Developing A New Breed Of Agile Fighter

This gives rise to another possibility where the system can utilise the aerodynamic forces available from the aircraft’s movable surfaces, activated by commands from the aircraft motion sensors, to augment the overall aerodynamic forces. This can be done to improve the aircraft’s aerodynamic, structural and operational capability. Such systems can augment the natural stability, or provide artificial stability, by applying the right corrections through the control surfaces to stabilise the aircraft. This feature is called active control technology (ACT).

For historical and very good safety reasons, aircraft have been designed to be naturally stable, with the aircraft’s centre of gravity, or balance point, in front of the centre of aerodynamic lift. This results in c1 vehicle which will resist change and, if disturbed, will return to a stable flying condition. Stable configurations have required designers to observe numerous fundamental constraints which result in severe loading and manoeuvring limitations.

The adoption of active control technology has permitted designers to dispense with natural stability and to adopt “artificial” stability with the centre of gravity located behind the centre of the aerodynamic lift. Stable and unstable configurations are shown in Figure 2. It can be seen immediately that with an unstable aircraft, the tailplane produces an upward force (positive lift) to balance the aircraft.

This upward force can be used to augment wing lift, the reverse being true for stable configurations. Unstable configurations thus allow smaller aerodynamic surfaces to be used for the same lifting capability and a reduction of about 10 per cent in wing size is possible. In supersonic flight, the centre of lift moves rearward over the wing, increasing stability, and in a stable aircraft more trimming force is needed from the tailplane and hence more drag.

Unstable aircraft thus need smaller tailplane deflections to trim, or balance, the aircraft and the tailplane is much more effective and can be reduced in size by up to 35 per cent. Reductions in the size of the flying surfaces reduce aircraft drag and give consequential reductions in engine size and fuel required. This is illustrated in Figure 3.

Overall weight savings of 10 to 15 per cent are realised if an unstable configuration is adopted in the design stage. There is also a corresponding reduction in costs. The unstable configuration is also more agile and results in an improved turning performance of about 20 per cent, which can be an overwhelming tactical advantage in a combat aircraft.

“What’s the catch”, you may ask. Unstable aircraft require continuous computerised artificial stability and cannot be controlled by a pilot, even for a few seconds. The safety of the aircraft and its crew must rely entirely on the electronic active control system. In recent years, advances in the performance and reliability of electronic equipment has allowed aircraft designers to contemplate the benefits accruing from fly-by-wire and active control systems.

Fly-by-wire systems were first introduced to augment natural stability and enhance aircraft handling over a wide flight envelope. These systems were not flight-critical and had an electrical or a mechanical back-up system to cope with electronic system failure. Tornado and Concorde are examples of such fly-bywire systems, but both are basically stable and do not enjoy the full aerodynamic benefits of artificial longitudinal stability.

Before the full aerodynamic benefits of unstable aircraft could be exploited, both aircraft designers and customers had to be convinced that practical active control systems could be designed and manufactured within acceptable cost and aircraft loss rates. The integrity requirement for flight control systems is less than one catastrophic failure in 10 million hours for a military combat aircraft.

Before British Aerospace were prepared to commit the design of future aircraft which would rely from the outset on active controls, they required that the integrity of such systems be demonstrated. In 1977, under contract to the Ministry of Defence (Procurement Executive), a demonstrator programme was undertaken with the aim of showing that full-time, full-authority active control systems could be implemented safely and the airworthiness of such systems assured.

The conventional way of meeting such high-integrity design requirements is to use multiplexed systems. This is a multiplex redundant system with quadruplex computing and primary sensing with duotriplex actuation. Such a system will operate after either two computing path failures or one hydraulic and one computing path failure.

It was necessary to use quadruplex redundancy, where all calculations and actions are carried out four times in parallel, to achieve the requirement of better than one catastrophic failure per 1 O million hours. Each computing lane compares its inputs and outputs with all other lanes and uses a “majority vote” of three out of four or two out of three to identify and ignore failing lanes.

