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Wired into the future

Julian Moxon/MARIGNANE

For a programme that is expected to see at least 642 helicopters delivered to the armed forces of four countries, it is perhaps surprising that there has been so little fuss attached to it to date.

For not only is the NH90 easily Europe's largest ever cooperative effort on a new military aircraft, but it is also the world's first fly-by-wire (FBW) transport helicopter and the first to be developed in line with NATO requirements.

The NH90 now appears likely to survive the budget-cutting axe that hangs constantly over all of the partners involved in its development. This is not least because of the extremely tight NATO-led management that has attended the programme from the start. Costings, timetables, workshares and specifications have remained more or less as they were when the NH90 memorandum of understanding was signed in December 1990. Confidence has increased as flight testing has advanced, with only minor problems emerging to date.

The Berlin air show in May saw the programme take a giant step forwards with the commitment from all four governments to order an initial 154 NH90s, with (unusually) a single contract due to be issued, they hope at the June 1999 Paris air show, both for the production start-up phase and for the production run. The four nations also confirmed that first deliveries will be made in 2003.

Arriving at this point has not been without its difficulties - hardly surprising when the complexities of a programme involving numerous different versions for the armies, navies and air forces of four countries are taken into account.

Hitherto, the NH90 programme has been based around the NFH and TTH versions (see boxes), designed respectively for marine and land-based operations. In recent months, however, the need to accommodate the differing budget and operational needs of the customers has seen the introduction of the "kits on option" idea. This is essentially a series of packages enabling existing and potential customers to tailor the helicopter to their specific requirements.

France and Germany, for example, have shown interest in "naval utility" versions which, in the case of France, would incorporate the 16 troop seats and rear ramp from the army version to create a shipborne transport helicopter capable of replacing the ageing Super Frelons still in French navy service. German and Italy, meanwhile, are discussing a common combat search and rescue version, while Germany wants a specialised VIP variant.

VARIATIONS ON A THEME

There will no doubt be further variations on the basic theme, especially when the export market is taken into account. A civil version of the NH90 is in the cards, the helicopter having already shown during flight testing that it will be an efficient people carrier, well suited for offshore and other utility applications.

Programme complexities stretch also to the division of work at the industrial level. For the development phase, Eurocopter France is responsible for the basic helicopter and its systems, while Eurocopter Germany takes care of the TTH army version and Italy's Agusta, with considerably naval systems integration experience from the EH Industries EH101, is responsible for the NFH.

NH INDUSTRIES NH90 SPECIFICATIONS
Length overall 19.56m
Main rotor diameter 16.3m
Tail rotor diameter 3.2m
Main rotor blade chord 0.65m
Tail rotor blade chord 0.32m
Height overall 5.44m
Height folded 4.1m
Maximum width 4.37m
Powerplant R-R/Turboméca
RTM322 or GE700/T6E
Power (each) 1,266kW

 

There are five prototypes, PT1 to PT5, the first two of which are flying, with the third due to fly at the end of 1998 and the fourth and fifth to follow in mid-1999 and early 1999, respectively (the reverse order is intentional).

PT1 took off on its maiden flight from Marignane on 18 December, 1995, and has now amassed more than 190h total flying time. This machine has now been sent to Agusta for initial testing of the second engine option, the General Electric T700/T6E, which is offered as an alternative to the Rolls-Royce Turboméca RTM322-01/9 on all versions. It is virtually certain that France will specify the RTM322 and Italy the T700, reflecting the workshares each has in the respective engines. The choice of engine, in any case, bears little relevance to the aircraft itself since both produce exactly the same power - 1,266kW (1,700hp) each at maximum continuous power, with a 30s rating of 1,690kW for one-engine out operation - and both have full authority digital engine control systems.

Prototype PT2 is dedicated to development of the FBW flight control system. Development has been progressive, with the helicopter being fitted with a mechanical system for initial flight testing following the maiden flight in March 1997. After 35h, an analogue FBW system was added, which was tested for a similar period. Now the digital system, which operates in tandem with its analogue counterpart, has been installed.

Prototype PT3, now under construction, will be used for core avionics development, bringing together the visualisation, navigation, communications and all other non-specialised equipment, while PT4 and PT5, respectively, are for TTH and NFH qualification. These are also now in the early stages of construction.

