Variable diameter rotor can improve the flight performance of the aircraft both in hovering and forward flying and a duct can improve the rotor performance further both in force and efficiency. In previous studies. those two parts are always studied in the one-factor-at-a-time method. and the duct is lack diameter change so that it cannot cooperate with the variable diameter rotor. https://www.nacrack.com/flash-offer-On-Cloudmonster-clearance/