The YUH-61A Tail Rotor: Development of a Stiff Inplane Bearingless Flexstrap Design

The Boeing Vertol YUH-61A tail rotor is a new, completely bearingless flexstrap design which uses fiberglass composite blades and straps to achieve significant gains in simplicity and survivability. This paper describes the design process by which such gains were made, with emphasis on new approaches used to assure aeroelastic stability. The discussion covers all the fundamental design considerations, the ways these influenced flap-lag stability and other aeroelastic issues, and every element of the aeroelastic design approach: theoretical methods, reduced-scale model testing, full scale isolated rotor testing, and testing on the prototype aircraft. The presentation includes some of the most important theoretical and experimental results. This rotor is particularly note-worthy because it incorporates blade sweep and tip weight to achieve flap-lag stability in a "stiff inplane" design (first inplane modal frequency above one per rev). These features provide stability by improving flap-lag couplings, making it unnecessary to separate the frequencies of the flap and inplane modes to avoid flap-lag instability. The resulting freedom of modal placement yields valuable design benefits.