With an eye to the environment, Eurocopter mulls system changes

 - July 29, 2008, 11:38 AM

Eurocopter is stepping up its efforts to make helicopters more environmentally friendly by eliminating hydraulic and engine bleed-air systems, introducing diesel engines for light rotorcraft and reducing helicopter noise.

Research and development on the elimination of hydraulic systems and the introduction of diesel engines began as part of the EU’s CleanSky joint technology initiative (JTI), a ?1.6 billion ($2.5 billion) effort launched in February. One CleanSky project is dedicated to replacing hydraulic and bleed-air systems with electric ones with the goal of eliminating polluting hydraulic fluid from helicopter maintenance and end-of-life operations. Second, an engine that doesn’t have to supply bleed air can be designed to achieve lower specific fuel consumption (SFC), engine manufacturers say.

“We have to optimize the entire electric chain–generation, storage and equipment power,” François Toulmay, Eurocopter’s head of European research programs, told AIN. Many systems already are electric but could still be improved, he added.

For example, starter-generators could use brushless technology. In flight controls, electromagnetic actuators (EMAs) could replace hydraulic servo actuators as part of a fly-by-wire control system. Compared with fixed-wing aircraft, a helicopter’s primary flight controls need relatively little power. Therefore, there is probably no need for local hydraulics, such as those found on electro-hydraulic actuators.

Another difference between fixed-wing aircraft and helicopters lies in redundancy. “On an airplane, flight controls are redundant thanks to the number of control surfaces–several ailerons, for example, with several actuators. On a helicopter, each actuator is critical and must contain its own redundancy,” Toulmay pointed out. Liebherr Aerospace, another Clean Sky stakeholder, is working specifically on EMAs.
Further, replacing the hydraulic jacks and lines in retractable landing gear with electric power would yield big benefits. “Landing gear retraction requires much fluid flow. Meanwhile, controlling the main rotor requires a lot of pressure. As
a result, hydraulic pumps are oversized to meet both pressure and flow criteria,” Toulmay explained. Since electricity can be stored, an electric system could be less “overdesigned.”

According to Toulmay, future helicopter electric network standards will follow the evolution of large transport airplanes, at 270 volts DC. Today’s baseline network is at 28 volts DC, with an optional 115 volts AC on medium or heavy helicopters. “We now ask our suppliers to be ready to provide 270V-capable equipment,” Toulmay said. De-icing systems already use this higher voltage.

The only system on Eurocopters that draws bleed air is cabin heating, and the company has not yet decided on a replacement for that system. Options include electric heating or drawing heat from the engines or transmission.

Diesel Power
Eurocopter is also working on diesel engines. “Compared with turbine engines, diesels have a lower SFC. Low-power turboshafts are less efficient than more powerful ones. Moreover, their best efficiency is at takeoff. Over a flight cycle, efficiency becomes relatively poor. Diesels have more consistent behavior through all conditions and are much more efficient over a flight cycle,” Toulmay said.

In addition, a turbocharged diesel loses little power in hot-and-high conditions. On the contrary, a turbine engine is designed with these conditions in mind, which is why there is some oversizing in turboshaft design.

Weight, however, is a challenge. “A diesel’s specific weight is three or four times that of a turbine,” Toulmay asserted. The bottom line is “how long does it take, in flight, to offset the extra engine weight with fuel savings?” Eurocopter’s target is one hour, which Toulmay acknowledged is an ambitious goal, but soaring oil prices and mounting concerns about CO2 emissions are making the case for the diesel engine. Eurocopter has set its sights on reducing average consumption and proportional CO2 emissions by more than 30 percent.

Under the CleanSky JTI, an EC 120-based demonstrator will fly in 2013, ideally on diesel power. “Our proof-of-concept will probably be a bit heavy.

We also anticipate challenges in vibration levels and adaptation of the mechanical transmission,” Toulmay said.

Eurocopter has not yet selected an engine for the demonstrator, but it is
in talks with the Safran group, which includes Snecma (commercial turbofans), Turbomeca (turboshafts) and SMA (light diesels). The last, which is producing its 230-hp engines at a low rate, will not be Eurocopter’s supplier. “These engines are not powerful enough and their SFC is too high for the demonstrator,” Toulmay said.
Rather, Turbomeca could offer a new design, Toulmay revealed. Turbomeca confirmed diesel technology is one option to cut SFC, but a spokeswoman for the company said that no strategy has been defined yet.

Eurocopter’s discussions with Safran are not exclusive, and the helicopter OEM might still request proposals from other engine makers.

Noise Reduction
To reduce noise, research on adaptive rotors continues, with the design of servo-flaps for main rotor blades to cut noise and vibration. The servo-flaps are actuated
by piezoelectric materials that make the blades oscillate around their longitudinal axis, thus dispersing the vortices generated at the blade tips. They are the main source of the beating sound (known as blade-vortex interaction noise) produced in descending flight, according to project leader Valentin Klöppel.

An earlier European research project with AgustaWestland, Friendcopter, has been the framework for the development of a new noise-measurement tool. The objective is to find “a methodology for data acquisition and analysis in the definition of low-noise flight procedures,” Eurocopter research engineer Marc Gervais explained, with the goal of calculating the noise footprint as precisely as possible.

Flight testing put an EC 130 through stabilized maneuvers such as takeoffs, approaches and overflights, and non-stabilized maneuvers that included accelerations, turns and decelerating, steep approaches.

An area of 110 acres was covered with 36 microphones, more than 10 times the normal complement for certification testing. The results are still being analyzed.
Part of the same effort is the development of a software tool, called Helena, for predicting noise footprint. Helena stands for Helicopter environmental noise analysis. “It takes into account the particular characteristics of noise directivity, including the issue of advancing and retreating blades, which makes helicopter noise so complex to study,” Gervais said.

Its developers hope Helena will be ready by the time Friendcopter concludes. Eurocopter plans to use it to assess the modifications made to helicopters under CleanSky. Two versions of Helena have been developed. The first one is for manufacturers and research centers. The second one is for users such as airports, operators and aviation authorities.