A hybrid for private pilots

Excel-Jet developer Robert Bornhofen has taken a different approach with his VLJ entry, the Sport-Jet. "Our customer is the pilot and the plane is designed for his or her comfort. We're not interested in air taxi," he explains. "Our focus is the general aviation pilot market." Working without outside investors and believing firmly that obtaining insurance will be the highest hurdle faced by VLJ buyers, Bornhofen's team has designed a hybrid aircraft that combines an aluminum tail and wings with a composite fuselage, a design he feels will maximize FAA's comfort level and thus expedite certification and insurability of the plane.

The four-place jet has only one engine, a Williams FJ-33, and is priced at $1.1 million. With a lower operational ceiling of 25,000 ft, Sport-Jet's maximum takeoff weight is a mere 4,900 lb (1,770 kg). But specs are otherwise fairly comparable to other VLJs — cruise speed is 350 knots/400 mph, range is about 1,000 miles (1,600 km) and only 2,300 ft (701m) of runway is required for takeoff. The single engine and lower ceiling were intentional choices, explains Bornhofen: "Certification and opera-tional rules get more complex above 25,000 ft and with multiple engines. The insurance issues really drive this industry."

The Sport-Jet, with a distinctive low-to-the-ground stance, was developed based on Bornhofen's prior experience designing a twin-engine aircraft with "superb" flight handling characteristics. Bornhofen says his market research indicated customers wanted a wider cabin, so the Sport-Jet's cabin width is the same as a Lear 35 jet and 14 inches (35.6 cm) wider than a Raytheon Beech Bonanza, a common twin-engine turboprop. The initial master model was produced traditionally by hand-shaping plywood frames and placing them in series to create the fuselage shape, from which carbon-fiber prepreg tools were fabricated. The prototype was made by Lancair (Redmond, Ore.).

The fuselage is an interesting combination of solid and cored laminate. Five solid laminate "rings," approximately 3 inches (75 mm) wide and 0.38 inch (9.5 mm) thick, are the principal load-bearing structures. The rings are joined together with three solid-laminate longerons of similar width and thickness, oriented parallel to the fuselage's long axis. The combination of the rings and longerons forms a sort of structural roll cage. Bornhofen explains that the ring and longeron plies are interwoven during layup to create a single, integrated, rigid part. Torayca AGATE prepregs were used on the prototype, but FiberCote's (Waterbury, Conn.) AGATE materials will be qualified as a backup for production, says Bornhofen.

The structural cage becomes a fuselage with the addition of the skin, typically a balanced layup of 3/core/3, produced in four open female molds with a combination of carbon and fiberglass. Skin parts are bonded to the cured fuselage cage. The elongated composite nose cone is produced separately and adhesively bonded with epoxy to the main fuselage. An aluminum tail cone surrounds the jet engine.

An aluminum spar/wing box structure, to which the removeable wings are bolted, carries the wing loads and is mated to the fuselage structure in a proprietary manner that avoids galvanic reaction. Bornhofen claims there is no weight penalty caused by use of aluminum for the wings and high T-tail, because aluminum delivers a stiffer part for the design weight. He prefers metal for those parts, citing better performance during lightning strike. In the case of damage, an off-the-shelf aluminum replacement wing or tail installs easily, a scenario with appeal to insurers. Control flaps and ailerons, which in the prototype are composite with a core of Rohacell foam (Degussa Rohm GmbH & Co. KG, Darmstadt, Germany), will likely remain composite in production.

While no advance sales have been made, the prototype is nearly ready for flight tests. FAA certification is at least 18 months down the road, says Bornhofen, and no firm decision on a fabricator has been made. "Piston aircraft are getting more expensive to buy and maintain every year while small jet engines are becoming more efficient," he concludes. "The cost/benefit is starting to look pretty good for small jets."