Variable Waist Cowl for Counter-Rotating Propeller Slipstream Control
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Solution Overview
Problem
Counter-rotating propellers face issues with mismatched slipstream and prop wash, leading to reduced performance and increased noise due to operating off design points, especially at low speeds and high power levels, causing rear propeller tip stall and energy wasting swirl.
Innovation Solution
A variable waist cowl structure is introduced between the front and rear propellers to control the slipstream, allowing for adjustment of the effective hub to tip radius ratio, ensuring proper alignment and reducing outer convergence of the prop wash, thereby maintaining optimal operating conditions across various flight regimes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If counter-rotating propellers are used to eliminate power wasting swirl, then power density is increased, but noise levels increase and propellers operate off design points
Solution Approach 1:
A variable geometry waist cowl is introduced as an intermediary device between the front and rear propellers. The cowl actively manages the slipstream flow from the front propeller, directing it to properly align with the rear propeller disk. This mediator structure enables the counter-rotating propeller system to maintain power density while reducing noise by ensuring both propellers operate at or near their design points through controlled slipstream alignment.
2Loss of energy
If the front prop tip slipstream passes outside the aft prop tip diameter, then partial prop wash outer swirl persists, but prop set performance is reduced
Solution Approach 1:
The waist cowl incorporates variable geometry capabilities that allow it to dynamically adjust its configuration in response to changing flight conditions and power levels. This dynamic adjustment enables the cowl to optimize slipstream alignment for the rear propeller across different operating regimes, ensuring that the prop wash properly covers the rear prop disk without excessive outer swirl, thereby maximizing prop set performance while minimizing energy losses.
3Speed
If the front prop tip slipstream passes radially inward from the aft prop tip, then the aft prop tip operates in low axial velocity flow, but tip stall and high noise occur
Solution Approach 1:
The variable geometry waist cowl changes the flow parameters of the slipstream by actively directing and shaping the air flow from the front propeller. By adjusting the cowl's geometry, the system modifies the axial velocity distribution and slipstream alignment angle to ensure that the rear propeller tips receive adequate high-velocity flow across different operating conditions, preventing tip stall and the associated high noise levels while maintaining efficient propeller operation.
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 solution ensures the rear propeller operates efficiently by maintaining proper slipstream alignment, reducing tip stall and noise, and eliminating energy wasting swirl, maximizing performance and efficiency throughout the operating regime.
Implementation Method 1
A variable waist cowl structure is introduced between the front and rear propellers to control the slipstream, allowing for adjustment of the effective hub to tip radius ratio, ensuring proper alignment and reducing outer convergence of the prop wash
Data Source
AI summary
A counter rotating propeller system comprises a structure configured to control the front propeller outer slipstream and prop wash. Specifically disclosed is a structure configured to vary the effective hub-tip radius ratio of the rear propeller.