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?? Serge Pod #05.07.2000 20:25

Serge Pod


Flights of fancy take shape


Tomorrow's aircraft are poised to break all the rules

http://www.janes.com/defence/air_forces/news/idr/idr000704_1_psm.jpg [not image]

A remarkable aircraft is being built in Victorville, California, under a contract from the Defense Advanced Research Projects Agency (DARPA). The Frontier Systems A160 unmanned helicopter is intended to demonstrate an endurance of well over 30hrs - some associated with the program are talking 48hrs or more - a service ceiling of 55,000ft (16,760m) and an unrefueled range of 3,700-5,500km.

These numbers are so far beyond current helicopter records (by a factor of two or more, in many cases) that they would strain credulity, were it not for the source. The designer of the A160 is Abraham Karem. The Leading Systems Amber, which Karem designed for DARPA in the 1980s, demonstrated 28hrs+ of endurance in a small unmanned aerial vehicle (UAV), together with unparalleled reliability. Amber is the direct ancestor of today's Predator. Frontier Systems' W570 contender for the Tier 2 Plus requirement (which led to the Global Hawk), designed for Loral, was arguably more advanced in concept than the contest winner.

The A160 is based on a reappraisal of the basics of helicopter design. Conventional helicopters operate within a very narrow rotor revolutions per minute (RPM) range. Their rotors are articulated to provide control authority and have flexible blades to save weight. These features lead to a complex and dynamic pattern of vibration; traditionally, rotors are designed for smooth and safe operation at a single speed point. They operate around 100% RPM whenever the helicopter is airborne.

The operating RPM is usually the highest speed possible, because this reduces the difference between the speed of the advancing blade (which is moving forward into the airstream) and the retreating blade, when the helicopter is in forward flight. The upper limit (450-500rpm on a small helicopter) is set to keep the tip speed of the advancing blade in the subsonic realm - about Mach 0.6 - at the helicopter's design cruising speed. Particularly at less-than-maximum speeds and weights, however, the helicopter rotor operates much faster than necessary. This reduces the lift/drag ratio of the blades and requires more power to turn the rotor.

http://www.janes.com/defence/air_forces/news/idr/idr000704_2_psm.jpg [not image]

The A160 rotor can be slowed down to as little as 40% of its maximum RPM, operating between 150-350rpm with tip speeds as low as Mach 0.25. The blades are tapered and change in thickness/chord ratio from root to tip to improve their lift/drag ratio. To avoid vibration problems, the rotor blades are light and stiff, and their stiffness in flap, lag and torsion is progressively reduced from root to tip, so that the tips are more flexible than the root. This is made possible by the use of tailored carbon fiber construction. The A160 rotor is hingeless and rigid, and has a larger diameter and lower disc loading than a conventional helicopter rotor with the same maximum lift.

The result is a dramatic improvement in aerodynamic efficiency at low speeds and weights. This can be combined with a fuel-efficient engine; a key to the performance of Karem's Amber vehicles was their use of high-performance reciprocating engines, specially developed by Leading Systems. (The demonstrator has a 300kW piston engine.) The result is a helicopter with very long range and endurance, and a respectable maximum speed of 140kt. Since noise is closely linked to rotor speed, the A160 will also be exceptionally quiet.

The A160 project has been under way since early 1998. Frontier Systems started by modifying a used Robinson R22 light helicopter to test the A160's autonomous flight control system software and hardware. This is designed to allow the A160 to be operated safely under the control of a non-pilot. The demonstrator was lost in an accident in February, but had flown successfully under autonomous control for 215hrs - an enviable achievement for a VTOL (vertical take-off and landing) UAV of any description.

The unique A160 rotor system is now being tested on a ground rig at Victorville. The development team is anticipating some vibration problems, but modeling shows that vibration should be confined to discrete bands, leaving a well-defined operating regime which is large enough to provide good performance.

The next decade should see the testing of other prototypes which break what were once considered iron-clad limits on aircraft design and performance. These include stealthy, agile aircraft with no conventional flight controls; and efficient supersonic-cruise aircraft with no sonic boom signature.

