How it all began:

Just like many other ideas, the GEN H-4 began as a scribble on just another sheet of paper. Gennai Yanagisawa, better known as ĎGení (a Japanese engine designer) was thinking of ways to use one of his engines. Several years ago while employed at Zenoa, Gen had engineered several two-cylce engines up to 250cc. Using two cylinders that he had designed while working for Zenoa, he designed a 125cc boxer engine. After 5 years of development on this new engine he had one of the best power to weight ratios available in the world. The engine was light, powerful and reliable, but its price put it beyond reality.

Gen's first thought was to put this miraculous little engine to use on an air craft. He figured more power and less weight on an any air craft would be to their advantage. The problem was what kind of air craft has only a 10 horse power engine? As a 10hp engine by itself can only propel the lightest aircraft. Gen, being the CEO of his own engineering firm, designed an engine unit with an extension shaft for use in hang gliders. It was tested by a well known hang glider pilot in Japan with outstanding results. But it was too early, hang gliding itself was not popular enough to generate interest in the engine unit at the time.

fig 1

By 1985, the engine had been tried in motor- paraglider units, model airplanes and helicopters. To look into further possibilities, an in-house project of a general purpose radio control helicopter was started (fig1). Engineering started as a floating platform for small scale crop dusting and photography. It was a simple co-axial unit, using only one engine, with no consideration for pitch, roll, or yaw control.

The platform was stable and powerful enough for its intended use. As the next development step; full directional control was needed in order to make the platform usable. Roll and pitch was controlled by tilting the rotors themselves and yaw was controlled using vanes. Looking back the answer is evident, but in 1987, after several flight attempts and crashes, and lots of head scratching, the lack of feedback from the unit became a limiting factor in the development. The engineering teamís spirits were down, unable to find a remedy. "The simplest answer is the best answer", Gen decides to make a man-carrying unit. 


Increasing the payload to 65kg by using two engines, the first prototype was completed in 1992 but did not produce enough lift to produce the desired result. After several testing sessions, the Boyís Dream Helicopter or BDH-1 (fig2) was exhibited at the 1994 AERO-SPACE show in Japan. The concept of a compact co-axial counter-rotating one man helicopter was highly praised and attracted worldwide attention.

To overcome the shortcomings of the first prototype, a three engined version (30hp) was developed  in 1996. The test pilot, Katsumi Terada, the first Japanese post-war private helicopter pilot, claimed that the BDH-2 (fig3) was easier to fly than any production helicopter he had ever flown. After extensive testing, designing, and re-designing, with new rotors, improved engine performance and a newly designed frame the BDH-3 was ready.


Although the BDH-3 flew well on 3 engines, the unit was not able to hover on 2. To compensate for loss of one engine the BDH-4 evolved after a total re-design. The 4 engines were located radially around the gear case making each engine easy to maintain and service. By placing engines above the gimbal assembly, the long, heavy driveshaft and universal joint was no longer needed, simplifying the design and making the total weight lighter.


This BDH-4 was exhibited at Air Venture 1997 drawing attention to the future possibilities of such helicopters. This unit used small electric motors turning small propellers to control the yaw, which was marginal in its performance. In 1998 a new system for yaw control utilizing an electric motor to differentiate the rotor rpm was completed. Now the yaw could be controlled easily by the pilotís left thumb.


The BDH-4 was re-named GEN-H-4 and a simple demonstration was done before spectators at Air-Venture 1998. In 1999 Engineering Systems CO hired Jon Plummer to be their "English Speaking" pilot and company representative in the United States. Jon proved to thousands of onlookers at Air-Venture 1999 that the GEN H-4 would actually fly and hovered it for approximately 6 minutes about 10 feet off the ground. Since then Jon has taken the GEN H-4 to several air shows around the United States and demonstrated it over 25 times.

Development is continuing in Matsumoto, Japan. Recently Engineering System CO. has developed a remote control version of the air craft (fig4) that is used for flight performance testing. Also in development is a more powerful version of the GEN H-4 with 15hp engines that will carry a larger pilot built more specifically for the American market.




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