A force balance is an important part of wind tunnel testing which allows the researcher to analyze the effect the flow of the air in the tunnel has on the object under test. It is the most vital instrumentation used in such testing. The force balance can take on many forms depending on the type of model being tested and the type of data that needs to be collected.
Most force balances measure the three aerodynamic force components: lift, sideforce, and drag (occasionally thrust, which is negative drag). Also, by working with the differences in the readings on the sensors, a balance can be used to determine pitch, yaw, and roll moments.
Here is a simple, two-component balance arrangement:
"S"-Type Load Cell (Lift) |___| |
| LIVE | |
^ | | |
| | Flow Direction | ____ |
Lift | <------ |_| | |_| "S"-Type Load Cell (Drag)
| | |_| |
<--- | (model sits in this | |
Drag |________space)________| | GROUNDED
The model that is being investigated would take the place of the box marked "LIVE". As a lift (or weight) force acts on the model, the lift load cell (shown here as an "S"-type cell, due to its shape) will undergo tension or compression; similarly, a drag (or thrust) force will tense (or compress) the drag cell. The amount of tension or compression is measured by a strain gauge on the cell, the output of which can be translated to the force acting in that direction. The sensors must be completely supporting the model; the force balance has a live and ground side just like an electrical circuit, and allowing anything but the sensors to touch both sides will shunt the force through the illicit contact point, and result in bad test data.
Most force balances are built in a redundant fashion, so that more than one sensor is used to measure each force. This helps improve the signal-to-noise ratio by better constraining the model.