Wind tunnel models use strain gage balances to measure the aerodynamic forces and moments produced by the model in the flow stream at various angles of attack and side slip. The balances are normally attached to the end of a sting in the wind tunnel test section and the model is built around the balance. Powered model testing has encountered great difficulties in maintaining high quality mesurement of these aerodynamic forces and moments on wind tunnel models, because fluid, at high pressures and temperatures (i.e., hydraulic fluids, high pressure air, Freon, water, nitrogen, etc.), passes through the balances to power the wind tunnel model aircraft.
Heretofore, attempts to pass high pressure fluids across the metric break (the junction between the non-metric portion of the balance connected directly to the sting of fixed portion of the mount and the metric portion of the balance connected to the model) of the wind tunnel model balances has taken two forms. The two forms are "flow-through" and "flow-around." Flow-through balances have the pressurized fluid passing through an internal portion, normally the center, of the balance. Flow-around balances have pressurized fluid moving around the outside of the balance.
Also previous balances have suffered from disadvantages in that mechanical hysteresis inevitably develops at the metric break since the metric and non-metric portions were two separate pieces mechanically connected together. This hysteresis resulted in poor accuracy of the measurement system. In addition, both of the prior art type balances discussed above are significantly affected by the effects of temperature, pressure and momentum of the pressurized fluid flowing through the balance and passing across the metric break. Also in the prior art, it was believed that thermal deviation effects could be derived from the temperature of the flexure beams but this proved to be only roughly accurate.
As a result, in the prior art attempts to develop a flow-through or flow-around wind tunnel balance, the pressure, temperature, and momentum effects could not be sufficiently well accounted for to achieve sufficient accuracy or accuracy comparable to current wind tunnel balances without flow. This in turn has forced those using these flow type balances to accept balance data with significant uncorrectable errors and/or increased wind tunnel time to reduce thermal effects.
It is therefore an object of the present invention to provide a new and improved wind tunnel balance which substantially overcomes the above-described deficiencies in the prior art.