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Manned Ornithopter Flights People often associate ornithopters with Leonardo da Vinci, the Renaissance genius who sketched some ideas for flapping-wing machines. But our story really begins in the 1920s. At that time, a lot of people were trying to fly with flapping wings, and most of those people lacked the knowledge to make a credible effort. That was until Alexander Lippisch got involved. A capable engineer, Lippisch later designed the Me 163 Komet, which was the only rocket-powered fighter plane used in combat. Alexander Lippisch Lippisch was intrigued by the flapping-wing flight attempts of a Dr. Brustmann. Brustmann's machine did not fly, but as a medical doctor he had actually measured the amount of power that could be obtained from a person's arm and leg muscles. Lippisch accepted the challenge to design an aircraft that could be driven solely by the muscles of the pilot. He chose flapping wings because of their potentially greater efficiency compared with the airplane concept. It should be mentioned that an ornithopter is not necessarily powered by human muscle. It is perhaps better to use an engine, rather than try to get by on the marginal amount of power that our muscles can deliver. For Lippisch, the primary goal was to see if an aircraft could fly on muscle power, and flapping wings were just a means to that end. So muscle power was used.
Because a tow launch was used for this 1929 flight, some authors have regarded it as an "extended glide", implying that the flapping action did not provide enough power for level flight after the release of the tow cable. Lippisch has stated "there was no doubt that the flights performed after the changes on the wings were powered flights and not gliding." To avoid any confusion about future muscle-powered flights, one should record the speed and height of the aircraft after tow release. If both are maintained, even for a short distance, that would demonstrate conclusively that the aircraft really is flying. [Note 1] Lacking such a detailed analysis, one might regard Lippisch's machine as the first successful manned ornithopter, but we will never really know for certain. Perhaps Krause could have flown even farther had he been so inclined, but muscle power output decreases rapidly over time. Therefore it is quite expected that a muscle-powered ornithopter, even if successful, would only be able to cover some limited distance. The same is true for muscle-powered airplanes. Only by greatly reducing the power requirement was it possible to make flights of several miles. Lippisch continued his ornithopter research. He and his students in the 1930s constructed a whole series of engine-powered, unmanned ornithopters. Generally they used small flappers in conjunction with larger fixed wings (not totally fixed because the whole machine would move up and down in reaction to the flapping). This meant they didn't need as much gear reduction, so the machines were easier to build. The flapper designs were backed up by extensive laboratory testing and theory. The longest flight on record was over 16 minutes! At the same time, the Muscle Flight Institute under Oskar Ursinus was acquiring more information on human muscle power. They found that a person using arms and legs together could produce over 1 horsepower in a short burst. Also significantly, the athletes could produce the greatest amount of power when working their muscles at about 1.7 cycles per second. For ornithopters, the flapping rate is related to the size of the wings. For the wings to operate at 1.7 Hz, they had to be smaller than what would be required to support a manned aircraft. [Note 2] The answer was found in Lippisch's small-flapper configuration. The small flapping wings could operate at 1.7 Hz, and a separate fixed wing could provide most of the lift. Because his talents were needed in the war effort, Lippisch was not able to continue his ornithopter work. Therefore it was Adalbert Schmid who constructed the small-flapper manned ornithopter that seemed to follow logically from what had been done so far. Adalbert Schmid
However, Schmid went beyond
the limitations of muscle-powered flight by putting an engine in
this ornithopter. [Ref. 2] With
a 3 hp Sachs motorcycle engine, and presumably wheels added, the
ornithopter was able to take off unassisted from the ground. It
made a quiet 15-minute flight at about 60 kilometers per hour. Then
a 6 hp engine was installed, increasing the speed to 80 kph. After
these historic accomplishments, Schmid's work was interrupted by
the war. By 1947, however, Schmid had built a second ornithopter. This one, a modified Grunau-Baby IIa sailplane, was constructed with flapping outer wing sections. Using a 10 hp engine, this double-seater was capable of speeds estimated at 100 to 120 kilometers per hour. [Ref. 3] More Recent Flights The use of an engine with appropriate gearing would enable progressively more bird-like wing configurations. Birds get some lift from the body and tail, but the flapping wings are comparatively large. A team at the University of Toronto Institute for Aerospace Studies, led by James DeLaurier, constructed one such machine in the 1990s. This ornithopter made a 14-second sustained flight in 2006. It was assisted by a small jet engine, but the flapping wings did most of the work. [Ref. 4]
One of the problems encountered during this project was that the aircraft tended to bounce on takeoff, due to the flapping of the wings. The jet assist allowed a smoother takeoff and helped compensate for the added weight of a rebuilt undercarriage. To provide a more comfortable ride, a manned ornithopter should not bob up and down while it is flying. DeLaurier's ornithopter incorporated a rigid center wing panel with motion opposite to the outer wing sections. This was intended to reduce the body oscillation somewhat. Another solution was implemented by Vladimir Toporov. In the mid-1990s, his student research group tested a muscle-powered ornithopter with four flapping wings. Like the other muscle-powered ornithopters, this machine made only short flights, because of how muscle power output decreases rapidly over time. But the four-wing design showed how it is possible to greatly reduce the body oscillation, an important step toward a practical manned aircraft. With further
development, the ornithopter should offer excellent fuel economy,
perhaps coupled with the ability to take off and land vertically.
On the other hand, the ornithopter is inherently complex, and that
might prevent its widespread application as a means of human transport.
In either case, further research will expand our knowledge and expertise
in the field of aeronautics. If nothing else, it has been shown
that people, after all, can fly by flapping wings! Notes: 1. The ornithopter was launched using an elastic cord. The cord imparted a certain amount of energy which got the ornithopter off the ground and got it up to speed. When an aircraft is tow-launched in this way, if the initial speed is higher than the stall speed (minimum flying speed) then the aircraft can continue for some distance without losing height, even if the aircraft is underpowered. In such a case, the aircraft speed would decrease throughout the flight as kinetic energy is used to keep the aircraft aloft. So to demonstrate a successful flight, one would need to show that the speed and height are both maintained for some distance. 2. I'm simplifying this complex issue somewhat. Large wings are required only if power is to be minimized. And the flapping rate is not entirely determined by the size of the wings. References: 1.
Lippisch, Alexander. Man Powered Flight in 1929. Journal
of the Royal Aeronautical Society. July 1960.
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Lippisch
constructed an ornithopter and began tests. A young pilot and athlete,
Hans Werner Krause, was selected to fly this machine. Since a small
engine could have done the job better, he didn't feel inclined to
expend much effort. Lippisch promised Krause a vacation if he could
reach a predetermined mark at about 250-300 meters from launch.
The goal was achieved. [Ref. 1]
On
June 26th, 1942, Schmid's ornithopter shown here made a flight of
900 meters at a constant 20 meters above the ground. The pilot,
Mueller, also supplied the power to flap the wings. This ornithopter
still relied on a tow launch, and despite the greater distance covered,
the extended glide argument could be made.
