Strain telemetry for load identification and center of gravity measurement

The location of the center of gravity is critical to the ability of an aircraft to sustain flight. When an aircraft is loaded improperly, its center of gravity can shift creating instability during takeoff, flight and landings. If the aircraft is properly instrumented, the effects of the loading process can be monitored to insure that the center of gravity location remains within an acceptable region of the aircraft. In aircraft applications, any additional weight or maintenance time can represent an unacceptable increase in operational costs. For purposes of limiting the weight impact of the load identification systems, several steps were taken in this study. Strain gages were used due to their small size and weight; a telemetry system was employed to eliminate the need for wires; and various techniques were implemented to eliminate the need for batteries in the telemetry system. With the batteries removed, the routine maintenance on the system is all but eliminated. Three telemetry systems were developed for potential use in telemetry for strain measurements. The first system used a voltage controlled oscillator (VCO) with a conventional Wheatstone bridge circuit and was only functional with the use of a battery. The second system used a resistor-capacitor circuit to produce strain measurement. This was powered through inductive coupling. The third system was a commercial telemetry system used in conjunction with solar cells. The identification of load through the use of strain gages was demonstrated through the use of a custom-made test fixture. Strain gages were installed on three supports which acted as the landing gear of the aircraft. A finite element model of the test rig was created to collaborate with the experimental data, as well as to aid in the determination of potential algorithms for the measurement of the load location. The results showed that telemetry systems can be an effective means for measuring strain and that strain measurement can be effectively used to identify the location and magnitude of an applied load. The system can be used to monitor the drift in the location of the center of gravity during the loading process of an aircraft