Julian Moxon/PARIS
The fall of the Berlin Wall in 1989 meant that, almost overnight, instant obsolescence rather than Russian armed might became the threat to Western weapons systems. No longer, for example, will anti-tank helicopters be expected to form part of a huge allied force, their main role being to attack the massed tanks of the Soviet Union.
Fortunately for its proponents, the Eurocopter Tiger, along with the rest of its genre, has capabilities, which perhaps make it even more relevant to the post cold-war environment. The proliferation of local conflicts demands increasingly sophisticated responses from multi-national forces, which need to be deployed rapidly. By their nature, such conflicts are difficult to contain, and require equipment enabling precision attack without killing non-combatants, or committing fratricide.
The first, and still the most popular, dedicated anti-tank helicopter, was the McDonnell Douglas Apache, which saw extensive service in the Gulf War, and which owes much of its design to US experience in Vietnam. The Bell Super Cobra remains a contender in international sales, while the more recent arrivals in the market, such as the Tiger, the South African Rooilvalk, and Italy's Agusta Tonal, bear testimony to the continuing interest by defence-procurement agencies in such machines.
Having been shelved at least twice since it was first mooted in the early 1980s, the Tiger has already survived its first battles, and remains in the procurement plans of France and Germany. Now, however, it faces perhaps its greatest test yet - to win the UK order for 91 armed helicopters, due to be decided in July.
The Netherlands' controversial decision in early April, to opt for the Apache was regarded as a setback, but not crucial to the UK choice, since there are substantial differences between the two requirements. The UK is also involved in development of the Trigat missile which is fundamental to the Tiger's anti-tank capability, and in the new MTR390 engine, and is being offered final assembly of future export machines as well as its own.
According to current planning, a total of 427 Tigers will be ordered by France and Germany for service entry in 1999 and 2000, respectively. Because of the Netherlands' earlier in-service date, strong pressure was applied by Eurocopter President Jean Francoise Bigay, before its decision to win a commitment on industrialisation this summer.
The intention remains, and production should be agreed before the UK decision (it wants delivery of its first 14 helicopters in 2001). The first major funding for tooling and production would then be released.
DESIGN
The Tiger is the first dedicated combat helicopter to be offered from day one with its own integrated anti-tank/air-to-air weapons system, and the first all-composite helicopter to be developed in Europe (and will be the first such machine in the world to enter production). Its design aims to combine the minimum visible and electromagnetic profiles with (in the anti-tank version) a totally passive target acquisition and weapons-delivery capability on the battlefield.
In terms of state-of-the art technology, it is worth comparing the Tiger with the US Comanche attack helicopter being developed by Boeing/Sikorsky and due to enter service early next century. Both weigh around 6t, against the Apache's 9t, with similar power-to-weight ratios, feature all-composite construction, rigid main rotors, and sophisticated mission equipment packages. The Comanche, however, will have more integrated avionics, although Tiger programme manager Alain Cassier says that it is "debatable" whether the amount of money spent on developing the Comanche system will actually provide an operational benefit.
Three customised versions of the basic Tiger are under development for France and Germany, and at least two more are being considered for the international market. France has ordered the Helicopter Anti-Char (HAC) pure anti-tank version and the Helicopter Appui Protection (HAP) escort/support derivative, with plans to buy 100 HACs and 115 HAPs (because of the different strategic environment, the mix was altered in mid-1993 from 140 HACs and 75 HAPs). Germany, which originally ordered the PAH-2 anti-tank variant, has now opted for a multi-role anti-tank/escort/support machine called the UHU (Unterstutzungs Hubschrauber). Funding problems appear to have finally been overcome, and the original commitment to buying 212 UHUs remains. The UK is also considering the multi-role variant, which would be largely similar in specification to the UHU. Some mission equipment, such as the helmet-mounted display, would probably be UK-supplied, however.
All these variants are based around the helicopter vehicle itself, complete with structure, dynamic assemblies, engines, hydraulics, fuel, electrical system, and so on. A modular "basic" avionics system is fitted linked to virtually all the avionics equipment through a dual-redundant MIL 1553B databus.
Each version then receives a specific mission equipment package (MEP). In the anti-tank Tiger this is called the "Euromep", and consists of the pilot night-vision system, mast-mounted sight and a pair of helmet-mounted sights/displays integrated with the anti-tank weapons (Trigat and/or Hot) and air-to-air missiles (Stinger or Mistral).
The combat support MEP consists of a head-up display, roof-mounted sight and two helmet-mounted sights, which are used to operated the 30mm gun, the rockets and/or the Mistral.
The Tiger's design follows popular anti-tank helicopter lines, with a tandem cockpit arrangement, necessary to keep frontal area to a minimum, and to provide maximum lateral visibility. Eurocopter says that such a configuration also provides a natural divide between controls and instruments, enabling precise task-sharing and job allocation.