This process can operate successfully to detect failures in two lanes, but given the resulting situation of one against one, cannot make a decision and a total system failure results after the next component failure. To reduce redundancy from quadruplex to triplex and still survive a second failure requires improved self-monitoring of each lane, so that in the one-against-one situation the “good” computing path can be identified. Until this barrier is overcome convincingly, quadruplex systems are likely to remain. The drive to triplex is being lessened by the improved packaging achieved through VLSI· (very large-scale integration) of electronics to reduce the aircraft installation overhead. The Jaguar demonstrator aircraft, a single-seat Jaguar GR Mk 1 , has been modified to embody the FBW flight control system in place of conventional mechanical controls. Since it was intended to fly the aircraft in a longitudinally unstable configuration which could not be controlled by a pilot alone, it was decided from the beginning to remove its mechanical controls completely.

The Jaguar is therefore unique in being the first aircraft to fly from the outset with a digital FBW system and no form of back-up system.

Digital computing was chosen over analogue because it offered improved accuracy and repeatability and can cope with the complex control laws needed to provide artificial stability. There is one potential gremlin in the choice of digital over analogue: digital computers use software. Due to the relative immaturity of software and its design and validation processes, there is a risk that generic software faults will cause a catastrophic failure. The production of dissimilar software by independent teams and its flight certification can be costly and is not adopted lightly.

Marconi Avionics, the suppliers of the integrated electronic flight-control system for the Jaguar FBW, had considerable digital experience from the Tornado autopilot and the YC-14 fly-by system and used different software design and test engineers to achieve an acceptable degree of dissimilarity. Because one of the aims of the program was to identify airworthiness criteria, the flight-control system was treated in all aspects as though it was intended for production.

The actuation, provided by Dowty Boulton Paul, is itself unique.

It uses a failure absorption principle, where detected failures are abs<>rbed rather than switched out as in the conventional actuator.

The flight control system, including the computers and their software, were subjected to stringent checking on a ground test at. Warton and very searching theoretical analysis at Marconi, Dowty and BAe. Wherever possible the project pilots were included in the process to monitor the overall performance of the system. Failure testing, including electrical and hydraulic system failures, as well as transient behaviour and performance, was assessed in all fields. . Jre states.

The rigour applied to the integrity appraisal was of the highest quality, and the team spirit built up during this period between the technical sponsors at the Royal Aircraft Establishment at Farnborough, Marconi, Dowty and BAe was very encouraging for the United Kingdom. Their endeavours have been well rewarded by one of the most successful demonstrator programmes of all time.

Having obtained flight certification, the aircraft first flew in October 1981. From the first flight, the handling proved to be excellent, instilling a high level of pilot confidence in the system. The aircraft combined pleasantly light control forces with good response over much of the flight envelope.

Initial flights were made by the aircraft with its standard stability levels of about + 8 per cent manoeuvre margin; the intention was to build confidence in the system. The aircraft was stable and so the fly-by-wire was applied to the control system. The stability figure quoted refers to a percentage of the mean aerodynamic chord and will vary from aircraft to aircraft for the same level of dimensional stability-the time to double the amplitude of an aircraft divergence.

The next phase was a relaxation, or reduction, in the stability levels, and demonstration of another feature of FBW systems, that of “carefree manoeuvring”. Combat aircraft must be aggressively manoeuvred to survive in air combat, and in the eat of combat it is not always easy to respect the aircraft design limitations.

FBW systems incorporate another set of control laws, in the case of Jaguar a stall departure/spin prevention system, which allows the aircraft to be flown right up to the limit within a small safe margin. The Jaguar programme required the operation of this system to be demonstrated and possibly the most dramatic trial of the programme was undertaken.

At a safe altitude of 40,000 ft the trial started with gentle slowdowns to full back stick with small lateral inputs and progressed to snatch pulls to full back stick with application of full lateral and rudder inputs. Even with the most aggressive pilot input, the system gives excellent incidence limiting and tight control. Few aircraft could have withstood this treatment without loss of control and possible aircraft loss.

The importance of this FBW feature is reflected in the pilot’s comment that “even a relatively inexperienced pilot would be able to extract the maximum performance from this aircraft. ” Aircraft losses alone in present-day service aircraft would probably justify fitting stall departure/spin prevention systems to improve their tactical capability.