Computing is the most important technology behind this renaissance in the flight sciences. Fly-by-wire (FBW) flight control systems using artificial stability were an early application of compact digital and analogue computers in the 1970s, and made it possible for the designers of the F-16 to relax the standards of natural stability that had governed aircraft design. Today, computer technology has reached the point where aircraft may be highly unstable about all axes, and the FBW system will integrate aerodynamic and propulsion control to fly the aircraft.

Computational fluid dynamics

Computer technology has also provided the innovative designer with valuable new tools. Computational fluid dynamics (CFD) continues to improve in its ability to model complex airflows and dynamic effects, and in the realism and resolution with which it can account for small-scale phenomena. The platforms needed to run CFD tools are becoming cheaper and more powerful. Wind tunnels and flight tests are still essential, but CFD allows more designs to be explored and developed in detail at far less cost than tunnel testing.

When it comes to building prototypes, information technology helps combine speed and low cost with high quality. Computer-aided design and manufacture, as demonstrated by the Boeing Phantom Works in a number of prototype programs (including the X-32 and X-36), makes it possible to proceed quickly on a prototype program without giving it a top-priority status that disrupts other work. Burt Rutan's Scaled Composites operation has also demonstrated the ability to produce aerodynamic demonstrators at short notice. (The newest Scaled prototype, the Adam M309 twin-engined light aircraft, was flown less than 10 months after contract award.)

Less expensive unmanned prototypes are also becoming more routine. Information technology provides more efficient datalinks and smarter control systems, often based on cheap commercial hardware. Unmanned prototypes are less expensive to build than piloted vehicles, and can usually be more faithful to the full-scale design. Except in the case of a large aircraft, the need to accommodate a crew station makes it difficult to produce a high-fidelity manned subscale prototype.

Rapid communications and integrated product teams (IPTs) have generally led to improved program management. Some of the Pentagon's Advanced Concept Technology Demonstration (ACTD) projects, such as the Global Hawk UAV, have produced positive results. What is equally important is that, where ACTDs have been less successful (as in the case of DarkStar), they have been completed within reasonable time and cost limits. There have been no recent programs comparable to the X-Wing, the X-29 or F-15 STOL/Maneuver Technology Demonstrator, each of which took several years, cost more than US$100 million and demonstrated technologies that the industry was not interested in exploiting.

Restrained costs and more consistent results have encouraged customers to fund more demonstration programs. Since 1990, NASA and the USAF (United States Air Force) have assigned more new X-vehicle designations than they did in the previous 30 years, and the Tactical Technology Office of DARPA has also sponsored several prototype programs.

NASA, the armed services and DARPA are working together as never before. NASA's alignment with the USAF and National Reconnaissance Office on space research is being replicated in air-vehicle programs, particularly when they have military and commercial applications. The latest batch of NASA Revolutionary Concepts (Revcon) programs include one project which meshes closely with a DARPA effort, and another which is based on an operational vehicle concept devised by the USAF Research Laboratory.

Canard Rotor Wing

The A160 is one of two DARPA-funded demonstrator programs that are well into the hardware stage and which are aimed at increasing the performance envelope for vertical-take-off aircraft. The DARPA/Boeing Canard Rotor-Wing (CRW) demonstrator is being completed at Mesa, and should fly early next year. The CRW combines two long-standing concepts - the hot-cycle tip-jet helicopter and the stopped-rotor helicopter - with the newer idea of a three-surface fixed-wing aircraft.

The 1,100kg-class demonstrator is powered by a low-bypass-ratio Williams International F112 engine (as developed for the AGM-129 Advanced Cruise Missile). For take-off, the exhaust is ducted via titanium pipes to nozzles on the tips of the two-blade rotor. The rotor has a simple gimballed hub, and cyclic and collective blade pitch control. No anti-torque rotor is needed, but small thrust nozzles on the rear fuselage are used for directional control.

The CRW accelerates to 60kt like a conventional helicopter, with all the exhaust going through the rotor. Transition takes place between 60kt and 120kt. During transition, the exhaust is switched to a conventional nozzle. Flaperons on the canard and tail surfaces are deflected downwards to lift the vehicle and unload the rotor. The gimbal freedom of the hub is gradually reduced by variable dampers. At 120kt, the rotor is entirely unloaded. The hub and pitch hinges are locked in the flight position and the flaperons move upwards, transferring most of the lift back to the wing.

