Whole Span Rotorcraft Blade Structurally Embedded Elastic Deflection Including Twist (SEEDIT) Sensor

Knowing a rotorcraft blade’s elastic deflections along the entire span, including twist, during flight would provide great insight into rotor dynamics, provide validation of structural codes, monitor blade structural integrity and provide blade clearance feedback relative to other blades or the fuselage. A full span SEEDIT sensor based on elongated strain gages is proposed, developed and validated for simultaneous flap, lag and twist elastic motion. Equations to convert the sensor output to deflections are derived from first principles. A custom software code designs the sensor for an arbitrary blade planform and structure and then produces the CAD files needed for a flexible circuit manufacturer. A full span sensor with a length of 54 inches (1.372m) and width of 3 inches (0.0762m) was manufactured for a Mach scale rotor blade. The sensor consists of a layer of Constantan resistive sensing traces and a layer of copper leads with Kapton insulation layers for a total thickness of 10.4 mil (0.264mm). The SEEDIT sensor was embedded in a Mach scale composite blade with a cambered airfoil, twist, sweep and varying spanwise chord. The sensor has 45 leads and is delivered to the blade manufacturer with the connector already secured to the leads for simple integration into the blade layup process. Fatigue testing demonstrated sensor life on par with the resistive material. The sensor was calibrated using motion capture cameras on the benchtop. For dynamic arbitrary motion up to 8Hz, the sensor accuracy at the blade tip was 4 mils (0.102mm) for lag, 70 mils (1.78mm) for flap and 0.23° for twist for blade elastic motions of ±500 mil (12.7mm) for lag, ±2000 mil (50.8mm) for flap and ±4.2° for twist. This is a first for flexible circuits with layers of different materials connected and for embedding a large full-span sensor within a rotorcraft blade. The sensor adds negligible complexity to the blade manufacturing process.