My big hope for the future of piston singles was diesel power. Much progress has been made, but as Cessna discovered with their JetA Cessna 182 engine failure, diesels are proving remarkably hard to deliver at a marketable cost and reliability.
In previous columns, I looked forward to the eventual replacement of our ancient technology, ‘Lycocontasaraus’ engines, with modern common rail diesels burning JetA1. The Cessna JetA was announced with much fanfare, and the first was expected to be delivered in June 2014. Then all went suspiciously quiet. No new 182s have been delivered in the past four years.
With the limited exception of the Austro engines in the Diamonds, JetA pistons are all but dead. In the meanwhile, battery powered aircraft are bounding over the horizon towards us with huge strides.
Hitherto, we have tended to scorn battery power as being a bit of a joke – useful only for providing perhaps 50 hp for a glider for a few minutes’ climb.
Not anymore. Making waves is the battery powered 330LE 300 hp Extra, using the SP260D electric motor. Full power with Siemens’ 260 kW motor, with its massive torque, will give excellent acrobatic flight. Endurance at full power is not much more than 15-20 minutes, but that’s plenty for an airshow routine – albeit one absent of the usual growl of the Extra’s Lycoming IO-540 piston engine.
Less spectacularly, but more practically, the ever innovative Pipistrel has produced a 115 hp electric powered training aircraft that will fly for at least an hour’s circuit bashing.
Pipistrel’s WattsUp two-seat trainer is yet another version (thankfully better named) of the hugely admired Sinus and Virus aircraft. The WattsUp is based on the Alpha trainer, but in place of the Alpha’s 80-hp Rotax 912, the WattsUp has a 115 hp Siemens electric motor.
Even more practically, Pipistrel is working with Siemens on a hybrid-electric powertrain for the Panthera four-seater, which is in flight testing for certification – but with a Lycoming IO-540.
What is significant is that the battery and liquid fuel powered Alpha trainers are essentially identical airframes, and thus there are no special allowances for the electric engine option. In fact, the electric option is more powerful than the Rotax. This really seems to be the coming of age moment for electric propulsion. They have the same maximum takeoff weight and useful load, so range is the only constraint, but it’s getting there – literally and figuratively.
AWS&T quotes Pipistrel’s Tine Tomasic: “The electric aircraft is not a cross-country aircraft and carries no baggage, but both can lift 400 lb.” The difference between the two is essentially limited to the powerplant, with the batteries fitted where the fuel tank is in the Alpha, behind the fuselage bulkhead.
With an endurance of one hour, plus a 30-minute reserve, compared with 3.6 hour, plus 30 minutes for the avgas-fuelled Alpha, the WattsUp is designed for airfield training. The aircraft can fly eight standard 1,000 ft circuits before it must land to recharge or replace the batteries. Its 17-kWh battery pack is dual-redundant and can be replaced in minutes, or recharged in 30 minutes, claims Tomasic. With a short takeoff distance and climb rate greater than 1,000 fpm, the aircraft is optimised for circuit work, recuperating 13% of the energy on every approach, which increases endurance and reduces landing roll.
Driving the two-blade fixed-pitch propeller via a gearbox, the Siemens electric motor is amazing for its lack of size and weight. Compared with the 60 kg Rotax, the electric motor weighs 14 kg and has a power density of 6.5 kW per kg, including the inverter, which shares the same water/glycol liquid cooling. “This is as far as we can push electric motor technology at the moment,” Tomasic says – and indeed that may be the problem with the market for these aircraft – most potential buyers are content to sit on their hands and wait for the next technological leap.
And what about electrical fires? Following Boeing’s lead after its 787 problems, the WattsUp’s lithium-ion pouch cells are packaged in tough containers. To prevent thermal runaways, the cells are cooled passively in the air and actively, with fans, on the ground. To avoid mishandling, connectors are untouchable. When installed, three boxes make a functional battery, while six provide dual redundancy.
