The use of composite tailored couplings in rotor blades to reduce vibratory hub loads was studied through design, structural and aeroelastic analysis, fabrication, and wind tunnel test of Mach scale articulated composite rotors with tailored flap-bending/torsion couplings. The rotor design was nominally based on the UH-60 BLACK HAWK rotor. The 6-foot diameter blades have a SC1095 profile and feature a linear twist of -12 deg. The analysis of composite rotor was carried out using a mixed cross-section structural model, and UMARC.

Five sets of composite rotor were fabricated, including a baseline rotor without coupling, rotors with spanwise uniform positive coupling and negative coupling, and rotors with spanwise dual-segmented coupling (FBT-P/N) and triple-segmented coupling. The blade composite D-spar is the primary structural element supporting the blade loads and providing the desired elastic couplings. Non-rotating tests were performed to examine blade structural properties. The measurements showed good correlation with predictions, and good repeatability for the four blades of each rotor set.

All rotors were tested at a rotor speed of 2300 rpm (tip Mach number 0.65) at different advance ratios and thrust levels, in the Glenn L. Martin Wind Tunnel at the University of Maryland. The test results showed that flap-bending/torsion couplings have a significant effect on the rotor vibratory hub loads. All coupled rotors reduced the 4/rev vertical force for advance ratios up to 0.3, with reductions ranging from 1 to 34%. The mixed coupling rotor FBT-P/N reduced overall 4/rev hub loads at advance ratios of 0.1, 0.2 and 0.3. At a rotor speed of 2300 rpm and an advance ratio of 0.3, the FBT-P/N rotor achieved 15% reduction for 4/rev vertical force, 3% for 4/rev in-plane force and 14% for 4/rev head moment. The reductions in the 4/rev hub loads are related to the experimentally observed reductions in 3/rev and 5/rev blade flap bending moments.

Through the present research, it has been experimentally demonstrated that structural couplings can significantly impact rotor vibration characteristics, and with suitable design optimization (coupling strength and spanwise distribution) they can be used to reduce vibratory hub loads without penalties.