Early aviation pioneers soon realized humans would not fly by imitating bird flight. A bird's movable wings provide both thrust and lift. Major scientific breakthroughs in flight occurred when scientists recognized:

1. fixed wings (as on modern airplanes) could generate lift and a separate power source (like an engine) could provide thrust;
2. the forces over the airplane would be the same whether an aircraft was stationary and air blown over it or if the airplane moved through the air;
3. scale models could be used.

Today, we know there are 4 forces which affect the flight of an airplane: lift, drag, thrust and weight. Thrust is the forward motion created by the propulsion system. The weight of the airplane acts in a direction towards the earth. Lift causes the airplane to rise in the air. Drag is the force caused by the resistance of air against the body. Drag acts in the direction opposite to the flight path.

Experiments were needed to understand how air affects aircraft surfaces and creates forces like lift and drag. Although the first experiments used nature's winds, they were too turbulent and random. Controllable mechanical devices providing a steady flow of air would give better results.

The first device invented was a "whirling arm". A model of the airplane or airfoil (wing shape) was mounted at the end of a whirling arm which spun around in circles. One of the problems with this device was the arm spun around in a circle and disturbed the air making it difficult to determine the velocity of the air over the model. This led to the invention of the wind tunnel in the 1870's.

You may know that two bicycle shop owners, Wilbur and Orville Wright, developed, built and flew the first manned powered aircraft in 1903. But in 1901, after a frustrating summer of unsuccessfully testing aircraft designs in the sand dunes near Kitty Hawk, North Carolina, Wilbur told his brother Orville, "Man will not fly for 50 years!". Wilbur believed other researchers' wind tunnel results that they were using were incorrect. The Wright Brothers spend the later part of 1901 testing airfoils (wing shapes) in a wind tunnel of their own design. One of their first designs was a wind tunnel which was operated by riding on a bicycle.

If you were designing and building a new airplane how would you test it? How would you know if it would have enough lift to fly? You would want a less expensive and safer way to understand the forces on your new design. It would be very expensive to build a full scale airplane model every time you had an idea for an airplane.

An engineer named Osborne Reynolds discovered the relationship between the forces on a full scale object and a model (like a commercial Boeing 777 airplane and a model of a Boeing 777 airplane).

This is one reason why wind tunnels are used. Wind tunnel testing is one step of the aircraft design process. Generally, a model of the object is placed in the wind tunnel and the aerodynamic forces (lift and drag), pressure and velocity can be measured.

Today, wind tunnels come in all shapes and sizes - some fit on a table, while others are several city blocks long. Regardless of the size, shape and type, all wind tunnels have the same basic components (parts). A wind tunnel is generally sort of a duct or pipe shape and air is either blown or pulled out of the tunnel by a fan or other drive system (a machine which creates force). One of the most important sections of the tunnel is what is called the "test section". This is the area where the model to be tested is placed. The controlled stream of air flows over the model. Test instruments and/or a balance record the data to be collected from the test.

Tunnels are often designated by the speed which they are operated - for example, a low speed subsonic tunnel or a supersonic tunnel. Some tunnels use water instead of air. These tunnels are often used for marine studies. Some of the largest wind tunnels in the world are located at NASA (the National Aeronautics and Space Administration). Entire full scale aircraft fit into the test section of the tunnel.