This year sees the start of production of the first true Eurocopter product for the civil market: the EC 135.

Andrzej Jeziorski/MUNICH

Cutaway by Giuseppe Picarella

ALL EUROCOPTER AIRCRAFT to date, apart from the military Tiger, have been either French, from Aerospatiale's stable, or German, of former MBB heritage. While the EC 135's early history goes back to the MBB BO 108 experimental programme, the French contribution to the new helicopter's development has since been substantial.

The BO 108 programme dates back to the early 1980s, and culminated with the manufacture of two prototypes funded by MBB and its suppliers, with Government support from the economics and research ministries in Bonn. The goal of the programme was to develop and test technologies which could be applied to a successor to the successful, but aging, BO 105 light-utility helicopter.

MBB hoped to achieve single-pilot instrument-flight-rules (IFR) operation with a cost-effective stability-augmentation operation, direct operating costs 25% below those of the BO 105, and improvements in performance, handling and maintainability.

The company used an airframe broadly similar to that of the larger MBB/Kawasaki BK.117, with a mostly composite structure, conventional tail rotor and a new, bearingless and hingeless, main rotor with elastomeric damping. The helicopter also had an anti-resonance isolation system, to reduce vibration and noise.

Aerodynamic improvements provided the BO 108 with a near-30% drag advantage over the BO 105, achieved by a 5° rotor-installation angle, with fuselage aerodynamics optimised to the cruise pitch attitude. The nose and tail sections were optimised and usable interior volume was enlarged, without an increase in the helicopter's frontal area.


The first prototype of the five-seat BO 108 was flown on 15 October 1988, and was powered by two 335kW (450shp) Allison 250-C20R-3 turbo-shafts mounted side by side above the cabin. This was followed in June 1991 by a second prototype, powered by 360kW Turbomeca Arrius 1Bs equipped with full-authority digital engine-control (FADEC).

The bearingless-main-rotor (BMR) system was so successful that a five-bladed version of it was selected by Boeing Helicopters and Sikorsky Aircraft for the RAH-66 Comanche combat helicopter. A similar, all-composite, bearingless tail rotor was also tried out on the BO 108 in 1990, but was not selected for the successor programme: the EC 135.

In January 1991, MBB officially declared that the BO 108 would be the successor to the BO 105. These plans were modified with the merger of the helicopter divisions of MBB (already incorporated into what is now Daimler-Benz Aerospace) and Aerospatiale a year later, to turn Eurocopter from a Groupement d'Interet Economique into a full-blown joint-venture company.

According to EC 135 programme director Elmar Compans, the French brought a valuable dose of objectivity into the programme, overcoming some of the more rigidly held ideas of the German BO 108 team.

Consultations with potential customers - operators of Eurocopter products and of competing types - showed that cabin volume should be increased and visibility improved, and that greater stress would have to be placed on mission flexibility (the cabin floor, for instance, should be flat and unobstructed to allow easy conversion from passenger to cargo roles). In late 1992, the design was modified to provide accommodation for five passengers, instead of the BO 108's three, and two crew. The Aerospatiale-developed Fenestron anti-torque system was adopted, and the EC 135 as it is today took shape.

The all-French tail "fan" is a third-generation version of the Fenestron, first used on the Aerospatiale Gazelle and Dauphin helicopters some 25 years ago.


The principle remains the same - lateral thrust is provided by a shrouded fan and augmented in forward flight by aerodynamic suction on the large tail fin. The shroud increases aerodynamic efficiency and safety, both by protecting ground personnel, and by protecting the rotor itself from collisions and foreign-object damage. It reduces the risk of grit and other objects being sucked up into the fan from the ground, reducing wear on the blades. This is also helped by the fact that the Fenestron fan is only about 1m in diameter - about half as big as an equivalent conventional tail-rotor.

The EC 135's Fenestron consists of an 11-blade stator and a ten-blade rotor, driven directly from the two-stage-transmission gearbox - a simpler gearing arrangement than that required by a conventional tail-rotor, with a corresponding reduction in maintenance costs, according to programme engineering chief Claudius Zwicker. The stator vanes straighten the flow in the fan efflux, reducing momentum losses. The carbonfibre-composite fan blades in earlier Fenestrons have been dropped in favour of metal ones, which are cheaper to produce.

