A basic knowledge of the forces that act upon an airplane and of the parts of an airplane will enhance your appreciation of the advances made during the years that the College Park Airport has operated.
Definitions of Forces
Weight
Weight is a force that is always directed toward the center of the earth. The magnitude of the force depends on the mass of all the airplane parts, plus the amount of fuel, plus any payload on board (people, baggage, freight, etc.). The weight is distributed throughout the airplane. But we can often think of it as collected and acting through a single point called the center of gravity. In flight, the airplane rotates about the center of gravity, but the direction of the weight force always remains toward the center of the earth. During a flight, the airplane's weight constantly changes as the aircraft consumes fuel. The distribution of the weight and the center of gravity can also change, so the pilot must constantly adjust the controls to keep the airplane balanced.
Lift
To make an airplane fly, we must generate a force to overcome the weight. This force is called the lift and is generated by the motion of the airplane through the air. Lift is an aerodynamic force ("aero" stands for the air, and "dynamic" denotes motion). Lift is directed perpendicular (at right angle) to the flight direction. As with weight, each part of the aircraft contributes to a single aircraft lift force. But most aircraft lift is generated by the wings. Aircraft lift acts through a single point called the center of pressure. The center of pressure is defined just like the center of gravity, but using the pressure distribution around the body instead of the weight distribution.
Drag
As the airplane moves through the air, there is another aerodynamic force present. The air resists the motion of the aircraft; this resistance force is called the drag of the airplane. Like lift, there are many factors that affect the magnitude of the drag force including:
- the shape of the airplane
- the "stickiness" of the air
- the speed
And like lift, we often collect all of the individual components' drags and combine them into a single aircraft drag magnitude. The direction of the drag force is always opposed to the flight direction, and drag acts through the center of pressure.
Thrust
To overcome drag, most airplanes have some kind of propulsion system to generate a force called thrust. The magnitude of the thrust depends on many factors associated with the propulsion system:
- type of engine
- number of engines
- throttle setting
- speed
The motion of the airplane through the air depends on the relative strength and direction of the forces shown above. If the forces are balanced, the aircraft cruises at constant velocity. If the forces are unbalanced, the aircraft accelerates in the direction of the largest force.
This material is adapted from NASA's Glenn Research Center Beginner's Guide to Aerodynamics.
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