By J. Mac McClellan, EAA Director of Publications
The HondaJet is on display at AirVenture Oshkosh again this year, and its unique configuration draws lots and lots of attention. When most people see the light business jet they ask themselves, "Why did Honda do that?"
Building an airplane was a long-held dream of company founder Soichiro Honda, and Honda found a bright young aerodynamicist, Michimasas Fujino, to bring the aviation dream to life.
So it was Fujino who asked himself the questions. And he has spent at least two decades researching jet aerodynamics in wind tunnels and using computational fluid dynamics (CFD) computer programs to prove his ideas work.
Fujino had the relatively rare opportunity of creating a new jet from scratch. Most airplanes are evolutions of an original successful design, and choices made by the people who created the original live on for decades as the airplane morphs into new versions.
Honda set its sights on the light end of the business jet market for its first airplane. Honda is really an engine company at its core, so it designed a brand new small turbofan, the HF120, rated in the 2,000-pound thrust class to power its first airplane. Honda has teamed with GE to certify and produce the engine.
The design goals of any business jet are pretty obvious: Everyone wants the most comfortable cabin with the most space that will fly fastest and farthest for the least cost. Of course, those objectives are actually in opposition. A big cabin adds drag so you slow down and need more fuel and thus more weight and can't go as far and so on and so on. Airplane design is a series of trade-offs.
But Fujino's testing showed that he could create a larger cabin than others in the light jet field without sacrificing speed and efficiency if he used some radically different technology. The most obvious is locating the engines atop pylons mounted on the wings. Another way Honda controls drag over a large cabin is with a shape that maintains laminar flow over much of the length of the fuselage.
Other business jets mount the engines on the aft fuselage. It's a compromise like all other choices but has some advantages. The aft fuselage structure must be strong to carry loads from the vertical and horizontal tail, so the structural strength is located there to accommodate the engines. But the engines cause drag by interfering with airflow over the wing root and around the fuselage. On more recently designed jets you can see how designers have moved the engine further up on the tail cone to get more distance away from the wing root and thus interfere less.
Fujino found that he could move the engines off the fuselage and mount them atop pylons located on the wing upper surface exactly at a point where a pressure wave—early formation of a shock wave—would interact with the pylon. By carefully shaping the pylon his wind tunnel and CFD testing demonstrated the pylon could actually reduce drag by managing the formation and growth of the pressure wave.
Moving the engines to the wings allowed Fujino to carry maximum cabin cross section far aft, providing more room than a traditional design. Enough room to have a comfortable lavatory in the rear, something very much in demand in light jets.
The reason the HondaJet aft cabin is so big is that there is no need to "area rule" the fuselage. To help reduce the drag caused by the interference of the engine, engine pylon, fuselage and wing, the fuselage on other jets has to be narrowed near the engines. Often the "area rule" shape is a pronounced concave section ahead of the engines, and that takes space away from the cabin. With the engines on the wing, the interference drag is gone so the HondaJet has no need for the fuselage narrowing "area rule" shape.
The unusual shape of the forward fuselage on the HondaJet is another drag-controlling technology created by Fujino. The drooping nose of the airplane is a natural laminar flow (NLF) shape, and so is the slope of the windshield. The forward section of any jet—particularly the windshield area that engineers call the canopy—is a potentially high drag area. Air must accelerate very rapidly to pass over the nose and flow back over the canopy and under body fairing. When air accelerates over a short distance it becomes turbulent instead of maintaining a smooth laminar flow.
The unusual droop of the HondaJet nose, and its rounded shape, plus the slope and shape of the canopy help to naturally maintain laminar flow. With this NLF shape Fujino found that his larger fuselage could have the same, or perhaps even less, drag than a smaller fuselage and cabin of conventional design.
For NLF to work the shape of the manufactured airframe must be maintained within extremely close tolerances. To maintain the design's precise shape Honda uses a carbon fiber honeycomb construction technology for the fuselage. The carbon fiber materials may save some weight, but the primary objective for using composites is to absolutely control shape to maintain NLF.
The HondaJet also has an NLF wing airfoil. An NLF wing can be fairly thick, allowing for a lightweight structure and room for fuel, but its shape, and the way the shape changes across the chord, is crucial.
When you're here at AirVenture, stop by the big Honda tent and take a close look at the jet. Walk around and notice the subtle shapes of the fuselage and its fairings. Also examine the shape of the engine pylons. They're not symmetrical, but are really airfoils that Fujino designed to minimize drag as the high-speed air flowing over the wing goes around the pylons.
HondaJets that conform to the final shape and structure have been flying, and actual in-flight testing shows the airplane can meet its goals of top cruise speed of 420 knots and an IFR range with four occupants of 1,180 nm.
The HondaJet is not yet certified and in production, and there are, as with any airplane development program, challenges ahead. But when you go by and look at the airplane, and inevitably ask yourself "why did they do that," you can see that every design choice has a very good reason behind it, and represents the vision of a young aerodynamicist who is not afraid to break with convention. I'd call that innovation.