The external balance is large system composed of beams, pivots, and weights designed to measure three forces and three moments: drag, side force, lift, rolling moment, pitching moment, and yawing moment. It is classified as a pyramidal balance because of the shape formed by measurement locations in conjunction with the balance moment center. Unlike internal balances, an external balance has a fixed balance moment center regardless of the model position which requires a moment transfer to the desired location on the model in post processing. The pyramidal balance is also unique in the fact that each force and moment is measured independent of the others.
The LSWT external balance was built along with the tunnel in the 1940’s and has remarkable range and accuracy. Drag and side force can measure between -1000 lbs and 1000 lbs and lift has a range of -1000 lbs to 3000 lbs and each force has an accuracy of 0.25 lbs. Pitching and yawing moments range from -2000 to 2000 ft-lbs and yawing moment ranges from -1000 to 1000 ft-lbs. Each of the moment measurements is accurate to within 0.25 ft-lbs.
A large amount of testing done at the LSWT uses the external balance for basic force and moment data. Anything from solar panels, to bicycles, to offshore oil rigs, to race car wings, aero vehicles, and more have utilized the data from the external balance to advance their designs. The LSWT uses a variety of mounts and struts to connect from the test section to the balance below, and while many models can use existing structures, occasionally a new mounting plate must be made.
The external balance also includes a turntable that get a model to any desired yaw angle within one run. Unfortunately, the speed of the turntable can be slower than desired for some customers, but the LSWT has a fast turntable with a diameter of 24 inches that can accommodate smaller models at a faster test schedule. The fast turntable is most commonly used for offshore oil rigs that perform multiple runs at different angles.
The LSWT team is very versatile, creative, and experienced and will tackle any project head on and do their best to create a mounting and data acquisition system that will fit your individual needs. If you have any questions about testing at the LSWT, please contact us.