U. Claréus, project manager, JAS 39 Aerodynamics, Saab Aerospace
During the course of fighter aircraft development since about 1915, and stimulated by martial inventiveness in time of war, a vast range of highly specialized warplanes have seen the light of day. Only a portion of these airplanes have been fielded and done operational services within the world’s air forces. And still fewer have left a great name for themselves as outstanding fighting vehicles. Many an aircraft buff sill volunteer to name some. For example Fokker D-7, Sopwith Camel, Focke-Wulf Fw-190, North American P-51 Mustang, MiG-15, Dassault Mirage III or whatever your personal preferences might dictate.
But predictably, they have something in common. Namely good basic aerodynamics, a strong and light airframe and a powerful and dependable engine. And together these elements are in balance and “adding up” into what can be described as a synergistic “one plus one is three” effect. Some prefer to call the outcome of a development process leading to such a gifted aircraft more a form of creative art on the part of the responsible engineers, and where harmony and beauty might seem to prevail, rather than as a dull piece of mere engineering and metal wrangling.
Over the years, many technological influences have also been in fashion and have strongly influenced fighter aircraft design. During the early days of the cold war, speed was essential and the quest for Mach 2+ performance was strong, to the detriment of dog-fighting capability and field performance. The introduction of air-to-air missiles ousted the internal gun in several fighters but later made a come-back as a result of lessons learned during the Vietnam war, as well as a rejuvenation of classic close air combat capabilities. The sixties also saw the development of VTOL and variable wing geometry as a means to more tactical ability, but those developments might be said to have fallen out of favour towards the close of the previous century, the JSF program not forgotten.
The impact of the tremendous capability increase of electronic computers and associated equipment, particularly fly-by-wire (FBW) control systems, have offered new roads to performance and handling qualities enhancements. Requirements for reduction of radar, IR, visible and audio signals from the aircraft, thus avoiding premature discovery by the enemy, have been strongly felt by the aerodynamicists, as well as the propulsion specialists.
The length of the development cycle and service longevity of new generations of fighters have increased, as well as cost, promoting collaboration between countries and industries in order to afford the very substantial investment a new aircraft incurs. But, as most interested people can observe, there are nearly always flaws in a “committee aircraft”, mostly because the specification invariably asks for incompatible characteristics to be met by a single design. There are numerous examples of this. Remember the MRCA (Panavia Tornado), SEPECAT Jaguar, AFVG (Anglo-French Variable Geometry Aircraft, that didn’t even pass the politicians hurdle), and perhaps the seemingly unpromising JSF program. This fate can also befall a single contractor, as the TFX (General Dynamics F-111A and B) failure bear witness to. Also combining different tasks into one design, as the Saab Viggen was an early example of, can be dangerous if basic lessons are neglected.
Henceforth, some viewpoints of the ingredients needed for success in the basic aeronautical elements in a modern fighter will be presented. Although I have not worked in the propulsion and airframe design departments, the views are those from one closely involved in the aerodynamic work of the Gripen since inception of this multi-purpose fighter aircraft project.
The fourth generation fighter spectrum.
The Gripen project started its development program in 1980 at a time when the American F-16 had made a strong impression. There was much talk of a “high-low mix”, at that time, meaning in reality “F-15/F-16” sharing the task of defence against aggressors and where “low” was the only choice for the not-so-rich allies. Also, everyone agreed that the fly-by-wire flight control system was a “must”, but if analogue or digital and how many channels needed, opinions crossed.
In Europe talk was going on, with the German TKF and British EAP as starting points, for a fourth generation fighter, which in due course of time would lead to “Eurofighter 2000/Typhoon”. In France, Dassault was committed to Mirage 2000 and 4000 and was in the very early stage of Rafale-discussions. In Israel, plans for the IAI Lavi, quite similar to Gripen in fact, had set in motion, but later fell prey to the cancellation axe.
Most of these projects had one feature in common, namely the delta canard layout.
At Saab, then in the concept phase of a new fighter, this line of thinking was also the case, which is not surprising. As pioneers of this very aerodynamic shape in the sixties, and with some ten years of Swedish Air Force (RSAF) experience with AJ 37 Viggen at that time, this was quite natural. However, it did not mean that other configurations were neglected.
The Americans obviously intended to stay with the aft-tailed layout, as F-14, F-15, F-16, F-17/18 and F-20 could witness. It had also been reported in the press (AW&ST) that American reconnaissance satellites had caught glimpses of new advanced Soviet fighters on the tarmac of an air base: Ram-J and Ram-L they were called in the CIA jargon and subsequently they became well known as the Su-27 and MiG-29.
