Supermaneuverability is the ability of aircraft to maintain pilot control and
perform maneuvers in situations and ways exceeding those that are possible by
pure aerodynamic mechanisms. This capability was first researched in
the United States, and eventually resulted in the development of the
McDonnell Douglas F-15 STOL/MTD as a proof of concept aircraft, the result of
research begun in 1975 at Langley Research Center, a full 8 years before
the Russian Mig-29 claimed this as a new, revolutionary capability. The USAF
abandoned the concept as counter-productive to BVR engagements as
the Cobra maneuver leaves the aircraft in a state of near zero energy, having
bled off nearly all of its speed in performing the Pugachev's Cobra maneuver
without gaining any compensating altitude in the process. Excepting
1-on-1 engagements, this leaves the aircraft very vulnerable to both missile
and gun attack by a wingman or other hostile, even if the initial threat
overshoots the supermaneuvered aircraft. In 1983 the Russian Mikoyan MiG-29 and
in 1996 the Sukhoi Su-27 were deployed with this capability, which has since
become a standard in their 4th and 5th generation aircraft. There has been some
speculation, but the mechanism behind the supermaneuverability of the Russian
aircraft is still unknown. However, post-stall analyses are increasingly
used in recent years to advance maneuverability via the use of thrust
vectoring engine nozzles. Russian emphasis on close-range
slow-speed supermaneuverability runs counter to Western
Energy–maneuverability theory, which favors retaining kinetic energy to gain
an increasingly better array of maneuvering options the longer an
engagement endures . Aerodynamic maneuverability vs
supermaneuverability Traditional aircraft maneuvering is
accomplished by altering the flow of air passing over the control surfaces of the
aircraft—the ailerons, elevators, flaps, air brakes and rudder. Some of these
control surfaces can be combined—such as in the "ruddervators" of a V-tail
configuration—but the basic properties are unaffected. When a control surface
is moved to present an angle to the oncoming airflow, it alters the airflow
around the surface, changing its pressure distribution, and thus applying
a pitching, rolling, or yawing moment to the aircraft. The angle of control
surface deflection and resulting directional force on the aircraft are
controlled both by the pilot and the aircraft's inbuilt control systems to
maintain the desired attitude, such as pitch, roll and heading, and also to
perform aerobatic maneuvers that rapidly change the aircraft's attitude. For
traditional maneuvering control to be maintained, the aircraft must maintain
sufficient forward velocity and a sufficiently low angle of attack to
provide airflow over the wings and also over its control surfaces. As airflow
decreases so does effectiveness of the control surfaces and thus the
maneuverability. On the other hand, if the angle of attack exceeds its critical
value, the airplane will stall. Pilots are trained to avoid stalls during
aerobatic maneuvering and especially in combat, as a stall can permit an
opponent to gain an advantageous position while the stalled aircraft's
pilot attempts to recover. The speed at which an aircraft is
capable of its maximum aerodynamic maneuverability is known as the corner
airspeed; at any greater speed the control surfaces cannot operate at
maximum effect due to either airframe stresses or induced instability from
turbulent airflow over the control surface. At lower speeds the redirection
of air over control surfaces, and thus the force applied to maneuver the
aircraft, is reduced below the airframe's maximum capacity and thus the
aircraft will not turn at its maximum rate. It is therefore desirable in
aerobatic maneuvering to maintain corner velocity.
In a supermaneuverable aircraft, the pilot can maintain a high degree of
maneuverability below corner velocity, and at least limited altitude control
without altitude loss below stall speed. Such an aircraft is capable of maneuvers
that are impossible with a purely aerodynamic design. More recently,
increased use of jet-powered, instrumented unmanned vehicles has
increased the potential flyable angle of attack beyond 90 degrees and well into
the post-stall safe flight domains, and has also replaced some of the
traditional uses of wind tunnels. Evidence
There is no strict set of guidelines an aircraft must meet or features it must
have in order to be classified as supermaneuverable. However, as
supermaneuverability itself is defined, the ability of an aircraft to perform
high alpha maneuvers that are impossible for most aircraft is evidence of the
aircraft's supermaneuverability. Such maneuvers include Pugachev's Cobra and
the Herbst maneuver. Some aircraft are capable of performing
Pugachev's Cobra without the aid of features that normally provide
post-stall maneuvering such as thrust vectoring. Advanced fourth generation
fighters such as the Su-27, MiG-29 along with their variants have been documented
as capable of performing this maneuver using normal, non-thrust vectoring
engines. The ability of these aircraft to perform this maneuver is based in
inherent instability like that of the F-16; the MiG-29 and Su-27 families of
jets are designed for desirable post-stall behavior. Thus, when
performing a maneuver like Pugachev's Cobra the aircraft will stall as the
nose pitches up and the airflow over the wing becomes separated, but naturally
nose down even from a partially inverted position, allowing the pilot to recover
complete control. The Cobra, as performed by non-thrust
vectoring aircraft, still depends on the aircraft moving through air; however, it
does not involve the aircraft's aerodynamic surfaces and normal laminar
airflow, but rather the whole airframe as a solid shape travelling through air,
and its center of gravity in relation to the thrust vector. Being performed under
conditions far beyond normal aerodynamic control and well into a stall without
thrust vectoring, this is a form of passive supermaneuverability, possible
because of aircraft design rather than thrust vectoring, which provides a way
to actively control the aircraft well outside of the normal flight envelope.
