by Jonathan M. Stern
Forces Acting on the Airplane
Four physical forces act on an airplane in flight (see Figure 2.12):
Figure 2.12. The four forces are thrust, drag, lift, and weight.
Thrust is created by the airplane's engine and propeller.
There are two types of drag: parasitic and induced. Parasitic drag results from the friction created by the air striking and flowing over the airplane. For example, parasitic drag is created when a passenger in an automobile sticks a hand out the window while the car is moving.
Induced drag is an unwanted by-product of lift creation. The greater the angle of attack, the greater the induced drag. The induced drag results from a component of lift that acts in a rearward direction.
For a given weight, the faster the airplane moves through the air, the lower the angle of attack necessary to maintain level flight. Accordingly, induced drag decreases as speed increases and increases as speed decreases.
Conversely, parasitic drag increases as speed increases. At typical cruise speeds, parasitic drag increases at a faster rate with increases of speed than induced drag decreases. Accordingly, in the range of typical cruise speeds, total drag increases with increases in airspeed. As the airplane is slowed near and approaching stall speed, decreases in airspeed actually result in greater amounts of total drag.
When the airplane slows below the point where this transition occurs, it actually takes more power to fly slower. In aviation parlance, this regime is known as the back side of the power curve.
The Earth's gravity pulls an airplane in flight toward the surface. The degree of this force is a function of the airplane's weight. Lift must counteract weight. When lift exceeds weight (assuming that the lift is produced in a direction opposite the Earth's surface), the airplane climbs. When upward lift equals weight, the airplane maintains altitude. When gravity exceeds lift, the airplane descends.
An airplane flying straight and level at a constant airspeed is in a state of equilibrium. Lift equals weight, and thrust equals drag. Vary any of these four forces and altitude and/or airspeed change.