The main advantage of the CRW, compared with earlier stopped-rotor concepts such as the X-Wing, is that it has a simple helicopter configuration (with no tail rotor) and a simple fixed-wing configuration, with the canard- and tail-borne mode to smooth the transition between the two. The rotor is also a pure lifting device - all the thrust is provided by the turbofan exhaust. The CRW is scalable in size and performance, according to Boeing. It is simple enough to be a practical solution for a 2-2.5 tonne UAV. Its speed can be increased into the high-subsonic regime by stopping the rotor-wing in a skewed position. Also, reaction-drive helicopters actually become more attractive at large sizes. The transmission of a normal helicopter is sized by torque - which increases rapidly with rotor size, because the power increases and the rotational speed declines. Boeing has looked at a CRW attack aircraft in the 11 tonne range, as well as at some transport applications.

There are some inherent design penalties in the CRW. The disk loading is at the upper end of the helicopter range - around 12-15 lbs/ft2, in the same order as the CH-53E. This could be a problem in operations that require hovering (rescue or ASW with a dipping sonar, for example) although it would not impede operations from unprepared surfaces. The low-bypass-ratio engine cycle is not ideal for subsonic cruise, particularly at low altitude, and the symmetrical aerofoil section is also a compromise, because it must develop lift in either direction. However, the CRW's combination of jet speed and helicopter-like vertical lift efficiency is unique.

Conventional runway-bound aircraft could also see some improvements. One of the most ambitious programs today is DARPA's Quiet Supersonic Platform (QSP), which is aimed at developing an efficient supersonic-cruise aircraft which does not produce a sonic boom. QSP has been a DARPA program since early this year, having originated as a commercial project to develop a supersonic business aircraft.

Gulfstream Aerospace and Lockheed Martin agreed to work on a supersonic business jet (SBJ) in mid-1998. After NASA cancelled its High-Speed Research (HSR) program - aimed at developing technology for a supersonic airliner - in early 1999, Lockheed Martin and Gulfstream approached the agency for support, but NASA was unwilling to fund what might be portrayed as a corporate luxury product. Congress felt differently, however, allocating US$15-20 million to support the project as a dual-use, commercial/military venture.

In March, DARPA asked industry to propose a demonstration program. This program will have two elements: the validation of key technologies and the concurrent design and testing of a flight test aircraft.

DARPA is not talking publicly about the QSP, but the agency's request for information (RFI) hints that the agency is ready to consider exotic approaches to the program, because its engineers believe that only a revolutionary design will meet the requirement. "The incremental application of new technologies, or the integration of existing technologies, will be insufficient to meet overall program goals," comments the agency.

Sonic-boom suppression

Low-sonic-boom technology is an important thrust of the DARPA work. Early this year, Lockheed Martin revealed a possible configuration for the SBJ design which it is studying in collaboration with Gulfstream Aerospace, providing some clues to the sonic-boom suppression techniques under consideration.

The twin-engined Lockheed design is 35m long. The 17m-span wing is sharply swept and tapered. The most unusual features are an elongated nose, with a conical tip, and an inverted-V tail surface which overlaps the wing. Both these features are probably dictated by the low-sonic-boom design of the aircraft.

Lockheed Martin declines to discuss details of the design, but a patent issued in 1998 to an engineer who is now a member of the Skunk Works SBJ team describes some of the principles behind it.

The goal is not to eliminate the boom - which is physically impossible - but to suppress the features that make it audible. A supersonic aircraft creates an N-shaped pressure wave, caused by an overpressure at its nose and an underpressure at its tail. The pressure rises sharply at the nose, declines to an underpressure towards the tail, and then returns quickly to the ambient level. The rapid pressure rises at the front and rear of the wave and are sensed as the characteristic double bang of a sonic boom.