Noise reduction has been a major consideration in the design of the EC 135, and the latest Fenestron incorporates some novel features. The number of rotor and stator blades differs deliberately, to reduce noise arising from the interaction of the fan and the flow-straightening vanes. The vanes are also angled with respect to the rotor radius, reducing the shock generated as each rotor blade passes. Furthermore, the rotor blades are unevenly spaced about the circumference.

This last measure has been taken to prevent all the rotor noise being generated at a single frequency, creating the intense whine characteristic of the Dauphin, for example. In the EC 135, the noise is generated over a broader frequency spectrum, at a lower amplitude.

This comes in addition to the Fenestron shroud cutting out noise which normally arises from the interaction between the main-rotor and tail-rotor blades.

The aerodynamic forces on the fin and the end plates of the horizontal stabiliser in forward flight mean that the tail rotor consumes "an order of magnitude" less power in the cruise than a conventional tail rotor, says Compans. They also provide lateral stability in the event of a tail-rotor failure in flight.

Compans adds that the Fenestron provides a further advantage, which is largely overlooked: "The Fenestron adds something of a Eurocopter corporate identity - so it's not just a technical contribution, but also a quite positive aesthetic one."

The other key feature of the EC 135 is the BMR main-rotor system derived from the BO 108 rotor, abandoning the traditional articulated rotor head. Instead, the system consists of four aerodynamically optimised, all-composite rotor blades, with integrated glass-fibre-composite flex-beams and control cuffs. The rotor shaft has a forged, one-piece, blade-attachment flange to which the blades are fixed by two bolts.

Conventional-rotor hub elements, such as transmission elements, bearings and centripetal-force bearing sleeves, are replaced by the elastic properties of the flex-beam. Flap and lead/lag hinges are also eliminated, their functions replaced by stiffness tuning in the flex-beam. Eurocopter claims that its BMR, which has accumulated about 1,200 flying hours to date, including BO 108 experience, is "the simplest and easiest-to-maintain main rotor in the world".

The maintenance advantage comes from the fact that it is built from less than half the parts, which make up a conventional rotor. It is also 22% lighter than traditional systems.

The BMR concept is one on which Eurocopter has been working since the mid-1970s, when an early version was test-flown on the BO 105. "We are now at about the tenth or eleventh version of the BMR," says Zwicker.


The early versions had problems with rotor-blade damping. Zwicker explains: "Every rotor blade has its own engine frequencies. As you run the engine up to speed, you pass through these engine frequencies, introducing dynamic forces which you have to damp, otherwise the rotor will be destroyed."

In the BO 105, the damping is provided by friction in the fixing of the blade to the rotor hub. After early ground tests of the BMR, it rapidly became clear that a damping device of some sort was going to be needed. The solution has been to fit elastomeric dampers directly to the control cuff.

The rotor blades themselves have parabolic tips, which decrease aerodynamic losses, reducing drag and noise. The curved tip acts in the same way as wing sweep, preventing shock formation at the blade tip and thus reducing noise caused by compressibility effects. The curved profile also spreads out the generation of the blade-tip vortex, producing a lower-energy vortex, which simultaneously cuts drag and reduces noise from blade/vortex interaction.

As a further noise-reduction measure, the main rotor runs at variable speed, as does the tail rotor. The rotor speed can be varied by some 4%, says Zwicker, running more slowly, and quietly, closer to the ground, and faster at high altitude, where external noise is not so critical.

Overall, says Eurocopter, the external-noise-reduction measures have produced impressive results. In fly-over tests with Arrius engines, at 500ft (150m), 125kt (235km/h) and 2,600kg, the aircraft registered 80.2dBA, comparing favourably with all major competitors. The helicopter is being offered with a choice of two power plants: either the Turbomeca Arrius 2B, or the Pratt & Whitney Canada PW206B.

During the BO 108 programme, experiences with the Arrius 1B led to an agreement between Turbomeca and Eurocopter to develop the Arrius 2B for production aircraft. At the same time, the company was looking for a second engine to offer on the helicopter and chose P&WC's PW206B. Eurocopter wanted a FADEC for both its power plants.

