Forces in Flight

There are, basically, four forces of flight: lift, drag, thrust and weight. The figure below shows how these four forces are related for straight and level flight. Lift force point upward, opposite to the weight. Thrust pushes the plane forward, as drag slows it down. The lift force must be greater than the weight and the thrust more powerful than the drag for the plane to fly.

Direction of Forces in Straight and Level Flight

Weight

Weight is present because of gravity. Gravity is a natural force that pulls the plane down towards the earth. Therefore, the direction of weight is down.

Lift

The force that pushes an object up against the weight is lift. On an airplane or a bird, the lift is created by the movement of the air around the wings. Air moves over the top and bottom of the wing iat different speeds to create lift. There are two ways to do this. The wing itself can have a curved upper surface and flatter lower surface. This forces the air flowing over the top of the wing to move faster. This creates lift. Another way is to use a flat wing and fly at an angle to the wind. The slanted wing causes the air to move more quickly over the top of it, creating lift.

Modern aircraft have a curved upper surface on the wing. The figure below shows two streamlines; one is going over the wing and the other under the wing. The faster air leads to low pressure on top of the wing and the slower stream under the wing creates a higher pressure. The two together produce lift.

According to Newton's Third Law, for every action there is an equal, but opposite reaction. Therefore, if the airfoil deflects the air down, the resulting opposite reaction is an upward push. Deflection is an important source of lift. Planes with flat wings, rather than cambered, or curved wings must tilt their wings to get deflection.

Generally, the faster you go, more lift is created. If speed is doubled, lift increases four times!

Race cars can use lift to help them. It is called negative lift because the shape of the airfoil produces lift that points downward. This helps the race car stay on the track as it goes around high speed turns.

Thrust

Thrust is created by airplane engines (or birds flapping wings). The engines can turn a propeller at high speed or can be a jet engine that pushes hot gases out the back. If the thrust is powerful enough it will overcome weight and drag and the plane will fly.

Drag

Drag works against thrust to slow an aircraft.

There are four types of drag:

1. Friction drag - As an airplane goes through the air, the air must go around the plane. The air is "rubbing" against the metal skin of the aircraft. This tends to slow the aircraft.
2. Form drag - The shape of the airplane can make more or less drag. If the plane is "streamlined" the air will pass around it with less drag. Think of a truck or a bus. The flat front is not streamlined. This creates more drag, and more fuel is used. Put your hand out the window of a car, palm forward, this is an example of the form of a bus or truck. Feel the drag!
3. Induced drag - When lift is created around a wing, drag is also created.
4. Wave drag - When an airplane is flying near or faster than the speed of sound the air flow around the aircraft changes and becomes an additional drag.

Summary

The four forces of flight, weight, lift, thrust and drag are well known by pilots. Stunt pilots use these forces to entertain crowds with their amazing tricks. They may stall (stop the lift) the plane in midair and let it fall and then at the last minute "pull out" and fly straight. Or, the pilot may point the airplane straight up and fly until the weight overcomes the thrust and the plane falls back toward the ground. The pilot then brings the plane in line to gain lift to fly straight again.

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Last modified: Wed Jul 28 11:58:49 PDT 1999