Where Darpa faltered, can private enterprise succeed? An outfit called Flight Of The Century (FOTC) has flown a modified Rutan Long-EZ with which it plans to set electric-aircraft speed and altitude records en route to its goal of developing an "infinite-range" aircraft and attempting an all-electric transatlantic flight to demonstrate its technology.
FOTC's ambition sounds similar to Darpa's goal for its recently terminated Vulture "infinite-endurance" unmanned aircraft program — and there are other similarities.
Battery-pack UAV hard-docked to aircraft (Graphics: FOTC)
FOTC's concept is to extend the range and endurance of an electric-powered aircraft by replacing battery packs in flight using a mid-air "refueling" technique. This would use "flying battery packs" — a UAV that would detach from the mothership once the batteries are depleted and fly down to a recharging station while a freshly charged battery pack is launched to rendezvous and dock with the aircraft.
For the Phase 1 infinite-flight demonstration, FOTC plans to use a trailing tether soft-dock method rather than attempt the more complex mid-air hard docking of two aircraft.
Deleted battery pack released as freshly charged UAV is launched
As steps toward its infinite-flight goal, FOTC says the range of an electric aircraft can be increased by the single in-flight jettison of a depleted battery pack — either by UAV or by parachute. Dropping half the battery weight 30-50% into the flight would reduce aircraft weight and extend range by up to 40%, the company says.
A greater than 90% increase in range could be achieved by sequentially dropping depleted battery-pack segments at regular intervals during the flight. Rather than a single, monolithic battery pack, FOTC reconfigures the same capacity into a series of smaller packs which are brought on line one by one, depleted individually, then dropped by parachute while the remaining packs are repositioned to rebalance the CG.
Breaking current large battery packs into 10 smaller units, and making 10 drops at equal intervals during the flight, will double the range of today’s electric aircraft, using today’s batteries, says FOTC. The batteries would be dropped over collection stations to be recharged and reused.
Using today's battery technology, rather than waiting for promised advances in power density, is key to the company's approach. The modified "Long-ESA" (for Electric Speed & Altitude) has the same 258-hp liquid-cooled permanent magnet DC motor and lithium-ion polymer prismatic pouch cells used by founder Chip Yates to set a 200 mph+ world speed record for an electric motorcycle.
In addition to trying for speed and altitude records, the Long-ESA will test FOTC's technology for dropping and repositioning battery packs in flight. It will also test whether it is feasible to recover kinetic energy from the freewheeling propeller via its electric motor during descending flight. "I want to fly higher than 30,000 feet, deplete my battery pack completely on the way up, and see how much I can recover on the way down," says Yates in a blog post.
Returning to Darpa's aborted Vulture. Although the agency eventually went with a single, large solar-powered unmanned aircraft designed by Boeing and Qinetiq, among the concepts considered were vehicles that could be refueled and serviced in flight. Aurora Flight Sciences proposed a vehicle made up of several smaller aircraft that would take off individually, then link up in flight. This allowed individual parts of the vehicle to be replaced in flight. All with the goal of staying aloft for at least five years — essentially indefinitely, given the plan was to ditch the aircraft at the end of its life.
Following Darpa's April decision to cancel the Vulture flight demonstration, it seems that proving infinite flight is feasible may be down to the Flight Of The Century team.