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When a model is placed in a uniform current of air in a properly designed channel or tunnel, the greater part of the errors due to towed tests are eliminated. The measuring instruments can be placed on a firm foundation, the air stream can be maintained at a nearly uniform speed and with little error due to eddies, and the test may be continued under uniform conditions for an indefinite period. While there are minor errors due to wall friction and slight variations in the velocity at different points in the cross section of the tunnel, they are very small when compared to the errors of towing. For this reason the wind tunnel is the accepted means of testing.

Eiffel's Wind Tunnel

The observer and weighing mechanism are supported above the air stream on a sliding floor, and a standard extends from the model in the wind stream to the balances on the weighing floor. These balances determine the lift and drag of the models, the center of pressure, etc.

The N. P. L. Tunnel

United States Navy Tunnel