Rotorcraft Tail Assembly Joint Tuning for Mode Decoupling
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Solution Overview
Problem
Existing tail assembly designs in rotorcraft fail to adequately decouple the chordwise pivot mode and beamwise bending mode from excitation frequencies generated by proprotor assemblies, leading to undesirable coupling and potential resonant vibrations and aeroelastic instability.
Innovation Solution
A tail assembly with a forward joint having tailored axial stiffness to manage the chordwise pivot mode below the excitation frequency and a forward joint with tailored bending stiffness to manage the beamwise bending mode above the excitation frequency, using spherical and elastomeric bearings to decouple these modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
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 causing undesirable vibrations
Solution Approach 1:
The stabilizer is divided into two independent vibration modes: chordwise pivot mode and beamwise bending mode. By designing separate structural paths and joint configurations for each mode, the patent enables independent frequency tuning of each mode without them coupling together, thus avoiding the harmful coupling effect while maintaining stability.
Solution Approach 2:
The patent changes the stiffness parameters of the forward and aft joints independently to tune the natural frequencies of the two modes. The forward joint stiffness is optimized for the chordwise pivot mode while the aft joint stiffness is optimized for the beamwise bending mode, allowing each mode to be positioned at different frequencies away from excitation sources.
2Stability of the object's composition
If the connection between the tail structure and the fuselage is stiffened to raise the chordwise pivot mode frequency above the proprotor excitation frequency, then the chordwise pivot mode frequency is increased, but the beamwise bending mode frequency cannot be sufficiently separated from excitation frequencies
Solution Approach 1:
The patent segments the frequency tuning control by assigning different joint stiffnesses to control different modes. The forward joint primarily controls the chordwise pivot mode frequency while the aft joint primarily controls the beamwise bending mode frequency, enabling independent frequency separation for each mode from excitation frequencies.
Solution Approach 2:
By independently adjusting the stiffness parameters of the forward and aft joints, the patent can raise the chordwise pivot mode frequency above excitation frequencies while simultaneously positioning the beamwise bending mode frequency at a different location, ensuring both modes are sufficiently separated from proprotor excitation frequencies.
3Ease of manufacture
If a single joint stiffness configuration is used for the tail assembly, then the structure is simpler to manufacture, but neither the chordwise pivot mode nor the beamwise bending mode can be independently tuned to avoid excitation frequencies
Solution Approach 1:
The tail assembly is segmented into multiple joint locations (forward joint and aft joint) with different stiffness characteristics. This segmentation allows each joint to be optimized for controlling specific vibration modes, providing the adaptability to independently tune frequencies while maintaining relatively simple individual joint designs that are manufacturable.
Solution Approach 2:
The patent implements different stiffness parameters at different joint locations along the stabilizer. The forward joint has stiffness optimized for chordwise pivot control while the aft joint has stiffness optimized for beamwise bending control. This parameter variation enables independent mode tuning without requiring complex single-joint mechanisms, balancing manufacturability with tuning versatility.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration prevents undesirable resonant vibrations and enhances aeroelastic stability by independently tuning the chordwise pivot and beamwise bending modes, ensuring the tail assembly operates safely and efficiently.
Implementation Method 1
The aft joint may include a clevis fitting coupled to the aft spar and a lug fitting coupled to the fuselage with the clevis fitting and the lug fitting coupled together by at least one spherical bearing assembly such that the clevis fitting and the lug fitting allow the stabilizer to pivot about the pitch axis.
Implementation Method 2
The forward joint may include a shaft coupled to the forward spar by first and second outboard bearings and coupled to the fuselage by first and second inboard bearings with the shaft having an axis extending in an axial direction. In such embodiment, the shaft may be a generally cylindrical hollow shaft such as a generally cylindrical tapered hollow shaft. Also, in such embodiment, the shaft may be formed from a metal.
Implementation Method 3
A tail assembly with tailored axial and bending stiffness configurations at the forward and aft joints, decoupling the chordwise pivot mode below the first excitation frequency and the beamwise bending mode above the second excitation frequency, using spherical and elastomeric bearings to control the stabilizer's motion.
Data Source
AI summary
A tail assembly for a rotorcraft having a fuselage. 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 decoupled from the beamwise bending mode of the stabilizer.