The reduction of scale force uncertainty in low thrust regions

Minimal scale force uncertainty in low thrust regions of an engine’s operating envelope is critical to meet flight test specifications as Air Force requirements become more stringent. In these low thrust regions, scale force accounts for over 90% of the total thrust uncertainty. Through experiments and experience, the elemental errors associated with scale force uncertainty have proven to be reducible. The primary contributors to scale force uncertainty in low thrust regions include such errors as thrust stand/load cell hysteresis and pre- to post-test zero shifts. Cell heating effects are also very important because they affect the test cell environment where scale force measurements are obtained. During engine altitude development testing in the C1 Altitude Test Facility at Arnold Engineering and Development Center (AEDC), these contributors were evaluated in an effort to obtain uncertainty levels of less than 1%. To date, data have been collected via thrust stand calibrations, cell temperature correlations, and pre- to post-test load cell readings. A root-sum-square uncertainty analysis was used to evaluate five operating scenarios to determine the optimal procedures for low scale force uncertainty at the 3,000 Ibf scale force level. The five operating scenarios were derived from experimental efforts to reduce hysteresis and zero shifts. The results show that scale force uncertainty levels can be reduced to 0.74% of 3,000 Ibf, down from an initial level of 2.41% of 3,000 Ibf. With these results, operating procedures for thrust stand calibrations and temperature limits on the test cell environment were obtained.