AIRCRAFT WIND MEASUREMENT CONSIDERING LIFT-INDUCED UPWASH

Flow distortion is a universal consideration in the measurement of win d. Usually the distortion results from deflection of the flow by the s upport. Instruments mounted on airplanes, however, experience an addit ional deflection associated with lift, which changes rapidly and conti nuously during flight. In front of the airplane, this deflection appea rs as upwash. Characteristic upwash contamination ranges from 0.5 to 2 .5 ms(-1), depending on wing loading, flight speed, and forward distan ce from the wing to the measurement location. On pressure-radome insta llations the distance from the wing is usually small and the potential for upwash contamination large. Sensors mounted at the end of long pr obes attached to smaller airplanes with light wing loading are less su sceptible to upwash contamination. Since wing loading also depends on the local vertical wind velocity the upwash velocity is directly corre lated with vertical wind velocity. Generally, empirical corrections ar e derived from flight tests. We develop a simple method from principle s of aerodynamics which explains these corrections in terms of upwash. The applicability of our approach to moderate upwash is demonstrated with data collected from the NOAA Long-EZ research airplane. More seve re upwash conditions require a more sophisticated correction beyond th e scope of this paper, but basically derived from the same principles.