ELECTROMAGNETIC NOISE MITIGATION ON AN ELECTRIC TILTROTOR RIG AND HOVER PERFORMANCE TESTING

Abstract

The electromagnetic noise on an electric motor-driven proprotor was resolved and hoverdata successfully acquired in the Glenn L. Martin wind tunnel. Electric motors have the potential to produce electromagnetic interference on surrounding systems. With the growth of electric vertical lift aircraft, electric motors are becoming more prevalent throughout the aerospace industry. To stay on the cutting edge of electric vertical lift research, the University of Maryland has developed an electric tiltrotor rig. This rig, henceforth called the Maryland Tiltrotor Rig (MTR), is a semi-span, floor-mounted, optionally powered rig with a static nacelle tilt mechanism, capable of testing 3-bladed proprotors up to 4.75 feet in diameter. The MTR contains an integrated electric motor inside its nacelle, in unavoidably close proximity to sensitive instruments. Whenever the motor was powered, severe noise contaminated all data channels, limiting the scope of useful testing to un-powered, freewheeling whirl flutter tests. The mitigation of this noise issue was the principle task of this work. The noise was diagnosed systematically, then solved using four filters, namely, a power filter, an edge filter, a signal filter, and a load cell channel filter. The first three filters required special hardware. Of these, the first two were custom built by LaunchPoint. The third was built in-house. It proved to be the most significant of all the filters. The fourth filter was a low-pass software code. The source of the problem was tracked down to high frequency pulses from the motor drive. The pulses repeated at 20 kHz and had 1.6 MHz ringing. With noise solved, the first successful hover testing of the MTR was conducted in the Glenn L. Martin wind tunnel. The collective was swept from 6 degrees to a high value of 30 degrees to put the rotor into stall with a blade loading up to 0.24. Thrust, torque, and tunnel circulation were measured. Figures of Merit and propulsive efficiencies were calculated from the measurements. A maximum Figure of Merit of about 0.8 was obtained. Both straight and swept-tip blades were tested. The accuracy of the data was assessed using blade element momentum theory, which included exact/large angles, airfoil decks, corrections for low Reynolds numbers, and corrections for tunnel recirculation. Now that is possible to conduct powered testing with the MTR, a wide range of future tests become possible, including powered whirl-flutter, nacelle tilt, and high-speed proprotor performance tests.