Julian Moxon/Marignane, FRANCECUTAWAY DRAWING/Giuseppe Picarella
In bringing two new machines to the market in the space of two years, Eurocopter appears to have pulled off something of a coup. By any standards, the eight seat, twin engined EC135 and now the five seat EC120 Colibri single have both been very well received, the smaller machine clocking up no fewer than 100 sales in the year since market launch in February 1997.
At a first glance, there seems no obvious outward reason for the Colibri's success. It is entirely conventional in configuration, the lift being provided by a three bladed main rotor, lateral control by the now traditional shrouded Eurocopter Fenestron tail fan, with power supplied by the 376kW (504 shp) Turboméca Arrius 2F turboshaft mounted on top of a conventional looking cabin.
But look again at the cabin. This reveals the extraordinary attention that Eurocopter has given to keeping the Colibri design simple, while ensuring that the machine appeals to the widest possible spectrum of potential customers. For example, access for large objects (such as a stretcher) is possible from three different openings - left, right and rear. A single-level floor runs through the passenger and baggage areas, the rear seats and divider being removable in 5min to create a 3m2 (32.3ft2) cargo area.
The single-level floor was seen as being of such importance that, to achieve it, Eurocopter divided the fuel tank into two halves horizontally, the upper half being located above the baggage hold, the slim lower half fitting under the baggage floor.
Such decisions, always in favour of the customer, were taken only after intensive market studies. "The aim was to offer a light, economic machine - but we had to ask the market exactly what that meant," says Colibri marketing manager Jean-Marc Royer.
A market study in 1992, both quantitative and qualitative, set the basic configuration of the Colibri in motion, establishing basic design of airframe and dynamics. In January 1996, a group of operators came to Marignane, southern France, for an in-depth review of the programme. "Then, we changed many things," says Royer, "but the result is that we believe we now have exactly the right product for today."
The original idea for the Colibri came from early market studies which indicated that there was a definite need for a new light helicopter, but that it had to be available by 1997.
Development talks with the two chosen partners, CATIC/HAMC and Singapore Technologies, began in April 1992, followed by full programme launch in January 1993. "This gave us just four years to get the helicopter into the market and operational," says Colibri programme director, Francis Combes. The maiden flight was in April 1995, with European certification at the end of 1997. "No-one imagined that it was possible to bring a new helicopter to certification in such a short time," he adds.
CHINESE LINKS
China joined the Colibri programme with a background of long term co-operation with Eurocopter on licence manufacture of the AS365N1 Dauphin for the local market. CATIC/ HAMC, which has 24% of the programme and Singapore Technologies, with 15%, are both risk sharing partners. CATIC is responsible for design and production of the cabin structure and fuel system, while Singapore Technologies produces the access doors, tailboom and composite structure for the Fenestron tailrotor.
Eurocopter is the technical leader of the programme, responsible for overall design, ground tests, design and production of the dynamic assemblies, avionics, hydraulics and electrical system, general integration, final assembly and specifications. Development of the aftersales and support network is also taken care of by the Franco-German consortium.
Teams from both partners spent nine months at Marignane "to ensure that all went home with a common idea", says Combes. "We harmonised weight, performance, and systems, defined the interfaces, and made sure we all put the holes in the right places".
One of the feedback items resulting from the 1996 review was the re-positioning of the "Moustache" landing skids, which customers decided needed raising by around 200mm. Another was the configuration of the doors. This was totally changed to provide a single large, hinged, door on the right hand side, with a smaller hinged forward door on the left and large, sliding rear door. " Making the rear panel into a door improved access to the baggage cabin for large items such as a stretcher, which can now be loaded both from the rear and the front", says Combes.
The cargo door under the tail boom is positioned to allow loading of a full-length stretcher, and there is a small shelf above the rear hold for storing equipment, such as a radio.
The rudder pedals were changed to "rocker" type devices which can easily be flipped forwards for longer legged pilots, enabling the pedal arms to be moved forwards about 40mm. "This meant we could move the pilot forward, push the front of the cabin forward, and create more space between the pilot and passengers." The main display panel was also enlarged to allow for bigger, or more, instrumentation.
One area in which there were design constraints that had nothing to do with any of these was the required introduction of crashworthy pilot and passenger seats and fuel tanks capable of withstanding 30g vertical and 18g horizontal impacts. These were required by the new US Federal Aviation AdministrationFAR27 and European Joint Aviation Authorities JAR27 regulations. The Colibri is the first light helicopter for which these requirements had to be satisfied. A further demand that the seat assembly could be removed in 5min for emergency medical services access to the rear hold demanded an ingenious, but simple solution. The two pilot seats are each mounted on a pair of simple, deformable, aluminium brackets, while all three of the rear seats are supported on a pair of telescopic arms containing pistons capable of absorbing the kinetic energy of the crash. "They had to be able to manage any combination between between one and three passengers," says Combes, "so if there was one sitting on the left side only, the system had to be self-adjusting".
