Different Ways Air Moves

A person who designs airplanes (aerodynamicist) must know several things. This section defines the words and concepts needed to understand how a fluid (air or water) moves in or around an object, like a wing.

Speed of Sound

Sound travels in invisible waves at about 760 miles per hour (mph). If you were far from an explosion you would see it before you would hear it, because it takes time for the sound waves to travel to your ear. The speed of sound is also called Mach 1. Twice the speed of sound is Mach 2.

A plane must be designed in a special way to go faster than Mach 1. As a plane gets close to this speed great pressures are created. The plane must be able to "break through" this sound barrier. This is called going supersonic. When this happens a shock wave is sent out in all directions. The first person to break through the sound barrier was Captain Charles Yeager in 1947. He was flying an experimental aircraft Bell XS-1. Today, many planes fly supersonic.

Friction

It is difficult to push a heavy box across a carpet. This is because of resistance called friction. If you push the same box across a smooth wood floor it is much easier, but there is still friction. When air passes over a wing of a plane there is friction. An aerodynamicist must understand how to design the wing to have the least amount of friction. The same rules apply when water passes over an object, such as the hull of a ship.

Layers

As a fluid (air, water) speeds over a surface (wing, hull) friction causes interesting things to happen. On the surface, the fluid stops moving. Another fluid layer forms on top of the first layer that moves slowly; then more fluid layers form on top of each other, each moving more swiftly until the top layer is moving at the original speed of the outside fluid.

Some surfaces allow a smooth layer to form. This is called laminar. Most of the time another layer develops; turbulent (opposite of smooth). Aerodynamicists must try to design surfaces (wings, hulls) that reduce turbulence or disorder.

Flow Separation

Sometimes the layers close to the surface of the wing force to move away from the surface. This is called separation. Pilots and engineers usually don't like flow separation because it can cause a stall. If a stall is not corrected the plane will crash. A controlled stall is sometimes used just before landing an airplane, to set the wheels down gently.

Buoyancy

When something is dropped into the water, it will either float or sink. If it floats, it is because the object weighs less than the water displaced (pushed away) by the object. If it sinks, it is because the object weighs more than the water displaced.

Buoyancy is usually only a problem in water. Most things do not have buoyancy in air. Very large balloons or blimps do have buoyancy in air. Ship and boat designers must know the rules of buoyancy when they plan or build water vehicles.

Shocks

As an airplane goes supersonic, shock waves are sent out in all directions. If an airplane is close to the ground the shock waves can shake buildings and break windows! Planes usually stay high in the air so the shock wave will be broken up by the winds before it hits the ground.

Laws

Scientists and engineers must understand certain laws of how air flows so that flight is possible. For example, they must understand that when air is heated it expands and increases in pressure.

In the 1700's Daniel Bernoulli found that when air goes quickly over a surface, such as a wing, air pressure decreases. When the speed (velocity) decreased, the pressure increased. He was able to put this into a math formula (Bernoulli's theorum). Today, wings are designed to increase the airflow over the top of a wing and decrease it under the wing. This causes lift, which keeps the airplane in the air!

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Last modified: Wed Jul 28 11:46:29 PDT 1999