The eurofighter development programme

The Eurofighter Development Programme Keith HARTLEY

The Eurofighter development programme has to conceive and validate an aircraft system design c a p a b l e of excellent performance over a very wide multi-dimensional envelope of flight and loading conditions and mission functions. Thisp a p e r reviews the broad sweep of these requirements and illustrates the nature of the process by reference to one particular facet of the performance requirement. urofighter Typhoon is being built against a multi-role fighter specification, initially based on the traditional Cold War European threat scenario but subsequently updated and re-endorsed for the post Cold War environment. This places a high premium on excellent air-to-air performance (both Beyond Visual Range - BVR and within visual range close combat) combined with good range and external load carrying capability and accurate weapon delivery in the air-to-surface roles. The aircraft size and shape have resulted from the compromises necessary to achieve this wide spread of high performance - aviation is the art of balancing conflicting aerodynamic, structural and avionic requirements to achieve the optimum blend of performance. Good supersonic acceleration requires high thrust to weight (Specific Excess Power - SEP) coupled with low drag while excellent sustained supersonic manoeuvre performance requires low induced and trim drag. We achieve this through relaxed longitudinal stability, at supersonic speeds; the laws of aerodynamics mean that the aircraft is therefore highly unstable in pitch at subsonic speeds. Thanks to modern flight control systems, we can contain and harness this instability to provide excellent subsonic manoeuvre performance at a reduced structural mass

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compared with conventional aircraft. The benefits of a delta wing are well known - good supersonic drag characteristics while retaining a large wing area and, hence, low wing loading for subsonic manoeuvre performance. These are best realised in an unstable aircraft when the control surfaces (elevons, on the wing trailing edge) work in the correct sense, i.e. increasing lift, during subsonic manoeuvre and approach and landing. The aircraft size is governed primarily by the volume of equipment needed to meet the weapons system performance requirements, e.g. scanner size for detection performance, multiple sensors, and so on. Compared with a conventional aircraft, size and mass are reduced thanks to the extensive use of carbon fibre and the structural benefits resulting from aerodynamic instability. So there you have it - an unstable canard delta, made from plastic, that gives us an excellent blend of performance characteristics matched to the customer's requirements. In the modern operational environment, success comes from the fight blend of airframe performance, weapons system performance, and away from the pilot's immediate desires! - reliability, maintainability and low life cycle costs: there is no point in having an aircraft which is a wonderful performer on the odd occasions it gets airborne, but otherwise

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spends most of its time in the hangar. The Eurofighter specification places equal emphasis on RMT (reliability, maintainability and testability) as on raw aerodynamic and weapons system performance. The procurement contract also includes a very comprehensive Integrated Logistical Support specification which covers all aspects of aircraft engineering, technical support and training. This aircraft is unlike any other produced in Europe, having a major emphasis on the whole operational package - fighting and supporting - considered throughout the whole design process.

The weapons system The aircraft has a variety of sensors. Air threats can be detected and classified by the radar (a third generation multimode pulse and pulse-doppler sensor), the infrared search and track (IRST), the Defensive Aids Sub System (DASS) passive warning sensors and the IFF interrogator, as well as being relayed across the data link network (multifunction information distribution system - MIDS). The sensors are complementary rather than duplicatory. Radar provides reasonable quality angular data, accurate range and track discrimination, and track identity; the IRST provides accurate angles but less accurate range; the data link provides accurate

........................................................................................................................................................................................................................... .ree..New_Euro but time-latent track and identity data; and the DASS provides angles, coarse range and identity. Processing within the weapons system fuses all the inputs to provide the best overall quality of all track data - kinematic and identity - for use in weapon aiming, priming and cockpit tactical displays. Clearly, there is potentially a huge amount of data available to the pilot. A state of the art cockpit design is essential if pilot workload is to be kept within bounds. For the first time ever in a European programme, the cockpit itself is a certifiable system, and a dedicated cockpit design group has been responsible for detailed Man Machine Interface (MMI) aspects throughout the design process. The resulting layout is as follows: A large, diffractive optics head-up display, certified as the primary flying instrument reference, showing high integrity flight path and tactical data. Three 15 cm square colour multifunction head down displays, with a variety of formats optimised for different phases of flight, tactical situations, and pilot-configurable settings. A comprehensive warnings system, combining audios, a warning panel and a voice warning system. This is an essential part of the cockpit design, since we display very little aircraft systems information to the pilot in normal operation - if it works, he doesn't want to know, if it breaks, someone (the voice warner) will tell him. Comprehensive throttle and stick controis; multimoding of these allows control of up to 45 functions from 25 controls. Direct Voice Input (DVI) - voice control of a wide range of functions from weapons system management, to format selection and manipulation, to alphanumeric data entry. The combination of DVI and throttle and stick controls - voice, throttle and stick (VTAS) - means that the pilot rarely needs to manipulate any controls other than those on the throttle and stick.

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Figure I. W e a p o n carrier - d e v e l o p m e n t aircraft DA5 Aerospace).