TUNING WORK

Head of NH90 engineering development Marcel Lafargue says there have been "no major problems" with the development programme to date. "We have had to work hard on tuning the SARIB anti-vibration system," he says, "and there is still a problem at three revolutions per second. We think it is to do with the chordwise centre of gravity position of the blades." He points out that it is a problem "-typical of 9-10t helicopters-we had the same with the [Eurocopter] Cougar". The difference is that "-we intend to get rid of it in the NH90," he says.

Pressure distribution over the engine air inlet has been improved with the introduction of a "partially dynamic" inlet. The new item adds weight, but is bringing improved fuel consumption and performance benefits that will probably see it become a standard fixture.

A still unresolved problem centres on vibration of the rear flight surfaces, caused by wake turbulence coming off the main rotor hub. Efforts are continuing to find the optimum shape for the cupola fairing that shrouds the hub, which in the NH90 is as close to the fuselage as possible to keep the overall height of the helicopter down. "You don't see this problem on helicopters with high-mounted rotors," says Lafargue.

TACTICAL TRANSPORT HELICOPTER
(TTH) SPECIFICATIONS
Maximum gross weight 10,000kg
Mission design weight 8,700kg
Maximum cruising speed 160kt
Economical cruising speed 140kt
Rate of climb 2,165ft/min
Hover ceiling (ISA) OGE 9,500ft
IGE 11,500ft
Maximum range (6,600ft/ISA+20C 880km
Maximum endurance (6,600ft/ISA+20C) 4.5h
Radius of action (2,000kg payload) 250km

 

The NH90 is an all-composite helicopter: the constant-section, damage-tolerant fuselage is built entirely from carbonfibre composites, with a high-temperature resin included in the material used for the engine covers. Construction of the cockpit takes place at Marignane, while that of the centre and rear fuselage takes place at Ottobrun, Germany, where the workshops, autoclaves and other composite-related facilities have long been in use for other Eurocopter products. This, however, is the first all-composite transport helicopter to roll out of the Marignane factory. It is also, claims Lafargue, the first such machine to have a diamond-shaped fuselage, the aim (as with the Boeing Sikorsky RAH-66 Comanche) being to reduce the radar signature.

Cabin useable height is 1.58m, with a minimum useable length of 4.8m and width of 2m. Two large sliding doors, each 1.6m long by 1.5m high, provide access, along with an optional rear loading door/ramp measuring 1.78m wide and 1.58m long.

The NH90 cockpit is conventional, although spacious, in layout, and is based around a four- or five-screen electronic display system and conventional controls, with provision for maximum visibility. All pilot and passenger seats are capable of meeting the required 22g crash standards, and for the first time occupants are protected from the rotor system entering the cockpit at this impact level. "It is possible because the shape, thickness and orientation of composites can be done in such a way as to make the additional weight acceptable," says Lafargue. "It would add at least 150kg if metal had to be used," he adds.

The swept-tip main rotor blades are of traditional Eurocopter design, with mixed glassfibre/carbonfibre skins and four internal torsion boxes, foam-filled leading edge and Nomex-filled trailing edge. The blade is not life-limited.

Four rotor blades "-is the best compromise for this size of helicopter," says Lafargue, "bearing in mind the need to balance the folding requirement with the hinge offset moment". The offset needs to be large to accommodate the blade folding system, but as small as possible to reduce the mass of the rotor head, and consequently drag. The blades are attached to the hub using the same bearingless elastomeric design as virtually all modern helicopters, although Lafargue points out that Eurocopter invented the "Spheriflex" hub design used on the NH90 and all Eurocopter machines.

The rear rotor, also Spheriflex, is as simple as possible, being a one-piece four-bladed unit. An Apache-like noise-reducing flattened-X type of tail rotor was ruled out. "I'm not convinced it reduces noise by very much, and it also demands a heavier, more complex hub," says Lafargue. "We preferred to play with the tip design, in both main and tail rotors, and overall we seem to have a remarkably quiet machine as a result".

Automatic blade and tail folding will be provided on the NFH version only, although manual blade folding will be provided on the TTH. The rotor is first aligned with the fuselage using a gearbox-mounted electric motor. An actuator then locks the blades before they are folded back along the fuselage by hub-mounted screwjacks, at the same time as the rear of the helicopter is rotated forwards through 180°.

For the first time, the system is totally electric. Previous helicopters, such as the Super Frelon, having used hydraulic folding. The electrical system, which is simpler, more reliable and cleaner, has been made possible with the development of smaller, more powerful electric motors. First benchtests are due at the end of the year, says Lafargue, with the system being installed on PT2 for initial tests and subsequently on PT5 for shipboard trials.

