The 1980s tailless-fighter concept could be a reality after 2000
Mastery of thrust vectoring is paving the way for tailless-fighter research.Guy Norris/LOS ANGELES
HIGH OVER THE dry lakebed at Edwards AFB, California, on 24 April, 1996, the pilot of a NASA McDonnell Douglas (MDC) F-15 flying at Mach 1.2 briefly tapped his rudder pedals and yawed the aircraft left and right using the Pratt & Whitney thrust-vectoring nozzles.
Observers at NASA Dryden Research Center, able to view the event via head-up-display images projected live on a big screen, saw nothing more than a slight shudder as the nozzles slewed. To the test team, however, the brief yawing motion was a vital first step towards the eventual development of a supersonic fighter which will use thrust vectoring in place of conventional tail surfaces.
"The idea of a supersonic tailless fighter has been one of our visions that goes back ten years," says Dennis Weiland, of the US Air Force Wright Laboratory's advanced flight-dynamics directorate. "The key to getting there, and the enabling technology required, is thrust vectoring. Ten years ago, there was no confidence in thrust vectoring. Now, we have got to the point where we have that confidence, and people are embracing that technology. Ten years ago, people would have laughed at what we're planning to do today."
Within minutes of the F-15's sonic boom rolling away, a Northrop Grumman B-2 took off for a routine test flight - a reminder that tailless aircraft are flown every day. While the flying wing, as exemplified by the B-2 and its ancestors, is a well-proven concept, what is not so well understood is how to transfer the benefits of a tailless design from a large subsonic bomber to a smaller, more agile, supersonic fighter.
SIGNATURE ADVANTAGE
Whatever the challenges, the benefits of a tailless configuration are worth striving for, according to Harvey Schellenger, chief engineer for the Rockwell/Daimler-Benz Aerospace (DASA) X-31 Enhanced Fighter Manoeuvrability technology-demonstrator. "Lower radar-cross-sectional area is one advantage of being tailless," he says. "A tailed aircraft can achieve very good levels of low observability, but it gets harder and harder to squeeze any more out of the shape once you're down to a certain level. Tailless aircraft take you to a much lower threshold."
By far the biggest benefits are in reduced weight and drag. Aerodynamically, the tailless aircraft is extremely efficient because the profile drag of a tail is eliminated. Drag (excluding lift-induced drag) can be as much as 60% less than that of a conventional aircraft. Structurally, aircraft weight can be distributed along the wing span, as in some of the higher-aspect-ratio tailless-fighter designs under study; wing-bending loads can be minimised and weight reduced.
Lower drag and lower weight combine to produce an aircraft with longer range for a given size or, alternatively, a smaller and stealthier aircraft for a given range. "It can save a lot of money because aircraft are bought by the pound," says Schellenger. "Most importantly, where you are designing for a specific mission, the aircraft can be smaller because it weighs less, the drag is less and therefore you need less fuel. We call it the multiplier effect. In general, for every 1lb [0.45kg] of tail eliminated, you take out the equivalent of 2lb from the rest of the airframe." Tails can typically account for 6% to 7% of the maximum take-off weight of an aircraft, he says. A tailless aircraft also has fewer control surfaces. Fewer actuators and fewer of their related hydraulic systems result in reduced maintenance and lower life-cycle costs.
FOCUS ON JSF
Tailless-fighter studies are part of the Wright Laboratory's efforts to "-support the technology requirements for the next generation of fighter aircraft, particularly those of the USAF", says Weiland, adding that the most immediate focus is on the US/UK Joint Strike Fighter (JSF). "All the JSF competing contractors are looking at some sort of tailless configuration," he says.
MDC, with Northrop Grumman and British Aerospace, has gone further than the other teams, publicly at least, in pushing a tailless solution. The MDC-led teams "semi-tailless" design has no vertical fins and uses thrust vectoring and upwardly canted horizontal stabilisers for yaw control. Boeing and Lockheed Martin are also believed to be looking at tailless options.
