Variation in Hover Aeromechanical Stability Trends with Bearingless Main Rotor Design

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Abstract
An experimental program has been conducted to measure the aeromechanical stability characteristics of a Bearingless Main Rotor (BMR) model. The model is based on a four bladed concept with a flexure, between the hub and each blade. which accommodates flatwise, edgewise, and torsional (pitch) motions. The flexure is enclosed by a torsionally stiff cuff that is cantilevered to the blade/flexure joint at its outboard end and shear restrained to the flexure at its inboard end. The shear restraint includes an elastomeric damper to stabilize edgewise motion. The model was tested in hover over a range of rotor thrust and rotational speed. Numerous rotor hub design parameters were varied to determine their effect on aeromechanical stability characteristics of the model. These included changes in fundamental flatwise natural frequency, blade built‐in cone and sweep angles, pitch link inclination, flexure prepitch, shear restraint to flexure attachment, movement of the pitch link attachment from the trailing edge to a leading edge location, or various combinations. The results in this paper show that most design changes, which were employed to increase aeromechanical stability margins, provided only negligible improvements or were detrimental in their effects. Movement of the pitch link attachment to the leading edge does provide improved stability characteristics at the higher collective pitch angles, which are usually critical, although the stablllty margins for this configuration were still small.

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