A Direct Comparison of Small Aircraft Dynamics between Wind Tunnel and Flight Tests

Abstract

The miniaturization of embedded electronics and sensors driven by the rapid development of mobile devices has enabled powerful avionics systems for very small aircraft. This enables a potential step forward in accurate flight data gathering for vehicles weighing 5 kg or less. Being able to flight test a small platform like this also allows the comparison of the results with reference data from ground testing in a standard sized wind tunnel of an identical airframe. With this process, the following questions can be answered: Firstly, would such a system then be able to collect accurate flight data for system identification (ID)? Is it possible at all to fly a small, remotely piloted aircraft precisely enough to record the required data, given its sensitivity to atmospheric turbulence, airframe noise, limitations of the remote piloting and so on? And secondly, if accurate data has been obtained, how well do the two experiments match? The small scale might potentially result in previously unknown or at least insignificant physical phenomena, which need to be taken into account when flight testing such a small platform. The changes in the inertial properties of the platform due to the added mass effect is one of these phenomena, which can typically be ignored for full scale aircraft. However, this has proven to be critically important for the successful analysis and comparison of the flight- and wind tunnel data obtained throughout this project. The avionics suite designed for this research was developed in house, since the weight restrictions of the small platform excluded any commercially available flight data recording packages. The suite features an lightweight airdata probe, control surface feedback sensors, a custom designed GPS receiver and many other advanced components previously not possible at this scale. A commercial reference INS was used to benchmark the system. The UAVmainframe also provides basic flight control functionality to aid the pilot in obtaining the required trim conditions and turbulence mitigation. Extensive data compatibility analysis and calibrations were performed on the recorded data using an Extended Kalman Filter (EKF) and various other methods to ensure the best possible data quality. The inertial properties of the test aircraft were determined by swing tests. The significance of the added mass contributions was discovered during these tests, which added up to 25% onto the `true' airframe inertial properties. In an effort to estimate these added mass terms, it has been found that the methods presented in literature to determine the corrections for full scale aircraft do not give the correct results for the small scale aircraft under consideration. Swing tests of a flat plate model of the test aircraft also did not capture the magnitude of the phenomenon correctly, which led to swing tests with a geometrically similar 3-d object of known inertial properties to successfully estimate the added mass corrections. Static derivatives were obtained from conventional wind tunnel testing, in conjunction with a high fidelity three dimensional inviscid solution using the PanAir code. A dynamic test rig was used in the wind tunnel to determine the dynamic derivatives. It allowed the instrumented airframe to rotate freely on a three axis gimbal, essentially 'fly' in the tunnel. The aerodynamic derivatives from these 3 DoF tests were estimated by performing system ID on the recorded data, where the model structures were modified for the reduced set of motion variables. Extensive flight testing was performed at the university's flight test centre. These tests showed the difficulty of testing such a small and light airframe due to wind and airframe noise, as well as the limitations due to lack of feedback received by the remote pilot. The pilot was aided by the flight control system to achieve a good trim condition, and pre-recorded input sequences, similar to the dynamic wind tunnel tests, were used to excite the longitudinal and lateral dynamics of the aircraft. One particular finding during the test campaign was that there is no such thing as totally calm conditions for this scale of airframe. Other findings include a high correlation between the pitch damping term and the pitching moment due to elevator, making it impossible to determine both at the same time, and that in flight the inertial properties of the test aircraft change to the values that include the added mass components, as compared to the dynamic wind tunnel tests, where the `true' inertias are used. By including these findings in the data processing, close agreement between flight and ground test data has been achieved.