Aerodynamic study of a small, ducted VTOL aerial vehicle

Abstract

The Perching Unmanned Aerial Vehicle (PUAV) is a 9-inch diameter ducted vertical takeoff and landing reconnaissance vehicle with the capability of fast-forward cruise flight. Currently in the development stage, the program is envisaged to yield a man-portable craft that a foot soldier can use to provide over-the-hill observation. Several prototypes have been constructed and tested, with mixed results. Concerns regarding duct aerodynamics led to the proposal for further aerodynamic study to investigate effects of inlet lip radius and surface area, diffuser area ratio, blade tip clearance and rotor position on thrust, power and efficiency. This report covers the theory of rotorcraft and ducted propeller aerodynamics, and outlines the tests performed and results obtained. It also presents specifications of the test vehicle and methods that can be used in future ducted aircraft studies. Large angle diffusers tested showed reduced thrust and efficiency and increased power compared to smaller diffusers, contrary to theory. Reverse flow within the core appears to disrupt uniform exit flow and yields a conically divergent turbulent wake. Results of this study will be used in the redesign of a duct core fairing, which will act to control the airflow and reduce the tendency for reverse flow at the center where blade thrust is absent. Future studies will also consider twisted, cambered and tapered rotor blades in an effort to better address spanwise thrust distribution and optimized airflow. The test apparatus and methods developed for this report, in addition to results of initial testing, will be instrumental to further development of small ducted UAVs. Findings and methods are not limited to exact duplicates of PUAV-like aircraft, but can be used in a wide range of applications including lift and thrust-producing ducts.