Wind Tunnel Balance Axial Force Measurement Using Tapered Corner Flexures
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Solution Overview
Problem
Wind tunnel balances face challenges in accurately measuring axial forces due to their limited sensitivity and fatigue resistance, especially under high static and dynamic loads, which restricts the testing of models to their maximum design loads and requires frequent calibration.
Innovation Solution
The design incorporates an internal balance with an axial force measurement section featuring a longitudinal slot and corner flexure groups with tapered flexures, enhancing axial force sensitivity and structural integrity while minimizing the interaction of other forces, allowing for higher load handling and improved data quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a multi-piece balance is used to increase load-carrying capability, then the balance can handle higher static and dynamic loads, but the balance experiences higher fatigue stress and requires frequent calibration
Solution Approach 1:
The balance is divided into multiple sections (first section with longitudinal slot, second section without slot) that are interconnected by corner flexure groups. This segmentation allows each section to be optimized for specific functions while maintaining overall structural integrity and load-carrying capability.
Solution Approach 2:
Different regions of the balance have different structural properties. The first section has a longitudinal slot with corner flexures for axial force measurement, while the second section is solid for moment and lateral force measurement. This local differentiation optimizes each region for its specific measurement function while maintaining overall reliability.
2Strength
If the balance size is increased to improve load-carrying capability, then higher static and dynamic loads can be handled, but the cavity cross-sectional size limits the balance diameter
Solution Approach 1:
The balance design transitions from a solid cross-section to a segmented structure with longitudinal slots, effectively utilizing the available cavity space in the longitudinal dimension while maintaining appropriate cross-sectional dimensions to fit within the model aircraft cavity.
3Measurement precision
If axial force sensitivity is increased to measure smaller axial forces, then measurement precision improves, but the balance becomes more susceptible to interference from other forces
Solution Approach 1:
The balance is segmented into distinct measurement sections with specialized structures. The first section with the longitudinal slot and corner flexures is specifically designed for axial force measurement, while the second solid section measures moments and lateral forces, reducing cross-interference between measurements.
Solution Approach 2:
Each section of the balance has locally optimized structural properties. The first section has reduced stiffness in the axial direction through the longitudinal slot to enhance axial force sensitivity, while the second section maintains higher stiffness for accurate moment and lateral force measurement, minimizing mutual interference.
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 increases the sensitivity and accuracy of axial force measurements, reduces the impact of other forces on axial force readings, and extends the balance's fatigue life, enabling testing under higher loads with consistent data quality.
Implementation Method 1
a flexure thickness that is tapered along the flexure length from each of the first and second flexure roots to a reduced thickness proximate the flexure midpoint
Implementation Method 2
strain gages mounted on the internal balance which may be provided as a multi-piece balance or as a single-piece balance
Data Source
AI summary
A balance has a balance centerline and an axial force measurement section having a longitudinal slot partially dividing the axial force measurement section into a first part and a second part longitudinally overlapping each other and interconnected by corner flexure groups. The axial force measurement section has an axial force measurement beam located approximately midway between the corner flexure groups. The axial force measurement beam has strain gages for determining axial force on the model. Each corner flexure group includes flexures each having a first flexure root and a second flexure root respectively joined to the first part and the second part. The flexures each have a flexure midpoint between the first and second flexure roots, and a flexure thickness that is tapered along the flexure length from each of the first and second flexure roots to a reduced thickness proximate the flexure midpoint.