Testing Blade Flex
Helicopters are a common sight in the sky and are used for many important tasks. Rotors are used to provide lift and the large blades on these rotor assemblies often rotate at a rate of 300 RPM or higher, depending on the helicopter type. The rotor blades must be strong, yet flexible – the attack angle of the blade is varied by the pilot to maneuver the helicopter in flight.
Helicopter rotor blades are dynamic devices, but a static test is typically performed during manufacture to verify the strength and mechanical properties. The rotor blade is mounted horizontally to a stationary fixture at one end, and a hydraulic ram is affixed to the other end. A series of LVDTs displacement sensors are arranged along the length of the blade and a strain gage load cell inserted between the hydraulic ram and the tip of the blade. As pressure is applied to the ram, it flexes the blade. The force applied to the blade is sensed by the load cell and the LVDTs measure the amount of bending in the blade. The ratio of applied force to the amount of bending over the length of the rotor blade is an important measure of its strength and flexibility.
The Validyne UPC2100 and USB2250 data acquisition systems are used to interface to the sensors. Long stroke LVDTs used to sense displacement can be wired directly to the UPC/USB. The UPC/USB interface which provides all required AC sensor excitation, demodulation, A/D conversion and records directly in inches of displacement. Similarly, the full-bridge load cell can be wired directly to the UPC/USB. All required DC excitation, amplification and A/D conversion is supplied as well as conversion directly into pounds of force for recording. Ultimately, a graph of force vs displacement at each measurement point along the rotor blade can be created and this is compared with historical data to insure that the blade has the required strength and flexibility. This can be done using the Easy Sense software or on LabVIEW using our free VI drivers.
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