Unusual one-off a production "maybe"

Honda R&D Americas Inc. (Raymond, Ohio) took the somewhat radical step in the mid-1990s of building a twin-engine jet aircraft around a new Honda turbofan jet engine, the Honda HF118. The engine, developed by Honda Aero Inc. (Reston, Va.), a subsidiary of Honda Motor Co. (Tokyo, Japan), is so promising that GE Aircraft Engines has formed a partnership with Honda to commercialize it. Yet, speculation now turns on whether Honda will pursue development of the aircraft itself, which was first flown in December 2003.

The HondaJet, with composite fuselage and metal wings and tail, caused significant buzz when it made its first public appearance at this year's Experiment Aircraft Assn. (EAA) AirVenture 2005 air show in Oshkosh, Wis. Its unusual over-the-wing engine-mount configuration sets it apart from other VLJs. HondaJet's designer, vice president of Honda R&D Americas Michimasa Fujino, spent considerable time analyzing the pros and cons of an aluminum vs. a composite wing: "From a structural design viewpoint, the bearing stress and interlaminar shear of composite materials are lower than aluminum. In our case, we had several fittings and brackets attached to the wing structure [for engine mounts] that required web or rib thickness to be increased to reduce the stress. In the end, using composites for VLJ wing structure is not a big advantage."

Lightning strike protection also was simpler with aluminum. A metallic mesh embedded in a composite causes slight surface roughness, which requires fairing and finishing work, Fujino contends, so machined aluminum was ultimately chosen.

Honda built the prototype in-house during the mid-1990s. Both metal (Nobinite, similar to Invar) and composite tools, some fabricated in-house, others made by subcontractors, were used for layup. The main fuselage was molded in left and right halves from solid laminates built up with 177°C/350°F cure carbon/epoxy prepreg from Cytec Engineered Materials (Tempe, Ariz.), made with Toho Tenax Co. Ltd. (Tokyo, Japan) G30-500 intermediate-modulus fiber and 5276-1 toughened epoxy. The fuselage skin was stiffened with stringers and frames, which were made separately on a flat table, then cut to shape with an automated cutting machine. After heating to 90°C/194°F, the cut shapes were press formed in a mold, but not allowed to fully cure. The shaped frames and stringers were then demolded and placed on the skin layups, and cocured with the skins in an autoclave, to form an integrated structure. Each frame and stringer was designed with the same dimension and shape, says Fujino, so that only two molds were required. Skin layup was balanced but varied throughout the fuselage to optimize resistance to imposed loads (exact fiber architecture is proprietary). The two halves were adhesively bonded, but metal fasteners were used to ensure a secure bond.

The cockpit and the tapered tail section were cored with aramid honeycomb, using the same carbon/ epoxy prepreg for the skins, to achieve compound curves. To avoid core crush during autoclave cure (at 85 psi/5.86 bar), the core was beveled along the part edges and a "picture-frame stabilizing method" was employed — adding a tie-down ply between the skin and the core to prevent core shift. "Our nose design has 10 percent less drag compared to turbulent-flow nose fuselage designs," notes Fujino.

A wing-to-body fairing also is carbon composite. Up to five passengers can be carried at a cruise speed of 400 knots/460 mph, with room left over for a lavatory. Ceiling is 41,000 ft, and range is 1,100 nm/1,265 miles (2,100 km). The over-the-wing engines maximize cabin space, says Fujino, since the contour doesn't have to pinch down to accommodate engine mounts.

Fujino says that "at this moment" there is no formal business plan for production. Nevertheless, this VLJ has good performance figures and interest is high.

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