The effects of varying diameter on coaxial propellers for the propulsion of multirotor systems

Rotorcraft are in use today as both unmanned and manned aerial vehicles, and are limited in performance by the size constraints on their propellers. In order to expand the nature and scope of missions that unmanned aerial vehicles can be used for, said performance limitations must be mitigated. As the payload requirements for rotorcraft increase, the thrust necessary to operate must also increase, which can be achieved to a certain extent by increasing the size of the propellers on the craft. However, large propellers increase the footprint of unmanned aerial vehicles, limiting their mobility in flight, their options for landing sites, and their transportability. When the use of increasingly larger propellers on the aircraft is no longer viable, an increase in the number of propellers is the next logical step. To increase the number of propellers without increasing the footprint of the craft, the propellers can be stacked so that they rotate coaxially. This investigation explains the reduction in the performance of downstream propellers in comparison to upstream and to single propeller arrangements. The loss of thrust is due to the wake of the upstream propeller applying a counter thrust to the downstream propeller. This study investigates methods to mitigate these thrust losses by alternating rotational direction, increasing separation distance, changing the balance of electrical power input, and varying the diameter of the propellers. Experiments were performed to compare the thrust output of contrarotating propellers and corotating propellers. Separation distance between the propellers was increased from the minimum possible up to a single diameter length and the effect on output was measured. The balance of electrical power input between the propellers was varied to determine which propeller should be powered first. The thrust and efficiency of systems of propellers with constant and increasing diameter are experimentally measured and compared to single propellers. Therefore, systems of propellers of increasing diameter may be viable for unmanned aerial vehicles whose mission requirements include maneuverability, transportability, and greater payload while accepting shorter fight times.M.S.Includes bibliographical referencesby Laura Ostar-Exe