The Herbst Maneuver, however, is believed to be impossible without thrust
vectoring as the "J-turn" requires a half-roll in addition to pitching while
the aircraft is stalled, which is impossible using conventional control
surfaces. Pugachev's Cobra can be performed with less change in altitude
if vectored thrust is used, as the aircraft can be made to pitch far more
rapidly, both inducing the stall before the aircraft significantly gains
altitude and recovering level attitude before altitude is lost.
Characteristics Although as aforementioned no fixed set
of features defines a supermaneuverable aircraft explicitly, virtually all
aircraft considered supermaneuverable have a majority of common
characteristics that aid in maneuverability and stall control.
= Post-stall characteristics = The key difference between a pure
aerodynamic fighter and a supermaneuverable one is generally found
in its post-stall characteristics. A stall, as aforementioned, happens when
the flow of air over the top of the wing becomes separated due to a high angle of
attack; the airfoil then loses its main source of lift and will not support the
aircraft until normal airflow is restored over the top of the wing.
The behavior of the aircraft in a stall is where the main difference can be
observed between aerodynamic maneuverability and
supermaneuverability. In a stall, traditional control surfaces, especially
the ailerons, have little or no ability to change the aircraft's attitude. Most
aircraft are designed to be stable and easily recoverable in such a situation;
the aircraft will pitch nose-down so that the angle of attack of the wings is
reduced to match the aircraft's current direction, restoring normal airflow over
the wings and control surfaces and enabling controlled flight. However,
some aircraft will deep stall; the aircraft's design will inhibit or
prevent a reduction in angle of attack to restore airflow. The F-16 has this
flaw, due in part to its fly-by-wire controls, which under certain
circumstances limit the ability of the pilot to point the nose of the aircraft
downward to reduce angle of attack and recover. Neither an extreme pitch-down
nor a deep stall is desirable in a supermaneuverable aircraft.
A supermaneuverable aircraft allows the pilot to maintain at least some control
when the aircraft stalls, and to regain full control quickly. This is achieved
largely by designing an aircraft that is highly maneuverable, but will not deep
stall and will recover predictably and favorably. To that design, features are
then added that allow the pilot to actively control the aircraft while in
the stall, and retain or regain forward level flight in an extremely shallow
band of altitude that surpasses the capabilities of pure aerodynamic
maneuvering. = Thrust-to-weight ratio =
A key feature of supermaneuvering fighters is a high thrust-to-weight
ratio; that is, the comparison of the force produced by the engines to the
aircraft's weight, which is the force of gravity on the aircraft. It is generally
desirable in any aerobatic aircraft, as a high-thrust-to-weight ratio allows the
aircraft to recover velocity quickly after a high-G maneuver. In particular,
a thrust-to-weight ratio greater than 1:1 is a critical threshold, as it
allows the aircraft to maintain and even gain velocity in a nose-up attitude;
such a climb is based on sheer engine power, without any lift provided by the
wings to counter gravity, and has become crucial to aerobatic maneuvers in the
vertical. High thrust-to-weight is essential to
supermaneuvering fighters because it not only avoids many situations in which an
aircraft can stall, but when the aircraft does stall, the high
thrust-to-weight ratio allows the pilot to sharply increase forward speed even
as the aircraft pitches nose-down; this reduces the angle the nose must pitch
down in order to meet the velocity vector, thus recovering more quickly
from the stall. This allows stalls to be controlled; the pilot will intentionally
stall the aircraft with a hard maneuver, then recover quickly with the high
engine power. Beginning in the late fourth-generation
and through Generation 4.5 of aircraft development, advances in engine
efficiency and power enabled many fighters to approach and exceed
thrust-to-weight ratios of 1:1. All current and planned fifth-generation
fighters will exceed this threshold. = High aerodynamic maneuverability =
Even though true supermaneuverability lies outside the realm of what is
possible with pure aerodynamic control, the technologies that push aircraft into
supermanuvering capability are based on what is otherwise a conventional
aerodynamically controlled design. Thus, a design that is highly maneuverable by
traditional aerodynamics is a necessary base for a supermaneuverable fighter.
Features such as large control surfaces which provide more force with less
angular change from neutral which minimizes separation of airflow, lifting
body design including the use of strakes, which allow the fuselage of the
aircraft to create lift in addition to that of its wings, and low-drag design,
particularly reducing drag at the leading edges of the aircraft such as
its nose cone, wings and engine intake ducts, are all essential to creating a
highly maneuverable aircraft. Some designs, like the F-16 are designed
to be inherently unstable; that is, the aircraft, if completely uncontrolled,
will not tend to return to level, stable flight after a disturbance as an
inherently stable design will. Such designs require the use of a
"fly-by-wire" system where a computer corrects for minor instabilities while
also interpreting the pilot's input and manipulating the control surfaces to
produce the desired behavior without inducing a loss of control. Thus
corrected for, the instability of the design creates an aircraft that is
highly maneuverable; free from the self-limiting resistance that a stable
design provides to desired maneuvers, an intentionally unstable design is capable
of far higher rates of turn than would otherwise be possible.
