Europe continues quest for ‘Green Rotorcraft’
As part of its Green Rotorcraft initiative, the Clean Sky public-private partnership for aeronautical research in Europe has issued its second call for proposals seeking such improvements as diesel engines, electric tail- rotor drive and waste heat conversion to electricity, all of which are expected to contribute to targeted drastic cuts in noise, CO2 and NOx emissions.
In Green Rotorcraft, one of Clean Sky’s six projects, the noise target is to shave 10 EPNdB. A turbine-powered helicopter should emit 26-percent less CO2 than currently, with the engine to account directly for 10 percent of the reduction. This means that specific fuel consumption (sfc) needs to fall by 10 percent and aerodynamic and system improvements should account for the remaining 16 percent. NOx emissions should be cut by 65 percent.
For a light helicopter powered by a diesel engine, the project expects CO2 emissions to fall by 40 percent compared with today’s EC120. In this case, the engine’s better sfc would account for 30 percent of the improvement. NOx emissions should be cut by 53 percent.
“The objectives take into account Acare [Advisory Council for Aerospace Research in Europe ] goals and what is doable within Clean Sky’s time and budget limits,” François Toulmay, Green Rotorcraft coordinator and Eurocopter’s program manager for EU framework research and innovation, told AIN. Acare’s goals for 2020 include a 50-percent cut in CO2 emissions per passenger-mile and an 80-percent reduction in NOx emissions. As for perceived noise, the envisioned reduction would cut current average levels by half. The E1.6 billion ($2.2 billion) Clean Sky joint-technology initiative is to last until 2015 or 2016, Toulmay said.
Powering a light helicopter with a diesel engine seems to be posing more difficulty than expected. The topic was already part of a first call for proposals but came to a dead end. “We did receive proposals but none complied with the allowed budget. We had beefed up the specifications, and this pushed the costs above budget,” Toulmay said. The joint undertaking will therefore repeat its call for diesel proposals later this year.
Organizers hope that such a proposal will eventually yield a demonstrator, derived from an existing aircraft. The engine maker the Clean Sky agency selects must already have some experience in general aviation and diesel technology–not necessarily in helicopters, said Toulmay. The main challenge with such a project is weight.
Another approach to piston power for light helicopters is more conceptual. “What would be the ideal high-compression engine for a helicopter? This is the question we want to explore,” Toulmay explained. AIN understands that alternative fuel, two-stroke or rotary architectures might be considered. A de-pollution device to cut NOx emissions could be added at the exhaust. The technologies might not be mature, but the objectives are more ambitious, Toulmay pointed out. These studies will be part of a third call for proposals.
Looking Beyond the Engine
Electric tail-rotor drive has the potential to produce major changes. The idea is to replace the mechanical transmission shaft with a local electric motor. Such a change “would allow the rotational speed of the tail rotor to vary widely since it would be independent of the main rotor,” Toulmay explained.
The configuration would reduce noise and fuel burn. For example, in cruise flight, the tail rotor is largely unnecessary since a fin is sufficient for yaw/antitorque control. The tail rotor does, however, work hard at low speeds and in hover.
Weight poses the biggest challenge for this project as well. “We need a very light electric motor,” Toulmay said. The Green Rotorcraft consortium is calling for a full-size ground demonstrator.
Harnessing waste heat might also improve efficiency, and the consortium is looking at two avenues to recoup between 1.35 and 13.5 horsepower.
“One is the Peltier-Seebeck thermoelectric effect, by which different metals or semiconductors are connected to transform temperature differences into electric current with no moving parts,” Toulmay said. This reversible thermoelectric effect is already implemented in portable refrigerators.
The other way is to use a conventional thermal machine with a fluid loop and generator. “It is difficult to make a robust device that is simple and light,” Toulmay conceded. Nevertheless, such a machine yields more power than the Peltier-Seebeck approach. The heat source would be in the engine bay or the oil system, where temperature differences are relatively low at 100 to 200 degrees C, so the machine might use a Sterling thermodynamic cycle.
Reducing drag is one means of improving the helicopter’s fuel efficiency. Hub drag is squarely in the crosshairs of Green Rotorcraft’s efforts since it accounts for 20 to 50 percent of total drag, said Toulmay. The hub has no fairing and is made of mechanical parts that rotate and interfere–“an effective air brake,” and it exacts a greater penalty on small rotorcraft.
The Green Rotorcraft consortium is considering optimizing the shape of the hub cap and adding fairings “on some parts of the hub.” Five rotor caps and five fairings will be tested in a wind tunnel.
Seeking also to limit efficiency losses from engine installation, the Green Rotorcraft Research engineers want to investigate how to improve the shape of the air intake and the inlet particle separator (to minimize pressure loss and distortion) and address hot gas ingestion and air-intake icing protection.
In addition, the consortium will analyze emissions with a view to refining research models. “CO2 emissions are proportional to fuel burn, but NOx and water emissions are not so straightforward,” Toulmay said.