of proposed aircraft and engine components.
During a test, the model is placed in the
of the tunnel and air is made to flow past the model.
of tests can be run in a wind tunnel.
Some tests are performed to directly measure the aerodynamic
on the model.
The most basic type of instrument used in this type of testing is the
To measure the force, we use some type of mechanical or
whose output changes when a force is applied to the system.
For rather simple,
wind tunnels, a mechanical spring gage may be sufficient to determine the forces on
the model. Multiple
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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