How it all began:
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.
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
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.
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
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.
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.
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