In the 1970s, researcher Richard Seebass proposed that the effect of the boom could be minimized by making the nose of the aeroplane blunt. This creates a pressure spike ahead of the forward shock. The pressure spike raises the local temperature and sound velocity, which "stretches" the forward shock and slows the rise in pressure. Seebass also suggested that the peak overpressure in the wave could be reduced by spreading the lift of the wing along the length of the aircraft. Finally, he concluded that if the boom was weak enough, phenomena associated with the molecular structure of air could make it inaudible on the ground.

In 1996, Seebass (by then a professor at the University of Colorado) analyzed the sonic-boom signature of an SBJ. His analysis showed that it was possible to achieve a peak overpressure of less than 0.45 lbs/ft2 and that such a low boom aircraft "could be certified to fly over land at supersonic speeds."

Morgenstern's design for McDonnell Douglas featured a canard foreplane with a movable leading edge, designed to create the same pressure spike as Seebass' blunt nose. He also proposed the use of a trailing-edge flap to increase pressure at the rear of the aircraft and reduce the intensity of the aft shock.

The Lockheed Martin design's conical nose is intended to create the forward pressure spike proposed by Seebass and Morgenstern. Its sharply swept arrow wing spreads the lifting surface along the vehicle's length. The inverted-V tail, Seebass comments, may be designed to generate extra lift towards the tail: the inboard trailing edge of a delta provides little lift, but a separate surface can do so.

Lockheed Martin may also be hoping to achieve a favorable interaction between the wing and tail, along the lines of the supersonic biplane proposed in the 1930s by German aerodynamicist Adolf Busemann.

Flying a demonstrator aircraft is crucial, because neither computational nor wind-tunnel work can show conclusively that the boom effects on the ground will be as predicted, or that the boom effects will be acceptable - which means virtually imperceptible. Although Lockheed Martin and Gulfstream say they are considering a subscale or even unmanned demonstrator, the sonic boom phenomenon is so complex that a near-full-size prototype may provide the only proof of the theory that will be sufficient to justify a multi-billion-dollar development program.

While airframe shaping is part of the QSP approach, DARPA is asking companies to look at other means of sonic boom suppression. DARPA specifically mentions the use of plasmas to reduce drag at supersonic speed. Russian work dating back to the 1970s indicates that it is possible to reduce the intensity of the shock wave from an aeroplane's nose by generating an electrical field in the airflow, which creates a plasma, or electrically charged gas stream.

The USAF Research Laboratory has sponsored a number of tests in an effort to reproduce the Russian plasma results. So far, the US researchers say they do not completely understand the mechanism involved. For example, there is no agreement as to whether the shock reduction is caused by heat, by the change in the molecular structure of the gas caused by the plasma, or both. Tests continue in the US and Russia, using a variety of plasma generators (some with inert gas injection) to create stable and streamer-like discharges.

http://www.janes.com/defence/air_forces/news/idr/idr000704_7_psm.jpg [not image]
It is safe to assume that other plasma-aerodynamics research has been carried out under classified programs, because of the technology's potential for reducing the drag of supersonic aircraft. Some of this work may be available to the DARPA effort.

Supersonic cruise bypass

Supersonic laminar flow control (LFC) is another area of interest, because it can reduce the aircraft's drag, fuel burn and weight. NASA explored LFC during its High-Speed Research program in the 1990s, and installed an experimental system on one of its F-16XL test aircraft. DARPA is looking at using aerodynamic shaping for LFC, rather than the complex, high-maintenance suction system tested by NASA.

New engines, including high-bypass supersonic cruise engines, will be considered. (Rolls-Royce, for example, has looked at a supersonic-cruise engine with a bypass ratio of 2:1.) NASA is supporting the propulsion side of the QSP program through one of its newly announced Revcon projects. Advanced Supersonic Propulsion and Integration Research (ASPIRE) is aimed at validating a two-dimensional, mixed-compression engine inlet for commercial applications. The major airframe manufacturers involved in ASPIRE are Lockheed Martin and Gulfstream.

A QSP-based military aircraft would have many applications. In the USAF's Expeditionary Aerospace Forces (EAF) doctrine, a long-range supersonic aircraft would be the first asset to arrive over the theater of operations, carrying out reconnaissance and transmitting imagery back to the force commander. During a crisis, it would be able to perform quick-reaction overhead reconnaissance and strike missions, supporting the USAF's renewed emphasis on long-range global strike capabilities.