It was well understood that a future fighter competition on the international market arena would be hard, but in Sweden one choice was obvious in the early eighties, and that was to opt for the “low” in terms of physical size and weight and especially cost. This did not necessarily mean “low” on performance, systems capability, weapons load and al the other requirements, if only the new design embedded all the new technologies that was in the offing or could be clearly discerned on the horizon. But all the classic elements discussed in the introduction had to be there too:
Aerodynamics, airframe/structure and propulsion, all had to be very good “on its own”, but together a mutual dependency, a synergistic relation, had to exist. In the following, it will be tried to show that this was also accomplished successfully for the Gripen fighter aircraft.
One way to measure such a statement can be to ask for how many and what kind of structural redesigns were found to be needed as a result of the hard realities brought to light in the flight test, in order to remedy unwanted or “out of spec” characteristics. Examples of major redesigns are plentiful in the aeronautical literature. It consumes time and money and has in many cases led to complete cancellations. For Saab Viggen a significant number of changes had to be made before the aircraft was ripe for service. Lack of an electronic flight control system (EFCS) and its great inherent flexibility is now seen as a big negative contributor to this unhappy state of affair.
Contrasting favourably to these circumstances, the Gripen flight test revealed no defects in aerodynamic, airframe or engine characteristics requiring structural modifications. Instead, they were all better than predicted in some significant ways.
One visible “fix” has been added to the aircraft, though. It consists of a small strake behind each canard surface, but their usefulness is restricted to angles of attack above the EFCS Manoeuvre Load Limitation Boundaries. Their purpose will be discussed later.
The choice of configuration, canard or tail, was far from obvious, initially. A substantial body of knowledge existed on the delta canard layout, gained from Viggen experience of course, but that was not entirely favourable for such a solution.
The drawbacks as well as the good features were evident. A sometimes heated debate on this topic had been going on inside the Swedish aeronautical community for years. Wind tunnel testing and project work on alternative aft tailed configurations had pointed out many advantages for that particular layout, where perhaps range and sustained turn rate were the most noticeable, granted the technological level of that time.
The close coupled delta canard configuration’s primary feature, its stable vortex flow up to very high angles of attack, meaning high maximum lift coefficient, had lately been realized by the Americans, instead using large strakes as forward wing root extensions together with conventional tail arrangement, as found on the F-16 and F-17/18.
The flow physics are essentially the same. The front surface, being a delta or highly swept strake, gives off a stable detached leading edge vortex that interferes with the vortex flow from the main wing and which mutually reinforces the vortex strength of each other, and therefore burst at a much higher AOA than a lone delta wing would do. This holds true for movements in the pitch plane, but generally not for the other axis, where such flow stability is more difficult to obtain, because of asymmetrical vortex bursting, so modern fighter aircraft generally “stall” first in the lateral and directional axis.
Still the canard layout offered much, if only the weak spots could be cured. First of all a movable canard surface, higher aspect ratio wing and good cross sectional area-ruling and high slenderness ratio had to be incorporated.
FBW and digital flight control computers with flexible control laws in conjunction with reduced or even negative static longitudinal stability, all these features promised to make the desired performance achievable and the cost reduction over the Viggen that the RSAF was anxious to obtain for its next generation of combat aircraft was thought to be within reach. A closer look at the fundamental aeronautical disciplines follows.
Unstable design features
The important issue of how much relaxed static longitudinal stability is optimal for various configurations was consistently, by internal as well as from external sources, answered with a neutral static margin for the wing-tail configuration and something like minus10 percent for the delta canard, and such a value was chosen, in regard to the reference point (25 percent Mean Aerodynamic Cord, MAC).
Adopting negative stability means that the center of gravity (cg) can be placed well back behind the aerodynamic center, which in turn for a canard layout opens up a greater degree of freedom in arranging the installation of internal systems and engine in such a way that an optimal cross sectional area distribution and thus low supersonic wave drag at the selected Mach number value, can be achieved
The wing can be located more forward on the fuselage and a long and slender tail cone, quite unlike the abrupt ending found on the Viggen, and without the horizontal tail adding unwanted volume to the area distribution, can be designed. This will contribute to a low aft body drag, and will also offer an extremely good position for large efficient air brakes, exhibiting marginal trim transients when deployed
To give a numerical value for this “installation effect” emanating from negative stability is impossible, but it is considered to be higher than the more straightforward and better known effects of higher trimmed lift coefficients, less induced (lift dependent) drag and reduced trim drag at supersonic speeds. The last mentioned effect is due to the more moderate positive stability in the supersonic region, as compared to the normal excessive “nose heaviness” of a subsonic stable aircraft.
