In most conventional aircraft, the functions of power and lift are
separated. Lift is provided by the rigidly outstretched wings and changes in geometry are
limited to the movement of surfaces such as ailerons and flaps. Propellers or jet engines
provide the power to move the aircraft forward through the air so that the wings can
generate lift. In flapping wing flight, however, the wings provide both lift and
propulsion.Clearly, flapping flight depends on the up and down
movements of the wings - an upstroke and a downstroke. But as anyone who has tried to make
a model ornithopter will know, it is not quite as simple as that! The relative wind comes
from below the wing during the downstroke, and from above the wing during the upstroke.
The wing must therefore constantly twist around its long axis so it has the appropriate
angle of incidence at each point in the flapping cycle.
Lift and propulsion are both produced during the downstroke. The wing is
powered downwards and forwards, with its leading edge tilted down. The wing generates lift
at right angles to the relative wind and thus there is a forward-directed thrust imparted
to the animal during the downstroke (see diagram alongside). The inner part of the wing
has less up and down motion so it behaves more like a fixed wing - the tip is able to do
more work.
Lift is produced in the upstroke as well. Despite the change in the
relative wind angle, the wing can still produce lift if the wing flips to a nose-up
position. However, this lift is accompanied by significant drag, and to diminish this
birds and bats partially fold their wings during the upstroke.
Reference
Pennycuick, C. (1972) Animal Flight. The Institute of Biology's
Studies in Biology no. 33. London: Edward Arnold. ( A concise and interesting
review of how the anatomy, physiology, and performance of flying animals is related to the
principles of flight.)
Good website
For an excellent review of progress with ornithopters, visit:
www.catskill.net/evolution/flight/home.html
