Variable Pitch Fan Over-Pitching for Engine Out Drag Reduction
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Solution Overview
Problem
Turbofan engines with very-high-bypass-ratio turbofan engines experience significant drag during an engine out (EO) condition due to air flow separation behind outer guide vanes, even when using variable pitch fans that are feathered to prevent spinning, as the feathered position still generates swirl causing drag.
Innovation Solution
Implementing a turbofan engine with a variable pitch fan that can be over-pitched beyond the feathered position and a nonrotatable outer guide vane design, combined with a rotation control device to prevent the fan from rotating in the opposite direction during an EO condition, minimizing air flow separation and drag.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the variable pitch fan is feathered to prevent spinning during engine out condition, then the fan will not spin, but the feathered fan blades still generate swirl causing air flow separation and drag
Solution Approach 1:
The patent changes the pitch angle parameter of the fan blades from the conventional feathered position to an over-pitched position (beyond 90 degrees relative to the plane of rotation). This parameter change eliminates the swirl generation that causes air flow separation behind the outer guide vanes, thereby reducing drag while still preventing fan spinning during engine out condition.
2Productivity
If the fan is allowed to spin freely during engine out condition, then the fan can maintain rotation, but air flow separation behind outer guide vanes causes substantial drag
Solution Approach 1:
Instead of allowing free spinning and accepting the drag penalty, the patent inverts the conventional approach by using active pitch control to set the blades at an over-pitched angle. This inversion transforms the fan from a passive free-spinning component to an actively controlled component that eliminates the harmful swirl and air flow separation, reducing drag significantly.
3Reliability
If a rotation control device is added to prevent fan rotation in opposite direction, then fan spinning is controlled, but device complexity increases
Solution Approach 1:
The patent integrates the rotation control function into the existing variable pitch mechanism, allowing the same pitch control system to serve dual purposes: controlling blade pitch angle for drag reduction and preventing fan spinning. This multi-functionality approach avoids adding separate dedicated anti-spin devices, thereby limiting the increase in system complexity.
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 reduces engine out drag by up to one-third compared to known systems, allowing for minimal air flow separation and improved engine performance during an EO condition.
Implementation Method 1
air flow separation behind the outer guide vanes (OGVs), and if this air flow separates behind the OGVs, such air flow separation may block the air flow from passing through the engine
Data Source
AI summary
There is provided a turbofan engine for an aircraft. The turbofan engine has a core with a fan cowl and a variable pitch fan (VPF) configured to only rotate in a first rotation direction. The VPF has a plurality of fan blades each configured to over-pitch to an over-pitch position relative to a feathered position. The turbofan engine has outer guide vanes (OGVs) axially disposed downstream of the VPF, and has a rotation control device to prevent the VPF from rotating in a second rotation direction opposite the first rotation direction, during an engine out (EO) condition of the turbofan engine. When the VPF is prevented from rotating during the EO condition, the fan blades are over-pitched to the over-pitch position relative to the feathered position, to achieve no or minimal air flow separation about the OGVs, and to reduce drag of the turbofan engine during the EO condition.