Ornithopter contributor to the creative work or subject Alphonse Pénaud
Wing design
If future crewed motorized ornithopters cease to be "exotic", imaginary, unreal aircraft and start to serve humans as junior members of the aircraft family, designers and engineers will need to solve not only wing design problems but many other problems involved in making them safe and reliable aircraft. Some of these problems, such as stability, controllability, and durability, are necessary for all aircraft. Other problems specific to ornithopters will appear; optimizing flapping-wing design is only one of them.
An effective ornithopter must have wings capable of generating both thrust, the force that propels the craft forward, and lift, the force (perpendicular to the direction of flight) that keeps the craft airborne. These forces must be strong enough to counter the effects of drag and the weight of the craft.
Leonardo's ornithopter designs were inspired by his study of birds, and conceived the use of flapping motion to generate thrust and provide the forward motion necessary for aerodynamic lift. However, using materials available at that time the craft would be too heavy and require too much energy to produce sufficient lift or thrust for flight. Alphonse Pénaud introduced the idea of a powered ornithopter in 1874. His design had limited power and was uncontrollable, causing it to be transformed into a toy for children. More recent vehicles, such as the human-powered ornithopters of Lippisch (1929) and Emiel Hartman (1959), were capable powered gliders but required a towing vehicle in order to take off and may not have been capable of generating sufficient lift for sustained flight. Hartman's ornithopter lacked the theoretical background of others based on the study of winged flight, but exemplified the idea of an ornithopter as a birdlike machine rather than a machine that directly copies birds' method of flight. The 1960s saw powered uncrewed ornithopters of various sizes capable of achieving and sustaining flight, providing valuable real-world examples of mechanical winged flight. In 1991, Harris and DeLaurier flew the first successful engine-powered remotely piloted ornithopter in Toronto, Canada. In 1999, a piloted ornithopter based on this design flew, capable of taking off from level pavement and executing sustained flight.An ornithopter's flapping wings and their motion through the air are designed to maximize the amount of lift generated within limits of weight, material strength and mechanical complexity. A flexible wing material can increase efficiency while keeping the driving mechanism simple. In wing designs with the spar sufficiently forward of the airfoil that the aerodynamic center is aft of the elastic axis of the wing, aeroelastic deformation causes the wing to move in a manner close to its ideal efficiency (in which pitching angles lag plunging displacements by approximately 90 degrees.) Flapping wings increase drag and are not as efficient as propeller-powered aircraft. Some designs achieve increased efficiency by applying more power on the down stroke than on the upstroke, as do most birds.In order to achieve the desired flexibility and minimum weight, engineers and researchers have experimented with wings that require carbon fiber, plywood, fabric, and ribs, with a stiff, strong trailing edge. Any mass located aft of the empennage reduces the wing's performance, so lightweight materials and empty space are used where possible. To minimize drag and maintain the desired shape, choice of a material for the wing surface is also important. In DeLaurier's experiments, a smooth aerodynamic surface with a double-surface airfoil is more efficient at producing lift than a single-surface airfoil.
Other ornithopters do not necessarily act like birds or bats in flight. Typically birds and bats have thin and cambered wings to produce lift and thrust. Ornithopters with thinner wings have a limited angle of attack but provide optimum minimum-drag performance for a single lift coefficient.Although hummingbirds fly with fully extended wings, such flight is not feasible for an ornithopter. If an ornithopter wing were to fully extend and twist and flap in small movements it would cause a stall, and if it were to twist and flap in very large motions, it would act like a windmill causing an inefficient flying situation.A team of engineers and researchers called "Fullwing" has created an ornithopter that has an average lift of over 8 pounds, an average thrust of 0.88 pounds, and a propulsive efficiency of 54%. The wings were tested in a low-speed wind tunnel measuring the aerodynamic performance, showing that the higher the frequency of the wing beat, the higher the average thrust of the ornithopter.
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