Studying the Art of Aircraft Building in the U.S.A.
On a darkened test course in the dead of night, one of Honda’s aircraft development team watches intently as the swirling smoke of a cigarette rises slowly up into the air and gradually tapers into a flat, straight stream.
“Okay! Let’s go!”
With a one-sixth scale model airplane attached to its roof, a car slowly began moving down the test course. Hoping to minimize the effects of crosswinds, the team had been waiting for the wind to stop, and the column of smoke was their clue to knowing exactly when it did. The car continued on, careful to avoid any additional acceleration that could affect to the “flight characteristics” of the model swaying in the moving air above it.
Honda began studying aircraft design in 1986, and this was one of the earliest projects to be undertaken by the Honda R&D Fundamental Technology Research Center (initially the Wako R&D Center, hereafter HGF). The research team had completed its theoretical calculations in order to establish some general forms and specifications for configuring an aircraft, and now was the time to expose those desktop calculations to some actual aerodynamic tests. Having no wind tunnel available, the team came up with the innovative (though definitely primitive) idea of mounting their aircraft fuselage model on the roof of a car and running it around the Tochigi Proving Ground test course. For reasons of secrecy, they decided to carry out this particular testing after dark.
Naturally, the movement of the car itself introduced additional G forces into the data, and these fluctuations had to be carefully measured and filtered out later in order to extract usable information about the real loads and aerodynamic forces affecting the aircraft model itself. Unfortunately, the resulting data was so imprecise as to be barely usable. Still, making do with such primitive techniques turned out to be good basic experience, especially for a team with no previous experience building aircraft. Being forced to think up formulas that could compensate for these extraneous forces helped deepen their understanding of how aircraft actually move, and gave them more insight than they might have gotten by simply measuring results in a proper wind tunnel.
Soon after HGF was founded, several members of the aircraft development team traveled to the United States to study aircraft construction at the Raspet Flight Research Laboratory connected to Mississippi State University. Their goal was to spend a period of just over two years educating themselves about aircraft design, fabrication, and flight testing. Raspet was chosen because it approached aircraft-building from this three reality principle.
The first study exercise this U.S. team undertook was to rebuild the wings and tail section of an existing single-engine turboprop aircraft by replacing the aluminum with composite materials. This presented an excellent opportunity to not only build these components themselves, but also to fly the resulting aircraft. As they set about converting the main wings, they also designed new landing gear, flaps, and integral fuel tanks. By doing everything themselves, they learned the basics of aircraft design and fabrication. Having completed this project successfully, the team then moved on to the next step - designing and building an entire aircraft.
Honda designed and built the tail and main wings of an existing propeller-driven aircraft, and took on the challenge of flight testing.
Honda’s First Attempt at an Original Aircraft
By 1988, the team had begun work on its first original aircraft. Their idea was to create a high-winged aircraft with shorter landing gear that would make it passenger car easy to board and alight from. Its distinctive styling would feature both forward-swept main wings and forward-mounted winglets (aka canards), and the wings’ extra-large triple-slotted flaps would enable shorter takeoff and landing distances. Finally, the fuselage would feature all-composite construction, and it would be powered by advanced, fuel-efficient turboprop engines,*1 that were then being developed by the Honda Aero Engine Division, and would be rear-mounted in a push-from-behind propulsion configuration.
However, when it came to the practical design phase, the team soon realized their initial vision would have to change in several respects. For starters, the advanced turboprop engines they were hoping to mount would likely be too difficult to build at the moment, and in any case falling fuel prices had - at the time - reduced the need for them. They quickly switched to using existing turbofan engines, but this created a new problem - all the engines then currently available were too large to fit on the tail section as originally planned, and the only place left for them to go would be directly over the high wings, at the points where they attached to the fuselage.
Although the forward-swept wings were kept in the design, the canards had to be dropped. This revised version was renamed the MH02, and the team quickly began building an experimental prototype. Finally, in March of 1993, the prototype made its first successful test flight. Unfortunately, the inadequacies of its configuration, notably the engine location, resulted in excessive high-speed aerodynamic drag and other issues that eventually forced the team to accept that this aircraft wasn’t ready, and they abandoned any plans of bringing it to market. Still, it was the world’s first all-composite jet aircraft, and this in itself was a remarkable accomplishment, even for an experimental model.
Although the MH02 had too many shortcomings to ever be a viable product, the process of developing it gave Honda a wealth of invaluable aircraft design experience. Creating and building an entire airplane from scratch - from the design of its basic configuration, structure and systems, to the actual fabrication of its fuselage, and testing it both on the ground and in the air - had presented tremendous challenges, and by the time the MH02’s final round of testing wrapped up in August of 1996 this process had helped Honda establish its own distinct aircraft design methodology.
