Thread: Chinese Secret "Tianchi" UAV was testing

Maybe Chinese doctrine for their FD-21D is now coming out more clearly

By David A. Fulghum, Bill Sweetman

The latest unmanned aircraft pictures from China show a reconnaissance truck with a joined wing and tail that could considerably increase range and payload and produce better handling at high altitudes.

U.S. analysts already are suggesting that the new Chinese UAV design — with its 60,000-ft. cruising altitude, 300-mi. radar surveillance range and low radar reflectivity if it uses the right composite structure — could serve as the targeting node for China’s anti-ship ballistic missiles. The ASBM threat against carriers finally has U.S. Navy officials worried.

Photographs emerging from Chinese Internet sources, depicting the aircraft on what is likely Chengdu Aircraft Corporation’s (CAC) ramp, show a new design featuring a novel joined-wing layout. In the same size class as the General Atomics-Aeronautical Systems Inc. Avenger, and powered by a single turbofan engine, the new UAV is the most advanced Chinese design seen to date and the largest joined-wing aircraft known to have been built.

The company also makes the J-10 strike fighter, the J-20 stealth fighter prototype and a Global Hawk-like maritime reconnaissance UAV called the Xianglong, or Soaring Dragon, which flew in December 2009. CAC officials say it has a wingspan of 75 ft., length of 45 ft. and a cruise altitude of 55,000-to-60,000 ft. Chinese sources credited it with a 7,500-kg (16,500-lb.) takeoff weight and 3,800 nm range. The forebody is bulged to accommodate a high-data-rate satcom antenna.

Joined wings — a subset of closed-wing systems — comprise a sweptback forward wing and a forward-swept aft wing.

In the new Chinese UAV (as in many such configurations) the rear wing is higher than the forward wing to reduce the effect of the forward wing’s downwash on the rear wing’s lifting qualities. The rear wing has a shorter span than the front wing and its downturned tips meet the front wing at a part-span point.

Advocates of the joined wing claim that its advantages stem from the fact that the front and rear wings are structurally cross-braced.

This allows a higher aspect ratio while keeping down weight and staying within flutter limits. A higher aspect ratio reduces drag due to lift, and because the wings are both slender and short-span (relative to a single wing with equivalent lift) the wing chords are short, which makes it easier to achieve laminar flow. The joined wing also can reduce trim drag.

Studies of joined wings go back to the earliest years of aviation, but modern work is traceable to Julian Wolkovitch, a California aerodynamicist.

Wolkovitch worked with Burt Rutan on an early design study, the Model 58 Predator agricultural airplane, and drew up plans to develop a flight demonstrator based on the fuselage of the Ames-Dryden AD-1 skewed-wing aircraft. However, the project was still unfunded when Wolkovitch died in 1991. (Rutan went on to build a different Predator design.)

More recently, Boeing used a joined-wing configuration in its contribution to the U.S. Air Force Research Laboratory’s (AFRL) SensorCraft project, aimed at developing an aircraft capable of carrying an airframe-integrated, 360-deg.-coverage, high-resolution radar and remaining on station for 30 hr. at 2,000 nm range.

A small, low-speed free-flight model known as VA-1, with a 14-ft. wingspan, was completed by AFRL in 2003 and test flown.

A model of Boeing’s Joined Wing SensorCraft was tested last year in NASA Langley’s Transonic Dynamics Tunnel under the Air Force’s Aerodynamic Efficiency Improvement program.

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