Fighter Concepts For The Nineties

However, military aviation is constantly advancing, and a new generation of combat aircraft is already being planned for the 1990s. Since the development of a project from the preliminary design stage to initial operational capability now generally takes 10 years or more, it is possible to predict many of the forthcoming changes in airframes, powerplants, arma­ment, systems and the cockpit area.

Some of these changes will require brand new develop­ ment programs, but others represent only straightforward ex­ tensions of features already flying in prototype form, if not in production aircraft.

For example, the Lockheed YF-12A pointed the way to slender delta airframes for Mach 3. The Northrop YF-17 show­ ed how the slender delta could be used as a leading-edge ex­ tension (LEX) on a conventional fighter planform, to sustain lift and maintain controllability to high angles of attack (AOA) in dogfights.

The practicality of variable-sweep, based on NASA’s out­ board hinge, was first proved by the General Dynamics F-111. An alternative approach to short-field performance was developed in the canard configuration of the Saab Viggen, in which the foreplane assists the wing in lifting the aircraft off the ground. That an aft lifting tail is possible was shown by the General Dynamics F-16, which has its CG behind the aerodynamic centre, and being unstable, relies on black box longitudinal stability.

Jet lift is an even more effective approach to improving airfield performance since its use can be extended to zero speeds; giving true VTOL capability. The British Aerospace Harrier and its Rolls-Royce Pegasus vectored-thrust turbofan pioneered the single-engine concept. This proved far simpler in both engineering and piloting terms than its rivals, and of considerable value in combat, either to enhance turn rate or to provide a large drag increment, forcing an opposing air­ craft to overshoot into a vulnerable position.

A less dramatic, but very useful, powerplant innovation is thrust reversal for short landing performance, first used as a production feature on the Saab Viggen series, the thrust of the Volvo RMB being reversed by three petal doors on the final nozzle. However, neither the Viggen nor the Panavia Tornado (which has clamshell doors on its Turbo-Union RB. 199 engines) uses thrust reversal in flight. This concept has only been tested (in the combat aircraft context) on a Grum­man F-11F Tiger, fitted with a Rohr thrust reverser on its Wright-built J65.

The McDonnell Douglas F-15 Eagle introduced a new generation of powerful yet lightweight engines, its Pratt & Whitney F100 turbofan producing a thrust equal to eight times the engine’s weight. Another advanced powerplant is the General Electric YJ101 “leaky turbojet”, which enables the Northrop YF-17 to go marginally supersonic without afterbur­ ning, and which formed the basis of the F404 engines of the McDonnell Douglas F-18 Hornet.

Turning to advances in combat aircraft equipment, the Grumman F-14 Tomcat showed what a modern weapons system can achieve in identifying and engaging multiple targets beyond visual range (BVR). The F-16 pioneered the use of electrically-signalled flying controls in a production aircraft, also the high-inclination seat for enhanced g­ tolerance, and the miniature isometric sidestick. In ex­perimental form, the F-16 has been fitted with forward fins to test new modes of lateral control, including sidestep manoeuvres.

McDonnell Douglas probably leads the world in cockpit design. Both the F-15 and F-18 have advanced displays and “switchology” enabling the pilot to fight while keeping his hands on the throttle and stick (HOTAS). It is probably fair to say that the same organisation also leads in the application of ad­vanced composite materials to aircraft structures, notably in the F-18 and AV-8B Advanced Harrier. The latter’s wing represents the largest piece of graphite/epoxy primary struc­ ture ever flown.

In terms of designing structures for sustained high temperatures, the Lockheed YF-12A and its SR-71 derivative are outstanding. examples. Most of their titanium skin operates at around 500 degrees F 260 degrees Cl during cruise. The YF-12 also featured internal carriage of its air-air weapons, as did the earlier Convair F-102 and F-106.

Internal carriage of air-surface weapons is instanced by the F-111, by the rotary bomb bay of the BAe Buccaneer, and the linear (aft ejection) bay of the Rockwell A-5 Vigilante. However, the inflexibility of internal storage is now turning attention to low-drag external mounting systems, as evidenc­ed by the lateral “Fastpacks” of the F-15C/D and USAF tests of conformal ventral bomb installations on the McDonnell Douglas F-4 Phantom.

