In addition to parasite drag, which is caused by the physical structure of the airplane, airplanes must also overcome “induced drag,” which is an unavoidable byproduct of lift. A wing’s lift doesn’t actually produce lift that is directed straight up. In fact, the lift is directed slightly backward.
Lift is generated in a perpendicular direction to the “chord line,” which is an imaginary straight line connecting the wing’s leading edge to its trailing edge. In flight, the wing is inclined slightly upward in the front, meaning the chord line is inclined at an angle, too. The lift force, therefore, is tilted backward slightly.
To return to high school physics for a moment, a force that is acting at an angle can be mathematically divided into its vertical and horizontal components. Most of an airplane’s lift is directed upward, but it has a backward component as well. That component is the induced drag.
The higher the airspeed, the lower the amount of induced drag. The higher speed increases the wing’s Bernoulli-type lift. The pilot can decrease the angle of attack by pitching the nose slightly downward. When the angle of attack is decreased, the chord line doesn’t tilt upward as much as it does at slow speed, and a flatter angle of attack—a negative angle of attack is possible even at very high speed—shortens the backward component of the total lift, or the induced drag.
By the Book
Thrust is force that must be generated to counteract drag. Some airplanes are equipped with piston engines that turn a propeller. Others are powered by jet engines, which heat large volumes of air by burning kerosene or some other fuel. The heated, expanded air accelerates out of the engine’s exhaust, creating power to drive the airplane forward.
Some large planes, particularly jets, feature angle of attack meters that display the precise angle between the relative wind and the wing’s chord line, but in smaller planes, pilots use airspeed as a rough measure of angle of attack—low speed means high angle of attack and a potential danger of reaching stall speed.
That’s not to say that total drag decreases at high speed. Total drag is very high at low speed, when induced drag accounts for most of it, and decreases as speed increases. But at some speed, which is different for each type of wing, the increase in parasite drag overtakes the decrease in induced drag, and drag increases with speed.