Compans says that Eurocopter maintained close contact with the engineering departments of both P&WC and Turbomeca during the engines' development, placing emphasis not just on power, but also on criteria such as noise, fuel consumption and emissions. In the end, he says, the competitive nature of the parallel development programmes helped motivate both companies to produce better results.

The Arrius 2B, which powers the first and third EC.135 prototypes, is rated at 340kW at ISA +20°C, compared with the PW206B's 335kW under the same conditions. Both engines are turbo-shafts, with single-stage centrifugal compressors, reverse-flow combustors, and single-stage high-pressure and low-pressure turbines.

The helicopter carries 605litres of fuel in a crashworthy, bladder-type, fuel tank under the floor, and an additional 118litres in an auxiliary tank. This gives it a range with maximum fuel of some 800km (430nm) - Paris to Berlin, roughly speaking.

The airframe structure is a mix of metal and Kevlar/carbonfibre-sandwich components. Composites constitute some 54% of the airframe's weight and were used both to keep weight low and to reduce vulnerability to corrosion. All the load-carrying structure is of aluminum alloy for lower cost and ease of production.


There is no supporting structure separating the cockpit from the cabin, improving accessibility. Two large clamshell doors allow easy loading through the rear of the cabin.

The passengers and crew sit on crash-resistant seats, which, like the fuel tanks and landing gear, can resist a 6G deceleration. Cabin height has been improved to about 1.2m since the BO 108 programme, by making the ZF-designed flat gearbox even more compact.

Noise and vibration levels inside the cabin are kept low by the anti-resonance isolation system, which provides dynamic separation of the rotor and transmission from the airframe structure. This gives passengers and crew a smoother ride, with vibration levels below 0.1G.

Combined with cabin-noise insulation, this system takes internal noise levels down to about 84dBA, depending on seat position - comparable with modern fixed-wing turboprops. Eurocopter's goal is to reduce internal noise to some 80dBA, focusing particularly on frequencies, which interfere with speech. It is aiming for a speech-interference level of below 75dB.

The cockpit has been designed for maximum visibility, with large windows and a compact instrument panel. Customer opinion seemed to favour, a mixture of conventional and electronic flight instruments, rather than a full-blown "glass cockpit". Alongside the traditional mainstay instruments, the pilot has a centrally mounted display system which gives fuel readings, temperatures, torque readings, cautions and so on. The helicopter's avionics make it IFR capable, and provision has been made for possible later integration, of a weather radar.

Compans says that Eurocopter is gearing itself up for more electronic-display-oriented instrument panels in future.

The standard flight controls are linked to a fully redundant hydraulic system designed by Feinmechanische Werke Mainz - this is effectively two systems operating in parallel, with either one capable of taking over from the other in the event of a failure. Similar redundancy is built into the 28V DC electrical system.

Eurocopter is aiming to slash maintenance costs by designing for less than 0.35 maintenance man-hours per flying hour (including the engines), and progressively doubling the time between periodic inspections. Inspections must now be carried out every 400 flying hours, or annually if less than 400h are flown per year. The manufacturer hopes to phase out alternate inspections as flying experience convinces the aviation authorities that such inspections are unnecessary.

Major inspections are to be carried out every 4,000h, or ten years, whichever comes first. The engine time between overhauls (TBO) is 3,500h, while the main transmission TBO is 2,400h, which could later be extended to 3,600h. The TBOs for the main rotor blades, hub and mast, as well as the Fenestron and hydraulic system, are unlimited.

The latest performance figures for a clean aircraft give a maximum take-off weight of 2,630kg, a maximum cruising speed of 140kt and a top speed of 155kt. The helicopter can operate at up to 19,700ft and climb at up to 1,750ft/min (8.9m/s).

Eurocopter plans to produce about 15 aircraft this year, and has already received firm orders exceeding this production figure. The first delivery will be to Deutsche Rettungsflugwacht, a Stuttgart-based private emergency-medical-services operator which already runs a fleet of Eurocopter BO 105s and BK.117s. Subsequent production output will depend on orders, but will speed up naturally along the learning curve.

The EC 135 is expected to receive certification to US Federal Aviation regulations part 27 standards from French, German and US authorities by the end of May.

Source: Flight International