Supermaneuverability


Supermaneuverability

Supermaneuverability is quality of aircraft defined as a threshold of attitude control exceeding that which is possible by pure aerodynamic maneuverability. It is a trait of some advanced fourth-generation and 4.5-generation fighter aircraft and is generally a specified requirement of current and future fifth-generation fighters.

Aerodynamic maneuverability vs supermaneuverability

Traditional aircraft maneuvering is accomplished by altering the flow of passing air over the control surfaces of the aircraft - the ailerons, elevators, flaps, air brakes and rudder. Some of these control surfaces are combined, such as in the "rudder-vaters" of a V-tail configuration; the basic properties are unaffected. When a control surface is moved to present an angle to the oncoming airflow, the control surface redirects the air in a different direction and, by Newton's Third Law, an equal, opposing force is applied by the air to the control surface and thus the aircraft. The angle and thus the directional forces on the aircraft are is controllable by the pilot 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 top of the wings (maintaining lift) and also over its control surfaces. If there is insufficient airflow, maneuverability is decreased and, as the velocity drops to less than that required to produce sufficient lift in the aircraft's current attitude, the airplane will stall.

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.

The speed below which the aircraft cannot maintain flight in its current configuration (throttle, attitude, configuration of brakes/flaps/landing gear) is known as the stall speed; below the stall speed, the aircraft becomes uncontrollable and/or cannot maintain lift at its current angle of attack; it will lose altitude and thus gather speed until either sufficient velocity is attained in the same axis as the nose of the plane to maintain lift and control, or the aircraft crashes into the ground. Stalls are therefore to be avoided in aerobatic maneuvering, especially in combat, as a stall permits an opponent to gain an advantageous position while the stalled aircraft's pilot attempts to recover. Stalls are also an indicator of insufficient maneuvering energy; an aircraft that is out of energy cannot turn or climb and is at a disadvantage to an opponent.

In an aircraft possessing supermaneuverability, the pilot is able to maintain a high degree of maneuverability below corner velocity, and at least limited attitude control without altitude loss below stall speed. Such an aircraft is capable of maneuvers that are impossible with a purely aerodynamic design.

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, an aircraft capable of performing maneuvers that are considered to be impossible using only aerodynamic maneuverability are evidence of an aircraft's supermaneuverability. Such maneuvers include Pugachev's Cobra and the Herbst maneuver (sometimes referred to as the "J-turn", but this term is ambiguous as it is also used in stunt driving on land).

However, 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 generation 4 and 4.5 fighters such as the Su-27 (which was the aircraft first used to execute the maneuver) and MiG-29 and their variants such as the Su-33 have been documented as capable of performing this maneuver using normal engines. The ability of these aircraft to perform this maneuver is based in inherent instability like that of the F-16, but unlike the F-16 which behaves unpredictably in the absence of its flight control system and in a stall situation maintains a level attitude (which is undesirable), the MiG-29 and Su-27 families of jets are designed for desirable post-stall behavior; the center of gravity is far forward of the wings, and thus the aircraft will dive nose-down without the flight computer or pilot countering this behavior with pitch control. 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, and naturally nose down.

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. The 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 (this can be caused by low speed, but its direct cause is based on the direction of the airflow contacting the wing); the airfoil then loses its main source of lift, the Bernoulli effect, 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 (known technically as the velocity vector), 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 design flaw; though highly maneuverable in normal circumstances, in a stall the aircraft will maintain a level attitude while literally falling out of the sky. 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 (thus allowing quick recovery by the pilot) and will recover predictably and favorably (ideally to level flight; more realistically to as shallow a nose-down attitude as possible). 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 surpasees 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 is crucial to aerobatic maneuvers in the vertical (which are in turn essential to air combat).

High thrust-to-weight is essential to supermaneuvering fighters because it not only avoids situations in which the aircraft can stall (such as during vertical climbing maneuvers), 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. This allows the pilot to point the nose of the aircraft quickly.

Beginning in the late fourth-generation and through Generation 4.5 of aircraft development, advances in engine efficiency and power enabled lightweight fighters to approach and exceed thrust-to-weight ratios of 1:1. All current and planned fifth-generation fighters will exceed this threshold, and in the case of the F-22 Raptor, do so by a large margin.

A favorable side effect of a high thrust-to-weight ratio is supersonic capability; the F-22's very high thrust-to-weight ratio of 1.26 in fact is a major contributor to its ability to supercruise. However, supersonic capability is not a requisite of supermanuverability.

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, is all essential to creating a highly-maneuverable aircraft.

Some designs, like the F-16 (which in current production form is regarded as highly maneuverable, but only the F-16 VISTA tech demonstrator is considered supermaneuverable) 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 (common on many fighters) exposes the elevators to even greater turbulence from underwing ordinance. 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. In addition, a full swept wing, which is favorable for speed, leaves no other place for the elevators than forward of the wings and thus delta-wing designs like the Saab Gripen use them by necessity.

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 (i.e. counter-rotate), 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 Raptor, for these reasons, does not incorporate canards in its design. However, many technology demonstrators and maneuverability testbeds such as the F-15 S/MTD incorporated them, 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-33 and Su-37 all use canards to supplement traditional tail-mounted elevators. All of these aircraft are reported to be capable of executing Pugachev's Cobra.

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 supermaneuvering capability 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 (i.e., upwards or downwards). 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 (generally that of the elevators) 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 and its runner-up in the Advanced Tactical Fighter competition, the YF-23. 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 Su-47 will use the technology as well.

Thrust vectoring is most useful while performing maneuvers such as the aerial J-turn, where the nose of the aircraft is pointed upwards (and thus the engine thrust counters gravity as well as providing attitude control). 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 (a 360° loop with negligible altitude change) and the controlled flat spin (360° of yaw around a point of rotation that lies inside the aircraft).

Maneuvering thrusters

In the majority of aircraft, all of the engine's thrust is directed either to the rear in conventional fixed-wing aircraft or, in the case of VTOL aircraft such as helicopters, downwards. Other possibilities exist, however. The Rolls-Royce Pegasus engine, which powers the AV-8 Harrier jump jet, allows the jet to hover; however the control surfaces are useless when the aircraft is in this mode. To provide maneuvering control while hovering, the aircraft incorporates a reaction control system; some of the engine's thrust is directed through channels to the wingtips, nose and tail, providing pitch, roll and yaw controls similar in function to the maneuvering thrusters on a spacecraft.

A similar system could conceivably be used on a supermaneuverable aircraft to provide full maneuvering control below stall speed, however there have been no designs since the Harrier to use such a system. The Harrier, though it can hover and maintain control at zero forward airspeed, is not considered a supermaneuverable design.


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