In-flight rotorcraft blade elastic twist sensor

Abstract

A novel sensor network for measuring rotorcraft blade elastic twist in flight using an expansion of strain gage theory is proposed and demonstrated. The embedded sensor has negligible weight, small power draw, high bandwidth (≥100 kHz), works in the high centrifugal force environment of the rotating blade and does not disturb the blade airflow or structure. The sensor network can also be used to measure lead–lag and flap bending. The blade is idealized as an Euler–Bernoulli beam in bending and a rod in torsion. The theory is rigorously derived from first principles and shows that a sawtooth shaped sensor will measure twist directly without any numerical integration. The network is modeled computationally for a blade undergoing arbitrary torsional and bending moments. The model shows the twist sensor is not affected by arbitrary loading or noise or local structural discontinuities. The twist sensor is then embedded in a Mach scale rotor blade. The elastic twist measurement from the sensor exactly matched the actual twist angle on the benchtop for small (±0.08°), moderate (±0.3°) and large (±2.5°) elastic twist angles over a 4.6 in span (16% of total span). For the large twist deflections, the blade also had flap bending deflections of ±0.34 in (±7% of span).