One of the most important parts of any rotorcraft is undoubtedly the rotor. But the blades are a close second. Afterall, for rotorcraft, the rotational movement of the blades is the only reason that the aircraft can even achieve lift and fly. For rotorcraft like helicopters, lift is achieved when the rotor blades (airfoil) meet the oncoming airflow and deflect them, creating a change in the direction of the airflow, which results in an area of low pressure forming behind the leading edge of the upper surface of the blade.
In turn, this pressure gradient causes the air flow to accelerate down along the upper surface. Simultaneously, the airflow under the blade is rapidly slowed or halted, causing an area of high pressure, also causing the airflow to accelerate along the upper surface. The two sections of the airflow leave the trailing edge of the blade with a downward component of moment, producing lift. In order to withstand the airflow and effectively be lifted, the blade must be strong and durable enough. Therefore, it’s important to test the blade flex.
Rotor blades are dynamic devices, so of course it’s important to regularly inspect and test their functionality. However, equally important is a static test to verify the strength and mechanical properties of the blades. Because the environmental conditions and operating conditions of the blades can vary greatly, it’s good to make sure, during the manufacturing process, that everything is up to standard.
The static test is done by mounting the rotor blade horizontally on a stationary fixture at one end, a hydraulic ram at the other with LVDT (linear variable differential transformers) displacement sensors arranged along the length of the blade, and a strain gage load cell inserted between the hydraulic ram and blade tip. Pressure is applied to the ram, flexing the blade. Meanwhile, the load cell senses the amount of force while the LVDTs measure the amount of bending. These two readings are calculated as a ratio of force to bending and help determine the strength and flexibility of the blade. The ideal ratio is typically set by the OEM (original equipment manufacturer).
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