The rotor system is driven (anti-clockwise)via a four-stage main gearbox provided by Agusta. Fokker supplies the intermediate tailrotor box and Eurocopter France the tailrotor box. The main gearbox features an integrated lubrication system, a new monitoring and diagnostic system and two accessory gearboxes with 30min run dry capability.

For ground operations, the NH90 is fitted with a 70kW Microturbo Saphyr 100 auxiliary power unit (APU), sized to be able to start the engines at an altitude of up to 13,000ft (4,000m) at ISA+35°C. The APU drives one of four hydraulic pumps and one of three 40kVA alternators (the other two are driven from the main gearbox) via the remote accessory gearbox, providing hydraulic and electrical power to the aircraft as well as air conditioning.

A further two hydraulic pumps are driven mechanically from the main gearbox, while the fourth is an electrohydraulic device "for a simple answer to providing hydraulic pressure even when there is neither a APU nor engine power", says Lafargue. This enables the rotor blades to be positioned correctly for starting, brakes to be applied and to operate the mid-fuselage harpoon for deck locking.

Environmental control is divided into three independent zones - cockpit, cabin and the main avionics bays. The first two use vapour cycle conditioning, and the latter filtered external air to keep the temperature below 70°C.

Engine exhaust infrared suppressors are now in development and will soon be installed on PT2 to validate their function. They are designed to divert the hot exhaust gas upwards through the low-mounted rotor blades, and are simpler and lighter than the larger ram-air type used on the Eurocopter Tiger, which in its forward battlefield role is more likely to be in the direct line of fire.

The NH90 is equipped with a "classic" tricycle-type landing gear, the main gear retracting into the side sponsons, which also house various antennae and the flotation collars. No extra fuel is carried in the sponsons, although Lafargue says the option "may be provided in the future".

CRASH RESISTANCE

Fuel is carried under the floor to preserve the centre of gravity as it is consumed, avoiding the need for compensation in the flight control system. Crash resistance, which is provided by a crushable zone and a non-deformable zone between the tank and outside skin, has been fully demonstrated in drop tests.

A maximum 2t of internal fuel can be carried, providing for the 4h NFH mission, with a 20min loiter time. Extra fuel capacity will be available in forward tanks (still to be developed) which will replace the nose-mounted anti-submarine/anti-surface weapons on the front of the aircraft's cabin.

The complexity of the tasks that will be assigned to NH90 crews, particularly in the naval role, has driven the development of one of the most integrated avionics systems ever seen in a transport helicopter. The aim is to keep crew workload down to an acceptable level through the use of automatic data processing and intelligent presentation.

The system is organised around two independent, dual-redundant 1553B databuses, one supporting the core avionics system, the other the mission system. A third, ARINC-standard bus, links these to the four (or five for the NFH) 200 x 200mm multifunction displays (MFDs), ensuring they remain available if the 1553 databuses are lost. The screens have been developed for the NH90 by Sextant Avionique and will be the largest on any NATO helicopter or fixed-wing aircraft.

The core avionics are common to both versions of the NH90, and carry out management of the multifunction control and display units, basic communications, navigation and guidance. The system also monitors the avionics and airframe systems.

Crew commands are keyed in directly through one of two control display units. Besides the MFDs, a central warning system, two remote frequency indicators and a single set of backup instruments are provided.

Incorporating the highly specific communications equipment for each army, even though all must comply with NATO interoperability requirements, has been another of the many headaches faced by the NH90 programme office at Aix-en-Provence.

The process has been greatly simplified by the decision of France and Germany to go for the same radio equipment which is used on the Tiger. A specification for a common set of communications equipment for the basic aircraft has finally been agreed and a contract awarded. All parties are now working on the individual specifications for their production aircraft.

Basic navigation equipment consists of two inertial reference systems with integrated global positioning system, a Doppler-radar groundspeed sensor, a pair of air-data computers and provision for a radio altimeter and microwave landing system.

COMPANY RESPONDSIBILITIES

EUROCOPTER FRANCE (41.6%)

Cockpit development, rotors and blades, core avionics and flight control system, powerplants and flight testing of basic prototypes.

EUROCOPTER GERMANY (23.7%)

Centre section, fuel system, avionics, tactical transport mission equipment package and flight tests of army prototype.

