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Computer drawing of a spring gage used to measure forces.

Aerodynamicists use wind tunnels to test models of proposed aircraft and engine components. During a test, the model is placed in the test section of the tunnel and air is made to flow past the model. Various types of tests can be run in a wind tunnel. Some tests are performed to directly measure the aerodynamic forces and moments on the model. The most basic type of instrument used in this type of testing is the force balance. To measure the force, we use some type of mechanical or electrical system whose output changes when a force is applied to the system. For rather simple, student-built wind tunnels, a mechanical spring gage may be sufficient to determine the forces on the model. Multiple gages can be used to determine the three forces and three moments present on the model. On this web page we will examine the operation of a spring gage.

As shown in the figure, a spring gage consists of a spring that is stretched by the application of a force, and some kind of scale to determine the magnitude of the force. The spring gage is based on the physical observation that the length L of a spring is linearly related to the applied force F through a spring constant that itself depends on the material and thickness of the spring. If the spring constant is specified by the letter K, then

L = (1/K) x F

F = K x L

If no force is applied to the spring, the spring has some length L0. We attach a bar to the spring that points to zero on the scale when no force is applied. When a force is applied, the spring stretches to some length L1 and the bar moves by a distance L1 - L0 . We calibrate the scale by applying a known force and marking the scale appropriately. In the figure, a force of 2 and 1/2 pounds has been applied at the right. For any other applied force the spring will stretch to a new value of L1 and

F = K x (L1 - L0)

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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