With the MH02 project officially ended - with no product to show for it - some in Honda strongly suggested that the company was done in the field of aircraft research, and doubted the wisdom of continuing with any further aircraft projects. After considerable discussion, Honda decided to carry on, but only on the condition that any subsequent aircraft research focus mainly on elemental technologies.
- Advanced turboprop engine - An advanced engine type with a double inverted propeller that was seriously studied by the aircraft industry in the wake of the 1980s second oil crisis.
Starting Again With More Focus - The HondaJet
Honda’s aircraft development team was relieved they had not been cancelled altogether, but they also knew that they’d never be satisfied spending their time researching aerodynamics, structural materials and other basic technologies. They knew that despite the setbacks, their dream of building their own business jet was still very much alive. Taking a hard look at the market, they carefully considered their options. Then, after a great deal of thought, they gradually began piecing together a new and clearer vision of exactly what sort of aircraft they wanted the company to build.
Small business jets at the time tended to have somewhat cramped cabins and limited luggage capacity. They could also use some improvements in terms of amenities, cabin quietness, interior and exterior styling, and overall design. The team believed that creating a more affordable aircraft that offered more space, a quieter cabin, and more appealing design would greatly expand the market for small business jets among both business travelers and the general public alike. Moreover, if this small jet could also be fast, efficient, cleaner in emissions, and better in fuel economy, then it would make sense for Honda to produce it. With these parameters in mind, they began formulating a concrete vision of precisely what kind of plane could achieve these goals.
It was decided that the new plane should offer 20–30% better fuel economy over existing aircraft, and that per-passenger operating costs should be comparable to that of first-class travel on domestic flights, assuming there were three to four passengers on board. The plane should also be small and lightweight, but at the same time provide enough cabin space so that passengers wouldn’t bump toes when seated opposite each another. Maximum speed should be at least 400 knots (741 km/h) and the cruising range should allow nonstop flights between, say, New York and Miami. The plane should be equipped with amenities like a private toilet and a luggage compartment large enough to hold golf bags for all passengers. And finally, the exterior styling and interior design should make this airplane not just a mode of transport, but also attractive enough to stimulate, and satisfy, the desire to own it. These were the targeted goals for the airplane that would eventually become the “HondaJet”.
However, in the real world, aircraft design can’t be based on ideals alone. Every design requires a specific set of calculations, and the final form is ultimately determined by a commensurate level of technological advancement. Simply setting challenging goals doesn’t make an aircraft viable, and sometimes the calculated results for overly high targeted requirements won’t work unless the technology is also up to par.
For this reason, initial concept drawings are never merely rough sketches, but instead are based on real engineering experience. For example, Newton’s second law of motion is clearly expressed as “F=ma”, meaning that net force is equal to mass times acceleration. However, when designing an aircraft, accurately determining the mass and estimating the force of air becomes critically important, and this requires a deep level of experience. The methods used to apply abstract theory to actual hardware, or determine acceptable tolerances and errors are also important. Fortunately, each member of the Honda aircraft development team came with considerable experience developing automobiles and other vehicles, and this engineering know-how helped them maintain a realistic development path.
MH02, the world’s first all-composite business jet
In Search of a Major Breakthrough
Honda’s unique Over-The-wing engine mount
The most challenging aspect of meeting the requirements of the new HondaJet was maximizing cabin space while reducing the overall size of the fuselage for improved efficiency. Bridging these two conflicting goals presented considerable technical difficulties. Most business jets of the time mounted their engines at the rear of the fuselage, and because of their size and weight, considerable structural reinforcement was needed to provide the necessary support, which inevitably took space away from what would usually be the rear of the cabin.
In some cases, engineers managed to “solve” this problem by simply lengthening the fuselage to find the space needed to mount the engines. Faced with this conundrum, many engineers simply tried to improve on existing structures wherever they could. However, the HondaJet development team knew that mere improvements would never be enough. It would require advanced technology, and a breakthrough.
The specific positioning of the engines soon became one of the most important points of focus. Mounting the engines directly on the main wings would offer the obvious advantage of freeing up the entire fuselage interior for a larger cabin space. Unfortunately, aerodynamic design theory tends to frown on placing anything on top of the main wings, mainly because it increases aerodynamic drag. In the earliest stages of development, the team wondered if some way could be found to simply cope with the problem and minimize this disadvantage.
Then someone said: “We need to completely change the way we’re looking at this.”
Perhaps some way could be found to optimally position the engines on the main wings while still managing to reduce aerodynamic drag.
A New Mindset Defies Conventional Wisdom
In baseball and golf there are times where a swing that would logically seem to require “pulling” the bat or club with the arms can be greatly improved by thinking of the movement as “pushing” with the arms instead. In a similar way, the HondaJet development team wondered if there might be some way of turning the drag disadvantage caused by mounting engines over the main wings into an advantage.
