Honda Flies the Real Jet
By J. Mac McClellan
December 22, 2010 — You may think you have seen the HondaJet many times at Oshkosh and other air shows over the past several years, but that exotic machine was just a warm-up. The first HondaJet that actually conforms to a certifiable and producible airplane made its first flight on December 20 at Piedmont Triad International Airport in Greensboro, North Carolina.
What Honda has been flying was a proof-of-concept jet that lacked the structure and systems required for certification. But that first article is very close in size and shape to the conforming airplane so that flying qualities and performance data collected from its hundreds of flights are useful in confirming that the real HondaJet will do what the company predicts.
Testing the configuration of the HondaJet in actual flight is unusually important because, well, Honda has never built an airplane of any type before. And the HondaJet has some novel design features that needed to be evaluated before the company went all in for full-scale development.
The father of the HondaJet is Michimasa Fujino, who is a well-regarded academic expert on aerodynamics. His theory - that the engine pylons located on top of the wings interact with high-velocity airflow over the wings’ upper surfaces to actually reduce drag - has tested well in wind tunnels. Computational fluid dynamics (CFD) software also showed that Fujino was on to something. But only a full-size flying jet could demonstrate for sure what lab tests showed, so that’s why Honda built the first test jet.
Where to place jet engines on an airplane is no slam-dunk decision. Structural loads must be considered, but most important is the air flow around the engine nacelles and pylons and how that flow reacts with other parts of the airplane. When the engines are located under the wing, their wake disturbs air flow over the wing, adds drag, and alters lift. If the engines are mounted on the fuselage there are “channels” between the engine and the fuselage, and between the nacelle and upper surface of the wing, through which air must accelerate to pass, thus causing drag.
For most designers the trend has been to move the engines farther from the wing or fuselage. For example, look at a Gulfstream 550 and see how the engines have moved way up high on the aft fuselage compared to earlier models. This location gets the engines farther from the wing and next to a narrower part of the fuselage. Or check out an Airbus 340 or 380 and note how far ahead and below the wing leading edge the engines are mounted on long pylons.
Fujino took a different route by locating the engines on the HondaJet well above the wing on tall pylons, and farther outboard than would be possible with conventional fuselage mounted engines. The HondaJet avoids much of the drag caused by forcing air between the engines and fuselage, or disturbing the air ahead of the wing. The remaining question is how much drag the engine pylons add, and Fujino is confident that it is less than any other engine location.
The HondaJet has a natural laminar flow wing (NLF), which is not new technology. The Citation CJ has a NLF wing, which helped that light jet deliver about 30 knots more cruise speed than the comparably sized and powered original Citation 500. But the HondaJet also has an NLF forward fuselage. Honda believes the sort of saggy belly shape aft of the nose and the very gradually sloped canopy shape will keep air flow laminar much of the way aft, something that really hasn’t been achieved on other jets.
First flights are important milestones, but Honda still has a long way to go to meet its goal of delivering the first HondaJet in 2012. The fuselage is made from composites, but that has been done by Beech with the Premier and Hawker 4000, so that should not be a certification question. The rest of the HondaJet structure and systems are more or less conventional, so there should be no showstoppers there. Now it’s all about the details. The HondaJet was born in a laboratory, then tested full size, and is now out to answer all of the questions the certification system asks. Congratulations to Fujino-san and Honda on making such bold design decisions and I wish the best of luck in the long slog of certification.