Whether the winning JSF contender ends up tailless, the Joint Advanced Strike Technology (JAST) effort, which preceded the JSF programme, has been the driving force behind recent tailless research. The first phase of quasi-tailless testing with the X-31 was aimed at supersonic thrust-vectoring research for the US Defense Advanced Research Projects Agency (DARPA), under its advanced short-take-off-and-landing/common affordable lightweight fighter programme, which was later merged into the JAST effort.
In the first phase of "tailless" testing for DARPA, from late 1993 to early 1994, 2G turns were flown with the X-31, using thrust vectoring at Mach 1.25 and altitudes at up to 38,000ft (11,600m). To simulate removal of the X-31's vertical tail, the other conventional control surfaces were used to counteract its stabilising influence, making the aircraft appear tailless to the flight-control computers. The thrust-vectoring paddles then provided directional stability and control in lieu of the fin and rudder.
Up until then, nearly all thrust-vectoring research, including that conducted using NASA's F-15 Short Take-off and Landing/ Manoeuvre Technology Demonstrator and MDC F-18 High Alpha Research Vehicle, had been aimed at demonstrating the control power of thrust vectoring at low speed. These included tests of the USAF's Lockheed Martin F-16 Multi-Axis Thrust-Vectoring aircraft, fitted with a General Electric axisymmetric-vectoring engine nozzle (AVEN).
The second phase of "tailless" X-31 tests was conducted directly for the JAST programme and involved slow-speed flying, with demonstrations of air-to-ground attacks, precision approaches to simulated carrier decks and close-formation flying. These tests were "...more challenging because it involved being able to control the aircraft in specific closed-loop manoeuvres", says Schellenger. "I think our demonstrations had a positive effect on everybody. We showed that tailless aircraft worked, and so can thrust vectoring, particularly since we tested it under 'no kidding' conditions...so close to the ground," he says.
Rockwell's involvement with tailless-fighter design goes back to the 1980s when it was competing for the USAF's Advanced Tactical Fighter programme. Its design employed thrust vectoring, for low-speed pitch and yaw control and an "active flexible wing" control system for high-speed flight (see box). Soon to be sold to Boeing, the firm, together with DASA, is in talks to extend the research life of the surviving X-31. The partners hope to fit the aircraft with a version of the AVEN nozzle adapted to its GE F404 engine and to investigate 100% tailless flight.
For the moment, it is NASA's F-15 Advanced Control Technology for Integrated Vehicles (ACTIVE) which is leading the way into tailless technology. The ACTIVE is a joint NASA, USAF, MDC and P&W programme, with the engine maker supplying both the F-15's F100-229 engines and pitch/yaw balance-beam nozzle (PYBBN) thrust-vectoring system.
THRUST-VECTORING BENEFITS
The programme is aimed at demonstrating the benefits of thrust vectoring to overall fighter performance, including improving range and reducing cruise drag and signature. A third phase now in detailed planning, the supersonic tailless-research (STAR) programme, if sanctioned, would take place from mid-1998 onwards. It would begin with a "quasi-tailless" period when the F-15's vertical tails would be reduced in size by up to 50%.
"When we have got a lot of confidence, it'll go all the way," says Weiland. "Our goal is to take off the tails, then get an F-15E pilot to fly it and show that it has as good as, or better, performance than that of an F-15E.
"The biggest challenge is not the hardware - the PYBBN is becoming a very mature design and the nozzle is performing flawlessly. The biggest challenge is integration. There are still many unknowns, such as the jet-interaction effects. We still have to work out what could happen if we have a nose vortex at high angles of attack, which would interact with the jet plume. We have got to find those holes in the flight programme and fill out the upper left-hand corner of the flight envelope," he adds.