In the reverse of the Apache layout (but the same as the Comanche) the Tiger pilot sits in the front, and the gunner in the rear. "This gives him better visibility for nap-of-the-earth operations, and landing," says Tiger chief test pilot Andrew Warner. It also places him close to the nose-mounted sight piloting system, limiting the parallax error between the sensor and the direct outside view. Placing the gunner in the rear also helps observation and target designation, and is best for the roof-mounted sight, which has a direct optical channel.
According to Warner, the Tiger's survivability "...relies on stealth and agility". While the Apache might have a lot of firepower, he says, "...it is less manoeuvrable, and because of its size and shape, considerably less stealthy". If the Tiger is hit, the philosophy is that it can survive "...by letting the round pass through the helicopter where possible without destroying vital systems".
For anti-tank missions, both of the crew members are rated pilots - the occupant of the rear seat must also be a qualified tactical commander/gunner, able to pilot the helicopter if necessary, but also responsible for managing the anti-tank weapons system. Only one weapon may be activated at any time, and air-to-air weapons have priority over anti-tank armaments.
"The pilot is responsible for safety", says Warner. "So he handles the aircraft, carries out self-defence and looks after communications". In the anti-tank version, the pilot also takes care of air-to-air combat, while in the escort/support role he will also be responsible for firing the rockets at ground targets.
CONSTRUCTION
The Tiger airframe is split into three parts: the front and rear sections are the responsibility of Eurocopter Deutschland (ECD), the centre section of Eurocopter France (ECF). The first main fuselage was assembled by ECD at its Ottobrun plant and completed in April 1990.
More than 80% of the Tiger's airframe consists of composite materials. Frames and beams are made from Kevlar/carbon laminates, while panels are of self-stabilised sandwich construction composed of carbon/Kevlar skins filled with Nomex honeycomb. Carbon-based composites account for 66% of airframe weight, Kevlar and aluminium alloys 11% each, and titanium 6% (the rest is accounted for by bolts and fittings).
A specially designed carbon/Kevlar sandwich floor in the cockpit contributes towards a survivable crash tolerance of 2,000ft/min (10.5m/s), while the airframe can withstand 1,670ft/min without major damage, and the landing gear 1,280ft/min with no distortion.
Stub wings on the centre fuselage have aluminium spars and carbon ribs covered in carbon skins. The engine floor, originally made from titanium, is now, all composite.
The entire tail section is made from composites, carbon single-block being used for the tail boom for bending stiffness and strength, while vertical and horizontal stabilisers are from carbon/kevlar.
ROTOR SYSTEM
The Tiger's 13m-diameter, four-bladed, hingeless main rotor is the key to its astonishing performance (Flight International, 1-7 March), which is evident in the helicopter's seemingly effortless ability to perform full loops, and negative g manoeuvres (the rotor has a -1g capability). Developed by ECD (now part of Daimler-Benz Aerospace), it is designed to provide a very high maximum lift coefficient, with a long blade chord to accept high-load factors. This and the 10% equivalent hinge offset, yield the very high manoeuvrability necessary for an escort/anti-tank helicopter (this requirement is in fact driven more by the escort version, which spends more of its time in nap-of-the-earth flight).
With only 24 main parts (apart from bolts, bushes, etc) the rotor weighs in at just 363kg. The fibre/elastomeric design comprises - the main hub, with a titanium centre piece and lower/upper all-composite plates; the conical/radial elastomeric bearings; the all-composite rotor blades and a viscous lead/lag damper. "We don't know if the damper is really necessary", says Warner. "We may remove it completely".
Flap, pitch and lead-lag movements are achieved entirely through elastic bending of the neck regions of the blades, while the torsional movements necessary for pitch control are transmitted by the control rods via the elastomeric bearings. Centrifugal forces are fed through conical bearings connected to the main rotor bolt. "It is a totally new concept," says Cassier, "and gives us more control power for less weight". The complete structure has infinite life and the elastomeric bearings a minimum life of 2,500h, maintenance being limited to visual inspections only.
The 2.7m diameter, three-bladed, high-set tail rotor follows a similar concept to the main rotor, using the Speriflex soft-in-plane system in which the composite blades are forked either side of the hub, with torsional movement again permitted through elastomeric bearings. A shrouded Fenestron-type tail rotor, although an Aerospatiale invention, and used in the Comanche, was not selected. "for the amount of power we wanted, we considered the Fenestron too large", says Warner.
TAIL-ROTOR POWER
The tail rotor is extremely effective, producing a 40¡ heading change within 1sec from a standing start, building to a 120¡/sec yaw. "Its comfortable to have that much power at the beginning of the programme...most helicopters require upgrading after service introduction to achieve adequate yaw power," says Warner. The rotor is driven by a wide thin-walled, hollow "supercritical" shaft, which is lighter and more ballistically tolerant than traditional designs, which have smaller diameters but are more sensitive to damage.
