EC 225 features new anti-vibration system

 - October 8, 2007, 10:43 AM

The recently EASA-certified Eurocopter EC 225 Super Puma has a new anti-vibration system that brings more performance, simplicity and lightness to the helicopter. Othertechnological innovations on themedium-twin helicopter include a new autopilot–with an engine-failure mode–and a new cockpit, which the company describes as providing a more intuitive man-machine interface.

The comfort of civilian passengers might come to mind first as the likely motivation for vibration reduction–especially in a helicopter that is often used in high-density passenger versions for offshore operations–but it was actually military operators who specifically asked for less vibration. (The military variant of the EC 225 is the EC 725.) The French air force needs the EC 725s it has ordered to be as steady as possible during search-and-rescue operations, Richard Dubreuil, Super Puma new development manager, told AIN during a visit to the company’s headquarters in Marignane, near Marseilles, France, early last month.

Thanks to technological advances  including active vibration control system (AVCS) modeling, dynamic structure optimization, lightweight actuators and computers that allow control algorithm integration, Eurocopter has been able to design an  AVCS that uses only three cabin resonators. Mounted inside the airframe (see drawing), these actuators apply additional dynamic forces on the fuselage to counter the rotor-generated excitation loads. A computer uses vibration measurements recorded in the helicopter by four accelerometers to control the injected loads. The resonators work like loudspeakers: an electric current induces an electromagnetic field that causes a spring-mounted weight to displace and generate the control load.

The task of reducing vibration has to embrace the entire helicopter design, dynamics manager Tomasz Krysinski noted. In other words, the purpose of anti-vibration systems is to attenuate those vibrations that the engineers could not eliminate.

Eurocopter’s decision to add a fifth blade to the 53-foot-diameter rotor of the EC 225 addresses this “fly right from the drawing board” philosophy. The helicopter needed more lift to counteract the increased max takeoff weight. One option engineers considered was augmenting the diameter of the four-blade rotor. “But we would have been subject to vibration problems,” Dubreuil said, recalling that the company had a better experience with the EC 155 with a five-blade rotor than with the four-blader. In addition, a new parabolic shape at the tip of the blade reduces the local Mach number. This limits one of the root causes of vibration–transonic effects.

There are several other sources of vibration, Krysinski told AIN. At the main rotor, these sources include stall effects, periodic variation of blade section angle of attack, non-uniform inflow and blade vortex interaction. And then there is tail-rotor shake. All of these sources create several vibration frequencies. The main one is the rotor’s rotation frequency multiplied by the number of blades. It is called “b omega,” where b is the number of blades. In the case of the newest version of the Super Puma, the rotation frequency is around 22 Hertz, so engineers designed the airframe in such a way that the helicopter’s natural frequencies are as far as possible from 22 Hertz. The “modal hole” between 19 and 27 Hertz avoids creation of additional vibrations.

According to Krysinski, on the EC 225 Eurocopter has reduced the vibration level of the Super Puma Mk 2 (the earlier version) by half. On a diagram, medium fuselage vibration appears to be constantly less than 0.1 g. Above 40 knots, vibrations decrease as speed increases.

The new system yields more than passenger comfort, its designer claims. It also has a notable weight advantage, thanks to the principle behind it. As the resonators vibrate at their natural frequency, they demand little energy for a relatively large displacement. The spring-mounted weights can thus be small ones.

Active Noise Reduction
The EC 225’s AVCS is therefore lighter than conventional, passive systems, Krysinski asserted. Dynamic absorbers, for example, that equip the Super Puma Mk 2 are small weights that can move on a section of the blade. On the NH90  military transport and the Tiger combat helicopter, a suspension system links the drive train to the airframe.

Active vibration control already exists on other helicopters. For example, Krysinski said, the so-called “active control of structural response” uses actuators in the main gearbox of the AgustaWestland EH-101. “But it is not as easy to integrate as our system,” Krysinski claimed.

Each resonator, complete with its control unit, consists of a small box (4 inches by 8 inches by 4 inches) that can be easily fitted between two frames. Two of them are located in the rear section of the cabin and the other one in the front (see drawing). The performance of the resonators is closely linked to their location, according to Krysinski; their action is applied where it is useful–on the cabin walls, close to the passengers. The whole system draws only one kilowatt of electric power.

Certification is simpler if an AVCS is integrated into the airframe rather than located in the drive train. The ease of certification and the fact that a complete system failure brings some discomfort but no danger contributes to lower costs. “The vibration level with a failed AVCS on the EC 225 is comparable to normal levels 20 years ago,” Krysinski told AIN, again emphasizing the importance of work done from the early stages of aircraft design. Electronic hardware and accelerometers can thus be low-cost units. Finally, the AVCS is cheaper than conventional, passive systems, Krysinski claimed. (Those passive systems have been removed from the EC 225.)

