Wing Rotation and the
Aerodynamic Basis of
Insect Flight
Michael H. Dickinson,
1
* Fritz-Olaf Lehmann,
2
Sanjay P. Sane
1
The enhanced aerodynamic performance of insects results from an interaction
of three distinct yet interactive mechanisms: delayed stall, rotational circula-
tion, and wake capture. Delayed stall functions during the translational portions
of the stroke, when the wings sweep through the air with a large angle of attack.
In contrast, rotational circulation and wake capture generate aerodynamic
forces during stroke reversals, when the wings rapidly rotate and change
direction. In addition to contributing to the lift required to keep an insect aloft,
these two rotational mechanisms provide a potent means by which the animal
can modulate the direction and magnitude of flight forces during steering
maneuvers. A comprehensive theory incorporating both translational and ro-
tational mechanisms may explain the diverse patterns of wing motion displayed
by different species of insects.
Insects were the first animals to evolve active
flight and remain unsurpassed in many as-
pects of aerodynamic performance and ma-
neuverability. Among insects, we find ani-
mals capable of taking off backwards, flying
sideways, and landing upside down (1).
While such complex aerial feats involve
many physiological and anatomical special-
izations that are poorly understood, perhaps
the greatest puzzle is how flapping wings can
generate enough force to keep an insect in the
air. Conventional aerodynamic theory is
based on rigid wings moving at constant ve-
locity. When insect wings are placed in a
wind tunnel and tested over the range of air
velocities that they encounter when flapped
by the animal, the measured forces are sub-
stantially smaller than those required for ac-
tive flight (2). Thus, something about the
complexity of the flapping motion increases
the lift produced by a wing above and beyond
that which it could generate at constant ve-
locity or that can be predicted by standard
aerodynamic theory.
The failure of conventional steady-state
theory has prompted the search for unsteady
mechanisms that might explain the high forc-
es produced by flapping wings (3, 4). The
wingstroke of an insect is typically divided
into four kinematic portions: two translation-
al phases (upstroke and downstroke), when
the wings sweep through the air with a high
angle of attack, and two rotational phases
(pronation and supination), when the wings
rapidly rotate and reverse direction. The un-
steady mechanisms that have been proposed
to explain the elevated performance of insect
wings typically emphasize either the transla-
tional or rotational phases of wing motion (3,
5– 8). The first unsteady effect to be identi-
fied was a rotational mechanism termed the
“clap and fling,” a close apposition of the two
wings preceding pronation that hastens the
development of circulation during the down-
stroke (9). Although the clap and fling may
be important, especially in small species, it is
not used by all insects (10) and thus cannot
represent a general solution to the enigma of
force production. Recent studies using real
and dynamically scaled models of hawk
moths suggest that a translational mecha-
nism, termed “delayed stall,” might explain
how insect wings generate such large forces
(11). At high angles of attack, a flow structure
forms on the leading edge of a wing that can
transiently generate circulatory forces in excess
of those supported under steady-state condi-
tions (7). On flapping wings, this leading edge
vortex is stabilized by the presence of axial
flow, thereby augmenting lift throughout the
downstroke (5, 11). Several additional unsteady
mechanisms have been proposed (6), mostly
based on wing rotation, but recent studies have
found little or no evidence for their use by
insects (11). Despite this lack of evidence, it is
unlikely that insects rely solely on translational
mechanisms to fly. Whereas delayed stall might
account for enough lift to keep an insect aloft, it
cannot easily explain how many insects can
generate aerodynamic forces that exceed twice
their body weight while carrying loads (10).
One persistent obstacle in the search for
additional unsteady mechanisms is the diffi-
culty in directly measuring the forces pro-
duced by a flapping insect (12). In order to
further explore the aerodynamic basis of in-
sect flight, we built a dynamically scaled
model of the fruit fly, Drosophila melano-
gaster, equipped with sensors at the base of
one wing capable of directly measuring the
time course of aerodynamic forces (Fig. 1A).
The forces generated by a pattern of wing
motion based on Drosophila kinematics (13)
are shown in Fig. 1, C through G. Both the
magnitude and the orientation of the mean
force coefficient (C
L
⫽1.39, inclined at 10.3°
with respect to vertical) closely match values
measured on tethered flies (14, 15). The in-
stantaneous forces are roughly normal to the
surface of the wing at all times, indicating
that at this Reynolds number, pressure forces
dominate the shear viscous forces acting par-
allel to the wing (Fig. 1C). The records show
a transient peak in aerodynamic force at the
start and end of each upstroke and down-
stroke (Fig. 1, D and E). The timing of these
force transients relative to stroke reversal
suggests that they result from some undeter-
mined rotational effect and not from a trans-
lational mechanism such as delayed stall.
Translational forces. In order to test
more rigorously whether rotational mecha-
nisms were responsible for the two force
peaks straddling stroke reversal, we estimat-
ed the forces that are generated solely by
translation (Fig. 2). We calculated mean
translational force coefficients (C
L
and C
D
)
from data obtained by moving the wing
through a 180° arc at constant velocity and
fixed angle of attack (14). To obtain a repre-
sentative mean value, we averaged the mea-
sured force coefficients over the interval in-
dicated by the dotted lines in Fig. 2A. The
values of the resulting translational lift and
drag coefficients are consistent with similar
measurements made on a two-dimensional
(2D) model wing at an identical Reynolds
number (7). The force coefficients of the 3D
wing are slightly smaller than the maximum
transient values generated by a 2D wing, but
larger than the 2D steady-state values (Fig.
2D). These results confirm the important con-
tribution of delayed stall in lift production
during the translational portion of the wing
stroke. The observation that the 3D force
coefficients are lower than the 2D peak tran-
sient values, but higher than the 2D steady-
state values, is entirely consistent with the
flow patterns generated during force produc-
tion. Whereas wing motion in 2D gives rise to
an alternating pattern of unstable vortices
termed a “von Ka´rma´n street” (7), the leading
edge vortex generated by the 3D model fly
wing was stable throughout motion (16). The
stability of the flow structure is manifest as
constant force generation during translation
(Fig. 2, A and B), in marked contrast to the
2D case (7). Thus, as has been previously
suggested, axial flow along the length of the
wing appears to stabilize the leading edge
vortex throughout translation (5, 11). Where-
1
Department of Integrative Biology, University of Cal-
ifornia, Berkeley, CA 94720, USA.
2
Theodor-Boveri-
Institute, Department of Behavioral Physiology and
Sociobiological Zoology, University of Wu¨rzburg am
Hubland, 97074 Wu¨rzburg, Germany.
*To whom correspondence should be addressed. E-
mail: [email protected]
RESEARCH ARTICLES
18 JUNE 1999 VOL 284 SCIENCE www.sciencemag.org1954
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