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› Back to ARC Wind Tunnel Division 
[ copied from a 1960s Task Corporation brochure. Task Corp. name was changed to Able.]
The Task Corporation Series D, six component internal strain gage balance is basically a floatingframe type. The primary frames consist of an inner rod, which fastens to the model support sting, and a cylindrical outer case, which is inserted into and attaches to the model. Forces and moments are resisted by individually removable elements, employing flexure pivots, connected between the inner rod and outer case of the balance.
The six force and moment sensing components of the balance consist of two normal force elements for determination of normal force and pitching moment, two side force elements for determination if side force and yawing moment, a dual chord [axial] force element and a dual roll moment.
The normal and side force elements are equipped with relaxation members at either end and are arranged to act in roll as a set of crossed flexures. Similar relaxation members provide compliance in the chord force direction. The rolling moment elements are provided with flexure pivots at either end which are designed to transmit pure rolling moment to the gage section. The dual chord element is located inside the dual roll element and transmits axial force from the outer case to the inner rod.
Accuracy
All points from any series of loadings of a single element produce data within ±0.25% of maximum load or ±0.5% of applied load when compared with the best straight line fit. This accuracy is inclusive of all scatter, hysteresis, and nonlinearity for both plus and minus loads.
Interactions
Percentage interactions on any gage member caused by the application of full design load on any other gage member fall within the following values: Seventy percent of the possible first order interactions will be less than or equal to 0.5%. An additional 20% of the first order interactions will be less than or equal to 1.0%. The remaining 10% of the first order interactions will be less than or equal to 3.0%. All interactions in excess of 1.0% will be linear within ±10% of the interaction output.
The percentage values of the first order interactions may change slightly if specifications require a small diameter balance or a very light or very heavy load range.
Thermal effects * [ not used in years ]
Transient temperature errors in any gage element do not exceed 1.0% of full load output when the sting temperature is constant or changing and the outer case temperature is changing at a rate of 50 deg F per minute between 60 deg F and
180 deg F.
At stabilized temperatures, the error in any gage element does not exceed 0.5% of full load output between 60 deg F and
180 deg F.
* Values slightly different for 0.75 MKII and 0.50MKI balance. Consult engineering for details.
Sensitivity
Full load output of any gage member is approximately 1 millivolt per volt input.
Gage characteristics
Bridge resistance is 120 ohms for normal and side force elements.
The basic bridge resistance of the rolling moment and chord force elements is 120 ohms. However, these components each operate with two full bridges wired in parallel. Thus the resistance is 60 ohms.
Operating voltage of each element is 6 volts. Lower voltage is recommended for balances 0.75 inch diameter or less.
Each balance is subjected to a performance evaluation to demonstrate satisfactory operation of all functions. It is expected that a more detailed calibration, tailored to the individual laboratory's needs, will be performed by the user. More complete calibrations can be performed by Task if desired.
Temperature Compensation Principle
The arrangement of chord force elements provides complete mechanical compensation for radial temperature gradients. The two chord force element strain gage bridges are wired to produce a signal when the forces on the two elements are opposite in sign. (This occurs only when an external axial load is applied.) If there is a length increment due to change in temperature added all along the inner rod or outer case, each axial force gage will experience an increment of load having the same sign and the signals will cancel. Thus the sensitivity of chord force to temperature gradients has been significantly reduced.
All gaged sections, with the exception of rolling moment, are of the bending type employing 4active flat wound, bakelite bonded, 120 ohm strain gages. Rolling moment is resisted by a torque tube to which are bonded similar gages also wired in the form of a 4active element bridge.
All strain gages are prematched in complete bridges for resistance and change in resistance with temperature. Bonding is accomplished using a combination of heat cycling and pneumatically applied pressure. After bonding, the individual bridges are again checked for temperature effects and trimming resistors are added where necessary.
Task Corporation also produces all necessary balance accessories including:
 Ring and plug gages  Master gages for balancetosting joint
 Balance gage  Exact duplicate of the balance exterior and sting fitting. Hole through center permits passage of leads when used in place of a balance.
 Calibration body  Loading sleeve used to apply loads for sensitivity and interaction calibration. Includes roll arm, precision circular level and flexures to which weight hangers attach.
 Calibration rig  Complete single or multiplane rigs for the application of calibrating loads outside of the tunnel.
 Calibration weights  Traceable to National Bureau of Standards are offered in the ranges 2#, 5#, 10#, 25#, 50#, 75# and 100#. Weight tolerance is ±0.1% of the total unit weight. More exacting tolerances are quoted on request.
 Coolant jackets  Required for balance operation up to stagnation temperatures of 1,500 deg F. These jackets are available for all balance sizes and require a minimal diameter increase.
 Overload stops  Are available for roll and chord force in all balance sizes. Stops for all six components may be provided in balances with external diameters of 2.0 inches and larger.
Standard Performance Calibration Procedure
1. Positive and negative design loads are applied to the balance outer case such that each force and moment element is
loaded individually. The load on each element is applied in increments not greater than onefifth of full design load.
Normal forces and side forces are applied orthogonally to the outer case, and axial force is applied along the axis z
of the outer case. Rolling moment is applied to the outer case by means of a moment acting in a plane normal to the
balance axis.
Each gage output is tabulated and sensitivity calculated for each element in terms of millivolts per volt per pound or
millivolts per volt per inch pound for both increasing and decreasing loadings. Interactions are plotted in terms of percent full
load output in the member being interacted upon, versus applied load.
2. With the sting taper maintained at approximately 60 deg F the balance case temperature is increased at a rate of a least
5 deg F per minute until the balance has reached 180 deg F. The outer case temperature is then maintained at 180 deg F until
the inner case temperature is stabilized, then the temperature is reduced at least 5 deg F per minute until the balance is
again stabilized at 60 deg F. the output of all gages and thermocouples are recorded every two minutes throughout the run.
A similar test is also performed wherein both the sting taper and balance sleeves are raised simultaneously in temperature
to 180 deg F.
With the balance at approximately 180 deg F, each component is loaded separately and the sensitivity in millivolts per volt
per pound or millivolts per volt per inch pound is calculated.
Prior to assembly, the individual balance elements are subjected to a sudden pressure change from 2 to 60 psia and the
outputs recorded.
Representative standard sizes and ranges of Series D balances are given in the accompanying table. The force and moment values shown are based on typical requirements. Other sizes and/or load ranges are available either as new designs or modifications of existing types. At present, there are in excess of 50 standard designs available.




















































































































* Approximate  Varies with type of attachment.