Thrust And Drag
An airplane does not fly, of course, simply by setting it out on the runway and waiting for a strong wind to blow over its wings. Instead, the airplane is caused to move forward, forcing it to rush through still air at a high rate of speed. The forward thrust for the aircraft comes from one of two sources: a rotating propeller blade powered by some kind of engine or a rocket engine. The propeller used to drive the Wrights' first flight at Kitty Hawk was a home built engine that weighed 180 lb (82 kg) and produced 180 horsepower.
The forward thrust provided by a propeller can be explained in exactly the same way that the lift of a wing can be explained. Think of a propeller as a short, narrow mini-wing pivoted at the center and connected to an engine that provides rotational motion. The mini-wing is shaped like a larger wing, with a convex forward surface and a flat back surface. As the mini-wing is caused to rotate around its pivot point, the wing sweeps through the air, which passes over the forward surface of the mini-wing faster than it does over the back surface. As a result, the pressure on the forward surface of the mini-wing is less than it is on the back, and the mini-wing is driven forward, carrying the airplane along with it.
The pitch and angle of attack of the mini-wing—the propeller—can be changed just as the angle of attack of the airplane's main wings can be changed. During level flight, the pitch of the propeller is turned at a sharp angle to the oncoming airflow, allowing the aircraft to maintain air speed with minimal fuel consumption. During takeoff and landing, the pitch of a propeller is reduced, exposing a maximum surface area to airflow and attaining a maximum thrust. At landing, the propeller direction can actually be reversed, causing the direction of force on the propellers to shift by 180°. As a result, the propeller becomes a brake on the airplane's forward motion.
The term drag refers to a variety of factors, each of which tends to slow down the forward movement of an aircraft. The most obvious type of drag is friction drag, a retarding force that results simply from the movement of a body such as an airplane through a fluid. The amount and effect of friction drag are very much a function of the shape and design of the aircraft.
Perhaps the most complex type of drag is that caused by the very air movements that lift an aircraft off the ground. The discussion of the Bernoulli effect above assumed that air flows smoothly over a surface. Such is never the case, however. Instead, as air travels over an airplane wing, it tends to break apart and form eddies and currents. The interaction of these eddies and currents with the overall air flow and with the airplane wing itself results in a retarding force: induced drag. One of the great challenges facing aeronautical engineers is to design aircraft in which all forms of drag are reduced to a minimum.
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