Comparing a 3-d printed hemispherical-head and Rankine-body probe shapes for very low speed flush air data system (FADS) measurements

Abstract

This study investigates the feasibility of using Flush Air Data Sensing (FADS) System technology for air data measurements at the very low-airspeeds, where many Unmanned Aerial Vehicles (UAVs) operate. FADS is a non-intrusive alternative to pitot probes, where the vehicle nosecone, wing leading edge, or other aerodynamic surfaces are configured with multiple pressure-ports distributed along the windward face. Although FADS technology has been used for a variety of high-speed aircraft, FADS has never been applied to very low-airspeed flight regimes. This study reports on wind tunnel tests of two 3-D printed shapes: 1) a cylindrical body with a hemispherical head, and 2) a Rankine-Body. These body shapes can act as a vehicle analog to a wide range of three-dimensional shapes and account for both blunt leading edge and trailing after body flow characteristics. For this study the "probes" were printed with 5 pressure ports and the associated flow channels aligned at 0o, +22.5o and +45o direction-angles along the vertical centerlines of the models. Sensed pressure data were curve-fit, developing quasi-potential flow calibration models for each probe, with coefficients compiled as a function of geometric angle-of-attack and tunnel airspeed. The calibration models account for end-to-end systematic effects, including the mounting sting flow compression, up wash, and tunnel blockage. Using the derived calibration models and the sensed pressure data, the effective angles-of-attack were re-calculated using the well-known "Triples" algorithm. The associated airspeed and dynamic pressure are estimated from the sensed pressure data using non-linear regression. The resulting estimates are compared to the tunnel reference conditions. Generally, both probe shapes performed well, with the redundant 5-port arrangement allowing for significant noise rejection. Both probes achieved RMS airspeed errors of less than 5%, angle-of-attack errors less than 1 deg., and dynamic pressure errors of less than 12 pascals, across airspeeds ranging from 5 to 25 m/sec. The sensed Airdata measurements at the lowest airspeeds (5 m/sec), exhibited similar accuracy to those sensed at the highest airspeeds (25 m/sec), verifying the applicability of FADS technology to very low airspeed flight regimes.