= Canard controls = A canard is an elevator control surface
placed forward of the wings. Sometimes, as with the B-1B, they are simply used
to stabilize flexible portions of the fuselage or provide very minute attitude
changes, but they are used often as a supplement to or full replacement of
tail-mounted stabilators. The theory behind canards as the sole
elevator surface is that no elevator configuration aft of the wings is truly
satisfactory for maneuvering purposes; the airflow over the wings creates
turbulence, however small, and thus affects elevators placed directly behind
the wings. Placement below the wings exposes the elevators to even greater
turbulence from under-wing ordnance. The original solution to such problems, the
T-tail, has been largely discredited as being prone to dangerous "deep stalls".
Other solutions like the V-tail place the combination rudder-elevator surfaces
out of the wings' airflow, but reduce the effectiveness of the control surface
in the pure pitch and yaw axes. As a supplement to traditional
elevators, canards vastly increase control surface area, and often increase
the critical angle of attack of the wings as the canard directs air more
directly toward the leading edge of the wing. They can also be designed to
operate independently, thus also acting as ailerons.
Canards are not a requirement, and can have disadvantages including reduced
pilot visibility, increased mechanical complexity and fragility, and increased
radar signature. The F-22, for example, does not incorporate canards, mostly for
stealth reasons. Many technology demonstrators and maneuverability
testbeds such as the F-15 S/MTD incorporated canards, even when the
production aircraft they were based on did not. Production fighters like the
Eurofighter Typhoon, Dassault Rafale and Saab Gripen all use a delta-wing
configuration with canard surfaces, while variants of the Su-27 including
the Su-30, Su-30MKI, Su-33 and Su-37 all use canards to supplement traditional
tail-mounted elevators. = Thrust vectoring =
Though a high thrust-to-weight ratio and high aerodynamic maneuverability are
found on both aerodynamic and supermaneuvering aircraft, the
technology most directly linked to supermaneuverability is thrust
vectoring, in which the geometry of the exhaust nozzle of a traditional jet
engine can be modified to angle the engine's thrust in a direction other
than directly to the rear. This applies force to the rear of the aircraft in the
opposite direction similar to a conventional control surface, but unlike
a control surface the force from the vectored thrust is dependent on current
engine thrust, not airspeed; thus thrust vectoring not only augments control
surfaces at speed, but allows the aircraft to retain maximum
maneuverability below corner speed and some attitude control below stall speed
while in maneuvers. Technology demonstrators such as the X-31, F-16
VISTA and F-15 S/MTD were built to showcase the capabilities of an aircraft
using this technology; it has since been incorporated into pre-production and
production fighters such as the F-22 Raptor. Eastern Bloc design companies
have also introduced this technology into variants of fourth-generation
aircraft such as the MiG-29 and Su-27 to produce the MiG-29OVT tech demonstrator
and Su-30MKI air superiority fighter respectively, and planned
fifth-generation Russian-designed aircraft such as the PAK-FA will use the
technology as well. In addition domestic Russian Su-30 fighters will be upgraded
with thrust vectoring engines. Thrust vectoring is most useful while
performing maneuvers such as the aerial J-turn, where the nose of the aircraft
is pointed upwards. It is generally considered impossible, in fact, to
perform a true J-turn maneuver without vectored thrust. Other maneuvers that
are considered impossible to perform under control using only aerodynamic
maneuvering include the Bell and the controlled flat spin.
Assessment Pilots can pitch the noses of
supermaneuverable aircraft up to extreme angles of attack allowing them to
potentially gain an advantage, by being able to lock on to a target and fire at
it, but at the expense of the altitude and/or speed of the aircraft.
Quantifying such a potential advantage using metrics was not really possible in
the early 1990s—traditional metrics such as instantaneous or sustained turn rate
plots did not highlight the differences which pilots described when flying such
aircraft. Several research projects were conducted through the 1990s and one in
particular, entitled "Performance Assessment of Fighter Aircraft
incorporating Advanced Technologies" by Antony Kutschera, not only reviewed the
suitability of existing metrics to measure the effects of
supermaneuverability, but went on to develop a new metric for quantifying
advantages and disadvantages while in flight. The new metric allows a
quantitative assessment of the aircraft to be made, in terms which designers,
pilots and tacticians alike can easily understand, unlike many newer metrics
which combined existing metrics to develop "magic" numbers which have
little meaning. Supermaneuverable aircraft currently in
use Su-30MKI
Dassault Rafale Eurofighter Typhoon
Mikoyan_MiG-35 Lockheed_Martin_F-22_Raptor
Sukhoi_Su-35 Sukhoi_T-50 PAK-FA
References