A force may be thought of as a push or pull in a specific direction. This slide shows the forces that act on an airplane in flight.
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.
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.
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:
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.
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:
The direction of the force depends on how the engines are attached to the aircraft. In the figure shown above, two jet engines are located under the wings, parallel to the body, with thrust acting along the body centerline. On some aircraft (such as the Harrier), the thrust direction can be varied to help the airplane take off in a very short distance. For jet engines, it is often confusing to remember that aircraft thrust is a reaction to the hot gas rushing out of the nozzle. The hot gas goes out the back, but the thrust pushes towards the front. Action <--> reaction is explained by Newton's Third Law of Motion.
A glider is a special kind of aircraft that has no engine. Some external source of power has to be applied to initiate the motion. During flight, the weight is opposed by both lift and drag, as shown on Vector Balance of Forces for a Glider. Paper airplanes are the most obvious example, but there are many kinds of gliders. Some gliders are piloted and are towed aloft by a powered aircraft, then cut free to glide for long distances before landing. During reentry and landing, the Space Shuttle is a glider; the rocket engines are used only to loft the Shuttle into space.
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.
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