# NASA - National Aeronautics and Space Administration

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Balance Basics

A wind tunnel balance is a device that measures the aerodynamic loads a
model experience during a wind tunnel test. A balance is just a multiple axis
force transducer. Balances are designed to measure some or all of the three forces and three moments a model experience. In aerodynamics terms, these forces and moments are called: Normal, Side, and Axial Force and Pitch, Yaw, and Rolling moment.

Balances come in many different designs and configurations. Most balances use strain gauged elements that relate applied loads to voltage signals. In the past, wind tunnel loads where measured using weight scales, much like the ones that existed in doctor's offices, and that's why today they're called balances.

Variations in Wind Tunnel Balances

• Size and Shape
• How it Attaches to the Model and to the Support System
• The Number of Forces and Moments it can Measure
• The Electronics, Type of Strain Gauges, and Wiring
• Composed of Single or Multiple Assembled Pieces

• Common Balance Types
( Strain Gauged )

• Internal Multiple Component Balance, with a tapered end, measures six axis loads
• Internal Single Piece Balance, with a cylindrical end, measures six axis loads
• Semi-Span Balance, Single Piece, measures five axis loads
• Ring or Rotor Balance
• Flow through Balance

• How a Strain Gauged Balance Works

Physical Elements
Balances are made of flexures that deflect with load is applied. These flexures are designed to respond to load in a particular axis. Balance that can measure multiple loads and moments have individual flexures that each measure load in one axis. Strain gauges are bonded to these flexures to measure the deflections.

Electrical Elements
Applied loads cause the bonded strain gauges to stretch. When a strain gauge changes length its electrical resistance changes. Individual strain gauges are wired in a whetstone bridge so that these small resistance changes can be measured as voltage signals.

• Choosing a Balance for a Wind Tunnel Test
• Determining the Health of a Balance
• Balance Capacity
• Calibrating a Balance
• Choosing a Calibration Load Envelope
• Generating a Calibration Reduction Matrix

• Nonlinear Behavior
• Interactions
• Accuracy
• Repeatability
• Time Dependent Behavior
• Temperature Dependence

• Ames Balance Calibration Lab does a basic inspection for each balance before use. It is recommended that customers do an inspection before using a balance. Although not comprehensive, a basic check will avoid aggravation that could result if a faulty balance is installed in a wind tunnel model.