Start-up Spectrum Aeronautical of Encinitas, Calif., joined the crowded very light jet segment last month when it unveiled a nine-seat, $3.65 million all-composite VLJ at the NBAA Convention. Managing director Linden Blue (known to many as the father of the Beech Starship) bills the 7,300-pound-mtow Spectrum 33–powered by a pair of Williams International FJ33 engines–as a “full-size airplane at half the weight.”

The program has been under way for about five years, and the company has built a Spectrum 33 prototype, registered N322LA, at its Utah facility. It is expected to fly by the end of next month, with FAA certification scheduled for late 2007 or early 2008.

Six aircraft–four flight-test aircraft and two ground-test articles–will participate in FAA certification trials. Blue told AIN that veteran test pilot Pete Reynolds will direct Spectrum’s flight-test program.

Blue said the use of composites and building techniques (the stiffened monocoque structure has no honeycomb core) make the airplane light enough to be classified as a very light jet, yet the Spectrum 33’s flat-floor cabin is nearly identical to that of the 12,375-pound Cessna Citation CJ2. In fact, the competing airplanes’ cabins are both 4 feet 10 inches in width and height, but the Spectrum’s 18-foot-long cabin exceeds that of the Cessna jet by six inches.

A company spokesman said the Spectrum 33 will have a three-screen glass cockpit with triple-redundant air-data/attitude heading reference system, though the company has yet to choose an avionics vendor. The cockpit will also have side-stick controllers and a pull-out keyboard and trackball, the company said.

Overcoming Composite ‘Problems’

The airplane will be built in three main sections–the one-piece fuselage, one-piece wing (tip to tip) and empennage. A proprietary automated tow-placement machine can complete the fuselage in about five hours, and no autoclave is needed to cure the structure.

Spectrum Aeronautical has partnered with Rocky Mountain Composites (RMC) of Spanish Fork, Utah, to solve what Blue described as “problems” with composite materials for airplanes. RMC has been working on aerospace composite solutions for nearly 20 years.

“Composites are much better than aluminum, which has been used in the aerospace industry since the 1930s,” Blue asserted. He said composites are the reason that Spectrum can develop a 10-seat twinjet with an mtow of 7,300 pounds. According to Blue, the Model 33’s one-piece fuselage, complete with integrated window and door frames, weighs a mere 309 pounds. The aircraft’s single-piece, “co-cured” wing weighs just 305 pounds.

Rocky Mountain made considerable progress in the late 1990s, developing an automated tow-placement machine that can wind a fuselage in as little as five hours. Moreover, Blue said, the process RMC devised is both precise and repeatable, which enables it to be used for certified aerospace applications.

Most important, Spectrum and RMC devised a way to eliminate the need for a honeycomb core. This was done with grid/sine wave “isogrid” stiffening in the fuselage. It is essentially a composite crosshatch inner skeleton, much like the internal geodesic wooden structure found on the World War II Vickers Wellington twin-engine bomber.

An interesting component on the Spectrum 33 is the composite spring main landing gear, developed in partnership with a firm that manufactures composite prosthetics. The landing-gear system is nearly identical to that found on retractable-gear Cessna piston singles, though the Spectrum’s composite spring is “much, much stronger and can bend much further.” The gear retraction method is also similar to that of the piston Cessnas.

The twinjet’s lightweight structure allows Spectrum to use two 1,568-pound-thrust FJ33-4As, which are smaller, lighter and less powerful than turbofans installed on comparably sized, current-production airplanes. Because the engines weigh only 290 pounds each, Spectrum engineers mounted them far aft near the tailcone, a configuration that Blue predicts will mitigate cabin noise.

Thanks to the smaller engines, the Spectrum 33’s fuel cost per nautical mile is projected to be 71 cents, about the same as that of the Eclipse 500. Specifications for the twinjet include a high-speed cruise of 415 knots, 2,000-nm VFR range, 3,895-pound useful load and 2,000-pound payload.