the crew. Trans (around) and Supersonic planes have wings designed differently because at sonic speeds the flow changes dramatically and what was efficient now has lost the ability to generate lift, prviously laminar flow is now turbulent (often because the layers are packed so very closely by the plane flying through them.) All planes are designed to increase laminar flow of air over them, they have to do it in different ways due to the characteristics of air flow at different speeds.

Not all turbulent flow is bad - a lot of planes have small "tiny rudders" on their wings at different places to help direct the laminar flow as wanted - like small orderly placed rocks in a stream, these appendages create small amounts of turbulence to control a larger laminar flow.