|U. Claréus, project manager, JAS 39 Aerodynamics, Saab
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 worlds 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
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 didnt 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
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
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
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 configurations 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
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
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
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
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 laws 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
A low speed wind tunnel program had immediately been instigated,
and for the first time the large low speed wind tunnel models
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
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,
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,
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 canards inherent good aerodynamics are:
· Stable detached leading edge vortex flow, high maximum
· 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
· Potential for future adaptations, like steep approach,
· Low buffeting levels made even better with leading edge
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
· Nor risk of being trapped in a superstall, control authority
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
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
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
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
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
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
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 aircrafts
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
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 canards
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.