The aircraft was progressively destabilised to +3 per cent, then to a per cent manoeuvre margin, and in the last condition justified the definition of active control technology where the system was providing artificial stability. In this condition, some 1/4 ton of lead was used to move the centre of gravity rearwards and the aircraft was close to its tipping limit. This was demonstrated when the pilot climbed out at the end of a flight and the nose of the aircraft rose alarmingly!

To investigate larger instabilities, it was necessary to move the centre of the aerodynamic lift forward to accentuate the negative manoeuvre margins. Large extensions, leading-edge strakes, were attached to the wings; these allowed instabilities in the order of -10 per cent manoeuvre margin to be flown. This level of instability is desirable for future combat aircraft.

The leading-edge strakes increased the wing area considerably. This wing area increase coupled with the unstable aircraft effects produced noticeable improvements in the aircraft’s land- ing and take-off speeds, which were some 20 knots lower in general. There were also improvements in turn rate and transonic acceleration. There was much pilot/engineer speculation about the added performance that the dash 58 Adour would have provided; standard Mk 104 engines are currently installed.

The aircraft was still very easy to fly and has become the benchmark for aircraft handling by one of the pilots, who now judges all aircraft against the Jaguar ACT, usually to their detriment. The largest instability reached in the trial was -12 per cent, caused by fuel slosh moving the centre of gravity further aft.

Much attention was paid to operational-type flying, dive attacks on ground targets, air-to-air tracking, in-flight refuelling, formation flying and aerobatics. The aircraft handled well, with both project pilots judging it a pleasure to fly. With some improvements in a small number of areas, it would produce an excellent handling standard for a combat aircraft. At the time of writing, the aircraft has made more than 90 flights, some of which were with Ministry of Defence test pilots who share the BAe pilot’s enthusiasm for the aircraft. The aircraft will fly several more times and then be stored until technology improvements in current equipment are available for demonstration.

FBW and ACT aircraft could be critically disabled by common-mode effects where all multiplex channels are simultaneously disabled. Common-mode effects concentrate the minds of the system designers and the generic software fault already mentioned is one such area. Two other areas are the effects of natural lightning and man-made electromagnetic interference. A special lightning rig was built at Warton to prove that the aircraft and its flight control system was immune from the effects of lightning. High voltage and high current pulses (80 kilovolts and 100 kiloamps) were injected into the fully operational aircraft with no effects. After further bench testing to represent the worst-case lightning strike the aircraft was cleared to operate in full lighting conditions. The aircraft was also fully tested for electromagnetic compatibility (EMC) in the HF band at A&AEE, Bascombe Down. This frequency band is the most critical because cable looms in small combat aircraft are resonant at these frequencies and induce maximum interference.

The demonstrator programme has now been completed very successfully, with every objective achieved. The flight-control system reliability was remarkable, with only six flight equipment failures – an excellent tribute to good design and qualification testing prior to flight. A high degree of confidence has been generated in the aircraft, its systems and its handling, such that future combat aircraft can be designed from the outset to take maximum advantage from artificial stability. Both aircraft manufacturers and customers are confident that active control systems can be implemented safely and cheaply with similar integrity to present-day aircraft.

Many of the considerations leading to FBW and active controls also apply to engine control and are as important though not always as immediately urgent as flight control. Rolls-Royce is engaged in various digital control demonstrations for future applications. In new programmes, it is hoped and expected that Rolls-Royce and British Aerospace will work closely on the safe and effective integration of digital engine and aircraft control systems.

The culmination of this work will be seen in the next European fighter aircraft (EFA). Both the British Experimental Aircraft Programme (EAP) and the collaborative EFA are being designed around an active control system whose pedigree can be traced to the Jaguar demonstrator.

Ed Daley worked on the Jaguar fly-by-wire programme for five years and from 1981 to 1984 was its project and technical manager. After joining English Electric Aviation in 1960 he was engaged in work on the navigation systems of the Canberra and Lightning aircraft. In 1974 he became head of navigational attack systems with responsibility for the Jaguar and Tornado and in 1977 was appointed avionics development manager for the Tornado at British Aerospace, Warton. Since October 1984 he has been leading a systems study team on the European fighter aircraft (EFA)