Contra-rotating Propeller Noise Reduction via Tip Speed and Pitch Control
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Solution Overview
Problem
Contra-rotating propeller gas turbine engines face noise penalties compared to ducted fan engines, and existing noise reduction methods are insufficient to meet stringent new aircraft noise restrictions.
Innovation Solution
A method of operating a contra-rotating propeller engine with variable pitch blades, where the tip speed of at least one propeller is increased by 20% +/- 5% during take-off, climb, or approach, and the blade pitch is adjusted to minimize noise by optimizing blade numbers and axial gap between propellers, reducing wake strength and noise generated by aerodynamic interactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the tip speed of propellers is increased during take-off, climb or approach, then noise levels are reduced, but the propeller blades experience higher aerodynamic loads and require adjusted pitch angles
Solution Approach 1:
The patent applies dynamics by making the propeller blade pitch angles variable rather than fixed. During take-off, climb, or approach phases, the pitch angles are adjusted to a first pitch angle that is lower than the cruise pitch angle. This dynamic adjustment allows the system to operate at higher tip speeds during certain phases while managing aerodynamic loads through pitch control, thereby reducing noise without compromising blade structural integrity
Solution Approach 2:
The patent changes the operating parameters by increasing propeller tip speed during take-off, climb, or approach phases compared to cruise conditions. Simultaneously, the pitch angle parameter is modified to a lower value during these same phases. This parameter change strategy enables noise reduction through higher tip speeds while the reduced pitch angle manages the aerodynamic loads that would otherwise increase with higher speeds
2Object-affected harmful factors
If unequal numbers of blades are used on front and rear propellers, then wake interaction noise is reduced, but aerodynamic performance and weight distribution become more complex
Solution Approach 1:
The patent applies asymmetry by using unequal numbers of blades on the front and rear propellers. Specifically, the front propeller has a different number of blades than the rear propeller, which prevents the formation of coherent wake patterns that would otherwise interact to create high noise levels. This asymmetric configuration disrupts the wake interaction mechanism while maintaining acceptable aerodynamic performance
Solution Approach 2:
The patent segments the propeller blade arrays into distinct front and rear configurations with different blade counts. This segmentation approach allows each propeller to be optimized independently for its specific role in the contra-rotating system, with the front propeller having a different blade number than the rear propeller to minimize wake interaction noise while maintaining overall system efficiency
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
Significantly reduces noise levels and far-field noise directivity by minimizing wake interaction noise while maintaining aerodynamic performance, allowing for higher altitude capabilities and reduced community noise.
Implementation Method 1
the blades are rotated closed relative to those that would be required on a design without a tip speed increase
Data Source
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AI summary
A method of operating a contra-rotating propeller engine that preferably comprises a 12 bladed front and a 9 bladed rear propeller. As is conventional, the engine is operated during at least a take-off phase, a cruise phase and an approach phase; during the cruise phase the engine operates with a generally constant propeller tip speed. The method is characterized by the step of operating the engine such that the tip speed of either or both of the propellers, during at least one of take-off, climb or approach, at least 10% greater than cruise tip speed. With a specific front to rear propeller spacing, increasing the tip speed reduces overall noise generated by the propellers.