Another important change vis-à-vis the Viggen is the location of the main landing gear in the fuselage, made easier by the mid wing position and blended body shaping. The external stores have larger space made available on the pylons beneath the wing, and the cantilever single strutted main landing gear leg still leaves an unobstructed, large area under the belly for voluminous stores carriage. Most advantageous is the fact that all large external stores can be placed at roughly the same lengthwise station and close to the cg. The flying qualities longitudinally have also proved to be remarkably independent of stores weight and whatever weapon mix
The cg range can be kept extremely small for all combinations of external stores and fuel conditions, being some 5 percent of MAC, for a mass variation of roughly 50 percent of maximum take off weight (MTOW).
Canard layout features
Engine air intake location is a topic of heated debate among aircraft designers. There are usually several options at the early design stage and pros and cons are easy to list for various arrangements. Many air inlet types were contemplated and some underwent both wind tunnel investigations and thorough studies at the drawing boards.
A fixed pitot type air intake, conventionally placed on both sides of the front fuselage, was till an easy choice, because of its simplicity and favourable cost. But everything considered, this type of intake offered most versatility. The pivots for the canards found a natural bed in this structural area and the aerodynamic carry-over loads from the canards onto the upper sides of the fuselage, acting as lift contributors, are substantial. An additional pylon location on the underside of the right intake was another bonus. This is primarily a station for various sensor pods of light weight. The left bottom side is partly occupied by the internal cannon (only for the single seater).
The aerodynamic advantages derived from the close coupled canard configuration, foremost its good vortex flow stability up to high angles of attack (AOA), that can be translated into a very high instantaneous turn rate, and which in conjunction with pivoting canards that are automatically trimmed to give optimal lift-to-drag (L/D) ratios for all cg positions, Mach and AOA, were not technically feasible for the Viggen generation of fighters. Only full span slotted flaps on the canards were present on the Viggen, for further improvement of its already excellent Short Take Off and Landing (STOL) characteristics).
One decisive feature in obtaining good, straight pitching moment characteristics from the type of plan-form was found to lie in the slightly aft sweep of the canard pivot. This was derived through an intensive wind tunnel effort that consisted of testing a formidable number of systematically differing plan-form shapes, both for the main wing and the front surfaces.
In order to successfully meet the often contradictory performance requirements stipulated by the RSAF, a good balance had to be struck between the important wing geometrical parameters, such as sweep angle, thickness, aspect ratio, twist, camber and area.
For example, a demand for high supersonic speed capability and/or low transonic buffeting levels during heavy g-loading will be eased by high wing sweep angle, but then range and manoeuvrability will be degraded accordingly. And a thin wing, good for high speed, might be a blow to rolling performance at high dynamic pressures.
The plan-form that eventually emerged was a good balance between zero-lift, wave, and induced drag and showing a maximum L/D of 9, some 25 percent and 60 percent higher than the previous Saab fighters, the Viggen and Draken respectively. Leading edge sweep angle, actually three different angles for the main wing, is higher on the canard surface to ensure stable flow, as the up-wash there can increase the local AOA substantially.
High angle of attack
The topic of air combat at high angles of attack has gained much interest since the seventies, when it made reappearance, perhaps helped by the not-so-reliable air-to-air missiles of that era. Air combat seemed to end up like a classic dog-fight, with decreasing speed and subsequent high AOA. Many early supersonic fighters had a tendency to stall out of the sky when entering this region of the flight envelope, to the dismay of its pilots, as recovery was often difficult, if not impossible.
The Viggen aircraft had gone through a program of spin testing in the late seventies, that verified the rather benign high AOA characteristics of the canard layout, a fact contrary to what was known on some contemporary aft-tailed foreign fighters. So this was also an argument favouring the Gripen canard layout. Early investigations in vertical spin tunnels and tests in different rotary rigs and subsequent simulations, also pointed to acceptable spin behaviour.
A very substantial flight test program that recently was concluded for both the single as well as the two seat Gripen versions has also fully verified the excellent recovery capability, both in manual test mode and in the normal automatic mode. There exists a requirement in the Gripen project specification for a spin recovery capability, and if this can not be shown, a spin prevention system must not allow a departure to happen. Flight testing has also verified that the EFCS matches this additional demand. Double insurance might be said to exist.