A supersonic transport would also support rapid movement of people and supplies between the US and the theater. A possible follow-on to an SBJ-sized aircraft would be a larger vehicle, which could be used as a 100-seat commercial transport or as a cargo aircraft.

Another area of advanced development in fixed-wing aircraft combines flight control, propulsion integration and low observables. By 2007, the USAF and NASA could be flying an experimental aircraft which not only has no tail surfaces, but has no conventional aerodynamic controls at all. Instead, it would be controlled by thrust vectoring. Ultimately, even the exhaust nozzles of such an aircraft would be physically fixed and smoothly faired into the skin.

Eliminating aerodynamic controls and surfaces would reduce the weight, drag and complexity of the aircraft and allow the entire perimeter to be designed for RCS reduction, without gaps or moving parts. As defensive systems acquire the ability to detect bistatic radar signatures, all-aspect RCS reduction will become more important, and the RCS contribution of vertical tails less acceptable. One challenge, however, is that the type of LO-compatible thrust-vectoring nozzle used by the Lockheed Martin F-22 is heavy, complex and expensive.

Under the Integrated High Performance Turbine Engine Technology (IHPTET) program, a team comprising GE and Allison Advanced Development Company has designed and is testing a nozzle which uses "fluidic" techniques to vary the effective nozzle area and vector the thrust, while the nozzle's physical shape remains unchanged. This has important advantages. It eliminates hot, highly loaded moving parts, which are a perennial maintenance problem. It allows the hot nozzle structure to be integrated into the surrounding cold aircraft structure, reducing its weight. The outer contours of a fixed nozzle can be fully blended at all times into the shape of the aircraft, improving LO characteristics. Its inner contours can be optimized for aerodynamic and thermal efficiency, rather than being dictated by the need to allow for variable geometry. In a fluidic nozzle, high-pressure air is bled from the compressor and ducted to injectors located around the throat. At full military power, when the greatest expansion ratio is needed, valves leading to the injectors are opened and the high-pressure air thickens the boundary layer in the throat, effectively reducing its area.

According to a NASA document, thrust vectoring can be achieved by using injectors in the divergent segment of the nozzle. Injecting high-pressure air into one side of the exhaust nozzle triggers an oblique shock wave in the exhaust, which deflects the jet away from the injection point. NASA tests have shown that the vectoring angle can be controlled by varying the mass flow through the injector, up to a maximum of 15.

The Boeing X-36 unmanned research aircraft may have incorporated a simple yaw-only fluidic nozzle. The nozzle had no visible moving parts, and its design was the only highly classified element of the X-36 program. The USAF Research Laboratory (AFRL) notes that fluidic nozzle technology can be used to promote mixing and reduce the peak temperature of the exhaust plume, reducing its infrared signature. In a related program, AFRL is working with contractors on "affordable exhaust washed structures" using high-temperature composite materials, many of them compatible with the radar absorbent structures which are used on the edges of LO aircraft.

Work continues in parallel on integrating thrust vectoring into aircraft control. In March 1998, NASA announced the start of the VECTOR (Vectoring, Extremely Short Take-off and Landing, Control and Tailless Operation Research) program, in which the surviving X-31 is to be fitted with a Volvo RM12 engine, a General Electric Axisymmetric Vectoring Exhaust Nozzle (AVEN) and an advanced air data system. Its vertical tail will be removed.

Sponsors include the US Navy and Sweden, which was interested at the time in a tailless version of the JAS 39 Gripen. The Extremely Short Take-off and Landing (ESTOL) research has been touted as a possible solution to future carrier-fighter requirements. The technique involves flying slow approaches at angles of attack above the stall, using thrust vectoring to de-rotate the aircraft for touchdown. The X-31 has now been delivered to the navy's flight-test center at Patuxent River in preparation for Phase II.

The USAF/NASA ACTIVE (Advanced Control Technology for Integrated Vehicles) program has been making steady progress since its first flight in 1996. The program uses the F-15B which was originally modified for the mid-1980s STOL/Maneuver Technology Demonstration (S/MTD) project, modified with pitch/yaw vectoring nozzles. ACTIVE program managers stress that the program is aimed at acquiring detailed data about thrust vectoring and at developing the control laws needed for it to work, rather than at demonstrating extremes of performance.

