Published Airpower Journal -
Spring 1988
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FIGHTER TECHNOLOGY OF THE FUTURE
Editor's Note:
Colonel Siuru's survey of possible technological innovations and potential capabilities
may seem a bit outside the Airpower journal's usual focus on the broader issues of using
one's fighting forces to best effect. It may also appear somewhat technically oriented to
our readers who are not in the pilot or engineer specialties. Both of these observations
may be true.
However, as has often been submitted in this journal, there is a relationship between doctrine and technology which if not carefully assessed and redefined periodically can lead a military force astray. This may be due to a doctrine outstripped by new technological capabilities or by an overly ambitious doctrine not in consonance with technical realities.
In this instance, one wonders what changes to tactical and especially operational doctrine we should be contemplating when and if supermaneuverability comes to fruition in operational forces. The advent of radar was initially seen as simply better "eyes" for extending a search area, but quickly made its impact felt in the entire realm of tactical and operational air doctrine. Some forces were quick to use the capability to advantage (modified their doctrine) while others were not. Now may be the time to consider the integration of super maneuverability into our doctrine. Your ideas are welcome.
THE manned fighter quite likely will be around well into the twenty-first century both in an air-to-air and air-to-ground role. To date, no unmanned, remotely piloted vehicle has shown the potential of attaining the potency of the marriage between a skilled pilot and a well-designed fighter, and this is not expected to change in the near future. Thus, the emphasis today is on technology that will allow fighters to survive and win in combat. There is great interest today in an area of technology that goes under the generic title of "supermaneuverability."
Credit for coining the word supermaneuverability goes to Dr W. B. Herbst, who introduced the idea in 1980. Doctor Herbst, of West Germany's Messerschmitt-Bo1kow-Blohm, defined supermaneuverability as the capability to execute maneuvers with controlled sideslip at angles of attack well beyond those for maximum lift. Today Doctor Herbst's definition is termed poststall maneuvering and is one of many important ideas included in the category of supermaneuverability.
The term supermaneuverability has been expanded to other concepts that can dramatically enlarge the flight envelope of an aircraft in terms of airspeed, turn rate, climb rate, acceleration, and so forth. Supermaneuverability implies capabilities and technology demands beyond those achievable through more efficient wings, better performing engines, or more sophisticated flight control systems. Capabilities such as increased usable lift, dynamic lift overshoot, thrust vectoring, and unsteady aerodynamic effects used in synergetic fashion are all means of obtaining greatly enhanced maneuverability,
Ever since the first fighter appeared in World War I, agility has been the key as to who survives in an aerial duel. Interestingly, the emphasis an agility has been rather cyclic since the fighters of World War I. Agility seems to receive the greatest emphasis during and right after a war when actual combat experience demonstrates its importance. Examples of very agile fighters include the Sopwith Camel of World War I, the P-51 Mustang of World War II, the F-86 Sabrejet used in the Korean conflict, and the F-15 and F-16 that were designed around experience gained in Vietnam. There seems to be a tendency to forget experiences during times of peace and to sacrifice agility in favor of greater speeds and more sophisticated electronics and weapons, which leads to heavier and more cumbersome fighters. Fortunately, the current high interest in supermaneuverability indicates this experience may not be repeated.
The best way to ensure combat survivability is to have both the best aircraft and the best pilot to fly it. In the past, the United States has leaned on the assumption that even though the Soviet and American fighter pilots are probably equal in ability, our fighters were more capable because we had a technology advantage. This is definitely changing. The experts believe that new Soviet fighters like the Su-27 Flanker, MiG-29 Fulcrum, and MiG-31 Foxhound are approaching the capabilities of our F-14s, F-15s, F-16s, and F-18s. To give our pilots the edge, new designs incorporating advanced technologies are needed as well as revised tactics to get the most out of the improvements. Enhanced maneuvering is high on the list of these technologies.
One of the things that has changed the tactics of air-to-air combat in recent years is the all-aspect missile. With the normal infrared (IR) heat-seeking missile, a pilot had to maneuver so that he was behind the enemy to make a kill since IR missiles had to home in on the hot engine exhaust. Today radar-guided missiles and missiles with much more sensitive IR sensors can home in on other parts of an aircraft. These all-aspect missiles can be fired from any direction, and fighters so equipped need only to get their noses pointed in the general direction of the enemy. The fighter pilot who can get his nose pointed within the required field of view first is the one most likely to survive.
While increased turn rate might seem to be the obvious answer, it is not always the best solution. For one thing, high turn rates mean high G-loads, and today's fighters are pushing the acceleration tolerance of even the most physically fit pilots. Also, high turn rates result in high drag, which can quickly decelerate an aircraft to the point where the aircraft has lost the maneuverability advantage that comes with high speed. As any fighter pilot knows, the name of the game is to be able to fire the first shot while still retaining enough speed to fly away to make another kill or to avoid being killed.