The EC 225 is close to current certification standards without the AVCS, according to Krysinski, but problems arise when vibrations reach 0.4 g–the level at which vibration begins to affect pilot vision. On the EC 225, even without the AVCS, maximum vibration level is close to 0.35 g but only at a transition speed, around 40 to 50 knots. Above this speed, vibration decreases rapidly to more acceptable levels of around 0.3 g, according to Krysinski.

Along with the AVCS, Eurocopter has developed Steadycontrol, a maintenance tool that optimizes the tuning of rotors and anti-vibrators. It is available on the manufacturer’s entire range of products, except the EC 145, which is not yet certified. Thanks to a neural technique, it should greatly ease rotor tuning after a modification. “Instead of six flights, only one is necessary,” Krysinski pointed out.

Other major advances are noticeable on the new Super Puma. The avionics suite differs from those on earlier versions of the helicopter. Four 6- by 8-inch Kollsman multifunction displays support a strong concept–bringing only the most relevant information to the pilot. For example, the compass-rose of the heading display is elliptical, its shape changing with the aircraft’s attitude, just as if the aircraft were overflying the rose. The pilot therefore has indications in three dimensions on a
single instrument. In addition, “We have worked hard to limit the amount of information shown simultaneously,” Dubreuil said.

The four-axis digital autopilot features new functions such as flight envelope protection and engine failure mode, in which the autopilot asks the human pilot to let it do its work alone. “The autopilot optimizes the management of the remaining power, changes blade pitch and reduces speed to maintain altitude; the aircraft loses less than 30 feet of altitude and is back on its initial altitude in five to 10 seconds,” Dubreuil explained.

Flight envelope protection uses the radio altimeter to prevent the aircraft from going below 80 feet agl in automatic mode. Should the pilot lose situational awareness during an automatic descent to 1,500 feet of barometric altitude, the autopilot will prevent the aircraft from hitting 1,600-foot terrain, the program manager explained. In such a case, the autopilot would arrest the descent at and maintain 1,680 feet.

On the new 2,101-shp Turbomeca Makila 2A engines, a “blade shedding” design is intended to prevent shrapnel from an uncontained disk failure cutting through hydraulic lines and control linkages. In case of an overspeed condition, turbine blades separate from the disk and remain within the engine. Although this is a notable safety improvement–there had been speculation that one Super Puma fatal accident in the North Sea followed an uncontained engine failure (see AIN, February 2002, page 73)–such a failed engine would have to be replaced entirely, Dubreuil clarified.

The EC 225’s main gearbox is designed to run dry for 30 minutes. During certification tests, the EASA wanted to see the aircraft fly for half an hour after a complete loss of main-gearbox lubricant before landing. “We demonstrated the gearbox can run with its backup system for 52 minutes,” Dubreuil emphasized. The backup system is based on a liquid-spraying subsystem, where the fluid is a glycol-based mix. Certification rules stipulate that manufacturers pass a 30-minute in-flight test.

Asked why Eurocopter did not make the fuselage wider, Dubreuil said the limited market would not have justified the high development costs. To offer more shoulder room in high-density seating configurations, operators can alter the alignment of seats in the same row, thereby placing the left seats a few inches aft of the right ones.

Although Eurocopter has started the FAA certification process, it has no firm schedule yet for obtaining U.S. approval. “We are planning to meet with FAA representatives in October and November,” Dubreuil said. Since the company has already obtained EASA certification “it will take one year to get the U.S. equivalent as soon as we go ahead,” he added. However, Eurocopter will wait for a first order from the U.S. before committing to a certification date for the 24,250-pound helicopter.

So far the manufacturer has received firm orders for 10 EC 225s, “four in the offshore transport market, two for VIPs and four for a still-to-be-announced customer,” Dubreuil said. The first helicopters to be delivered– by year-end–are those in VIP layout. Eurocopter will hand over the first offshore version, to UK-based Bristow Helicopters, in the middle of next year.

Eurocopter officials see the Sikorsky S-92 as the main competitor of the EC 225/725. The EC 225 is set to replace the Super Puma Mk 2 in the company’s product lineup. Prices are similar, with the offshore version commanding approximately $18 million. No firm schedule has been set yet for concluding production of the Mk 2. The less-expensive Mk 1, with recently updated avionics, will remain in Eurocopter’s product line.

The EC 225 received its long-awaited type certificate from the EASA on July 29. The helicopter is so far allowed to fly at a maximum altitude of 20,000 feet and is certified for IFR operations. The operating envelope is limited, though, to temperatures between -15 degrees C and 40 degrees C. Eurocopter has two more flight-test campaigns scheduled to extend the temperature envelope by the middle of next year. The company will extend the upper limit of the envelope to 50 degrees C and  the lower limit to -30 or -45 degrees C, Dubreuil told AIN. EASA approval for flight into icing conditions should then be awarded.

The newest version of the Super Puma flew for the first time in November 2000, when Eurocopter was still planning certification for late 2002. The 18-month delay stems partly from a redesign of the engine’s compressor.