Data on jet interaction are quickly building up, says NASA ACTIVE programme manager Don Gatlin. Expansion of the thrust-vectoring envelope was to be complete by July 31 "-and we expect to start closing up nozzle-interaction data in late August or early September", he says. Initially, vectoring was limited to 2-3° in yaw, at up to Mach 1.95, while pitch vectoring is limited to 5-6° "-because of the aircraft's aerodynamic ability to counter [the pitch forces] if we have a problem", Gatlin says.
The PYBBN provides up to ±20¡ vectoring in any direction and was tested at swivel rates of up to 79¡/s to its maximum 20° pitch travel during subsonic test flights running up to the supersonic vectoring trials. The nozzle itself is limited to a 180kN (4,000lb) dynamic force "-and, in certain flight conditions, it will reach this 180kN force limit before it will reach the 20° vectoring limit," says P&W PYBBN programme manager Roger Bursey.
The fan-duct cases on the aircraft's F100-229 engines have been modified to withstand the additional loads, while the rear fuselage and main engine-mounts have been strengthened. Similar installation work is being performed on the USAF's F-16D Variable Stability In-Flight Simulator Aircraft (VISTA), which is also being fitted with a PYBBN-equipped F100-229. Modification work started in July, says Weiland, the USAF deputy programme manager for both the ACTIVE and VISTA programmes.
During Phase 1 of the ACTIVE programme, control of the nozzles is pilot-selectable via a vehicle-management computer, which does not form part of the F-15's standard flight-control system. For Phase 2 of the programme, starting in February 1997, the aircraft will be modified with a fully integrated flight- and propulsion-control system. This will be connected to the stick and rudder, and operation of the thrust-vectoring system will be transparent to the pilot. Phase 2 is due to run to the end of June 1998, when the STAR programme will begin.
X-36 DESIGN
The joint NASA/MDC X-36 programme is the only one designed from the outset to test every element of tailless-fighter technology and to integrate them into one airframe. It is also said to be the first fighter-type aircraft to be designed from the start without vertical or horizontal tails.
The 28%-scale, remotely piloted, test aircraft was rolled out at MDC's St Louis, Missouri, plant on 19 March after more than two years of design and manufacturing. It arrived at NASA Dryden in June for a six-month flight-test programme at Edwards AFB. The canard-configured aircraft has both stealth and supersonic design features, such as a chined forebody and swept cranked-arrow wing, but will be flown only to a maximum of Mach 0.6.
Most of the flight-control authority resides with the four surfaces on each wing, rather than with the still-classified thrust-vectoring system, which operates in yaw only. There are split ailerons at each wingtip, with independent upper and lower surfaces, which can act as drag rudders and ailerons simultaneously. The upper surface can be deflected by 60° up and 30° down while the lower surface, can be moved by 30° up and 60° down. The split ailerons are divided into inboard and outboard surfaces for redundancy and to keep actuator loads to a minimum.
Inboard of the split ailerons is "a typical flaperon" for pitch-and-roll control, says MDC X-36 programme manager David Manley. "We were looking for yaw power without a tail, and we basically ended up with what the B-2 team did," he says, referring to Northrop Grumman's use of split-aileron drag rudders for yaw control. "The B-2 has a lot more span, so we had a tougher job," he adds. Large foreplanes were added to the X-36 to provide nose-down moment to counteract the pitch-up generated by the chined forebody. As the X-36 was designed to be tailless from the start, "...our design philosophy does not actually require thrust vectoring to control the vehicle", says NASA X-36 programme manager Dr Larry Birckelbaw. The aircraft is designed to be controllable, and maintain acceptable handling qualities, after engine failure, or with the thrust-vectoring nozzle locked in one position.
A key aspect of X-36 flight-testing will be the performance of the flight-control system and the control laws "-that couple the whole thing together", says Birckelbaw. An MDC computer with seven Texas Instruments digital signal-processor chips will run flight-control software at 117 million instructions a second.
Source: Flight International