In April 1999, the ACTIVE program completed the first phase of its flight tests with an Intelligent Flight Control System (IFCS) using neural net technology. The IFCS is designed to refine its own control laws in real time, adjusting the gains in the system to achieve the maneuver commanded by the pilot. Unlike a conventional flight control system, where the control laws are determined in software and adapted according to the aircraft's configuration (weight and external stores, for example), a neural-net-based FCS can, in theory, control the airplane in any configuration. It will even work with an unknown configuration - as would be the case if the aircraft was damaged.

ACTIVE has been successful enough for USAF, NASA and industry to propose an ambitious follow-on program. "Our experience is that man-rating the system pushes the technology further than a low-cost point demonstration," comments NASA program manager Gerard Schkolnik.

A team including AFRL, NASA, Lockheed Martin and Pratt & Whitney has defined a new thrust-vectoring test aircraft, the X-44A, based on an F-22 airframe, engines and systems. "There are a lot of reasons why the F-22 is an excellent aircraft to build on for a flight demonstration," says Schkolnik. "It has the infrastructure that is needed to take thrust vectoring out of the 'nice-to-have' arena into the 'must-have' zone." The X-44A has been referred to as the MANTA, or Multi-Axis No-Tail Aircraft. An X-44A feasibility study is still in progress, but the assignment of a designation to the program indicates that there is already high-level support for the concept. The X-44A would have pitch/yaw vectoring nozzles and - as currently envisaged - would not only be tailless but would have no moveable aerodynamic surfaces. The result would be a structurally simpler, lighter airframe, with increased fuel volume and fewer gaps to cause RCS problems. The X-44A "will probably not fly before 2007", says Schkolnik. On current plans, though, the F-22 program will retire the first and second flying prototypes, which have a restricted flight envelope, in 2001-02. The X-44A technology, combined with fluidic nozzles and QSP's supersonic-cruise aerodynamics, point to a generation of high-performance, very stealthy aircraft, with exceptionally high aerodynamic efficiency.

Future strategic aircraft

http://www.janes.com/defence/air_forces/news/idr/idr000704_8_psm.jpg [not image]

Other technology programs are aimed at improving the efficiency of larger subsonic aircraft. This technology is a candidate for a future strategic airlift aircraft, a new tanker or even a carrier for cruise missiles or UCAVs - a replacement for the B-52, in essence. The USAF's most recent policy statements - notably, a White Paper on airpower - have placed increasing emphasis on the importance of long-range strike aircraft, and it is far from inconceivable that a new bomber program could be launched if George W Bush is elected President in November.

In another example of a USAF/NASA link, one of the newly announced NASA Revcon projects is closely associated with a specific USAF need.

Boeing's Phantom Works has been working for some years on joined-wing aircraft and has proposed a radical intelligence, surveillance and reconnaissance (ISR) platform with a diamond-wing layout. The USAF Research Laboratory refers to this vehicle as the Sensor Craft. It is a UAV, designed specifically to incorporate very large electronically steered antenna arrays in the leading edge of the forward wing and the trailing edges of the aft wing. Each array is aligned to cover a quadrant, and can act as a passive or active sensor or as a datalink antenna. The Sensor Craft would also carry electro-optical sensors, including a long-range infrared search and track system for use against air targets and a three-dimensional laser imaging system.

The joined-wing layout proposed for the Sensor Craft is a "co-planar" design in which the front and rear wings lie in the same plane. The engine and fuel would be accommodated in the fuselage, and the primary control surfaces would be located on the trailing edge of the forward wing, with spoiler-type ailerons in the extreme wingtips. NASA and the USAF plan to modify an S-3 Viking into a piloted joined-wing prototype.

Joined wings and blended wing-body configurations are being considered for future airlift and tanker requirements. Lockheed Martin has proposed a variant of the joined-wing configuration, the "box wing"; this differs from the classic joined wing in that the tips of the front and rear wings are connected by endplates. As well as reducing potential interference problems at the tips, the endplates provide locations for outboard refueling booms: Lockheed Martin points out that such a design would allow the USAF to replace its KC-135s with a smaller number of new aircraft.

