Concept Boat competition
|Most insects have four wings. The exceptions are the
flies, which have two wings. In addition to performing the movements required
for flight, the wings carry touch and strain receptors, so they can be considered as
sensory structures too. The wings are driven by proportionally large power muscles, but a
set of 13 smaller control muscles act on each wing joint to change the position and angle
of the wing during the beat cycle.
|| In the two-winged flies, the hind wings have become converted into
halteres, small club-shaped organs that oscillate up and down as gyroscopic sense organs
during flight (Hengstenberg, 1998).
| Unlike the relatively steady-state
aerodynamics experienced by conventional aircraft wings, flapping wings
such as insect wings experience rapid changes in airflow as the wing beats
up and down - unsteady airflow.
Insects use a variety of special
aerodynamic effects to facilitate flapping flight (Brookes, 1997). The techniques differ
according to the size and organisation of the different insects, but include bringing
their wings together at the top of the upstroke ('clap') and then peeling them apart,
starting at the leading edges, to generate a circulation of air into the enlarging gap
('fling') and generating significant lift (Weis-Fogh, 1975). This process is made possible
by having stiff leading edges to the wings and more flexible surfaces behind them.
Pleating of the thin membranous wing can influence the patterns of deformation that occur
under aerodynamic loading.
As the insect wing continues on its downstroke, a leading edge
vortex develops along the top of the wing, significantly contributing to lift.
In some species, the leading edge vortex spirals out towards the tip of
the wing, while in others there appears to be no spanwise movement of the
At the bottom of the downstroke in some insects the vortex is
before the upstroke begins. Another boost in lift can be experienced at the
beginning of the upstroke as the wing passes through the wake of the
downstroke (Dickinson, 2001).
Transient bursts of lift can also be generated by the wings as
they change their angle of attack at the top and bottom of each stroke -
rotational lift (Wootton, 1999).
Subtle changes in the degree of wake
capture and rotational lift can be used by the fly to induce turns
(Dickinson, 2001). Free-flying butterflies use varied combinations of
these unconventional aerodynamic mechanisms in successive wing beats to
achieve takeoff, steady flight, manoeuvring, and landing - their
apparently irregular 'fluttering' flight is in fact highly co-ordinated (Srygley
and Thomas, 2002).
(For a picture of Dragonfly wings
and a summary of dragonfly flight, click here.)
Brookes, M. (1997) On a wing and a vortex. New Scientist,
Dickinson, M. (2001) Solving the mystery of insect
flight. Scientific American, 34-41 (Jun).
Dudley, R. (2000) The biomechanics of insect flight:
form, function, evolution. Princeton University Press.
Grodnitsky, D.L. (1999) Form and function of insect
wings: the evolution of biological structures. Johns Hopkins
Hengstenberg, R. (1998) Controlling the fly's gyroscopes.
Nature, 392, 757-758.
Srygley, R.B., and Thomas, A.L.R. (2002) Unconventional
lift-generating mechanisms in free-flying butterflies. Nature, 420,
Weis-Fogh, T. (1975) Unusual mechanisms for the generation of lift in flying
animals. Scientific American, 233(5), 80-87.
Wootton, R. (1999) How flies fly. Nature, 400,
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