AGUSTA (28.2%)

Main gearbox, iron bird test rig, hydraulic system, naval mission equipment package and flight tests of naval prototype.

FOKKER SERVICES (6.5%)

Tail section, landing gear design, sliding doors, windtunnel testing.

 

NATO FRIGATE HELICOPTER (NFH)
SPECIFICATIONS
Maximum gross weight 10,000kg
Mission design weight 9,100kg
Maximum cruise speed 157kt
Economical cruise speed 140kt
Rate of climb 2,165ft/min
Hover ceiling (ISA) OGE 8,525ft
IGE 10,820ft
Maximum range 950km
Ferry range 1,200km
Maximum endurance 5h
Flying time 90km from base 3.3h + 20 min reserve

Primarily designed for autonomous anti-submarine and anti-ship warfare, but also for support, vertical replenishment, search and rescue and troop transport. Capable of all-weather operations from small or large ships in extreme weather, mission preparation being helped by the use of the harpoon deck lock, designed to exert a 5t downwards force on the airframe to keep the parked aircraft in position. The NFH has a 360í track-while-scan radar with target recognition capability, electronic warfare system, forward-looking infrared, stores management system and a data recording system. A wide range of anti-ship/anti-submarine weapons and torpedos are offered as options. The NFH is also fitted with a dedicated tactical control system, responsible for the same functions as for the TTH, but additionally equipped with an attack function for over-the-horizon targeting. A dedicated network is provided which distributes video imagery of sonobuoy releases and dipping sonar operations to the cockpit and cabin multi-function displays.

FLY-BY-WIRE SYSTEM

The NH90 is the first transport helicopter in the world to be fitted with a fly-by-wire (FBW) flight control system (FCS), the only other rotorcraft so equipped being the Comanche combat helicopter and Bell Boeing V-22 tiltrotor transport.

Development of the system follows a 15-year research programme which, ever since 1991, has been based around a FBW-equipped Eurocopter Dauphin helicopter which is used for control law and system architecture development and for evaluation of the pilot interfaces with the helicopter.

Pierre Albert Vidal, NH90 FCS manager, says civil certification requirements specify that a system with no mechanical backup, such as that to be used on the production version of the helicopter, demand that the probability of catastrophic failure should be lower than 10-9/h, while single failures should not lead to catastrophe. He adds that an FBW system was also the only way to ensure the required "Level 1"handling qualities, based around the new ADS33 aeronautical design specifications.

Thanks to their FBW systems, only the Comanche and NH90 will be able to meet these standards, which call for such manoeuvres as pirouettes, sidestepping, flying around a mark while pointing towards it and rapid stopping of the helicopter. All such manoeuvres must be carried out within strict pre-set parameters relating to time taken, workload, altitude loss and so on.

For the first time in a European military helicopter specification, the requirement encompassed Level 1 flying qualities for all NH90 operational missions. Meeting it requires high- authority control laws, a requirement which Vidal says is met more easily with FBW than with conventional mechanical controls.

Another FBW driver relates to the "widebody" fuselage of the NH90 and a problem that can result from a low-mounted aft stabiliser. This would normally have demanded a large stabiliser surface area to clear the turbulence encountered in the fuselage wake - which would, in turn, increase the pitch-up effect of the rotor wake at low speed, leading to unacceptable nose-up attitude during shipboard landings. The decision was thus taken to reduce the stabiliser area to the minimum necessary for trimming in cruise at a forward centre of gravity, and to use pitch stability augmentation to meet dynamic requirements.

This led to a relaxed-stability approach with an exceptionally reliable pitch stability augmentation system that would work even if the flight control system was severely damaged - a requirement best met with full-time FBW which, by definition, has to be totally reliable and damage tolerant.

"We made numerous trade-off studies," says Vidal, "and we found we got substantial advantages with FBW, which is lighter, less mechanically complex, saves space and is no more expensive to produce and maintain than a mechanical system." FBW, he adds, also "-improves pilotability, because the natural response of the aircraft is improved, diminishing pilot workload, so that the crew is better able to carry out its mission, which is important in a two-pilot helicopter of this complexity". An FBW system is also more survivable than a mechanical FCS, he adds, because it allows for different control paths which can be distributed either side of the airframe.