The WattsUp has Pipistrel’s fourth-generation battery management system (BMS). It provides three ways to quickly disconnect the batteries – via two independent CANbus databuses, or by pulling a circuit breaker in the cockpit. The BMS provides active voltage balancing during charging, as well as cell failure protection, and monitors internal impedances to forecast failures 20-30 cycles in advance, Tomasic says.
Pipistrel has spent ten years developing electric power. It flew its first electric aircraft in 2007: the Taurus Electro motor glider. The once-off Taurus G4 was a dual-fuselage derivative, with a 150-kW motor and massive 100-kWh battery pack, that won NASA’s Green Flight Challenge in 2011, exceeding the equivalent of 400 passenger miles per gallon.
Over these four generations of electric aircraft, Pipistrel has learned much about electric propulsion. This includes ensuring the safety of energy storage from thermal, crash and handling hazards, and solving the problem of how to stop electromagnetic interference from a large electric motor drowning out radio comms. “At 100 kW, electromagnetic interference problems are significant, and grounding is an issue in composite aircraft,” says Tomasic.
Perhaps the biggest hurdles will inevitably be regulatory. With a 550-kg gross weight, the WattsUp is designed to light sport aircraft (LSA) standards, but gaining approval is complex. In the USA, the LSA category allows manufacturers to certify aircraft by complying with industry standards, but is restricted to piston engines. In Europe, LSA rules allow electric propulsion, but require European Aviation Safety Agency (EASA) certification. So, the first WattsUp deliveries will be to Australia, which uses the European LSA definition but accepts US-style LSA certification.
Meanwhile, as it works on European LSA certification, Pipistrel plans to petition the FAA for an exemption to the piston-engine restriction on LSAs. As a precedent, the FAA has previously approved exemptions to the LSA’s maximum-gross-weight restriction to enable spin resistance in the Icon A5 amphibian and roadable capability in the Terrafugia Transition flying car.
For those who think the WattsUp is still just a novel LSA training plane, the big breakthrough will come when Siemens and Pipistrel launch their hybrid-electric drivetrain for their four-seat e-Panthera.
The hybrid-electric e-Panthera is essentially a firewall-forward modification of the Lycoming IO-540-powered Panthera four-seater that is scheduled for certification in 2017. As with hybrid cars, an internal-combustion engine drives a generator, which provides electric power via the control system to both a battery pack and the main propulsion motor. This drives a five-blade, low-rpm propeller. Both conventional and hybrid aircraft are intended to have a 1,315-kg maximum takeoff weight, 175 kt cruise and range greater than 1,000 nm at 155 kt.
The hybrid drivetrain mounts to the firewall via the same attachment points as the Lycoming engine powering the conventional Panthera. The powertrain produces 200 kW for takeoff, and 100 kW in cruise. For redundancy, the motor has dual windings, with two inverters for each. “We use a keep-alive principle in the inverter,” says Tomasic, with each inverter able to handle all the power. The aircraft has a single-lever power control, with the system deciding where the power comes from.
Batteries are installed in the wing, which forms the enclosure, behind the fire-protected fuel tanks. Lithium-ion cells are packaged into six modules per side in the actively ventilated enclosure. As a last resort, in the event of a thermal runaway, the batteries can burn through the carbon-fibre skin and the wing structure will continue to carry standard loads, says Tomasic.
As with the WattsUp, there are no airworthiness regulations yet for electric or hybrid-electric propulsion systems. But FAA Part 23 and EASA CS-23 certification rules for general aviation aircraft are being rewritten around industry standards being developed by ASTM International, which has a committee drawing up specifications for the design, manufacture and integration of the new powerplants.
Unfortunately Tomasic does not say what the expected fuel consumption of the hybrid e-Panthera will be. The reality is that while good old fossil fuel is still readily available – even at more than R20 a litre, it will probably still be cheaper and easier just to operate a pure Avgas burner. Like Toyotas much derided Prius, the cost of the hybrid powerplant, and especially the cost of battery replacement after three to five years, will probably not be compensated for by fuel savings.
But Elon Musk has gained huge traction with his Tesla, and if you also want to be that cool, perhaps you should plan to have a hybrid in your hangar.