As remarked previously, the only externally visible “fix” to the airframe are a pair of small strakes behind the canard surfaces. This type of “flow augmentation system”, often serving the purpose of directional and lateral stability enhancement at high AOA, is not uncommon on fighters; suffice to mention the Eurofighter and the Mirage 2000.
A spectacular Gripen aircraft departure and ensuing crash at a public air display in 1993, was the cause of modifications and revisions to the EFCS control laws in order to cure certain ailments there, one example being pilot induced oscillations (PIO). Among the changes was one pertaining to canard deflection angles at high AOA in combat mode, to increase margins for the trailing edges surfaces to run into a geometrical limitation, and thus possible longitudinal stability loss and eventual departure.
Yaw and roll stability at high AOA is strongly dependent on canard incidence, and slightly above the MLL boundary, stability drops off rapidly, becoming unstable earlier for canard deflections in the region of minus 10 to minus 25 degrees. Obviously, this incidence range was avoided. Instead small positive values of canard deflection were used in the control law’s schedules. This was beneficial, as it meant that the trailing edge surfaces were positive, that is rear end down, thus giving more positive lift. But now it was realized that in some conditions, a physical, geometrical limitation to the elevons might be encountered, which momentarily caused loss of stability.
A low speed wind tunnel program had immediately been instigated, and for the first time the large low speed wind tunnel model’s electrical engines, that normally were used only to provide discrete incidence changes to facilitate operations, where now deflected continually during a run.
A bunch of “fixing devices” was tried, and success was instant with several of these. The earlier rapid drop of stability was now completely over-bridged, and the plots showed good, continuous behaviour, indicating at dramatic improvement of the flow characteristics in that a delay of separation occurred to slightly higher alphas. A new canard trim schedule could now be introduced that eliminated the risk of the control surfaces being limited in its travel.
The flow phenomenon, commonly called “dynamic lift”, perhaps more aptly called aerodynamic hysteresis, has been the object of intense interest in some countries for decades, not the least has this been the case in Russia. Its best public known, practical application may well be the awesome aerobatic display performed by test pilot V.G. Pugachev and his “cobra” turn in a Sukhoi Su-27.
When these hysteresis effects manifested themselves during high AOA/spin tests in the specially modified second Gripen prototype, they came as no surprise. Years prior, low speed wind tunnel tests with pitching motion of the model had already demonstrated the presence of marked unsteady flow effects, hysteresis, in the post stall alpha regime. Normal force hysteresis was most evident, but all the other components, except side force, had their share.
In the high AOA and spin tests that has taken place since 1996 and recently concluded successfully, the normal tactic was to initiate the tests with a near vertical climb with speed dropping off to near zero and a rapid increase of AOA up to extreme angles, and the aircraft could then be “parked” at 70 to 80 degrees of alpha. When giving adverse aileron input there, a flat spin with up to a maximum of 90 degrees per second of yaw rotation started and could then be stopped by pro aileron input. Recovery followed, whenever commanded.
A very recent test performed in a specially high AOA equipped twin seat Gripen version has recorded a noticeable increase in maximum normal force coefficient over the static data base value, jumping up to 3.2, nearly doubling the static number 1.8.
Wind tunnel and flight test data correspond reasonably well, but it must still be said that modelling these effects are difficult, so normally in high AOA simulations they are neglected. In the future, their inclusion will hopefully improve simulations of more complex behaviour, like departure entrance.
The salient points in the Gripen aerodynamics are:
Digital fly-by-wire control system and relaxed, negative static stability in pitch (cg far aft) have made the disposition of the delta canard layout, internal as well as external, much easier, whereby:
Optimal cros sectional area ruling, thus wave drag reduction, has been fully realized.
Main landing gear stowed in fuselage, therefore external stores close to cg, small cg-shift that improves flying qualities.
Wing far forward, enabling long tail cone, meaning base drag and local area distribution favourable, and efficient air brake location on tail cone with small transients when deployed.
The direct fall-out of relaxed static stability are:
· Higher trimmed lift.
· Reduced lift dependent drag.
· Reduced supersonic trim drag.
Delta canard’s inherent good aerodynamics are:
· Stable detached leading edge vortex flow, high maximum lift coefficient.
· Positive trim lift on all lifting surfaces.
· Floating canard offers stable aircraft if EFCS fails.
· Good field performance (take off and landing), enhanced by special aerodynamic breaking mode.
· Battle damage tolerance good, “overlapping” control surfaces.
