AIAA R-091-2003 Recommended Practice Description
Following is a brief summary of pertain information regarding terms and methods used for applying balance instrumentation when using the AIAA Recommended Practice R-091-2003. These “formats” are what is used for accepted terms, order of gages, units, etc.
AIAA Format Typical Standards:
Gage orders are,
- Force balance gage order is NF1, NF2, SF1, SF2, RM, AF.
- Moment balance gage order is PM1, PM2, YM1, YM2, RM, AF.
- Direct-Read balance gage order is NF, PM, SF, YM, RM, AF.
Gage names are,
- NF = Normal Force
- SF = Side Force
- RM = Rolling Moment
- PM = Pitching Moment
- YM = Yawing Moment
- AF = Axial Force
- NF1 = forward normal gage
- NF2 = aft normal gage
- SF1 = forward side gage
- SF2 = aft side gage
- PM1 = forward pitching moment gage
- PM2 = aft pitching moment gage
- YM1 = forward yawing moment gage
- YM2 = aft yawing moment gage
A suffix “r” indicates output. “NF1” is Component Load for forward normal force in lbs, “NF1r” is Bridge Output in microvolts per volt (uV/V).
Computed loads use engineering units (EU) of pounds and inch-pounds.
Excitation is in volts.
AIAA Format and the Legacy Ames Format
Years ago Ames calibration format used two sets of equations, one for positive loads, another for negative loads for primary gage sensitivities. This lead to data reduction matrix format which was different from other balance calibration labs. To reduce confusion, a Balance Working Group developed the AIAA recommended practice for a common standard among users of internal strain gage balances.
Calibrations in previous years are still on file at the Ames Balance Calibration lab and are still in the legacy Ames format. Following is a brief of that format.
The first term in the AIAA format shown in the example below,
EU = K2*X^2 + K1*X where X is uV/V (used for both used for positive and negative loads)
The Ames legacy format would use two different equations for the first term, for example,
Positive loads: EU = K2POS*X^2 + K1POS*X where X is uV/V
Negative loads: EU = K2NEG*X^2 + K1NEG*X where X is uV/V
Computations must know when the loading crosses the x-axis. Reason for two different slopes accounts for differences in mechanical stress of strain gages mounted in circular flexures.
Also, coefficients for the Ames legacy format must be rescaled so K2POS, K1POS, K2NEG, and K1NEG are in EU per Counts. This rescaling is done with calibration shunt resistors and ratiometric calculations convert the coefficients so they are applicable in the computations program (SDS). While analyzing data, one has to interchange the units when comparing results to the calibration lab values.
Coefficients for the AIAA format may be left in the same Balance Calibration lab units of EU per uV/V. Calibration shunt resistors are still used to rescale the instrumentation data because the raw units are in SDS counts. The rescale ratio from the Balance Constant Conversion Form converts the units of raw data from SDS Counts to microvolt per volt.