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Agile thinking

While the design emphasis was on air-to-air combat - both beyond and within visual range - the result is a capable multirole machine

Acceleration is described as "tremendous" and agility is claimed to exceed that of today's best fighters, but pilots say the Typhoon is easy to fly. This is no accident, but the result of careful integration of aircraft configuration and flight control.

In flight testing, Eurofighter says, the aircraft "frequently" supercruises - flies "well above" Mach 1 without engine reheat - and has shown itself capable of exceeding M2 with reheat. The aircraft has been flown to M1.6 with external fuel tanks, a "significant achievement", the company says. "We are getting tremendously efficient aerodynamic performance," says managing director Brian Phillipson.

Evidently there is no shortage of raw performance in the Typhoon, but much of the flight testing has been devoted to proving that the aircraft is safe and easy to fly to the limits of that performance. "In the Tornado, the limits are on the pilot's kneepad. In the Eurofighter, the pilot will not care about limits. It's all done by computer," says development phase director Martin Friemer. "The pilot can fly the entire envelope and never exceed a limit."

Several design elements combine to give the Typhoon its combat capability. The unstable delta/canard configuration combines low supersonic drag with high subsonic agility, while the low wing loading and high thrust-to-weight ratio combine to provide high manoeuvrability, short take-off capability and a heavy payload for air-to-ground missions.

The Eurofighter configuration is optimised for air combat performance. Beyond-visual-range combat requires high acceleration and climb rate to give maximum launch energy, and therefore range, to the aircraft's air-to-air missiles. Manoeuvring into an attack position without losing that energy requires a high supersonic sustained turn rate, while escape by manoeuvring to reduce the effective range of the opponent's missile requires a high instantaneous turn rate. Close-range combat, meanwhile, requires high agility.

Europe's three new-generation fighters - including the Dassault Rafale and Saab/BAe Gripen - are all delta/canards. Although strake/trapezoidal wing planforms similar to today's Lockheed Martin F-16 and Boeing F/A-18 were considered, designers quickly settled on the delta/canard as offering the best combination of high agility and small size.

Combining a delta wing with a canard foreplane offers aerodynamic and structural advantages, but minimising the disadvantages requires careful integration. "A lot of aerodynamic studies and windtunnel testing are required to understand the complex flow mechanism and to maximise the benefits while keeping detrimental effects as low as possible," according to Friemer.

A delta wing offers low supersonic drag, because of its low thickness-to-chord ratio, and is also both structurally and volumetrically efficient because the long root chord results in a relatively thick wing. This reduces structure weight and increases fuel capacity relative to a trapezoidal wing.

The disadvantage of a delta is that it generates high drag at even moderate angles of attack. This can be minimised by making the configuration unstable in pitch, as Dassault did with the Mirage 2000.

In a longitudinally stable configuration, the centre of pressure is behind the centre of gravity (cg). As the aircraft pitches up, lift increases and acts to push the nose back down. In manoeuvres, the trailing-edge control surfaces must be deflected upward, reducing lift, to trim the aircraft.

In an unstable configuration, lift acts forward of the cg and therefore amplifies any pitch excursions - intentional or unintentional.

Instability reduces the trim drag of a delta because, in manoeuvres, the trailing-edge surfaces are deflected downward, towards the optimum wing camber. Leading-edge devices, when deflected downward, further optimise camber and allow the delta wing to generate high lift with less drag.

An unstable aircraft is also more responsive in pitch, increasing its agility. Although the Euro-fighter becomes stable supersonically, when the centre of pressure moves aft, it is still less stable than a conventional configuration and therefore retains the advantages of reduced trim drag and better turn performance.

A delta alone does not provide the required agility, however. Adding a foreplane generates lift ahead of the cg, helping destabilise the aircraft, and the interaction of the foreplane vortex with the wing flow increases total lift to more than the sum of both surfaces. The same vortex interaction shifts the wing aerodynamic load distribution inboard, reducing bending moment and structural weight.

Eurofighter says the delta/canard configuration results in a shorter, lighter aircraft, but admits it has less attractive features. The foreplane, for example, can amplify the already non-linear aerodynamic characteristics of the delta. Full-authority fly-by-wire is required to handle the aerodynamic non-linearities and to artificially stabilise the aircraft.

Experience with highly unstable configurations, provided by the German F-104 Control Configured Vehicle and UK Active Control Technology Jaguar research aircraft, was fed into the Experimental Aircraft Programme (EAP). First flown in 1986, the EAP demonstrator incorporated many of the design features of what was to become the Eurofighter, and made "an extremely valuable contribution" to the programme, says Friemer.

Eurofighter designers wanted as much instability as possible, for minimum drag and maximum agility. "We aimed for 15% early on, but settled for 8%," says Friemer.

The maximum pitch instability possible was determined by the capability of the flight controls. "We needed highly dynamic actuators, which drove the level of instability we could achieve," he says.

The aircraft has a quadruplex-redundant digital flight control system (FCS). Developing the system has been a challenge, admits Phillipson. "The FCS integrates the armament and fuel systems to control a small, highly unstable aircraft over a huge range of stores and cg movement," he says.

FCS testing is through its high-risk phase, Phillipson says, having overcome "very significant challenges" posed by the cg variations with large external stores. "Eighteen months ago we were sucking our teeth, we did not know how to handle the instabilities, but we got through and it's hanging together well," he says. The aircraft has been flown with 1,000 litre (265 USgal) supersonic and 1,500 litre subsonic external fuel tanks. "The integrity of the fuel and stores management systems is critical," he says.