Supermaneuverability can also be important in allowing an aircraft to avoid an enemy missile. With very high agility, the fighter would be able to outfly the missile and break lock with the missile's guidance system. Aircraft with greatly enhanced maneuverability could perform some very erratic evasive maneuvers.
Incidentally, supermaneuverability is not limited to manned fighters. Most of the supermaneuverability concepts could also be used on unmanned craft such as air-to-air missiles. Thus, we could have supermaneuverable missiles trying to destroy supermaneuverable aircraft and supermaneuverable aircraft evading supermaneuverable missiles.
The enhanced maneuvering concept receiving the greatest interest today is the idea of "poststall maneuvering," that is, flying at very high angles of attack, perhaps even up to 70 to 90 degrees for short periods of time. Poststall maneuvering will allow fighters to make drastic changes in direction within extremely short distances and times. As an example of poststall maneuvering, let us look at an engagement between two fighters, one with poststall maneuvering capability and one without it (fig. 1). The supermaneuverable fighter could turn much faster than a conventional aircraft and dissipate much less energy in the process. Quite conceivably, it would have the adversary in its weapon system field of view several critical seconds before the other aircraft has completed its turn and is in firing position.
Normally, even the best designed wings will stall at angles of attack above 20 degrees. Stalls usually result in "departure" normally leading to loss of control. To make poststall maneuvering work, the aircraft will have to be controllable at very high angles of attack. Lack of controllability at high angles of attack occurs because normal control surfaces lose their effectiveness. Airspeed is often quite low when there is a high angle of attack, and the violent vortices in the wake of a stalled wing have a drastic effect on the vertical and tail surfaces. This means that conventional aerodynamic control surfaces such as rudders and elevators will have to be helped by other techniques such as vectored engine thrust to maintain control.
One method to enhance maneuvering capability is to simply use all lift inherent in a particular design, although the word simply might be an oversimplification. For example, many fighters could fly at higher angles of attack without stalling and thus generate more lift, but they are limited by such detrimental aerodynamic phenomena as buffeting, wing rock, nose slice, and poor directional stability. Some of the phenomena can be corrected by subtle changes in aircraft design that result from wind-tunnel testing and computer simulations.
A measure of supermaneuverability can be obtained through dynamic lift overshoot. Here the idea is to increase the angle of attack so rapidly that the airflow remains attached to the wing well beyond the angle it would normally separate, thus providing a momentary increase in lift that could be used for enhanced maneuvering.
One method to achieve dynamic lift overshoot is to use a rapidly rotating airfoil, that is, one that oscillates or pitches and plunges at high frequencies. Although this concept is still in a very exploratory stage, wind-tunnel tests, computer simulations, and experience with helicopter rotor blades have demonstrated the potential of this idea.
While perhaps not strictly fitting the definition of supermaneuverability, there are other ideas that could give future fighters the capability needed to survive in combat.
One way of obtaining unconventional maneuvering is by using thrust vectoring, that is, changing the direction of the thrust produced by an aircraft's engine. Incidentally, thrust vectoring is one improved agility technique that is already in use on an operational military fighter, the AV-8 Harrier, a vertical and/or short takeoff and landing (VSTOL) aircraft used by the US Marine Corps as well as the Royal Air Force and Navy. While the Harrier was aimed at VSTOL capability, pilots soon found that by swiveling the Pegasus engine's four nozzles in flight, some unique and useful maneuvers are possible. Thus "vectoring in forward flight" (VIFF) was born. For instance, by using VIFF the Harrier can decelerate more rapidly than other aircraft and can do it without reducing engine rpms that will be needed for subsequent acceleration or without extending telltale speed brakes.
Two-dimensional, rectangular nozzles with horizontal doors for thrust deflection are an alternative to swiveling nozzles. Besides deflecting thrust, the nozzles can reverse the thrust to reduce landing distances or to increase in-flight maneuverability. While rectangular nozzles cannot deflect the exhaust to the degree found in the Harrier, the thrust-vectoring capability is still substantial. The thrust vectoring available from the two-dimensional nozzle is especially valuable for maneuvering at high angles of attack and low speeds where ordinary aerodynamic control surfaces lose their effectiveness. For this reason, some form of thrust vectoring will undoubtedly be an integral part of any supermaneuvering technique.
Thrust vectoring brings with it another important capability--a short takeoff and landing (STOL) ability. This feature is needed in future fighters as well as in other military aircraft because in any future major war, aircraft will probably have to work out of severely bomb-cratered airfields.
