To alleviate the thrust losses in radial direction experienced in conventional saucer-shaped Coanda Unmanned Aerial Vehicles (UAV), a rectangular, linear arrangement of the Coanda surface is adopted in the novel design. This arrangement minimizes the area change in the radial direction to recover such thrust losses. A prototype of the proposed UAV structure is 3D printed and assembled with a single 9-inch propeller. In addition, a custom-built dynamometer is designed specifically to measure propeller thrust under different loading conditions. Performance characteristics of the UAV are evaluated through static testing on the dynamometer which uses a load cell to measure the overall thrust. Downwash airspeed is measured via a commercially available pitot tube system. These results are then validated through Computational Fluid Dynamics (CFD) simulations using ANSYS Fluent® while applying the Multiple Reference Frame (MRF) approach in steady state. Using the k-ε Realizable turbulence model, CFD simulations are conducted at rotational speeds ranging from 4000 RPM to 8000 RPM to simulate experimental loading conditions. CFD simulations provide good overall agreement with experimental results having errors less than 10% for overall thrust and downwash airspeed. Numerical comparison between the novel Coanda design and a conventional saucer-shaped design, having similar radial dimensions, is conducted at rotational speeds ranging from 2000 RPM to 12,000 RPM. Simulation results show that the novel Coanda design offers an overall 17% improvement in thrust per the side surface area, demonstrating an effective reduction of the thrust losses in the radial direction. Further design revisions are discussed in the future work section to achieve better performance.

Background:​ The application of Unmanned Aerial Vehicles (UAVs) has witnessed an unprecedented rise across a wide variety of industries notably the commercial and military sector [1]. Based on specific mission requirements, the objective of most UAV designers is to enhance the aerodynamic performance of their design such that it yields enhanced payload capacity, endurance and range. An increasingly popular avenue of research in multi-rotor UAVs is harnessing propeller downwash to generate additional lift through the Coanda effect [2]. A lifting force is induced from the Coanda effect when a moving fluid remains attached to a convex surface thereby creating a low-pressure gradient. UAVs which apply the Coanda effect are generally saucer-like in shape and valued particularly for their Vertical Take-Off and Landing (VTOL) capabilities [3]. This design configuration generates lift in two ways:​ 1) through the rotating propeller(s) located at the center and 2) by radially re-directing propeller downwash over convex Coanda surfaces. In a conference paper published in 2002, Robert Collins presented the application of the Coanda effect in saucer-shaped UAVs which subsequently led to a patent being granted 2003 [4,5]. Geoffrey Hutton in collaboration with GFS Projects, introduced a family of Coanda UAVs aptly named GFS (Geoff’s Flying Sauers). The GFS UAVs have a circular canopy as a housing for the propulsion system with an orthogonal arrangement of the Coanda surfaces [6]. In 2006, Jean-Louis Naudin made further improvements to the GFS UAVs by introducing an electric engine and made his design, GFS-UAV (N-01A), available to UAV enthusiasts everywhere [7]. The work of Collins, Hutton and Naudin paved the way for many designers to apply their own innovative approaches to Coanda based UAVs in order to extract higher thrust [8].

Research Aims:​ The scope of our study is to evaluate the performance of a novel Coanda UAV design. This innovative design, conceptualized by Dr. Jin Wook Lee, aims to reduce fluid momentum losses experienced in a conventional saucer shaped Coanda UAV in the radial direction [9]. The design calls for, a rectangular, linear arrangement of Coanda surfaces which minimizes the area change in the radial direction.

Methodology:​ The preliminary study established an experimental set-up which comprised of a commercially available thrust-stand and rotating cup anemometer. Data such as thrust, torque, propeller efficiency and downwash velocity are gathered from the experimental set-up. A full 3D CFD simulation is developed using commercial CFD software ANSYS Fluent® while applying the Multiple Reference Frame (MRF) approach in steady state. The steady-state 3D simulations for two turbulence models (k-ϵ Realizable and k-ω SST) provided good overall agreement with the experimental results having thrust errors less than 10%. Two key shortcomings are identified in the preliminary experimental setup. First, the commercially available thrust stand can not be modified to arrest a full-scale UAV prototype structure. Second, the rotating cup anemometer is found to be an inaccurate device for measuring propeller downwash. This necessitated the need to develop a custom-built thrust stand solution catered to Coanda based research and capable of measuring forces under different loading conditions. A pitot tube connected to a robust manometer module is used to measure downwash airspeed. The Coanda UAV study involves validating the experimental results of the novel prototype with CFD simulations while applying the MRF approach in steady state. A full-scale prototype of the novel design is the 3D printed in symmetrical segments. These segments are mechanically fastened to each other using guides, insets, and connectors. Using the k-ϵ Realizable turbulence model, the CFD simulations validated the experimental trials conducted under four different loading conditions having thrust and airspeed errors less than 10%. The CFD design tool is then used to evaluate the aerodynamic performance of the novel design as well as comparing it to conventional Coanda design with similar dimensions. The CFD design tool is then used to modify the novel design further to bring its performance on par with the current Coanda UAVs.

Research Significance:​ The Coanda UAV study results show that the novel Coanda design offers an overall 17% improvement in thrust per the side surface area, demonstrating an effective reduction of the thrust losses in the radial direction. The novel design also opens new potential for adding various Coanda UAV-related upgrades. These include the potential application of a Ring-Wing Airfoil propulsion system which would that the incoming air is parallel to the UAV’s baseplate instead of being perpendicular when using a propeller. This would potentially address of the most severe limitations experienced in all UAVs:​ the negative lift generated by the UAVs baseplate. The application of lightweight noise reducing materials coupled with rectangular surfaces would also improve the stealth capabilities offered by Coanda UAVs by reducing the acoustic and radar signatures respectively.