Active Pitch Links and Trailing-Edge Tabs for Rotor Track and Balance

This study compares the effectiveness of an on-blade active trailing-edge tab and an adjustable pitch link mechanism for rotor track-and-balance. The active tab is an extendable trailing-edge-plate over a spanwise section of each blade. A simulation model of the UH-60 Black Hawk rotor with seeded imbalance is developed, with the pitch links/tabs minimizing the 1/rev (1P) vibratory loads using a weighted least-squares optimization method. The trailing-edge tab has the ability to significantly reduce the 1P in-plane forces in hover but shows more modest reductions in cruise. The tabs are unable to reduce 1P vertical forces in hover but are very effective in reducing them in cruise, and are also moderately effective in reducing 1P in-plane moments over the range of airspeeds. Best reductions in 1P loads are achieved by employing an active tab mechanism (adjusted at different airspeeds) over a passive mechanism (with constant setting over the airspeed range), with the active tab yielding additional gains in 1P in-plane forces and moments in hover and in 1P vertical forces in cruise. Compared to the trailing-edge tab, the pitch link shows significantly larger reductions in 1P in-plane forces over all airspeeds and simultaneously reduces 1P vertical force to comparable levels in cruise. Reductions in 1P in-plane moments with the pitch link are comparable to those with the trailing-edge tabs in hover but show a slight improvement in cruise. In contrast to the trailing-edge tab, minimal differences are observed between active and passive pitch links, except for the case of 1P inplane forces in hover. In hover, for both tab and pitch link, an examination of the load reduction process indicates that a seeded radial shear and root torsional moment imbalance is cancelled by the generation of net blade root chordwise shear and root flap bending moments, respectively, on orthogonal blades. In cruise, a similar process is observed, but the generation of net radial shears and root torsional moments on opposing blades were also contributors to total reduction in 1P hub in-plane and pitching moment vibrations for the trailing-edge tab.

Reference

Krishnamurthi, J., and Gandhi, F., "Active Pitch Links and Trailing-Edge Tabs for Rotor Track and Balance ,"

Proceedings of the AHS Specialists’ Conference on Aeromechanics Design for Transformative Vertical Flight, San Francisco, California, Jan 16-19, 2018.