Weapons and stores As you might expect in an aircraft with high excess power and a large wing, there is plenty of scope for carriage of external stores. The medium range missiles (currently Raytheon AIM-120 AMRAAM) are carried in semi-recessed bays on the fuselage. Each wing carries up to 4 pylons. The outboard pylon is always fitted, and is a low drag device for carriage of the short-range missiles (AIM-9L or ASRAAM). The centre of the remaining 3 pylons is plumbed for fuel 1000 litre or 1500 litre tanks. The outer of the three pylons is stressed for 10001b bombs or other stores, while the inner pylon can carry large stores like Paveway 3 laser guided bombs or Stormshadow stand-off missiles. Another 1000 litre fuel tank can be carried on the centreline pylon, which, like the other pylons, includes MilStd 1760 weapons bus interfaces. If selfdesignation of targets with a laser is required, a suitable pod (the aircraft is currently designed for TIALD and the LORAL pod from the F-18) is carried in place of the front left under-fuselage AMRAAM. A huge range of configurations can be carded, including AIR

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up to eighteen 500 lb Mk 82 bombs, a variety of laser guided stores, antiarmour stores and anti-radiation missiles (figure I).

The development programme The development programme is a direct consequence of these characteristics and the need to demonstrate compliance with all the aircraft and system aspects of the specification. With a wide flight envelope required, and large structural envelopes to dear, plus all the weapons system performance measurement and development, plus the need to demonstrate and validate reliability and maintainability criteria, it is clear that the development programme demands a significant level of resource and effort. We have 7 flying prototypes in the programme at present; another complete airframe is completing the ground-based fatigue testing. The aircraft are dispersed between the different Eurofighter partner companies, in rough proportion to the agreed workshares; this means two aircraft each in UK, Germany and Italy, and one in EUROPE

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Spain. The first three aircraft are primarily airframe and engine development assets, expanding the flight envelope in the various configurations flutter and handling qualities, completing stores carriage, release and jettison envelope work and aircraft performance measurement. These aircraft are fitted with partial avionic systems electronic cockpits, data buses, but not the main weapons sensors. The remaining four aircraft are primarily weapons system development vehicles, most of them being fitted with the majority of the avionics, sensors and equipment; their tasks are mainly sensor performance measurement, weapons integration, and system moding (figure 2). The first two aircraft (one in Germany, one in UK) were fitted initially with modified Tornado RB199 engines: there is a significant reduction in risk if a new airframe is flown with a tried and tested engine. This initial flying allowed us to clear the flight envelope up to 650 knots/Mach 2, and to 7.25g. Both aircraft have now been modified to remove the RB199s and replace them with development standard Eurojet EJ2OO engines, as already fitted to the other prototypes. Within the next 18 months, these prototypes will be joined by the first 5 production aircraft. These aircraft are being instrumented for testing during build, (and are hence known as the Instrumented Production Aircraft IPA's). They add production standard resources to the development programme at a time when it will be particularly busy, building up to initial service release. These aircraft also provide an asset for our Customers to use in the development of tactics - this is in addition to the operational test and evaluation (OT&E) type of evaluation being done in the development programme between industry and the Customers' own test centres. The development of the EJ200 engine is described in a companion paper in this issue [1]. In simple pilot's terms, -

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the engine is all one could wish for excellent throttle response, carefree handling, and performance in both dry and reheat power that is exhilarating. Even the apparently trivial details add to its appeal - the intakes "howl" at the pilot in a most stimulating way whenever he gets close to maximum dry power, or in reheat. This might sound amusing but meaningless, but anything that adds to the pilot's empathy and closeness to his aeroplane adds immeasurably to his performance.

Carefree handling The development programme has far too many significant aspects for me to describe them all in detail. Instead, I will describe just one - the carefree handling programme - as an illustration of the development process. There are several reasons for designing-in a very capable flight control system that includes automatic limiting of angle of attack (AOA) and normal 'g', including: It can dramaticallyreduce pilot workload. By preventing inadvertent exceeding of structural limitations, we can reduce design margins and thus achieve lighter structures. We can retain good handling qualities over a range of configurations, flight conditions and centre of gravity (CG) positions, while retaining the performance advantages gained from relaxed pitch stability. The Flight Control System (FCS) is a digital manoeuvre demand system, quadruplexed for integrity and redundancy. In pitch it provides pitch rate demand - i.e. the pitch rate response of the aircraft is proportional to pilot stick displacement, although we vary that in some parts of the envelope. AOA and g limiting functions are imposed on top of the pitch rate demand laws. The FCS is provided with data on aircraft store configuration, fuel state and disposition, 3 axis attitude and rates, and can thus calculate centre of gravity position and adapt and optimise the control laws 4 8