This is done by linking a seat belt sensor to the telescopic arms through a mechanical system. "If we had not done it that way, the system would have been one and a half times heavier," he adds. The two-level fuel tanks, which carry a combined total of 411 litres, have been subjected to a crash test at over a 15m drop with no ill effects.
To improve maintainability, Eurocopter has paid considerable attention to the design of the engine platform and to the integration of the overall engine/transmission/tailrotor system. This centres on the positioning of the engine, which is located off-centreline, to the left of the main rotor mast - a first in a light helicopter, claims Eurocopter. The new layout yields several advantages: the balance of the helicopter is improved because the main rotor downwash provides slightly more downward force on the right side than the left, and by offsetting the engine, the right side of the platform becomes available for installation of an optional sand filter within the existing engine cowling. Having the air inlet within the cowling also eliminates the need for a gearbox oil cooler because the incoming air circulates around the housing. "It also meant that we could provide an air conditioning option as part of the options package," says Combes.
Another important result was that the drive to the tailrotor was now able to leave the main gearbox directly in line with the tailboom, and lower than would have been possible if the engine were centrally mounted. The tailrotor driveshaft can therefore run through the tail boom itself rather than through a mounted shroud on top, simplifying the overall design, and keeping it light.
The carbonfibre composite tailrotor shaft is an advance on the supercritical design used for the Eurocopter Tiger anti-tank helicopter. Two tubes with wide diameters are linked and supported by a self-adjusting bearing located at about one-third the distance between the rotor mast and tail fan (the unequal lengths being chosen to reduce harmonics). The single bearing replaces typically five on a traditional design such as the Ecureuil. In its military guise, the design is far more damage tolerant, while, for the civil role, it provides greater strength for less weight - advantages which would not have been possible in the days when steel was the only suitable tailshaft material.
The main gearbox consists of two modules. The main reduction module transmits the rotational torque to the hydraulic unit, gearbox lubricating pump, tailrotor shaft, rotor brake and second reduction stage, while an epicyclic reduction module, transmitting power to the main rotor. The design holds no surprises, apart from the re-profiling of the sun and planet teeth in the epicyclic module "to further reduce noise", says Combes.
Eurocopter has developed considerably the Fenestron fan-in-tail rotor design since it was first seen on the Aerospatiale Gazelle in 1968. On the EC120 and EC135 the design has taken a major step towards extra quietness through the introduction of an eight-bladed asymmetric fan, ensuring that the tailrotor system is totally free of resonance, helping keep noise down and reducing maintenance.
The blades are distributed in two two-bladed pairs, one pair being slightly closer together, the other further apart, each located directly opposite the other. The extra blades allow for a lower fan tip speed of 180m/s (590ft/s), avoiding the transonic noise generated by the higher tip speed typical of conventional tail rotors.
The rotor system is optimised to make the most of the camber that has been introduced into the tail fin. "The design aims to give the pilot a foot position corresponding to the least power that is consumed," says Combes. He points out that one effect of the Fenestron, which was introduced to shield the fan from the ground and to prevent people inadvertently walking into it, is that it produces a drag inducing "cylinder" of air at right angles to the direction of rotation. The effect of this has been reduced by introducing a set of stator blades, which although increasing weight, recover 15% of the thrust lost through the helical vortex coming off the blades. "It results in a very good compromise, producing a rotor that is 6.6dB below ICAO[International Civil Aviation Organisation] noise limitations", he adds.
To reduce complexity, and eliminate long hydraulic lines, pitch control for the Fenestron fan blades is mechanical, using a flexible cable running through the boom to the pedals. "On a small helicopter, a cable is right. We have no need for rudder pedal servos, and the design cannot lose hydraulic power," says Combes.
THREE-BLADED ROTOR
The decision to use a three-bladed main rotor followed noise and weight considerations. "We do not agree with the idea of having a large number of blades on a small helicopter, because it means they must have a finer chord and can only be made from metal. Ours have a wider chord, which reduces rotational speed and therefore noise," says Combes. The blade spar is made from carbonfibre composite, and the skin from glassfibre reinforced plastic - a compromise between the all-carbonfibre Dauphin and all-glassfibre Ecureuil designs which allowed Eurocopter to design for the right stiffness, and therefore natural frequency. The blade is foam filled, with a carbon leading edge and torsion box. The profile runs from a thickness-to-chord ratio of 12% at the root to 7% at the tips, where parabolic shaping reduces noise from Mach effects.
The Colibri's Spheriflex hub follows the now traditional Eurocopter design, which is based around an all-composite spherical main bearing. It is mounted on the main shaft using a tapered cone support which is less complex than previous designs and reduces fretting of the head on the shaft during rotation.
The hub itself is cast from a new titanium alloy which is less susceptible to corrosion and fatigue than previous alloys. The sleeves that locate the blades on to the hub are of a new metal-matrix composite material - with silicone intrusions developed by Aerospatiale. "It is difficult to design an all-composite component which takes account of the torsional stresses," says Combes, "and steel is too heavy, so we decided to use this new material, which allows for a design that follows precisely the stress pattern. We think we made the right decision, as only one modification was needed before it went into production."
