Agility+Stealth = X-36: formula for an advanced fighter

Conventional aerospace wisdom holds that stealth and high performance--like matter and antimatter--cannot be friends. Gains in one area usually lead to sacrifices in the other.

Take, for example, the F-117 stealth fighter of Gulf War fame. Its sharp edges, abrupt angles, and flat surfaces deflect radar waves to deliver impressive stealth--while simultaneously flouting the laws of aerodynamics. Though amazing for its time, the F-117's subsonic, non-aerobatic capabilities emphasize invisibility over agility. By contrast, there's the F-16, a sexy-sleek, supersonic air-superiority fighter possessed of outstanding maneuverability--and a quite noticeable radar signature.

In April, engineers from NASA and McDonnell Douglas unveiled an aircraft designed to prove the conventional wisdom wrong: the X-36, a subscale, remotely piloted demonstrator of an extremely agile, very stealthy fighter. Developed in 28 months at a fire-sale price of $17-million, its job is to prove several fundamental but high-risk technologies--all under development during the past five years by engineers at both NASA and industry. The technologies include:

- A low-profile, tailless planform with unique split-aileron control surfaces

- A classified thrust-vectoring system

- High-rate, digital, fly-by-wire flight controls.

"These are all brand new technologies," says Larry Birckelbaw, X-36 program manager at NASA, "and we recognized that any potential customers would be hesitant to incorporate them into an advanced aircraft design without them being sufficiently proven. That's the purpose of the X-36."

The X-36, a 28%-scale model of a hypothetical 40,000-lb class aircraft, actually employs no radar-absorbing materials in its structure. However, the basic configuration would be a stealthy one were the vehicle full size. In addition to being stealthy, the design explores reductions in weight and drag, and improvements in range, maneuverability, and survivability for possible application to future designs.

Aside from its duties in the air, the X-36 also provided McDonnell Douglas with a test bed for demonstrating several innovative engineering and precision manufacturing techniques (see the "Phantom Works" sidebar), such as:

- Low-temperature cured composites

- Low-cost tooling molds

- Unitized, high-speed machined assemblies

- Advanced CAD/CAM, straight from "screen to machine."

Cat-agile, but no tail. The design's most obvious departure from current fighter design is its lack of an empennage. "There are some obvious benefits to removing the tail," says Dave Manley, program manager at McDonnell Douglas. "One is the smaller radar signature; the others are reduced drag and weight." Lopping off the tail saves an immediate 10% in mass and produces a "fairly significant" drop in RCS (radar cross-section). But most importantly, eliminating the tail cuts drag. The X-36 experiences 20 to 30% less aerodynamic drag than an equivalent-size F-15.

On a conventional aircraft, of course, the tail supplies important stability and control forces. These must be generated in another way on the X-36. That other way requires a variety of aerodynamic surfaces. These include two large forward canards for pitch control, and on the outboard trailing edge of each side of the main wing, two pairs of split ailerons. The ailerons' upper and lower surfaces can move separately or together to act as drag rudders (for yaw control) and as ailerons at the same time.

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