· Potential for future adaptations, like steep approach, fuselage aiming.
· Low buffeting levels made even better with leading edge flaps.
Spin recovery known to be acceptable for close coupled delta canard (not necessarily so for a long coupled canard configuration):
· Proven spin recovery capability for complete cg and AOR range.
· Nor risk of being trapped in a superstall, control authority exists.
Gripen airframe/structural layout
A notable characteristic for all fourth generation fighters seem to be the ability to pull up to 9 g in order to out-fight the opponent in an air combat engagement. For the previous generation of fighters, like the Viggen, 7.33 to 8 g was the rule, i.e. the limit load factor with a corresponding ultimate load factor of 12.
Combined with the demand for still higher operational service life, typically by a factor of two or more, going up from 1800-2000 hours for the Viggen to 4000 hours for the Gripen, the demand on the structural engineers to keep the aircraft together have certainly not lessened. Design criteria like “damage tolerance” are also typical features nowadays.
Carbon fibre composites (CFC) seemed to offer a way out, for all the tough demands put up on a new fighter in the early eighties. In-house research at Saab and by outside partners in this field had been running for more than a decade. So this type of material was considered mature enough, then. The CFC technique has kept its promise as a high strength construction material of low weight, despite the often higher resulting costs, compared to conventional aluminium designs.
A delta wing as on the Gripen offers a light but strong and stiff structure in conjunction with the use of CFC on the outside skins and main spars, even when the relative thickness of the wing is small.
The question of stiffness is vital, as the single-spar aluminium winged Viggen had shown years before. Not initially meeting the severe requirements on roll rate at high dynamic pressures, more hydraulic cylinders for the moving of the inner trailing edge elevons had been added. And the wings broke.
Early into the Gripen project, the industry discussed the trouble that McDonnell Douglas faced in complying with US Navy roll rate demands for the F/A-18. Aileron reversal had occurred during testing of roll rate at high speed/low altitude. The rather high aspect ratio wing lacked enough stiffness and had to be strengthened, adding weight.
At Saab, an intense cooperative work between the aerodynamicists and the strength department was instituted. Flight mechanics simulations had established the required minimum values for the flex-to-rigid ratio of the rolling-moment-due-to-aileron-deflection-derivative, for meeting the very stringent supersonic roll rate demands. The British Aerospace designed CFC wing was fully up to expectations, as flight tests revealed early, allowing a high rate of roll at the critical Mach/altitude/load factor values stipulated.
The Gripen wing is a multi-spar, single upper and lower skin design with three fuselage bending moment main fittings, blended into a mid wing body position. Aerodynamic benefits accrue from this location, considered to generate little or no interference drag from the wing-body juncture. Mid wing symmetry has always paid off in aircraft design, performance wise. The Saab Draken proves it, as does the MiG-21 and several Sukhoi fighters.
A delta wing also offers a fairly large volume for fuel and has in general good static and dynamic aero-servo-elastic stability properties, even with large external stores on the wing weapons pylons.
Careful area-ruling was adhered to during the design phase, and constant improvement suggestions flowed from the aerodynamicists to the airframe design engineers. A particular case of point was the front fuselage that was of circular shape initially, but had to yield to a complicated super-elliptical geometry. Significant gains in wave drag and also high AOA behaviour, were among the pay-offs, but the manufacturing department expressed concern over escalated costs.
At one moment early into the design phase, it was found necessary to start all over again with “fresh drawings” as alarm over rising supersonic drag became strong enough.
A goal of a 25 percent reduction was set up, very optimistically indeed. A significant reduction of the maximum cross section area and a corresponding lengthening of the fuselage to increase slenderness ratio was decided. During subsequent development the goal could not be fully realized, as compromises always have to be made in the design process, to satisfy all the various departments in the organization. The final result, not only accentuating the aircraft’s pleasing aesthetic lines, flight testing more than confirmed that the eventual drag predictions were met, or bettered.
Delta wing, multi spar, carbon fibre composite, offering large fuel volume and low weight.
Strong and stiff wing with good aeroelastic properties. High flex-to-rigid ratios for aerodynamic control derivatives.
Fuselage mounted main landing gear means good external stores capability and small cg-shift, thus easier to meet Flying Qalities requirements.
Optimal cross sectional area distribution and mid winged blended body with low drag.
Improvements in jet engine technology have to a large extent been the driving force for the marked increase of capability for every new generation of fighter aircraft.
