Hyperstatic Truss Connecting Rods for Fan Engine Suspension
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Solution Overview
Problem
Bypass turbojet engines face a mass penalty due to oversized connecting rods needed to withstand both tensile and compressive loads from fan blade breakage, which also incur an aerodynamic penalty from obstructing airflow.
Innovation Solution
A hyperstatic truss of connecting rods with varying strengths, where internal rods handle both tension and compression, and shrouds only handle tension, allowing independent optimization of cross-sectional area and profile for reduced mass and drag.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If connecting rods are sized to withstand both tensile and compressive loads from fan blade breakage, then mechanical reliability is improved, but mass increases
Solution Approach 1:
The connecting rod is divided into two functional segments: an internal rod that handles both tension and compression loads, and an external shroud that handles only tension loads. This segmentation allows each component to be optimized for its specific function, reducing the overall mass while maintaining the required reliability for withstanding fan blade breakage loads.
Solution Approach 2:
Different parts of the connecting rod structure are given different mechanical properties and functions. The internal rod is designed with properties suitable for both tensile and compressive loading, while the external shroud is designed specifically for tensile loading. This local differentiation of quality allows the structure to be lighter while still meeting the reliability requirements for extreme load conditions.
2Strength
If connecting rods are made with larger cross-sectional area to withstand compressive loads, then compressive strength is improved, but aerodynamic drag increases
Solution Approach 1:
The connecting rod is segmented into an internal rod and external shroud, where the shroud provides aerodynamic protection without needing to bear compressive loads. This allows the aerodynamic profile to be optimized independently of the structural requirements for compressive strength, reducing drag while maintaining strength.
Solution Approach 2:
The external shroud acts as an intermediary that provides aerodynamic protection to the internal rod. It carries tensile loads and protects the internal rod from aerodynamic drag, allowing the internal rod to be optimized for structural strength without being directly exposed to aerodynamic forces that would increase drag.
Data Source
AI summary
A hyperstatic truss including connecting rods, used for suspension of a first ring, forming part of an engine case, inside a second ring concentric to the first ring, the connecting rods being secured at one end to the first ring and at the other end to the second ring. The tensile stiffness of the connecting rods is greater than the compressive stiffness thereof. The truss for example can be used for suspension of a ducted-fan turbine engine with an elongate bypass duct.