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
electrical system whose output changes when a force is applied to the system.
Modern wind tunnels typically use electrical strain gages to
determine forces on the model. Multiple
can be used to determine all the three forces and three moments.
On this web page we will examine the operation of a strain gage.
As shown in the figure, a strain gage consists of a long, thin electrical wire which is
looped back and forth and securely attached to a piece of material, called the substrate.
When a force F is applied to the substrate, the substrate and the attached wire
are stretched. On the figure, the stretching is highly exaggerated; the relative size of the
strain gage is shown by the picture in the middle of the figure. As the wire is stretched, the
length l is increased and the cross-sectional area A is decreased. The
electrical resistance R in the wire is given by:
R = (rho * l ) / A
where rho is the resistivity of the material of the wire. Resistivity is the inverse of
the conductivity of the material. Stretching the wire causes the resistance to increase.
During operation, a constant known voltage V is applied across the gage and the
current i through the gage is measured by a dial in the
control room. From
for an electric circuit the current depends on the voltage and resistance
i = V / R
Stretching the wire changes the resistance in the wire which changes the output current for the circuit.
After calibration using a known amount of force, the output current is inversely proportional
to the applied force F.
i ~ 1 / F
We now have an instrument that can be used to measure the force in either an
There are however some conditions that must be carefully monitored to insure an
accurate measurement of force. The assumption is made that the stretching of the wire
is the same as the stretching of the substrate. This is only true as long as the wire is
rigidly attached to the substrate. Care must be taken in building, operating, and handling
of the gage to insure that the wire stays attached to the substrate.
The substrate and wire can also be "stretched" by heating of the element. The change
in length of the wire due to heating is registered
as an applied force when no force is applied. To eliminate the temperature effects
on the gage, multiple gages are typically attached to the model in a
Wheatstone bridge circuit. The operation of the
Wheatstone bridge circuit is given on another page.
- Beginner's Guide Home Page