Dasa leads the flight controls joint team, formed in 1994 to overcome problems experienced developing the FCS. Now the work - and risk - is shared equally between Dasa, BAe and Marconi Electronic Systems, which supplies the flight control computers (FCCs), software and other elements. "There was real improvement after we were integrated into one team," says Stefan Levedag, flight control systems leader at Dasa.

The FCS is designed to withstand two "hardover" failures and remain operational. Each of the four FCCs runs software programmed in a "safe" subset of Ada. The computers communicate via high speed, high integrity links to cross-check all sensor inputs and command outputs to detect and isolate any failures. The FCCs also communicate via databuses with the aircraft's avionics and utilities control systems.

The computers drive both the primary flight controls (flaperons, foreplane and rudder) and secondary flight controls (leading-edge slats, inlet cowl flaps, airbrake and nosewheel steering). Sensors include four rotating-vane air data probes under the nose and an inertial measurement unit using skewed-axis gyros to provide quadruplex redundancy.

All four FCCs drive each of the primary flight control actuators, using high-current electrical links to both command and power the servo valves. "These are the first direct-drive actuators in a production aircraft," says Levedag. The actuators, supplied by Liebherr, are "small, simple and rugged," he says.

Both flaperons and foreplane are used for pitch control, with the foreplane used to initiate manoeuvres, and the flaperons to trim the wing. At all other times, the foreplane follows a 'drag-optimum' schedule with angle of attack (AoA) and Mach number to provide the best flaperon trim position. The leading-edge slats, which increase both wing area and camber when extended, are also scheduled with AoA and Mach number. The inlet cowls are open only at low speed and high AoA, to improve airflow into the engines.

A major feature of the Typhoon is its "carefree handling" (CFH) capability. This FCS function protects the aircraft from any inadvertent, or intentional, control demand which would exceed its aerodynamic or structural limits. Eurofighter says CFH allows the pilot to demand maximum performance confident that the aircraft will not be overstressed or depart controlled flight.

Aerodynamic and structural limits, how quickly they be approached and by how much they can be overshot, are all programmed into the FCCs, which then limit their response to control demands to keep the aircraft within the permitted envelope. "CFH lets the pilot fly to the margin, but we allow some overshoot so the pilot can approach the margin aggressively," Levedag says. The pilot can override g limits, but this is considered unlikely.

The ability of carefree handling to control g limits precisely has allowed designers to reduce the ultimate load factor to 1.4, from the normal 1.5, resulting in a lighter aircraft. The airframe is designed for a 6,000h life.

The FCS is also responsible for all automatic flight control, and is more than a simple autopilot and autothrottle. Advanced functions include automatic combat air patrol and attack manoeuvres and an automatic recovery capability: a disoriented pilot can push the "panic" button and the aircraft will stabilise itself wings level in a gentle climb.

One criticism levelled at the Typhoon is its apparent lack of stealth features. Eurofighter argues that several aspects contribute to the design's low observability. First there is its small size, with materials and coatings used to reduce the radar signature further. "We are contractually bound to a frontal-hemisphere radar cross-section smaller than any aircraft in production at the moment," says Eurofighter.

Then there is the systems philosophy, which allows passive operation using an infrared search and track sensor, datalink, electronic support measures, digital terrain system and helmet-mounted sight and night-vision equipment. Together, these give the Typhoon pilot the ability to detect and engage air and ground targets without using radar, Eurofighter says.

Small size and light weight are key elements of the Eurofighter design concept, as enshrined in the original Turin Agreement characteristics of a 10t (22,000lb) empty weight and 50m2 (540ft2) wing area. In fact, production aircraft will have an increased empty weight of 11t, the result of structural strengthening to meet revised requirements which emphasise the Typhoon's multi-role capability.

Meeting the weight target mandated extensive use of composites. As a result, about 70% of the surface area is carbonfibre-reinforced plastic. The wing, vertical stabiliser and forward fuselage are mainly carbonfibre, while the centre and aft fuselage have composite skins over aluminium and titanium substructures.

A high-strength, damage-tolerant fibre/resin system is used, and the wing's multiple carbonfibre spars are co-bonded to the lower skins. Plans to use lightweight aluminium-lithium were dropped because of "manufacturing quality" issues, says Friemer, and a conventional alloy is used for the fuselage frames, chin intake, wing leading edges, wingtip pods and fin leading and trailing edges.

The foreplane is titanium, superplastically formed and diffusion bonded (SPF/DB) for low weight, high strength and optimum aerodynamic profile. A vertical shear web between the engines is also SPF/DB titanium. The high-strength, temperature-resistant metal is also used for the wing/fuselage attachments and the outboard flaperons, which must withstand the exhaust plumes of the air-to-air missiles always carried on the outermost wing pylons (the wingtips being permanently occupied by pods housing the defensive aids subsystem, including towed decoys).

The delta planform allows the Typhoon to carry substantial air-to-ground weaponry in addition to its standard air-to-air armament of internal 27mm cannon, four medium-range missiles semi-submerged under the fuselage and two short-range missiles outboard. Including the fuselage stations, there is a total of 13 hardpoints.

Fuel is carried in the wing and two fuselage tanks, and the aircraft can carry three 1,000 litre supersonic or two 1,500 litre subsonic external fuel tanks. A retractable inflight refuelling probe is housed forward of the cockpit. "Air refuelling is now common, which is an indication of the importance of air-to-ground capability to all four nations," says NETMA deputy general manager Christian Biener.

The customer seems happy with the vehicle. "The aircraft may not reach its requested weight - but only by 20-30kg. And we will get 200-300kg more fuel," says Biener. "There is plenty of margin. We are quite sure we are going to have the specification overfulfilled."

Phillipson agrees: "We know the platform aerodynamics very well. There have been no surprises; everything is on the predictions. It's remarkable how good this aircraft is."

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