Dynamically Tuned Rotorcraft Tail Assemblies for Mode Separation
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Solution Overview
Problem
Existing tail assembly designs in rotorcraft suffer from undesirable coupling of chordwise pivot and beamwise bending modes, leading to potential resonant vibrations and aeroelastic instability due to insufficient separation of natural frequencies from proprotor excitation frequencies.
Innovation Solution
A tail assembly with a forward joint having tailored axial stiffness to manage chordwise pivot mode below the first excitation frequency and a forward joint with tailored bending stiffness to manage beamwise bending mode above the second excitation frequency, decoupling these modes and enhancing aeroelastic stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the connection between the tail structure and the fuselage is softened to lower the chordwise pivot mode frequency, then the chordwise pivot mode frequency is reduced below the proprotor excitation frequency, but the chordwise pivot mode and beamwise bending mode become coupled together
Solution Approach 1:
The forward joint is segmented into two independent stiffness components: axial stiffness (Kx) for chordwise pivot mode control and bending stiffness (Kz) for beamwise bending mode control. This segmentation allows independent tuning of each mode's natural frequency, preventing unwanted coupling between modes while maintaining aeroelastic stability.
2Stability of the object's composition
If the connection stiffness is increased to separate the modes, then mode coupling is reduced, but the chordwise pivot mode frequency rises above the proprotor excitation frequency
Solution Approach 1:
The axial stiffness parameter (Kx) and bending stiffness parameter (Kz) of the forward joint are independently adjusted to achieve the desired natural frequencies. By changing these stiffness parameters separately, the invention positions the chordwise pivot mode below the excitation frequency while maintaining mode separation, thus ensuring aeroelastic stability.
3Device complexity
If a single stiffness value is used for the forward joint, then the structure is simpler, but both chordwise pivot and beamwise bending modes are affected equally making independent tuning impossible
Solution Approach 1:
Different stiffness properties are assigned to different directions at the forward joint: axial stiffness (Kx) governs chordwise pivot motion while bending stiffness (Kz) governs beamwise bending motion. This local differentiation of stiffness quality in different directions enables independent tuning of each mode without increasing overall structural complexity.
Data Source
AI summary
A tail assembly for a rotorcraft having a fuselage and a rotor assembly that generates first and second excitation frequencies. The tail assembly includes a stabilizer having an aft spar and a forward spar. An aft joint couples the aft spar to the fuselage and defines a pitch axis such that the aft joint allows the stabilizer to pivot about the pitch axis. A forward joint couples the forward spar to the fuselage. The forward joint has an axial stiffness configured to tailor a chordwise pivot mode of the stabilizer and a bending stiffness configured to tailor a beamwise bending mode of the stabilizer. The chordwise pivot mode of the stabilizer is below the first excitation frequency, the beamwise bending mode of the stabilizer is above the second excitation frequency and the chordwise pivot mode of the stabilizer is decoupled from the beamwise bending mode of the stabilizer.