The horizontal stabiliser position is now settled, the surface having originally been placed forward of the tail boom to avoid a pitch-up which occurred during transition. This had a bad effect on hover performance, however. With the stabiliser removed, Warner then showed that the Tiger could be flown at 150kt (280km/h) and 3g, after which it was moved back to the original position and reduced in size by 40%. He gives the rigid main rotor much of the credit for this, "...because it has a short natural frequency and high natural self-damping, yielding excellent dynamic stability in pitch". The Tiger is also, he points out, very easy to fly, with no automatic flight-control system. "It is the easiest handling helicopter I have ever flown," he says.
A totally new main gearbox drives the rotors, "...which means we have lots of growth potential", says Cassier. To fit within the Tiger's narrow profile, the design had to be thin, while also being extremely reliable, durable and light. The answer was to incorporate three reduction stages, the two inputs being combined at the second stage to drive the main rotor through an epicyclic arrangement. The left-hand input is also equipped with a declutching mechanism to drive the helicopter's electrical generation and air-conditioning systems with the left-hand engine running and the rotors stopped.
The gearbox has a 30min dry-run capability, but has been operated dry for 1h at cruise power with no damage. Designed by Eurocopter France, it has been made stronger and more reliable by using a new "deep nitriding" technology throughout the gear system. Integrated bearing races have also been introduced, which are lighter and more reliable.
To reduce vertical, pitch and roll vibrations induced by the rotor, a spring-mass anti-vibration system, located between the gearbox and fuselage, has been developed for the Tiger. This replaces the traditional "barbecue" system in which the gearbox is suspended from a cage, vibrations being absorbed by grill-shaped dampers supporting the bottom. Essentially, the new "SARIB" system mounts the box on a beam, which is supported by nodal points, the anti-vibration forces being provided by tuned mass dampers. "We have been surprised at the result," says Cassier: "It takes out main rotor forces and moments extremely well".
Driving the ensemble are two Turbomeca/MTU/Rolls-Royce MTR390 turbo-shafts developed specifically for the Tiger, which provide an excess of power which not only gives the helicopter the required performance, but ensure that it remains operational with one engine dead. Low fuel consumption, good acceleration, and ease of maintenance were also called for.
"This is a new generation of engine," says Warner, "in which all the operating limits can be reached at the same time." While this is good for performance, it becomes more difficult for pilots (especially in combat) to check gas-generator speed, torque and turbine limits simultaneously on three separate and very important gauges. A "power-margin indicator," showing the margins remaining when the first limit has been reached, has therefore been incorporated into the lower corner of the primary flight display.
LIGHTWEIGHT ENGINES
The MTR390 weighs only 169kg, measuring just over 1m in length, providing 873kW (1,170shp) of maximum continuous power, and 958kW at take-off with a super contingency rating of 1,160kW. For durability, both the Turbomeca-supplied compressor stages are centrifugal, and are driven by a single high-pressure turbine stage from MTU.
Drive to the rotors comes from a two-stage power turbine developed by Rolls Royce, which passes through a reduction gearbox, giving an 8,000RPM out-put speed. The engines are controlled by a Turbomeca/Elecma-supplied full-authority digital control, with manual back up.
Between the engines is a titanium wall to reduce the chance of ballistic damage to both engines while, to reduce infra-red signature, the exhaust is diluted first through a mixer, upwards and out, to be dispersed by the main rotor.
Two main internal fuel tanks are fitted, with extra tanks in the stub wings providing a total capacity of 1,354litres (360gal).
The heart of the Tiger's basic avionics system is a pair of Mil Std 1553 databuses (one master, the other slaved), a pair of control and display units, two interchangeable multi-function colour liquid-crystal displays in each cockpit (mounted vertically in the pilot's, side by side in the co-pilot/gunner's), the automatic flight-control system, moving map generator, and navigation system. Two remote terminal units collate information from the helicopter, its electrical and fuel systems, engines and hydraulics, and present the data to the pilot. Dual redundant mission computers serve as bus controllers and control the Tiger's sights and weapons.
The navigation system comprises the air data units, GPS, dual-redundant strap-down inertial reference system, four-beam doppler radar, radio altimeter, and low air speed sensors.
Responsibility for almost the entire man-machine interface, including primary flight-reference system, mission and weapons computers, and displays was conferred to the Livotec consortium consisting of France's Sextant Avionique, with Germany's VDO Litef.
Flight controls are mechanical, and based on autopilot-controlled duplex primary hydraulic servocontrols. During an extensive trade-off analysis, Eurocopter considered a triplex fly-by-wire system, but found, according to Warner, that "...the weight of the boxes was more than that of half-a-dozen composite rods".
Five prototype Tigers, are being fielded - the first four are now flying, the first machine (PT1) having taken to the air in early 1991. This is being used specifically for vehicle development. PT2 and PT3 are initially for basic avionics and system development (PT2 will be converted into an escort/support version, PT3 into the anti-tank version). The final two prototypes will respectively be built as dedicated escort support and anti-tank machines, and will be used for troop tests after development. The PT5 is due to fly in February 1996.
Source: Flight International
