Externally mounted balances can be installed over or under the wind tunnel test section, depending on the tunnel design, space availability, and testing requirements. The less complex balance will measure 3 aerodynamic forces (Lift, Pitch and Drag), while for 3D model testing, the 6 aerodynamic forces and moments will be measured.

External balances are generally frameworks that are interconnected with mechanical links and flexures that allow the entire structure to move according to the aerodynamic forces applied by the wind to the model.

The displacement is measured either by weighbeams, or more often now, precision load cells. The balance is designed to separate out the individual aerodynamic forces and moments and minimise any interaction between them.

The design of these balances is concerned with the ideal position for the ‘Virtual Centre’ for testing – probably mid-centre line for aircraft testing, and near the test section floor for road vehicle testing. Model tare-weight, expected maximum aerodynamic loads, and range of motion required are also factors that are needed for each individual customer.

ATE has a range of ‘typical’ loads and designs, but in many cases the load range, position of model testing resolution centre, space restrictions and other customer testing requirements, often mean that each balance is unique.