Lift is the force that pushes a object up, against the natural force of gravity. It enables a plane, or other object, to climb into the air and remain aloft during flight. For example, an object held flat against a stream of air, is pushed backward. However, if the object is rotated forward toward the ground, the air can now push it up as well as back. This is the principle upon which a kite works. When flying, the kite is angled against the wind. The movement necessary to produce flight is running, and the string keeps the kite from going backward. The force of horizontally moving air is being used to overcome the downward pull of gravity. The faster the movement, the greater the upward force. The kite lifts into the sky.

The same thing happens in water. A water skier moves forward pulled by the boat. The force of the water pushing up overcomes the pull of gravity so he can stand on top of the water. If the movement stops, the skier sinks because the buoyancy of the water is not great enough to counterbalance the pull of gravity. Combining the two, a water skier attached to a kite with enough forward motion, can achieve lift and fly.

This moving air is what kites, birds, and planes use to get away from earth. Moving air has been harnessed to be useful for thousands of years : sailboats and windmills. Moving air can also be destructive: hurricanes and tornadoes. Thousands of years ago the Chinese experimented with kites and used these aerodynamic forces to fly them. But there were still more secrets to be discovered before man could fly like the birds.

How is lift produced? Bernoulli s principle works whether the fluid is in a container or it is freely moving. And it applies to liquids and gasses, although he never thought of air as a fluid. His discovery explains why the wings of birds and planes are sucked upward and why a piece of paper goes up when you blow over it.

Bernoulli never thought to apply his discovery to flight, but this is how it applies to wings. The distinctive shape of a wing is known as an airfoil. Lift is produced by the motion of an airfoil, or wing, moving through the air. The special shape, or camber (the curved upper surface and a straight or less curved under surface), of the airfoil is what produces a change in air pressure above the wing by deflecting or turning the air. As the airfoil moves through the air, air goes above and below. The upper surface has a longer surface distance. In order for the two airstreams to meet at the trailing edge of the wing at the same time, the top air must go faster. Increased speed equals decreased pressure, equals lift.

Another way of understanding the deflection of air by an airfoil, is by applying Newton s Third Law of Motion. The airfoil deflects the air going over the upper surface downward as it leaves the trailing edge of the wing. When the wing is at an angle, this deflection is even greater. According to Newton s 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. There are some planes that do not have cambered wings. Their wings are flat and are designed to fly at very high speeds. These planes get lift only by deflection. Planes flying upside down, even with cambered wings, can deflect enough air to get lift.

The amount of lift a wing can produce is governed by several factors. 1. The weight of the object to be flown. 2. The size of the under surface of the wing. The larger the wing the more lift produced. The ratio is approximately 1:1. 3. Another way to increase the lift is to change the angle of the wing as it faces the air. This is controlled by the pilot of the plane and is called the plane s attitude. By tilting the leading edge up, the distance the upper air stream must flow is even greater: upper air speed increases, thus lowering pressure above the wing and creating more lift. This is called the angle of attack. Two times the angle of attack equals two times the lift. Increasing the lift means the plane can climb faster or fly at a slower speed.

This works only to a point. If the angle of attack is too great, all lift disappears and the object begins to fall to the ground. This point is called the stalling point. Another way to describe the lower pressure created on top of the wing, is as a partial vacuum. This sucks the wing up keeping the airstream close to the wing. However, as the angle of attack increases it becomes more difficult for this vacuum to hold the airstream in place. When the airstream breaks away from the wing, the lift disappears and the object reaches the stalling point. When the angle of attack becomes too great, the air flow is no longer smooth. It breaks up into eddies or small whirlpools on top of the wing. These eddies decrease lift and the plane drops or stalls. The plane will crash if the angle is not reduced. Planes fly with a wing angle somewhere between 4 and 15 degrees. Stall occurs when the angle is greater than 15 to 20 degrees.

There is still other ways to get maximum lift without reducing the angle of attack. The lift produced by an airfoil depends upon the speed at which the wing moves through the air. During take off and landing, it is necessary for a plane to fly as slowly as possible. By employing three types of high-lift devices, the air moving across the wing can be made to move even faster, thereby, increasing the vacuum which pulls the airstream back down on the wing. Lift will be produced again. This is accomplished by forcing the air through a smaller space, or slot, just ahead of the upper surface of the wing. Planes have mechanical slots on the leading edge of the airfoil, birds have feathers, but both are used to increase the speed of air moving over the upper surface of the wing. Another device is the flap, a hinged section at the back of the wing. During regular flight, it remains flat with the wing. However, during landing and occasionally during take off, the flaps are lowered. This increases the camber giving extra lift. During landing, the flaps also help slow the plane. The third device is a slat, a hinged section neat the front tip of the wing. When the plane slows down, the slats move forward to increase the camber, thereby, increasing the lift.

Speed is the most important element in producing lift. Speed can be increased by increasing the forward speed of the wing itself as it travels through the air. This causes an even more dramatic change in lift. If you double the speed, you get 4 times the lift. Triple the speed and you get 9 times the lift.

The weight of the object desiring to fly, determines how much lift is necessary. Contributing to this, is whether the object is going up, down, or in level flight. When going up, lift must be greater than weight. In level flight, lift must equal weight, and going down lift is less than weight.. Minimum speed for lift is dependent upon the design of the flying object. Understanding the principles behind lift, was the secret of flight that eluded man for thousands of years.

Web Hosting Provided By The National Business Aviation Association.

Explore Space ... Not Drugs!
Hear what astronauts have to say about staying drug-free.
Last modified: Tue May 6 17:20:13 PDT 1997

Copyright © 1997 by Cislunar Aerospace, Inc. All Rights Reserved.