Getting an aircraft to launch and land vertically is not difficult. Getting one to launch vertically and then complete a long-endurance flight, however, is an entirely different bag of cats. But a team from NASA's Langley Research Center believe that they have developed a flight system that can do both tasks equally well. The secret: hybrid power.
The GL-10 "Greased Lightning" is a small-scale tilt-wing, tilt-tail, long endurance, VTOL aircraft prototype. A series of nine props mounted on the swivelling wings and tail structure provide lift, while a pair of tiny diesel engines stored in the fuselage run continually to recharge the plane's Li-ion batteries — which provide the electric prop motors with the energy needed to get the whole shebang off the ground. It's much the same premise as the Chevy Volt's operation, except the diesel engines don't ever turn off.
This combines the best aspects of both fuel types: the instant torque generated by electric power supplies, melded with the incredible energy density of diesel. Where a purely electric UAV with the same power capacity would only have enough reserves for about a half hour of flight, the GL-10's diesel tanks can — though theoretically at this point — keep it in the air for up to a full day.
As Langley's Operational Report on the GL-10 explains:
The GL-10 prototype aircraft efficiently combines two challenging mission objectives: long endurance and Vertical Take-Off and Landing (VTOL) flight. At full scale the design exploits the advantages of a hybrid diesel electric drive train, namely, scalable Distributed Electric Propulsion (DEP). Exploiting this advantage allows an efficient airframe design with lightweight propulsion providing sufficient power required for vertical takeoff as well as efficiently providing significantly lower power required in forward flight for 24 hour endurance.
This increase in efficiency causes a proportional increase in control system and aerodynamic modelling complexity. Propulsion and aerodynamic interactions add to the aerodynamic modelling and control design complexity. Additionally, the aircraft must traverse a difficult flight mode between hover and wing born forward flight, the transition flight mode.
This distributed propulsion approach, though complex aerodynamically, offers advantages: excess power in transition, propulsive control power in all flight modes, beneficial downwash over the wing and tail during low speed transition providing some control surface control power, and downwash that creates a lower wing incidence angle during transition avoiding stall.
The current GL-10 iteration is a 1:2 scale model, measures 3.2m from wingtip to wingtip, and weighs just 27kg. It made its maiden flight (albeit a tethered one) on August 19 and should undergo untethered testing later this year. Even if it proves successful, there isn't much chance that we'll see a full-sized version in the skies above anytime soon — NASA is using this prototype exclusively for propulsion research.
Still, the Langley team believes the system they have designed could easily be adapted to aircraft of any size, from hobby R/C planes to hulking commercial airliners. It's just a matter of scaling up the system. [AIAA via Extremetech]