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  • J-20's Stealth Signature Poses Interesting Unknowns
    Posted by David A. Fulghum 4:01 PM on Jan 13, 2011

    David Fulghum and Bill Sweetman/Washington
    Anti-stealth and stealth detection technologies will bring into question all stealth designs, including China’s new J-20. How much invulnerability will current low-observability techniques retain as air defense systems adopt even larger and more powerful active, electronically scanned array (AESA) radars?

    Airborne detection of stealth aircraft may already be an operational capability. Raytheon’s family of X-band airborne AESA radar family (in particular those on upgraded F-15Cs stationed in Okinawa) can detect small, low-signature cruise missiles. Moreover, Northrop Grumman’s lower-frequency, L-band AESA radar on Australia’s Wedgetail airborne early warning and control aircraft is larger and potentially more capable of detecting stealth aircraft at longer ranges.

    Better images emerging from China point clearly to the J-20’s use of stealth technology, but there are still major uncertainties and unanswered questions.

    The overall shape resembles that of the F-35 and F-22, with a single “chine line” uniting the forebody, upper inlet lips and wing and canard edges, a curved surface above that line and flat, canted body surfaces below it. The wing and canard edges are aligned – the wing and canard leading edges are parallel and the trailing edge of the canard is aligned with the opposite wing trailing edge. The same basic philosophy has also been adopted in British, Swedish and Japanese studies for stealth fighters.

    The aim in all cases is to endow a practical, agile fighter configuration with a “bow-tie” radar signature, with the smallest signature around the nose and the greatest (still much lower than that of a conventional aircraft with curved or vertical-slab sides) to the side. The fighter’s mission planning system, using a database of known radar locations, then derives a “blue line” track that weaves between radars and avoids exposing the side-on signature to those radars more than transiently.

    The diverterless supersonic inlet avoids a signature problem caused by a conventional boundary layer diverter plate – the F-22 has a conventional inlet, which is likely to require extensive radar absorbent material (RAM) treatment.

    The biggest uncertainty about the design concerns the engine exhausts, which as seen on the prototype are likely to cause a radar cross-section (RCS) peak from the rear aspect. One possibility is that a stealthier two-dimensional nozzle will be integrated later in the program:  however, the nozzles on the current aircraft show some signs of RCS-reducing saw-tooth treatment, suggesting that the PLA has accepted a rear-aspect RCS penalty rather than the much greater weight and complexity of 2-D nozzles.

    Other details of the design are unknowns. Stealth development has been dogged by detail design challenges. All the antennas on the aircraft have to be flush with the skin and covered with surfaces that retain stealth properties while being transparent in a specific frequency. Maintainability becomes a complex trade-off:  some systems requiring frequent attention will be accessed via landing gear and weapon bays, and others by latched and actuated doors that can opened and closed without affecting RCS, but the latter are a heavy solution.

    Perhaps the toughest challenge in stealth design is the need to manage RF surface currents over the skin. Early stealth designs used heavy, maintenance intensive RAM. The F-22 introduced a much lighter surface treatment, but it has proven unexpectedly difficult to maintain, causing corrosion issues. Lockheed Martin now claims that the F-35 will be robust and affordable to maintain in service, with a combination of a high-toughness sprayed-on topcoat and a conductive layer cured into composite skin panels.

    The Chengdu J-20 design has struck many analysts and observers as familiar and somewhat different that the Lockheed-Martin F-22, F-35 or the Sukhoi T-50.

    “The J-20 is reminiscent of the Russian MiG 1.42 both in terms of planform, and also with regard to the rear fuselage configuration,” says Douglas Barrie, senior fellow for military aerospace at London’s International Institute for Strategic Studies. “The most obvious difference is the greater forward fuselage shaping as the basis for low observable characteristics, along with the different engine intake configuration. The MiG program was cancelled by the Russian government around 1997.” However, the similarity to the MiG concept may suggest some collusion with the Russian aviation industry.

    Tags: ar99, stealth, J-20

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