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HondaJet Fights Engine Drag

January 7, 2011

It is revealing that at its heart Honda is an engine company. Honda makes all types of engines and is now in the jet engine business partnered with GE. And when it came time to design its first jet, Honda has taken a unique path on how to locate the engines and minimize the drag their presence creates.

Where to locate the engines on a jet is always a challenge because the presence of the nacelles, and their proximity to the fuselage or wing, creates an enormous amount of drag at cruise speeds, particularly above 350 knots. Air in the slipstream must accelerate to pass around the nacelles and that creates drag. Worse yet, the slipstream is forced through a small space between the nacelle and fuselage. Air really resists flowing through the “channel” between the engine and fuselage and the pileup of air refusing the flow through spreads out over a large area generating loads of drag over most of the aft fuselage.

If engines are mounted low on the aft fuselage like they were on some earlier designed jets such as the Sabreliner or Citation 500, there is also drag interference between air flowing over the top of the wing and the lower part of the nacelle.

To help diminish drag around the engines many designers apply the “area rule” that was established during jet flight research in the 1950s and 60s. According to the area rule drag does not increase dramatically if the total cross sectional area of a body stays about the same when the shape changes. Engines sticking out add to the cross section area of the fuselage so the area rules says the cross section of the fuselage must be reduced at and aft of the engines.

You can see this area rule on most business jets with the “wasp waist” concave shape of the tailcone. Learjets and Falcons have pretty dramatic area rule shapes, and the Beech Premier has really big concave hallows inboard of the engines. The area rule works, but only to a point.

Michimasa Fujino, who is president and CEO of Honda Aircraft, invested more than 20 years in the laboratory and wind tunnel to find a better solution to the drag created by engine location. His idea is to move the engines pretty far out on the wing and mount them on tall pylons above the upper wing surface. His design for the HondaJet leaves plenty of room between the engines and fuselage so the slipstream can pass through smoothly without a big drag increase. And the engine pylons are tall enough so that air flowing over the wing is not greatly impeded by the lower part of the nacelle.

Because Fujino did not need to deal with the “channel” between the nacelles and fuselage the HondaJet cabin can carry its full width far aft with no worries about the area rule. This extra space in the aft cabin provides room for the largest lavatory in the class, and that is a very big selling point with business jet buyers.

Fujino’s testing also determined that the engine pylon reacts favorably with a pressure disturbance—a small shock wave—that forms on top of the wing at normal cruise speed. The pylon acts like one of those shock cones that were used on some high-speed early jet designs such as the Convair 990 to reduce drag.

The HondaJet also uses a natural laminar flow (NLF) wing shape to reduce drag. Laminar flow always creates less drag than the turbulent flow of a normal wing shape and NLF technology is pretty well proven. Cessna uses an NLF wing on the Citation CJ. The original CJ is about the same size and weight as the old Citation 500, and has about the same amount of thrust, but the CJ cruises around 30 knots faster with most of the drag reduction coming from the NLF wing.

But the HondaJet is the first business jet to attempt to achieve a long run of laminar flow over the forward fuselage. The fuselage is built from composites so its shape can be controlled precisely and the surface is very smooth. That sort of sagging underbelly on the forward fuselage, and the shape of the canopy, are keys to achieving a high degree of smooth laminar flow over the HondaJet fuselage.

Though the individual design features of the HondaJet are not radical in themselves, they have not been applied to one airplane before. That’s why Honda built the demonstration airplane it has been test flying for the past several years. That test airplane which most of us have seen at major airshows around the world for years comes very close to the final external size and shape of the HondaJet, but its structure and systems do not conform to a final airplane. Honda says the test airplane has confirmed what the wind tunnel and computer analysis predicted and the HondaJet will meet its speed and range goals.

The first flight of the actual HondaJet prototype was a big deal in December because it was the necessary and major step to move from a test program into the path toward certification and production. As with any development program, Honda has lots of work ahead, and at least minor setbacks are probably unavoidable. But the first flight of the conforming prototype moves the project from the lab into the real world of airplane development and production. Congratulations to Fujino-san for his innovation and his perseverance to make it a reality.

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