Influence of ground effects on body forces for bi-copter UAV
Creators: name: Norton, Daniel
Publisher: name: Graduate Studies
Year: 2017
Subjects: subject: Engineering--Aerospace
subject: Engineering--Mechanical
subject: Ground effects
subject: Body forces
subject: UAV
subject: Dihedral angle
subject: Bi-copter

description: The Navig8 are a family of dimensionally similar Unmanned Aerial Vehicles (UAVs) designed for high-speed autonomous flight in confined spaces. The Navig8 UAVs generate their lift through the use of dual shrouded fans, and are capable of controlling the pitch of their bodies while hovering. The use of shrouded fans increases the efficiency of the Navig8, which allows the UAV's design to have smaller fans while simultaneously decreasing its power usage. The autonomous control of a Navig8 UAV requires an understanding of the forces acting on its body while operating in close proximity to the ground, which is known as the ground effects region, as well as the forces acting on their fans and shrouds. Computational Fluid Dynamics (CFD) simulations were used to determine the forces acting on the Navig8 UAV's body, fans, and shrouds as a function of height and dihedral angle using a dimensionally-similar model. Heights were defined as the distance from the bottom of the shroud to the ground, and dihedral angles were defined as the difference between the fan's axis of rotation and vertical. Positive dihedral angles direct flow away from the UAV's body. Heights ranged from a maximum of 7.0 fan diameters to a minimum of 0.75 fan diameters. Dihedral angles ranged from a maximum of 8 degrees to a minimum of 0 degrees. The simulated results were compared to the results calculated from existing methods to estimate body and fan forces while operating in the ground effects region. Recommendations were made to the design and operation of the Navig8 based upon the results. The simulated forces acting on the Navig8's body were found to have two primary competing sources: 1) fountain forces and 2) suck-down forces. Fountain forces are caused by the flow from the two fans impacting the ground and creating wall jets. These wall jets intersecting each other beneath the UAV's body then fountain into the body which results in an upwards force. Suck-down forces are caused by a vortex forming between the downwards flow from the fan and the upwards fountain flow. This results in a general downwards force on the body. Suck-down forces are primarily important at altitudes below 1.5 fan diameters, where the gradient of body lift with respect to height is positive, which results in unstable operation. Body lift peaks at approximately 25% of the combined fan and shroud lift out of ground effects at an altitude between 1.1 and 1.5 fan diameters. The body forces decrease from their peak with increasing altitude and drop below 5% by an altitude of 3.8 fan diameters. Increasing the dihedral angle of the fans increases the maximum altitude for which body forces are significant, but has no noticeable impact on the maximum body forces. The change in fan lift varied by less than 2% and the change in shroud lift varied by less than 3% within the experimental range of heights and dihedral angles. The simulated body forces did not agree with those calculated from available estimation methods. Simulated body forces were higher in magnitude than the estimated body forces from available methods, and these methods were unable to properly account for the dominance of suck-down forces at elevations below 1.5 fan diameters.