Boeing and NASA are collaborating on a demonstration of the blended wing-body (BWB) concept, originally developed at by McDonnell Douglas in Long Beach. The BWB represents a step beyond the classic flying wing, as represented by the B-2, and is primarily aimed at very large aircraft. It combines a broad delta-shaped body, with an inherently better relationship of volume to surface ration than a cylindrical fuselage, with slender outer wings. This configuration also provides more uniform spanwise loading than even the B-2, further saving weight. The BWB uses modern flight control technology and trimming fuel tanks to operate with an aft center of gravity, reducing trim drag. Unlike the B-2, the BWB has slats and flaps on the outer wing, allowing it to fly with a higher wing loading.

The BWB is also designed to take full advantage of very high-bypass ratio engines, with a BPR of up to 20 compared with six to eight on current engines. These engines are more efficient than today's engines, but the gains are quickly wiped out in a conventional installation by nacelle drag and the weight of longer pylons and landing gear. On the BWB, the engines are built into the trailing edge of the center section.

Tailored composite structures

One of the most challenging aspects of the BWB design is the structure of the non-cylindrical pressure cabin. Tailored composite structures, producing large, lightweight skins with high bending rigidity, are essential. The systems will also be unusual. The high-BPR engines, with their small cores, cannot provide enough air for cabin pressurization; instead, the BWB will have an electrically powered secondary system.

A 14% scale model of a 247ft (75m)-span BWB is being designed and built by NASA and Boeing. Powered by three Williams WR24 engines, the model has a span of 10.5m and is designed specifically to explore the low-speed stability, control and handling behavior of the BWB. Flight tests are due to start in early 2002.

Lockheed Martin, meanwhile, has studied very large box-wing aircraft, including a military superfreighter - an ultimate replacement for the C-5 - with a payload in excess of 350,000 lbs. Some of the detail design challenges may be less daunting than those that face the BWB, and - with a narrow-track landing gear attached directly to the fuselage - the box-wing may be more compatible with existing airports and infrastructure.

The existence of the A160 and similar aircraft is proof that aviation technology is showing surprising vitality for a centenarian. Budgets and the discretion with which they are used, rather than technology, will set the limits to the military use of aerospace in the 21st century.

http://www.janes.com/defence/air_forces/news/idr/idr000704_1_p.jpg [not image]
Boeing concept for an attack aircraft based on Canard Rotor-Wing technology. It could be a candidate to replace USMC AH-1Z helicopters.
(Source: Boeing)

http://www.janes.com/defence/air_forces/news/idr/idr000704_2_p.jpg [not image]
Lockheed Martin claims that an advantage of the box-wing concept is that it can carry multiple refueling booms, increasing the number of fighters that one aircraft can support.
(Source: Lockheed Martin)

Boeing patent drawing shows the joined-wing "sensor craft" under study for the USAF Research Laboratory. The engine and fuel are carried in the center fuselage, and phased-array antennas in the wing leading and trailing edges provide full 360° coverage around the vehicle.
(Source: Boeing)

Frontier Systems' A160 demonstrator has a streamlined body and provision for an under-nose sensor turret. The landing gear is retracted in this view.
(Source: Frontier Systems)

A key technology for the A160 is the ability to design rotor blades of long span, high aspect ratio and varying planform and taper - like a sailplane wing - which are nevertheless rigid.
(Source: Frontier Systems)

Drawing of a configuration considered by Lockheed Martin and Gulfstream for the QSP program. The slender, long-chord wing, extended conical nose and inverted V-tail are influenced by its low-sonic-boom design.
(Source: Lockheed Martin)

http://www.janes.com/defence/air_forces/news/idr/idr000704_7_p.jpg [not image]
Blended wing-body shapes such as this Lockheed Martin design are candidates for transport, tanker and cruise-missile-launcher missions.
(Source: Lockheed Martin)

http://www.janes.com/defence/air_forces/news/idr/idr000704_8_p.jpg [not image]
Lockheed Martin has studied box-wing aircraft as large as this 160-tonne payload cargo aircraft.
(Source: Lockheed Martin)


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