Wind Tunnels Tests
Wind tunnels have been used since the earliest days of flight: the Wright brothers built their own wind tunnel as part of the development of the world’s first powered aircraft. Since that time, a great deal of practical experience has been gained and the most sophisticated tests now encompass models that are structurally flexible with active control surfaces and used in tunnels that permit real-time motions of the main body, allowing full aeroservoelasticity experiments to be carried out. We do not go that far in our wind tunnel programs, but we routinely test wings and full airframe configurations over a wide range of speeds and angles of attack (AoA), with variations in control surface deflections, both with and without propeller power to establish full lift, drag, and moment results. The primary result we seek is the stall speed of the aircraft, usually in the landing configuration since this plays such a critical role in the safe operation of small UAVs. If the estimation of landing speed has been overly optimistic, then flight operations can quickly lead to damaged airframes during difficult landings (hopefully the concept and preliminary design calculations will not have been so far out as to lead to designs that cannot generate enough lift given the available power to fly at all!). Secondly, we seek drag performance at the likely flying speeds so as to estimate glide angles and cruise performance. Finally, tunnel data can be used to calibrate previous computational runs and the initial spreadsheet calculations used at the start of the design process. This can be particularly important in improving the designs of subsequent aircraft. In all cases, we take great care to avoid any instances of flutter or divergence in the tunnel tests, first by having estimated likely onset speeds prior to test and second by careful control of the tunnel operating speed during measurements.