There are certain laws of nature or physics that apply to any object that is lifted from the Earth and moved through the air. To analyze and predict airplane performance under various operating conditions, it is important that pilots gain as much knowledge as possible concerning the laws and principles that apply to flight. The principles of flight discussed in this chapter are intended primarily for beginning pilots, and are not intended as a detailed and complete explanation of the complexities of aerodynamics.
FORCES ACTING ON THE AIRPLANE IN FLIGHT
When in flight, there are certain forces acting on the airplane. It is the primary task of a pilot to control these forces so as to direct the airplane’s speed and flightpath in a safe and efficient manner. To do this the pilot must understand these forces and their effects. [Figure 1-1] Among the aerodynamic forces acting on an airplane during flight, four are considered to be basic because they act upon the airplane during all maneuvers.
These basic forces are :
• Gravity (Weight)
While in steady-state flight, the attitude, direction, and speed of
the airplane will remain constant until one or more of the basic forces
changes in magnitude. In unaccelerated flight (steady flight) the opposing
forces are in equilibrium. Lift and thrust are considered as positive forces,
while weight and drag are considered as negative forces, and the sum of
the opposing forces is zero. In other words, lift equals weight and thrust
equals drag. When pressure is applied to the airplane controls, one or
more of the basic forces changes in magnitude and becomes greater than
the opposing force, causing the airplane to accelerate or move in the direction
of the applied force. For example, if power is applied (increasing thrust)
and altitude is maintained, the airplane will accelerate.
As speed increases, drag increases, until a point is reached where drag again equals thrust, and the airplane will continue in steady flight at a higher speed. As another example, if power is applied while in level flight, and a climb attitude is established, the force of lift would increase during the time back elevator pressure is applied; but after a steady-state climb is established, the force of lift would be approximately equal to the force of weight. The airplane does not climb because lift is greater than in level flight, but because thrust is greater than drag, and because a component of thrust is developed which acts upward, perpendicular to the flightpath.
Airplane designers make an effort to increase the performance of the airplane
by increasing the efficiency of the desirable forces of lift and thrust
while reducing, as much as possible, the undesirable forces of weight and
drag. Nonetheless, compromise must be made to satisfy the function and
desired performance of the airplane.
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