Correlation of High-Speed Tiltrotor Stability Predictions with Test Data and Parametric Study

Abstract

High-speed stability of tiltrotor was studied. The University of Maryland’s Maryland Tiltrotor Rig (MTR) was chosen for the analysis due to availability of properties and test data, and its interesting high-stability behavior observed in the Glenn L. Martin wind tunnel in August 2022. A Rotorcraft Comprehensive Analysis System (RCAS) model of the MTR gimbaled hub was built in addition to the University of Maryland Advanced Rotorcraft Code-II (UMARC-II) model from previous work. The objective is threefold: i) validate RCAS tiltrotor stability predictions, ii) shed light on the high-stability behavior of the MTR, and iii) find ways to lower the instability speed of the MTR for future wind tunnel tests. Trim collective for freewheeling and stability predictions were compared with wind tunnel test data up to 200 knots. RCAS and UMARC-II predictions showed good agreement with each other and the test data. Predictions show that MTR is stable up to 215 knots (490-knots full-scale flight) although the wing is only 18% thick (current technology is 23%). A parametric study was carried out. The impact of wing stiffness, pitch-flap coupling (δ3 angle), lag stiffness, blade chord, number of blades, pylon mass, pylon center of gravity (c.g.), pylon location, and rotor speed was studied. MTR’s pylon c.g. is unconventionally behind the wing elastic axis. It was found that this significantly improved stability. This behavior is not specific to MTR; full-scale aircraft stability can also be improved by moving the pylon c.g. backward if wing beam is the least stable mode. A combination of forward pylon c.g., reduced rotor speed, and increased blade chord reduced the instability speed by more than 55 knots to near 160 knots, helping researchers obtain high-quality test data in the upcoming Glenn L. Martin wind tunnel tests.