Voting Result: 3.11711389918
Overview for COLIBRI
According to the requirements of the AIRBUS CARGO DRONE CHALLENGE the main aim is to merge the design elements of a multi-rotor and fixed-wing aircraft in the most efficient way.
In general four top level requirements shaped the design of the COLIBRI:
· Separate lift-cruise propeller requirement (minimum 4L+1C)
· All electric requirement (battery only)
· Range requirements (60km/5kg , 100km/3kg)
· Payload bay requirements (dimensions, accessibility, etc.)
Efficient hover requires a large propeller disc area, respectively a low propeller disc loading. Usually large diameter rotors do not go well together with aerodynamics in forward flight. A large number of smaller propellers distributed around the fuselage or implemented as ducted fans are also troublesome and less efficient. Hence I tried to fulfill the minimum requirement of four propellers and make use of as much disc area as possible.
The chosen solution is the most unique feature of the COLIBRI. It uses four mono-propellers for VTOL and retracts them into the fuselage after transition to cruise flight.
Mono-propellers (a.k.a. single-blade propellers) already have been successfully used in Aviation; especially with motorized gliders that retract the propeller into the fuselage after reaching the desired altitude for soaring.
Compared with two-blade propellers, mono-propellers require significantly less space (volume) in the fuselage. Respectively a larger fuselage would increase structural weight and wetted area. In case of the COLIBRI the propeller disc area is quadrupled for a given propeller bay length. This allows the use of four 28” (711mm) diameter propellers in combination with a very compact and aerodynamic shaped airframe.
Furthermore the retracted lift-propellers will reduce cruise drag significantly and I think this is the only way to fulfill the demanding requirement of 60km/5kg resp. 100km/3kg cargo delivery with a separate lift-cruise and all-electric propulsion system.
The added complexity for the mechanism of retractable propeller-booms and associated lids is acceptable due to the expected gain in cruise performance. The design decision is comparable with the one of fixed vs. retractable landing gears.
Another advantage is that the propeller-booms do not need to be detached for transport and storage and are well protected inside the fuselage.
Last but not least it is envisaged that the propeller-booms will (automatically) retract after landing to ensure safe cargo handling around the aircraft.
For cruise flight two propellers are located on the wingtips. They rotate in the opposite direction of the wing tip vortices. Studies by NASA have shown that a wing’s induced drag can be reduced and the propeller’s propulsive efficiency can be increased significantly this way. In combination with the clean fuselage (lift-props retracted) superior cruise efficiency is achieved.
Additional the cruise-propellers can be used in all phases of the flight for directional control, but especially during hover (cross wind).
Payload Bay Concept:
The payload bay is located at the center of gravity under the main wing spar. The bay is accessible from the top via the payload-hatch to quickly load/unload goods. The payload-module itself is detachable and interchangeable from the lower side. Several Payload Module options are available.
BR Michael Gloetzer