Development of the FBW system is being carried out in parallel with that of the NH90 itself, so the first prototype is equipped with a mechanical FCS. The second started flying in March 1997 with the analogue FBW system, which will provide the backup to the all-digital system (there is no mechanical back-up on production aircraft). The same prototype has recently begun testing the all-digital system.

Functionally, the NH90 FBW system is divided into a primary FCS (PFCS) and automatic FCS (AFCS). The PFCS is dedicated more to hands-on flight, and the AFCS to hands-off operation. The PFCS is the focal point of the FBW system because it is responsible for pilot interfaces, basic and degraded control laws, actuator control and actuation - all of the functions relating to the handling qualities of the aircraft. "The PFCS can be considered as the electronic equivalent of a conventional mechanical FCS enhanced with a large authority stability and control augmentation system", says Vidal.

For maximum redundancy and survivability, a quadruplex in-line monitored system was chosen, as opposed to cross-channel monitoring (which relies on voting to decide correct functioning). There are two dual digital and two dual analogue channels, integrated into two identical flight control computers (FCCs), the analogue channels providing back-up - but with limited flying quality - to the digital.

The use of an analogue system provides yet another redundancy feature, says Vidal. "In a pure digital system, redundancy is achieved by using different software in each channel. When you add analogue, you can do it by using different hardware and by taking components from different sources". Analogue is also more survivable against radiation damage, he adds.

Each dual channel consists of two lanes, each being responsible for computing the position of the actuators which move the rotor blades to provide collective and cyclic pitch control.

The problem of providing actuator redundancy has been overcome in a novel way, since a conventional swashplate was chosen, which only allows for single actuators operating in each control sense. Quadruple redundancy for actuator control has therefore been provided within the actuator itself, with extreme attention paid to prevention of internal jamming.

Four torque motors in each actuator operate two rotary hydraulic control valves mounted on the same shaft. The four (analogue) actuator control channels feeding them, each composed of two lanes (one "command", the other "monitor") must agree. Undetected failure of any one channel or motor is minimised by a "force fighting" system in which the superior force of at least two other motors overrides a faulty channel or motor. A detected failure results in that channel being switched off.

The two hydraulic control valves feed into separate cylinders of a tandem-bodied actuator. Jamming of any motor can be overriden by a built-in clutch, while jamming of a control valve is overriden by an intermediate sleeve which actuates within the control valve. Any jamming incident is detected by specific devices on the actuator ram and on the motor shafts.

Sidestick controls were rejected, says Vidal, not only because they gave "no appreciable improvement in handling quality", but because of the need, particularly acute in a helicopter, for synchronisation of the pilot and copilot controls to ensure that stick position accurately reflects the rotor position.

"When the programme was launched, industry was not able to provide a synchronisable sidestick controller at a reasonable volume, weight and cost," he adds. Conventional sticks were therefore used, with artificial force feel provided by one trim actuator per axis. For redundancy, information on the stick position is provided to the FBW system by two different types of sensor.

The control laws selected for the NH90 provide ADS33-specified attitude response in pitch or roll, blending progressively to a rate response as speed increases. Besides the basic flying qualities, the control laws provide for nap-of-the-earth flying, and for system degradation. The helicopter pilot is informed constantly of which law he is flying.

Designed to carry 14-20 troops or more than 2.5t of cargo, up to 12 stretchers or a light transport vehicle. Performs strategic air reconnaissance, medical evacuation, special operations, electronic warfare, airborne command, parachuting, VIP transport and flight training. Equipped with night vision system (tactical forward looking infrared), night vision goggles, helmet mounted sight and display, global positioning system, digital map and weather radar. The TTH is equipped for nap-of-the-earth operations near the forward battle line in any environment.

TTH mission equipment is based around four modules: the tactical control system (TCS), dedicated mission flight aids (MFA), electronic warfare system (EWS) and tactical communications (TCOM) system. The TCS, which is the heart of the mission system, interfaces with the core avionics and integrates all mission sensors, communications and navigation equipment and generates the tactical situation display, providing for flightplan management, feasibility calculation and so on, as well as handling the recording and transmission of tactical mission data.

The MFA manages the TTH sensors and navigation equipment to help with tactical mission planning, while the EWS takes care of the aircraft's protection by managing the missile launch detector, radar and laser warning receivers, chaff/ flares and infrared jammer. Finally, the TCOM deals with tactical communication with ground forces, maintaining contact through frequency hopping, pre- defined radio silences, secure voice and other techniques.

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