Other ways to use unconventional aerodynamics to achieve enhanced maneuverability were investigated in the control configuration vehicle (CCV) and the advanced fighter technology integration (AFTI) programs (fig. 3). In these programs, modified F-16s demonstrated some very new ways to fly. Normally an aircraft flies in "coupled modes" so that when it turns it also rolls and when it climbs the angle of attack increases. In the CCV and AFTI F-16s, the maneuvers were decoupled. When decoupled, the aircraft can rise vertically without raising its nose, raise or lower its nose without climbing, make a wings-level turn, or fly straight ahead while pointing its nose off centerline, and perform several other interesting maneuvers. The decoupled maneuvers demonstrated by the modified F-16s would be especially attractive for fast and precise pointing before firing weapons in air-to-air combat. The extra few seconds and increased accuracy could give the pilot the necessary edge to survive.
The joined wing is another concept that could provide enhanced maneuverability (fig. 4). A joined-wing aircraft has its tail wing swept forward to be joined with the rearward swept main wing so that the wings form a diamond when viewed from the top or head-on. Besides providing a lighter, stiffer aircraft with decreased drag, this concept makes some interesting flight motions possible. To move sideways without rolling, the control surfaces on the front and rear wings could be deflected in unison to provide equal but canceling rolling movements. To make rapid pitch-up maneuvers, the front and rear surfaces could be deflected in opposing directions. Moving all surfaces downward results in lift augmentation that allows the aircraft to rise essentially vertically.
Some degree of enhanced agility can be achieved by using high technology to improve already proven aircraft designs. Take, for example, the mission adaptive wing (MAW). With flexible composite materials and actuators buried inside the wing, the wing's surface contour can be changed without using conventional flow-disrupting empennages such as flaps, spoilers, and ailerons. This means that the wing is less prone to stall at high angles of attack during high g turns and that high lift-to-drag ratios needed for enhanced agility are possible.
The above discussion of concepts has frequently mentioned the importance of being able to effectively control an aircraft during unconventional maneuvers. Controllability and maneuverability go hand in hand, the formal definition of agility being the sum of the two factors. A highly maneuverable fighter that is difficult to control will not be successful, and the opposite is also true. The F-86 and MiG-15 are examples of the need for agility. The MiG-15 could easily outmaneuver the F-86, but it was harder to control. Therefore, F-86 pilots were able to achieve impressive kill ratios over the MiG15 by controlling the F-86's flight path better to get into position to make a kill.
An integral part of enhanced maneuverability is relaxed static stability. Most aircraft are designed to be inherently stable so that they automatically return to straight and level flight, for example, after a wind gust or a Pilot command. While good static stability means a forgiving airplane, it is incompatible with the superior maneuverability desired in a fighter. Today's newer fighters are normally designed with relaxed static stability, that is, with little, zero, or even negative static stability. Without the sophisticated stability augmentation systems used in modern fighters, pilots could not maintain control of their aircraft.
Future aircraft with superior agility will integrate many technologies such as propulsion, aerodynamics, and controllability obtained through advanced digital fly-by-wire and later, fly-by-light control systems. The latter uses fiber optics in lieu of wires.
While technology can be used to produce supermaneuverable fighters, it might be the physiological capabilities of the human pilot that could put the upper limit on maneuverability. For example, the pilot can become disoriented when his aircraft moves against intuition and experience. It may take extensive training to get used to flying sideways, flying at attitudes well into the stall regime, or being able to point the nose up or down without climbing or diving. Control systems may have to be designed so that the pilot only provides the initial command while the computer performs the rest of the maneuver sequence.
Then there is the problem of gravity-induced loss of consciousness (G-LOC). This occurs when there is a rapid or sustained increase in Gs and the body's defensive mechanics cannot maintain sufficient blood pressure in the brain. G-LOC occurs suddenly, with the pilot being unconscious for approximately one-half minute, enough to spell disaster in a high-performance aircraft. Even when the pilot recovers, he could still be disoriented for quite awhile and be unable to handle the high stress of close air combat and perhaps not even to fly safely.
There must be solutions to the physiological problems associated with supermaneuverability. G-suits will have to be more responsive. Because G-LOC depends on how high the head is elevated above the heart, the pilot's seat could be reclined. Inclinations of about 65 degrees are needed, so the seat would have to be articulated so the pilot can sit more erect for normal flight and then recline for combat maneuvering. Other solutions could include special drugs. For instance, carbon dioxide injected into the oxygen seems to help, and even the use of "smelling salts" may speed up the recovery of consciousness.
