Manufacturers and researchers appear to be in agreement: The way to develop electric propulsion for aircraft is to start small. But with the pace at which technology is developing, electric-powered aircraft may not stay small very long.
In September 2010,Innovation Works (IW) and Aero Composites Saintonge (ACS) flew a single-seat, 375-lb. Cri-Cri modified with four electric motors in place of its two 9-hp piston engines. By the end of this year, EADS IW and ACS plan to fly the E-Fan, a two-seat training aircraft purpose-designed around electric-powered ducted propellers.
The Cri-Cri and E-Fan are battery-powered, but in June 2011 EADS teamed with Siemens and Diamond Aircraft to fly the DA36 E-Star, an HK36 Super Dimona motor glider modified to test a hybrid-electric drive system. A year later, in June 2012, the team flew the improved E-Star 2 with an 80-kw (107-hp) serial-hybrid drive system based on a small Wankel engine, generator and batteries.
The E-Fan and E-Star are among “E-aircraft” research projects under way at EADS as it evaluates different approaches to reducing aviation carbon-dioxide emissions. Another is the E-Thrust concept study withinto a distributed propulsion system in which a turbine engine powers six electrically driven fans integrated into the wings of a commercial airliner to reduce weight and drag.
EADS and Siemens also have partnered with the Technical University of Munich to establish the PowerLab at the nearby Ludwig Boelkow Campus in Ottobrun. This four-year project is dedicated to developing and testing lightweight, high-efficiency generators and motors in the 300-600-kw class. “This is a good level at which to enter real aviation,” says Peter Jankers, PowerLab project head. A follow-on demonstrator aircraft could have from one 300-kw to four 600-kw motors. “We could easily get to megawatt class,” he says.
Work on innovative propulsion systems is part of EADS's research to support the environmental goals laid out in the European Commission's Flightpath 2050 report, which was prepared by the Advisory Council for Aviation Research and Innovation in Europe. This sets a target, by 2050, of reducing emissions of CO2 by 75%, nitrogen oxides by 90% and noise by 65% compared with levels in the year 2000.
The Cri-Cri, the first electric aerobatic aircraft, is tiny, but EADS believes the tandem-seat E-Fan could be matured and marketed as a practical general-aviation trainer. The electrically driven shrouded propellers provide a total static thrust of about 340 lb., the energy provided by two battery packs in the wings. The centrally mounted mainwheel is electrically driven also, for taxiing without using engines and to boost acceleration on takeoff.
The E-Star 2 also is a step closer to a marketable product. Its serial-hybrid drivetrain has a lighter, more compact electric motor from Siemens, a generator driven by a small Wankel from Austro Engine and EADS IW-prepared battery packs in the wings. Aircraft empty weight is reduced by about 100 kg (220 lb.). The motor weighs 13 kg including gearbox and control electronics, and runs off the generator, producing a continuous 65 kw. This is boosted by the batteries to 80 kw for takeoff and climb. The combustion engine runs at a constant 30 kw to generate power and recharge the batteries. Siemens believes series hybrid power will make its way soon into small aircraft and is scalable to commercial aircraft with 50-100 seats, reducing emissions 25%.
The PowerLab project, meanwhile, is targeting development of electric generators and motors with power densities of 10 kw per kilogram—twice that of the motor in the E-Star 2. Such a density would make a megawatt-class power system “quite reasonable to fly, and is not too far away,” says Jankers. Begun this summer, the PowerLab project is looking at how to generate, convert, distribute, buffer and store electrical power as well as design safety into the system and integrate two distinct types of electrical machine: fast-running, high-efficiency generators and low-rpm, high-torque motors.
The project will involve detail design studies and laboratory demonstrations of electrical propulsion, with the goal of “building a foundation for electric flight,” Jankers says, by creating a community centered on the Boelkow campus with expertise in aircraft hybrid propulsion. Although the technology is targeted at hybrid power, PowerLab is focused on the electric part, particularly how to use and manage battery cells and electrical machines “in a more intelligent way,” and understand their limits and how to protect the system.
“We need to understand the technology, and see where the gaps are,” he says.