Current Experiments

Other clues to the possible form of the ’90s fighters can be found in research programs and published data on project studies.

Most development efforts still appear to be directed toward further improvements in manoeuvrability (in the broadest sense) for relatively conventional fighters and ground attack aircraft designed for maximum speeds of Mach 2 or less. However, other options are being kept open.

The US has a joint Air Force/NASA program known as the National Hypersonic Flight Research Facility (NHFRF), calling for two air-launched rocket-powered lifting bodies to test aerodynamic, structural, powerplant, and control system con­cepts at speeds up to Mach 8. On a more modest scale, there is interest in developing a Light Experimental Supercruiser, effectively a miniature Concorde able to fly supersonically without afterburning.

Boeing has proposed a Microfighter, up to 10 of which would be carried inside a 747 transport and launched and recovered as necessary. Lockheed is working on a Stealth fighter, presumably using radar-absorbent materials (RAM) and with engine intake and exhaust on top of the fuselage to minimise radar and IR signatures. This may be the project referred to by the USAF as the Tactical High Altitude Penetrator (THAP).

In the struggle for improved manoeuvrability in future projects, canard configurations appear to be gaining in pop­ularity, while the forward-swept wing is enjoying a come­ back thanks to the directional properties of composite materials, which can overcome its torsional divergence problem.

Although several existing fighters use manoeuvre flaps, these are only a crude substitute for a wing section of op­timum camber, and various attempts are consequently being made to produce a wing that changes its profile smoothly to suit flight conditions. The USAF Arnold Engineering Develop­ment Center (AEDC) has already tunnel-tested a GD­ designed Self-Optimising Flexible Technology (SOFT) wing on a half-model F-111. Grumman is working on a comparable scheme, termed the Mission-Adaptive Wing (MAW).

Another AEDC project is the parasol-mounted Favorable-Interference Wing for future supersonic fighters. Germany’s Dornier has exhibited a fighter model with a novel wing configuration, effectively a high aspect ratio biplane in which the tips of the upper and lower wings are joined together.

Turning to aircraft systems, considerable effort is being directed toward improved flying controls with lighter weights and better integration of pilot inputs with those from black boxes. Due to fears of the possible effects of electromagnetic radiation (from radar transmitters or nuclear explosions) on electrically-signalled flying controls (“fly-by-wire”), attention is turning to “fly-by-light” in which the signals are transmitted by fibre optics.

Development work is also proceeding on variable­ geometry ejection seats, to combine the relatively upright position needed for cruise and ejection with the fully­ reclined posture essential for sustained high manoeuvres. One such seat under test by the USAF Flight Dynamics Laboratory (AFFDL) is equipped with three inflatable bladders to adjust the pilot’s position on demand.

Forthcoming weapons will be able to engage targets out­ side the limits of a gunsight or conventional head-up display, and it will therefore be necessary to give the pilot new means to provide target data to the fire control system. Various helmet-mounted displays and sights are now in production. Taking this concept one stage further, work is proceeding on an aiming system that follows the movement of the pilot’s eye, rather than his helmet. The USAF 6570th Aerospace Medical Research Laboratory (AMRL) is testing a Remote Oculometer developed by Honeywell, which tracks the move­ ment of the cornea using nearly light. Development work is also in hand toward long-range aircraft identification by a combination of TV and radar.

In the powerplant field, the most important short-term ad­vance is expected to be variable-cycle engines (VCE), which will combine the best characteristics of turbojets and tur­bofans. Two current experimental examples are the GE23 and PWA-685, both proceeding under a joint USAF/USN program.

In the V/STOL field, Rolls-Royce is pressing for funding for augmented vectored thrust, ie, supersonic derivatives of the Pegasus, the most likely application of which is the US Navy’s V/STOL-B project. However, work is also in progress at AEDC on a General Dynamics canard configuration with overwing engines exhausting through rectangular nozzles over trailing edge flaps. This “powered lift” approach to thrust vectoring has the advantage that off-the-shelf engines can be used. Regardless of V/STOL considerations, there is considerable interest in using two-dimensional nozzles to permit thrust vectoring in combat.

