In cooperation with Cleveland Pneumatic Tool Company, National Water Lift has developed an aircraft weighing system which will provide accurate aircraft weight information, yet is easy to install and maintain. The major difficulty in aircraft weighing systems, the accurate determination of the strut load, has been solved through the use of a stable and reliable sensing platform. Once an accurate strut load has been determined, the scaling of the transducer outputs, and display of the measured quantities are easily accomplished.

Although many techniques of determining vertical load are technically possible, the use of the oleo strut pressure as a measure of vertical load has long been the obvious solution to the aircraft weighing problem. However, friction contributed by the seals at the pressure barriers in the oleo struts, have caused large errors in oleo pressure readings and have produced uncertainties which are unacceptable for “primary” weighing systems.

Cleveland Pneumatic Tool has determined that this type of friction can effectively be eliminated, simply by introducing a small amount of relative motion across the
o-ring interface, in a plane parallel to that of the o-ring. The required motion can be produced by oscillating the oleo piston with respect to the oleo cylinder.

When this principle was applied to an actual aircraft landing gear, it was found that after only a few low amplitude oscillations, oleo pressure was proportional to the strut load, within a very small tolerance range; thus providing a load indicator with the required accuracy to be used as a primary weighing system.