The idea they came up with was to separate the airflow around the engine from the airflow around the wing, and then combine the two to create the more desirable airflow they were looking for. This approach applied a classic pre-computer era method of creating flow components using complex functions and then combining them in a linear manner to solve the flow analytically.
Specifically, the team’s goal was to minimize the shock wave created as the speed of the aircraft increases and the airflow over the wings reaches the speed of sound, by combining the deceleration area in front of the engines. Since these shockwaves tend to create significant wave resistance, suppressing them would have the advantage of significantly reducing drag.
This groundbreaking change in thinking shattered the conventional wisdom of aerodynamic design theory, and the team ran numerous simulations to test their idea, with promising results. They also found that any flutter problems caused by placing heavy objects on the wings could be mitigated by locating the engines’ center of gravity nearer to the nodes of vibration.
Ridicule Changes to Respect in the Wind Tunnel
While this new idea initially seemed sound, it still needed to be verified in simulation. The team build a scale model of the aircraft, with the engines positioned on top of the wings, and took it to Boeing’s transonic wind tunnel for testing. Transonic speed refers to the range in which the airflow over the surface of the aircraft is a mixture of different speeds, some reaching the speed of sound and others much lower subsonic speeds. Honda didn’t have a wind tunnel capable of reproducing these conditions, so they went to Boeing to conduct their tests.
Initially, Boeing’s wind tunnel engineers were singularly unimpressed by this odd aircraft configuration, with its engines positioned over the main wings: “Don’t these guys know anything about building aircraft....?!” However, after a week of testing, with promising results, followed by another week of testing, with even more positive results, attitudes began to change: “These Honda guys are really smart”
The Honda test plane was then moved to a special high Reynolds number (high turbulence) transonic wind tunnel located at NASA (National Aeronautics and Space Administration) and it also performed well there. The effectiveness of this new configuration - with its engines mounted onto the upper surfaces of the wings - was inspired by classic aerodynamic theory, conceptualized using the latest in numerical fluid dynamics simulations, and then proven in transonic wind tunnel testing.
Defying the Textbooks
It turned out that mounting the engines to the tops of the wings actually improved performance compared to the usual position at the rear of the fuselage. Those who knew best about aircraft design found this hard to believe. Even many knowledgeable folk within Honda found it difficult to understand, and it sounded even more confusing when it was explained in technical terms. Some worried that presenting such an aircraft to the world could provoke criticism that Honda was “just a car company trying to make an airplane without understanding aeronautical technology.” Right or wrong, they were concerned that such negative publicity could seriously damage Honda’s long-established brand image as an “advanced technology company.”
In part to quell such fears, Honda’s aircraft development team decided to compile their new concepts into a technical paper to be presented to the American Institute of Aeronautics and Astronautics (AIAA). They figured that objective evaluation by outside experts might at least protect the company from being slandered by those who didn’t understand what they had created. They had no idea if this paper would even be accepted by the AIAA, but it was the only thing they could think to do.
Normally, it takes about a year for technical papers to be reviewed, but in 2002 the HondaJet paper was accepted by the AIAA in only a matter of weeks. To the company’s great relief, the response was overwhelmingly positive, and the concepts described in the paper were applauded as “some of the most significant new discoveries in aircraft design.” Having earned this recognition, the HondaJet’s unique new shape and the technology used to position the engines above the wings became widely known throughout the U.S. aerospace industry. Any further naysaying soon faded away and support for the new design began to take hold.
So it was that the HondaJet, with its innovative engine layout, was able to reconcile the seemingly contradictory goals of both a larger cabin and a smaller fuselage. Other new features included a lighter weight composite fuselage, a unique natural laminar flow wing configuration developed to minimize aerodynamic drag, an optimized nose shape that achieved more natural laminar flow, and even the winglets added to the wing tips. In this way, Honda had succeeded in incorporating multiple new technologies into an innovative, high-performance design that flew in the face of traditional aeronautical engineering textbook warnings about the folly of introducing more than two innovations at once.
Engine placement on top of the wing maximizes fuselage space.
Natural laminar flow is applied to the wing and fuselage nose shape to minimize aerodynamic drag.
A Successful First Flight
Having completed the HondaJet’s basic design, the development team then moved on to designing aircraft’s more intricate structures and systems. Once the airframe assembly, strength testing, and systems testing were all completed, in 2003 they installed a pair of Honda-made HF118 turbofan engines and began a series of ground tests using a POC (proof-of-concept) version of the aircraft. Then, after completing some taxi testing at both lower and higher speeds, the HondaJet took off for the first time and successfully completed its maiden test flight. As it happened, the date was December 3, 2003, just a few short weeks before the 100th anniversary of the Wright Brothers’ historic first flight at Kitty Hawk.
Successful first flight by POC aircraft equipped with the HF118