for any particular configuration. It also allows us to automatically adjust, for example, instantaneous g limit with fuel usage or stores release. Primary flight control is via the split elevons and the foreplane (or canard). The FCS uses the rudder for turn coordination and sideslip control and it also controls operation of the leading edge flaps. The scheduling for leading edge flaps is complicated but in general it is optimised for best lift/drag ratio when subsonic and for minimum drag when supersonic. Flight testing of the carefree handling qualities is carried out progressively. A considerable amount of wind tunnel testing to determine the basic aerodynamic coefficients is followed by a lot of mathematical modelling and manin-the-loop simulation. I am often asked about the confidence we can achieve from simulation at this depth: from our experience on this and earlier programmes uging similar flight control systems, I have been very impressed with how well the simulation has matched our flight experience. Only in areas of marked aerodynamic non-linearity have we seen any detectable difference between prediction and reality, and even then those differences have been minor. In our current high AOA programme, the only discrepancy so far has been an increase of one degree in predicted sideslip under an unusual combination of 1AS, Mach Number, and control input; this difference was so small that the pilot did not notice it, and even when prompted to recreate the situation found it difficult to detect. Given that one of the primary objectives of the initial flight test carefree handling programme was to validate the simulation and aerodynamic modelling at these extremes, we are very pleased with the results For the initial series of tests, the aircraft was fitted with an anti-spin gantry. Although we planned no deliberate departures from controlled flight nor predicted any within our planned

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Figure 2. DA2 in air-to-air refuelling trials. (Doc. British Aerospace) test envelope, it was a sensible precaution to take. All our modelling indicates that if the aircraft did depart then recovery would be achievable but we had no wish to demonstrate this in flight. If such things occur in service, we will have failed in our carefree handling objective; therefore, the additional cost in flights and programme time for demonstrating natural recovery from a spin is difficult to justify. The first couple of flights in the programme were used to demonstrate correct operation of the anti-spin chute. Incidentally, although I don't have time to go into the detail here, this was the one area of the programme where we had trouble - suffice to say it took us more than double the planned flights to validate the spin chute. The next phase of the programme involved expanding the flight envelope beyond our existing limits (then generally around 25 ° AOA) out to the full back stick limits (around 28 ° - 290 at current CG and configuration). This was done initially in benign manoeu-

vres - lg slowdowns, and progressive windup turns - before going to more dynamic manoeuvres - 2 second "snatches" to full back stick, then snatching to full back stick as fast as possible. Each manoeuvre is evaluated in real time by the ground team. The aircraft telemeters data to the ground station, where it is compared with the outputs from the complete aerodynamic model. The aerodynamic model is running simultaneously with the aircraft and uses the aircraft-telemetered values of speed, height, start AOA and sideslip, stick inputs, etc., to predict the aircraft's response. These two responses - aircraft actual and aerodynamic model predicted - are displayed on the same monitor to the telemetry team engineer, who can see in an instant whether we are achieving reasonable and sensible matches with our predictions. The process - when operated by a well-trained and very competent telemetry team - is extraordinarily efficient. Indeed, the time limiting factor tends to be the ability of the |4

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pilot in the cockpit to keep ahead of the proceedings and ensure the aircraft is positioned for the next test point as quickly as possible. Having confirmed that our modelling is good, and established confidence in the aircraft's handling qualities, we then move on to more aggressive manoeuvring. It is a quirk of human nature that as soon as you give a pilot a clever means of keeping out of trouble he will promptly become much more careless in his flying behaviour and put in gross control inputs which he would never dream of doing in a conventional aircraft. The carefree FCS therefore has to be able to cope with some very extreme manoeuvre inputs. Because we cannot hope to devise and flight test every conceivable control input a pilot might make, we try to create and demonstrate the worst combinations of pitch, roll and rudder and inertia-coupled inputs we can. This phase of testing therefore covers rolling pulls - full roll stick for a couple of seconds followed by a snatch to full back stick with the roll EUROPE

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The Eurofialhter slick still applied, rolling breaks snatch to full back stick and then full roll stick, sometimes with proverse rudder, sometimes with adverse; and diagonal snatches - full roll and full back stick together as fast as the pilot can do it. We've now completed the first phase of the programme, with manoeuvres up to 29 ° AOA. At the current FCS software standard and aircraft CG, that has given us great confidence to move onto the next stage. This needs revised FCS software - the first version of which is available in October which contains improvements to the

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"fine tuning" handling qualities and will allow us to explore further as CG's move aft. We intend to iterate in this way until we achieve our intended production envelope of around 33 ° 35 ° AOA. The final phase of the programme will include the worst mishandling we can create, including zero speed tailslides and hammerheads.

Lessons There are several good lessons from the carefree handling programme so far: Invest up front in good quality wind tunnel work, mathematical modelling

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and man-in-the-loop simulation - it significantly reduces risk in the eventual flight trials. Telemetry is essential in allowing a good progression of test points throughout a sortie, and well-configured telemetry integrated with the test plan can lead to significant savings in the flight programme. A well trained, well motivated telemetry team is crucial to achieving the best quality results from the flying, as well as maintaining high levels of safety. Be prepared for the unexpected - e.g. our only programme problem came with the anti-spin chute, an apparently tried and tested item. •

REFERENCE [1] Hilton D., The Eurojet EJ200, Air & Space Europe, (3) (1999).

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