Abstract
Abstract. A method is described that estimates the error in the static pressure measurement on an aircraft from differential pressure measurements on the hemispherical surface of a Rosemount model 858AJ air velocity probe mounted on a boom ahead of the aircraft. The theoretical predictions for how the pressure should vary over the surface of the hemisphere, involving an unknown sensitivity parameter, leads to a set of equations that can be solved for the unknowns – angle of attack, angle of sideslip, dynamic pressure and the error in static pressure – if the sensitivity factor can be determined. The sensitivity factor was determined on the University of Wyoming King Air research aircraft by comparisons with the error measured with a carefully designed sonde towed on connecting tubing behind the aircraft – a trailing cone – and the result was shown to have a precision of about ±10 Pa over a wide range of conditions, including various altitudes, power settings, and gear and flap extensions. Under accelerated flight conditions, geometric altitude data from a combined Global Navigation Satellite System (GNSS) and inertial measurement unit (IMU) system are used to estimate acceleration effects on the error, and the algorithm is shown to predict corrections to a precision of better than ±20 Pa under those conditions. Some limiting factors affecting the precision of static pressure measurement on a research aircraft are discussed.
Highlights
Static pressure measurement on an aircraft is inherently problematic because the pressure changes as the air accelerates around the wings and fuselage, as predicted by the Bernoulli equation
On the University of Wyoming King Air (UWKA) research aircraft, static pressure errors can be as large as 2 % of qc at research aircraft speeds (∼ 100 m s−1), and this error transfers directly to an error of 1 % (1 m s−1) in airspeed
We show that the static pressure error can be determined from the differential pressure measurements, assuming that sensitivity factor f can be determined
Summary
Static pressure measurement on an aircraft is inherently problematic because the pressure changes as the air accelerates around the wings and fuselage, as predicted by the Bernoulli equation. In addition to causing errors in pressure-derived aircraft altitude, the static defect leads directly to errors in airspeed and other measurements that need dynamic corrections, such as temperature, for example. When lateral airspeed components occur because of turbulence, or by rudder application causing sideslipping, this assumption may not be correct, as is shown later in this paper. These errors are likely to change with the deployment of wing flaps, landing gear, or the addition of external housings and fairings used to accommodate instruments. We first use trailing sonde data to determine the probe sensitivity factor, and compare resulting error estimates with accurate altitude measurements from a GPS-aided inertial measurement unit (IMU), allowing for an independent check of the precision of the algorithm and an examination of the effects of aircraft acceleration
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