Techniques are needed to detect when the pilot becomes unconscious and automatic flight controls must take over. Because things happen so rapidly in high-performance aircraft, detection must be done instantaneously and preferably before complete pilot blackout. Techniques must have low false alarm rates so that override does not occur while the pilot is conscious and still in control, especially during combat.
Some of the methods currently being researched include detecting the drooping or lolling of the pilot's head that is associated with loss of consciousness. There is also the monitoring of the pilot's grip on the controls. A more sophisticated measurement involves sensing the loss of blood pressure pulse in an artery near the brain with a special sensor mounted in the pilot's helmet. Another technique involves monitoring the pilot's eye-blink rate. It is well known that just before a person blacks out, the eyes stop blinking automatically and there is a fixed stare.
Several detection devices would be used in "jury" fashion to reduce false alarms. Furthermore, this could be augmented by monitoring the G history of the flight and determining when the aircraft is in a high G environment and when override might be needed because of the possibility of blackout.
The development of any new aircraft can be extremely expensive. Some of the unproven techniques for achieving enhanced agility could be dangerous if tested in manned experimental aircraft, Therefore, much of the initial development will be done with simulators that provide realism approaching that experienced in a real fighter cockpit. To see how various supermaneuverable concepts might fare in actual combat, two or more simulators can be tied together so that the simulated aircraft "flown" by experienced pilots can interact. Different maneuvering concepts can be changed on the simulator usually by rewriting software rather than designing and building new expensive hardware. Thus, new ideas can be tested fairly inexpensively and without endangering an aircraft or its pilot.
One safe and relatively cheap way to flight-test new ideas is to use a remotely piloted research vehicle (RPRV). These subscale, unmanned aircraft, which are remotely controlled by a "pilot" on the ground, are built at reduced scale and need not be man-rated. One successful RPRV was Rockwell international's Highly Maneuverable Aircraft Technology (HiMAT) RPRV built a few years ago, which produced much important design data for future fighters.
No matter how much computer simulation is done or how many RPRVs are flown, the best concepts will still have to be flight-tested with a live pilot behind the stick. For example, the CCV and AFTI F-16s mentioned previously tested some unique maneuvering techniques, and the mission adaptive wing has been grafted to a F-111 for flight-testing. Now the Grumman-built X-29 is flight-testing some other ideas.
Another "X" airplane that will be used in supermaneuverability developments is the X-31 Enhanced Fighter Maneuverability (EFM) program. The primary emphasis of this joint US-West German program will be on poststall maneuvering at very high angles of attack. Rockwell International and Messerschmitt-Bolkow-Blohm plan to have the X-31 flying by 1989.
Enhanced maneuverability, ranging from minor changes in current aircraft to revolutionary new aircraft, will be needed if our fighters are to survive and win in future aerial conflicts. While the technology community is developing a plethora of potentially valuable supermaneuverability concepts, an equally important part of the equation is the development of tactics and doctrine that can make the best use of the technology. Thus, experienced air tacticians and fighter pilots must have an important role in the development of effective and usable supermaneuverability.
References
Gallaway, Capt Charles R., and Russell F. Osborn. "Aerodynamics Perspective of Supermaneuverability." Paper presented at the Third Applied Aerodynamics Conference of the American Institute of Aeronautics and Astronautics (AIAA), Colorado Springs, Colo., 14-16 October 1985.
Herbst, W. B. "Future Fighter Technologies." Journal of Aircraft, August 1980, 561-66.
______________. "Supermaneuverability." Jahrestagung 1983 de Deutschen Gesellschaft fur Luft- und Raumfahrt e.V., Munchen 17. bis 19. Oktober 1983.
Lang, Col James D., and Maj Michael S. Francis. "Unsteady Aerodynamics and Dynamic Aircraft Maneuverability." Paper presented at the Advisory Group for Aerospace Research and Development (AGARD) Symposium on "Unsteady Dynamics: Fundamentals and Applications to Aircraft Dynamics," 6-9 May 1985, Gottingen, West Germany.
Wolkovitch, Julian. "The Joined Wing: An Overview." Journal of Aircraft, March 1986, 561-66.
Col William D. Siuru, Jr.USAF, Retired (PhD, Arizona State University), is senior research associate, Space and Flight Systems Laboratory, University of Colorado at Colorado Springs. He was director of flight systems engineering. Aeronautical Systems Division at Wright-Paterson AFB, Ohio, at the time of his retirement after a 24-year military career. Colonel Siuru held a variety of technical and management positions in Air Force Systems Command, taught in the Engineering Department at West Point, and served as commander, Frank J. Seiler Research Laboratory, US Air Force Academy. He has written six books and many articles and was a frequent contributor to the Air University Review.
Disclaimer
The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.
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