Eurocopter has solved in an ingenious way a problem inherent in using spherical bearings on a small helicopter. Such bearings are necessarily large, leading to a blade flapping hinge location which is outboard of the ideal place for the right balance of control sensitivity and stability. In the Colibri, the blades are mounted to the sleeves at a slight angle, which has the effect of producing "Kcoupling" between the pitch and flapping angles. This reduces the control moment because the pitch angle is effectively reduced as the flapping angle increases. "It puts control sensitivity exactly where we want it," says chief flight test engineer Bernard Certain.
The 376kW Turboméca TM319 Arrius engine has single-crystal turbine blades in the high pressure turbine, the resulting higher operating temperature, and hence thermal efficiency, contributing towards a fuel consumption of 85kg/h in economy cruise. A pair of centrifugal compressor stages driving single- stage gas generator and power turbines keeps the engine very compact.
In a further major development, the engine parameters on the Colibri are displayed on a new instrument from Sextant Avionique - the Vehicle and Engine Multifunction Display (VEMD). This uses active matrix liquid crystal displays which allow what Eurocopter says is an "unprecedented" viewing angle, and readability even in direct sunlight.
The system is based around two screens, both displaying parameters in "needle" format for greater clarity, the upper display showing those linked to engine generator and rotor speed, such as turbine gas temperature, engine power margin (with respect to altitude/temperature limitations) and torque. Fuel quantity is also displayed, as is the numeric value of outside temperature. The lower screen is used for engine oil pressure and temperature, generator current and voltage, fuel flow and remaining flight time. Parameters which have either not reached or have exceeded their operational values are underscored in red. The VEMD operates in four main modes - normal ground and engine starts, normal flight, flight report, and maintenance. The different modes are displayed on the screens by using push buttons on the VEMD or on the collective grip.
The novelty behind the VEMD is that it is an "instrument of first limitation", says Combes, which puts the main engine parameters for any particular phase of flight at the top of the list of priorities. So for startup, the three classic parameters are shown - torque, turbine temperature and compressor speed, changing automatically to the first limitation mode as soon as 60% of Ng (gas generator speed) is reached.
Any condition which is out of the ordinary puts the display back to the "three needles" configuration. A further display provides an engine power check during cruising flight - essentially a means of monitoring engine condition. The display also provides a weight/performance function.
Eurocopter has placed more emphasis on customer support than with any previous helicopter. The aim, says technical support director Michel Hancart, is "to get the cost of maintenance right down, second, to maximise the number of maintenance tasks which can be carried out by the operator, and third, to provide a network of localised customer support centres".
Maintenance has been simplified by ensuring easy access to every major component, minimising the number of complex tools required and eliminating bench testing. So, for example (after training), the operator can now remove the gearbox main entry pinion, change a turbine module and perform many other tasks that previously would have required a visit to a major repair centre.
Attention has been paid to documentation, with every technical publication relating to the Colibri (except the flight manual) provided on CD-ROM as standard. Every task is described in full colour three dimensional or orthogonal format, with interactive displays divided vertically so that a drawing can be presented on one side, with instructions on the other, or two drawings can be manipulated together.
MAINTENANCE
Scheduled maintenance time per flying hour for the Colibri is 0.25 man hours, which Eurocopter claims is 30% less than for equivalent helicopters. The main and tail gearboxes are cleared initially to a 2,000h time between overhauls, increasing to 3,000h as running time is accumulated. "We have set direct maintenance costs for the first 5,000h of the Colibri's life at $111/h", says Hancart.
The Colibri is marketed with five mission packages - corporate passenger transport (weighing 37kg), police (27kg), training/ liaison (27kg), medical evacuation(25kg) and utility (18kg). All are available as separate packages designed around the basic aircraft, and consist of instruments and equipment specific to each role.
US certification was received on 28 January, one week after first delivery, to the Japanese company Nosaki. Eurocopter plans to deliver between 30 and 40 Colibris in 1998, building to between 60 and 100 in 1999.
The first prototype will be on display at the Asian Aerospace show in Singapore from 24 February, with production examples priced at $795,000 to June 1999.
Empty Weight (kg) 895
Useful load (kg) 785
Maximum take-off weight (kg) 1,680
Maximum cargo sling load (kg) 700
Maximum operational weight (kg) (External load
Configuration) 1,770
Fuel capacity (litres) 411.5
Ferrying tank (optional) 80
Performance at max take off weight
Never exceed speed (kts) 150
Fast cruise speed (kt) 125
Economical cruise speed (kt) 103
Fuel consumption at economical cruise speed (kg/h) 87
Rate of climb (m/s) 6.73
Maximum range - no fuel reserves/economical cruise speed (km) 732
Endurance/no reserve/65kts 4.18hrs
Hover ceiling/IGE (ft) 10,000
Hover ceiling/OGE (ft) 8,300
Source: Flight International