What is meant by “capability” here is hard to define exactly, as there exists a certain amount of cross-fertilization between the classic mechanical technologies and the newer electronic equipment suite, comprising both hardware and software. Certainly, speed has not increased since the sixties or seventies, as demand for that, which for half a century was so dominating, seems to have slacked to zero, nowadays. Versatility of role in whatever scenario that might be projected militarily, has instead taken a front row. So a well balanced aircraft, large or small, is needed to fulfil a mission successfully.
That selection of engine is critical has been demonstrated repeatedly in the annals of fighter aircraft development. The engine once chosen for the Viggen had fared its share of problems. Being a civil-turned-into-military engine, it had for instance run into severe surging at large AOA, angles not normally operated by airliners, and also resulting in complete loss of aircraft. It was eventually fixed by costly engine and intake modifications. Although powerful in afterburner mode sand with low specific fuel consumption (SFC), due to its high by-pass ratio, a military engine must be hundred percent reliable in the hands of a hard fisted combat pilot, fully occupied with his tasks.
For Saab, the hundred percent military General Electric F404 engine represented the natural answer for a light weight fighter propulsion unit. A version without afterburner of this engine had previously been thoroughly studied in connection with a later cancelled project.
Small in size, with a thrust-to-weight ratio (T/W) of 8, and actually flying in the Northrop F-17/18 and F-20, more thrust was still asked for. The version that Volvo license manufacture under the designation RM12 had maximum thrust boosted to 18000 lbs, up from 16000 lbs thrust in the US Navy engine version.
In the Gripen the engine was placed at the fuselage rear end, well behind the nominal cg-location, a fact that was not possible on previous Saab fighters like the Draken and Viggen, where the resulting longitudinal instability otherwise would have been unacceptable. Those aircraft had their engine close to the cg, occupying valuable space in the mid fuselage region, a place that in the Gripen unstable design profitably could be used to house the main landing gear and integral fuel tanks.
The Viggen had pioneered the use of reverse thrust in a military fighter. The extremely tough STOL requirements were the main reason for its inclusion there. It had drawbacks of course, primarily added weight and increased base and wave drag and a costly ground and flight testing, including an aircraft lost after touch down, because of initial directional stability problems on the ground, caused by asymmetrical reversed thrust.
For Gripen, reversed thrust, although contemplated, was not necessary, even if field performance requirements were only mildly relaxed compared to its predecessor. This was accomplished through a higher thrust-to-weight ratio at take off and higher trimmed lift coefficient at landing, despite 1.5 degrees less AOA at the approach, as compared to the Viggen.
The Gripen automatic landing mode triggers at nose wheel ground contact, and provide large deflections of canard, elevon and air-brakes and also application of a nose-wheel brake, as deceleration means.
Thus, a smooth boat-tail, inherent in the engine convergent-divergent nozzle, constitutes the end of the fuselage that therefore exhibits very low base drag values.
It is not enough that the figures of merit for an engine, like SFC and T/W are satisfactory. The engine must also stand up to care-free handling and wild aircraft manoeuvres in the full flight envelope. The experience so far of the engine in this respect has also been entirely satisfactory. The very long bifurcated engine air duct endows good flow characteristics, like small pressure losses and low swirl, and the inlet guide vanes equipped engine exhibits remarkable tolerance at high AOA.
Only once, during a recent spin test, where spin entrance was gained through wild tactical manoeuvring in full afterburner, was a surge recorded at extreme AOA and side-slip angles, but there followed an instant engine recovery to full power.
A military engine from the outset.
Prior service experience in fighter aircraft.
High thrust-to-weight ratio, low SFC and small size.
Reliable. Fast response and high tolerance to hard pilot treatment.
Easy engine change and maintenance resulting in low life cycle cost.
The introduction of electronic flight control systems and acceptance of longitudinal static instability, i.e. the center of gravity behind the aerodynamic center, opened up the realization of the delta canard’s full potential in a way that was not possible for an earlier generation of fighter aircraft, such as the Saab Viggen.
The potential had its root most of all in the well balanced “architecture” of internal systems disposition and outside geometry so that a blend of the classic aeronautical elements of aerodynamics, airframe and propulsive system, each being excellent, was together found to manifold in true synergistic style.
The truthfulness of this proclamation can be judged by the easy compliance of flight performance requirements and the avoidance of any structural modifications to the airframe or engine. Instead, many software changes to the EFCS have been introduced. The flying qualities requirements have in due time been fully met through the use of this flexible tool. This is also a much cheaper solution than the old method of partly rebuilding the aircraft to conform to the specification, as had been necessary for the Viggen.
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