The advent of Concorde and the Tu-144 has encouraged a revival of interest in hypersonic interceptors. For speeds above Mach 4, ramjets appear inevitable. It is anticipated that one task of the Martin X-24C lifting body will research into the operation of such engines up to Mach 7, pending the availability of the NHRF vehicle mentioned earlier. Fighters using this form of powerplant will probably take off, climb, and accelerate under conventional gas turbine power, and then switch the airflow from the intakes to ramjets when high speeds are attained.

The field of air-air armament has been neglected in re­cent years, most existing weapons being derivatives of long­ established designs. However, a new generation of cannons will be available in the 1990s, most likely with two or three barrels, and firing plastic-cased ammunition. Air-air missiles will be considerably more manoeuvrable than current types. American “air-slew” technology envisages weapons that can be launched forwards, then turn through 180 degrees to home on to a pursuing fighter!

However, all conventional air-air weapons have major drawbacks. The use of cannon poses serious aiming problems in the case of a manoeuvring target. Guided weapons can only be carried in limited numbers, and may be decoyed by chaff or flares. If high-powered lasers could destroy the op­ posing aircraft, then problems of aiming and fight endurance would be eliminated. Unfortunately, current indications are that the laser equipment required would be so large that it would only be applicable to bomber defence. Moreover, the enemy aircraft might be protected from the laser by a simple protective coating, which would reflect most of the transmitted energy.

Principal Projects

The US is currently engaged in a series of development projects that will form building blocks for the new fighter generation. One such project is the General Dynamics AFTl- 16, an Advanced Fighter Technology Integration program directed by AFFDL and based on the F-16, but with triplex digital task-tailored flight controls. Forward-mounted ventral fins will permit direct side-force control (DSFC), giving wings­ level steering with up to 1. SG laterally, and the ability to point the fuselage up to five degrees on either side of the line of flight.

The AFTl-16 will also have an enlarged head-up display providing more than twice the angular coverage of the stan­ dard HUD. As a further option, the aircraft may be fitted with a high-acceleration seat with a range of seat inclinations from 30 to 65 degrees. Flight testing is scheduled to start in June 1981 at Edwards AFB.

On a smaller scale, but of greater significance for the longer term, Rockwell has constructed two HiMAT (Highly Manoeuvrable Aircraft Technology) remotely-piloted research vehicles (RPRV). Funded jointly by the USAF and NASA, the HiMAT vehicles are intended to test high-risk fighter technologies without the expense of man-rating the aircraft.

The vehicle is of close-coupled canard layout, and of modular construction to facilitate changes to wing planform and powerplant. Graphite composites represent ap­proximately 26 per cent of the structure’s weight. Both the wing and the canard are aeroelastically tailored by orienting the composite plies so that under load the surfaces distort into the optimum shape.

The plan is for the HiMAT vehicle to be launched from a B-52 at high altitude, and then controlled by a pilot on the ground to fly through various manoeuvres, ending with a skid-landing on the Edwards AFB lakebed. In an emergency, the vehicle can be flown by a pilot in the TF-104G chase aircraft, or left to orbit on its autopilot. Flight testing will be con­ducted by NASA’s Dryden Flight Research Center (DFRC).

The main aim of the HiMAT program is to pave the way to greatly enhanced transonic manoeuvrability. The vehicle (essentially a scale model of a Rockwell fighter project) is designed to sustain turns at Mach 0. 9 at 25,000ft (7600ml, and 6g at Mach 1. 2 at 30,000ft (9150m). As further options, the HiMAT vehicle may be fitted with forward-swept wings, two­ dimensional nozzles for thrust vectoring, and variable­ incidence wings.

Notwithstanding the attractions of RPRVs, there is growing support for piloted technology demonstrator aircraft, possibly funded jointly by the US and one or more European countries, since Britain, West Germany, and France are all in the market for a new fighter for the 1990s.

One of the most promising contenders for demonstrator funding is the McDonnell Douglas Model 265-2 Vectored Lift Fighter (VLF), in which the pilot would have two extra modes of control. Firstly, he would be able to command direct side force for level turns, using in combination the two vertical tails and a vertical canard. Secondly, he would be able to obtain quick response in pitch through direct lift (ie, holding the aircraft altitude constant), by combining the movement of the beavertail elevator with that of variable-incidence outings.

