Soft-In-Plane Proprotor Systems for Tiltrotor Aeroelastic Stability
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Solution Overview
Problem
Tiltrotor aircraft face limitations in maximum airspeed due to forward airspeed-induced proprotor aeroelastic instability, which is exacerbated by the natural frequencies of vibration in their structures, particularly in airplane mode, where the proprotor blades' frequencies align with excitation frequencies, leading to instability and potential mechanical failures.
Innovation Solution
The proprotor system is designed with proprotor blades that can independently flap and change pitch, featuring a first in-plane frequency less than 1.0/rev, coupled with a pitch control assembly and lead-lag dampers, allowing operation between 40% to 80% of design RPM, and utilizing composite materials and bearing assemblies to manage centrifugal forces and vibrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the proprotor blades operate at conventional frequencies (greater than 1.0/rev), then the structural strength and rigidity are maintained, but forward airspeed-induced proprotor aeroelastic instability occurs leading to mechanical failures
Solution Approach 1:
The patent changes the critical parameter of proprotor blade in-plane frequency from conventional values greater than 1.0/rev to a reduced value between 0.6/rev and 0.9/rev. This parameter change decouples the blade frequency from excitation frequencies generated during forward flight, eliminating the resonance condition that causes aeroelastic instability while maintaining structural integrity through careful design of the bearing assembly and blade root configuration.
2Speed
If the proprotor system is designed for high speed operation in airplane mode, then forward thrust capability is improved, but forward airspeed induces proprotor aeroelastic instability that limits maximum airspeed
Solution Approach 1:
The patent implements a parameter change in the proprotor blade in-plane frequency to a range of 0.6/rev to 0.9/rev, which eliminates the resonance condition that previously limited maximum airspeed. This allows the tiltrotor aircraft to operate at higher forward speeds in airplane mode without experiencing forward airspeed-induced proprotor aeroelastic instability, thereby increasing the maximum airspeed capability while maintaining blade stability.
3Strength
If the proprotor blades have natural frequencies that align with excitation frequencies, then the structural response to operating parameters is amplified, but this leads to aeroelastic instability and potential mechanical failures
Solution Approach 1:
The patent changes the natural frequency parameter of the proprotor blades from values that align with excitation frequencies (greater than 1.0/rev) to a reduced range of 0.6/rev to 0.9/rev. This frequency detuning prevents resonant amplification of structural response to operating parameters and environmental conditions, thereby eliminating the condition that leads to aeroelastic instability and mechanical failures while maintaining necessary structural strength.
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 design decouples the proprotor blades' frequencies from excitation frequencies, reducing instability and enabling safe operation across both helicopter and airplane modes, enhancing the aircraft's speed capabilities and reducing mechanical stress.
Implementation Method 1
a lead-lag damper coupled to the hub
Implementation Method 2
A bearing assembly is disposed between the inboard and outboard arcuate sections of each loop yoke. Each bearing assembly includes a flapping bearing disposed generally within the inboard arcuate section of the respective loop yoke and coupled to the hub
Implementation Method 3
a centrifugal force bearing disposed generally within the outboard arcuate section of the respective loop yoke
Data Source
AI summary
A soft-in-plane proprotor system for a tiltrotor aircraft having a helicopter mode and an airplane mode. The proprotor system includes a hub, a plurality of proprotor blades and a plurality of loop yokes, each coupling one of the proprotor blades with the hub and each including first and second longitudinal sections extending between inboard and outboard arcuate sections and a bearing assembly disposed between the inboard and outboard arcuate sections of each loop yoke. Each bearing assembly includes a flapping bearing disposed generally within the inboard arcuate section of the respective loop yoke and coupled to the hub, a lead-lag damper coupled to the hub, a centrifugal force bearing disposed generally within the outboard arcuate section of the respective loop yoke and a blade anchor coupled between the lead-lag damper and the centrifugal force bearing. The blade anchor is also coupled to the respective proprotor blade.