Mistuned Fan Blades for Supersonic Flutter Resistance
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Solution Overview
Problem
Gas turbine engine compressor rotors experience aerodynamic instability due to supersonic flutter, which cannot be mitigated by simply accelerating through a speed range and can lead to undesirable stress load levels, unlike subsonic or transonic stall flutter.
Innovation Solution
A mistuned fan design with alternating first and second fan blades having different airfoil thickness distributions, where the thickness of the airfoil at specific span distances corresponds to regions of high and low strain energy, creating distinct natural vibration frequencies to prevent supersonic flutter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fan blades are made with uniform airfoil thickness distribution, then manufacturing is simplified and structural consistency is maintained, but supersonic flutter occurs due to identical natural vibration frequencies of adjacent blades
Solution Approach 1:
The patent applies local quality by introducing frequency modifiers at specific span-wise locations (high strain energy and low strain energy regions) rather than uniformly across the entire airfoil. This creates localized thickness variations that selectively adjust natural vibration frequencies of adjacent blades, providing flutter resistance while maintaining manufacturing simplicity in non-modified regions
Solution Approach 2:
The patent introduces asymmetry by creating alternating patterns of first and second frequency modifiers with different thickness differentials at corresponding span-wise locations. This asymmetric modification breaks the symmetry of uniform blade designs, creating frequency mistuning between adjacent blades to prevent supersonic flutter while maintaining overall structural consistency
2Reliability
If frequency modifiers are added to adjust natural vibration frequencies, then supersonic flutter is prevented through frequency mistuning, but manufacturing complexity increases
Solution Approach 1:
The frequency modifiers are applied only at specific span-wise locations corresponding to high and low strain energy regions, rather than across the entire airfoil surface. This localized approach minimizes manufacturing complexity by limiting modifications to discrete areas while achieving the desired frequency mistuning effect for flutter prevention
Solution Approach 2:
The patent changes local geometric parameters (airfoil thickness) at specific locations to create frequency modifiers. By adjusting the thickness differential parameter at high and low strain energy regions, the natural vibration frequencies are tuned without requiring fundamental changes to the overall airfoil manufacturing process
3Reliability
If airfoil thickness is reduced at high strain energy locations, then natural vibration frequency is adjusted for mistuning, but structural strength may be compromised
Solution Approach 1:
The frequency modifier at the high strain energy location is designed with a controlled thickness differential that is sufficient to adjust the natural vibration frequency for frequency separation, yet limited in magnitude to maintain adequate structural strength. The local modification is optimized to achieve the minimum necessary frequency adjustment without excessive thickness reduction
Solution Approach 2:
The patent compensates for potential strength loss from thickness reduction at the high strain energy location by introducing a frequency modifier at a low strain energy location. The combined effect of modifiers at both locations achieves the desired frequency separation while the low strain energy location modification helps maintain overall structural integrity
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
The mistuned fan design effectively reduces the occurrence and impact of supersonic flutter by creating a sufficient frequency separation between adjacent fan blades, preventing unwanted resonance and stress load issues.
Implementation Method 1
the first and second frequency modifiers generating different natural vibration frequencies for each of the first and second fan blades
Implementation Method 2
Supersonic flutter (which can be either stalled or unstalled, as shown in FIG. 6) occurs in the high speed regime of the compressor or fan where tip speed is very high
Implementation Method 3
The first span distance corresponds to a span-wise location of high strain energy and the second span distance corresponds to a span-wise location of low strain energy
Data Source
AI summary
A compressor rotor for a gas turbine engine is described which includes sets of blades having different airfoil thickness distributions, each including a frequency modifier forming a thickness differential relative to a baseline blade thickness. The frequency modifiers provide different natural vibration frequencies for each of the blades, and facilitate modifying natural vibration frequency separation between adjacent blades of the compressor rotor.