The VLF demonstrator would be powered by two General Electric J85-21 engines (as in the Northrop F-5E), would have fly-by-wire, and a 65 degree high-acceleration seat. A slightly unstable CG position would be used, to minimise trim drag. Direct lift/side force control would be effected by an isometric (force-sensing) thumb switch on top of the control column handle. One valuable fall-out from these additional modes of control would be that the fuselage (and hence the gun) could be pointed off the line of flight in both azimuth and elevation. This form of target tracking could be done either manually, or automatically via the fire control system.

In air-to-ground gunnery, fuselage aiming gives more time on target. For example, in low-level strafing runs, the nose can be depressed without changing the line of flight. In air-to-air gunnery, firing opportunities are multiplied and lengthened. Simulator studies have indicated a SO per cent improvement in targets killed by bombing and a 200 per cent rise in air-to-air hits.

Following tests with such demonstrator aircraft, a new generation of operational fighters will enter service around 1990. They may well fly no faster than current types, but they will be able to use smaller airfields, be far more manoeuvrable, use less thrust to go supersonic and sustain turns, and employ novel forms of flight control.

As technologies advance, the possibilities in avionic systems border on science fiction. Will radar really be able automatically to identify aircraft at long range? In night operations, will radar paint a picture of the outside world on the fighter’s canopy? Will the pilot select and launch a missile simply by saying (for example) “Sky Flash One” and “Fire”? Will his scalp be wired to the autopilot, so that he can fly by thought control? When it comes to replacing the Mirage replacement, the prospects will be fantastic!

Highly Maneuverable Aircraft Technology. . . HiMAT

Mission

To enhance transonic maneuverability of future U. S. fighter aircraft of the 80’s and 90’s. To bridge the gap between wind tunnel model testing, simulators, and other ground tests and the increasingly expensive full­ scale manned flight testing.

Basic Program

Rockwell International, as the prime contractor, has developed and built two such aircraft for NASA. In March 1978, Rock­ well’s Los Angeles Division delivered the first aircraft with the first flight expected in the summer of 1979. In June 1978, Rockwell delivered on schedule HiMAT vehicle 2. HiMAT represents a $17 million investment in the application of advanced technology by NASA and the Air Force Flight Dynamics Laboratory, at Wright-Patterson Air Force Base, Ohio. The flight test program will be carried out at NASA’s Hugh L. Dryden Flight Research Center, Edwards, California.

Research Flight

Hi MAT is to be launched by a B-52 at about 45,000 ft. altitude. Unmanned, it will be “flown” by a NASA test pilot in the ground facility. The ground cockpit will contain all the normal flight controls – throttle. stick. rudder pedals, and sensor displays on the instrument panel. A computer will convert the pilot’s actions into electronic signals tele­ metered to the craft, where an onboard computer sends signals through the digital “fly-by-wire” system to the flight con­trol surfaces. Telemetry equipment aboard HiMAT will relay hundreds of bits of “real­ time” information per second back to ground computers. The duration of flights will range from a few minutes to more than an hour. Landing on the dry desert lakebed will be made on skid-type landing gear similar to that used by the record-setting Rockwell X-15 rocket-powered research plane of the ’60s.

Performance

The craft is capable of speeds of more than 1-1/2 times that of sound. The HiMAT vehicle is designed to demonstrate maneuvering performance 60 per cent better than today’s advanced fighters. The main empha­ sis during flight testing will be on high-G maneuvers in the transonic flight envelope (600 to 800 mph), where superior perfor­ mance is most difficult and most significant.

Research Concept

The concept of using remotely piloted research vehicles (RPRV’s), developed by NASA, makes it possible to test high-risk technology at a low cost and at a faster rate (time from design to hardware) because the research aircraft does not have to be man­ rated. always an expensive and time-con­summing process. Some of the new concepts that will be tested in flight include advancements in aerodynamics, advanced compo­sites and metallics, flight and propulsion control system designs, and other high-risk technologies. The application of advanced technologies will focus on enhancing maneu­verability, both individually and on an integrated basis.

Dimensions

Almost half the size of most fighters, HiMAT depicts the look of the future. Its length is 22. 5 ft. , wingspan measures 15. 6 ft. , and height is 4. 3 ft. Powered by a GE J85-21 afterburning engine, its launch weight is 3,400 lbs.