This review discussed single stereotyped flight maneuver of the fruit fly, Drosophila …

• Synopsis
• Introduction
• The visual control of body saccades
• Organization of musculo-skeletal system
• Mechanosensory feedback
• Aerodynamics of saccades
• Conclusions
• Acknowledgments
• References
• Figures

Biology Articles » Zoology » Ethology » The Initiation and Control of Rapid Flight Maneuvers in Fruit Flies » Aerodynamics of saccades

Aerodynamics of saccades

- The Initiation and Control of Rapid Flight Maneuvers in Fruit Flies

Studies on both real and robotic insects over the past 12 years have revealed much about the aerodynamics of insect flight (Sane, 2003). The unusual aerodynamics of flies and many other insects results from the peculiar reciprocating motion of their wings. Rather than moving forward while flapping their wings up and down like a bird, flies hover while beating their wings back and forth. At the end of the downstroke, the wing pitches up and flips over, so that it maintains a positive angle of attack during the upstroke, with the leading edge forward but the ventral surface facing up. A reverse rotation at the end of the upstroke readies the wing for the downstroke. Depending on the precise form of this back-and-forth pattern, the wings can generate forces by the variety of different mechanisms.

The primary means by which a fly wing creates aerodynamic force is dynamic stall. Due to its large angle of attack, the wing separates flow creating a prominent leading edge vortex (LEV) (Dickinson and Gotz, 1993; Ellington et al., 1996). Unlike similar structures created by a 2-dimensional translating wing (Dickinson and Gotz, 1993), a revolving wing (i.e., one that sweeps around a fixed base) creates a stable LEV that remains attached throughout the stroke (Birch and Dickinson, 2001; Usherwood and Ellington, 2002; Birch and Dickinson, 2003). The term "dynamic stall" is therefore misleading, because the flow, although separated, is time-invariant with respect to the wing once the LEV has formed. The constant circulation that results from the stable LEV is responsible for steady force production of sufficient magnitude to sustain flight. In addition to dynamic stall, flapping wings can generate force by additional means including rotational force, wake capture, and added mass (Dickinson et al., 1999a; Sane, 2003). By changing the shape and inclination of the wing stroke and the speed and timing of wing rotation, an individual insect can dramatically alter the relative contributions of the various aerodynamic mechanisms from one stroke to the next (Srygley and Thomas, 2002).

Equipped with a better understanding of the basic relationships between wing motion and force production, it is now possible to study the aerodynamics of specific flight maneuvers such as saccades (Fry et al., 2003). When hovering, fruit flies move their wings back and forth in almost perfect mirror symmetry (Fig. 4A). The mean stroke plane is nearly horizontal, and the wings follow a "U-shaped" trajectory. The aerodynamic forces resulting from this pattern of wing motion were measured by playing the kinematics through a dynamically-scaled robotic insect (Dickinson et al., 1999b). The wings generate a large force peak near the middle of the upstroke and a smaller peak near the middle of the downstroke. Although the upstroke produces more lift due to a stronger vertical plunge, the horizontal force (thrust) generated during the upstroke and downstroke is nearly equal and opposite, consistent with the low forward velocity. By comparing measured forces with a multi-component quasi-steady model (Sane and Dickinson, 2002), it is possible to quantify the relative importance of different aerodynamic mechanisms. In the case of hovering, dynamic stall accounts for about 80% of the mean force produced and predicts the overall time course of measured forces.

Flies use remarkably minor alterations in wing motion to generate saccades. In addition, the forces measured relative to the animal's body axis change very little throughout the maneuver. The alterations of lift and thrust during the saccade result from the changing orientation of the body, just as a helicopter can increase thrust by pitching downward. Thus, understanding how the fly controls body orientation is central to the analysis of flight maneuvers. To rotate about its yaw axis, a fly must overcome its moment of inertia as well as frictional damping. The torque required to do so, T, may be approximated as:

where I and C are the moment of inertia and frictional damping about the yaw axis, and is yaw position. Prior models of fly flight have assumed that viscosity dominates the dynamics of rotation so that an animal would instantly reach terminal angular velocity (Land and Collett, 1974; Reichardt and Poggio, 1976). However, the measured time course of T, measured by playing the saccade kinematics on the robot, is similar to that of the fly's angular acceleration, not its angular velocity (Fig. 4C). This suggests that the dynamics of this small insect are dominated by body inertia and not friction. Estimates of I and C based on body morphology closely match those based independently on the free flight kinematics and forces. In both cases the predicted time constant (I/C) is between 0.5 to 1sec, or roughly 10 to 20 times the duration of a single saccade. Thus, a fly would never approach terminal angular velocity during a saccade. This dominance of inertia has important consequences for the generation of saccades and flight control in general. A fly cannot rely on air friction to stop its motion at the end of a turn. Instead, it must create counter torque in the opposite direction to terminate a saccade.

Following a trigger from the visual system, how does a fly alter its wing motion to first initiate and then terminate a saccade? Two specific changes in wing motion correlate most strongly with measured yaw torque: a backward tilt of the stroke plane and an increase in stroke amplitude (Fig. 4D). The backward tilt of the stroke plane elevates flight force during the upstroke by increasing the aerodynamic angle of attack. An increase in stroke amplitude further augments force by elevating wing velocity. At the onset of a saccade, the outside wing undergoes these changes, thereby creating torque to rotate the body at the start of the turn. After about 20 ms the inside wing exhibits similar changes, thereby generating counter-torque to terminate the saccade. Although the changes in wing kinematics and moments are subtle, analysis of the resulting forces indicate that they are nevertheless sufficient to rotate the fly's body through the turn (Fry et al., 2003),

If a visual expansion triggers the production of torque that starts the saccade, what is responsible for triggering the counter-torque that terminates the maneuver? Does the entire turn/counter-turn behavior represent a single pre-programmed reflex, or are the two phases of the behavior triggered by separate sensory signals? Several lines of evidence suggest that the halteres may play a crucial role in triggering the counter-turn. The fictive saccades of rigidly tethered animals are much longer than free flight saccades, whereas those of loosely tethered animals, free to rotate around their yaw axis, are intermediate (Mayer et al., 1988). The most parsimonious explanation for this result is that sensory systems detect the rotation at the onset of each saccade and initiate a compensatory counter signal to terminate the turn. Because of the intrinsic dynamics of phototransduction and motion processing, the visual system is much less sensitive to rapid rotation than is the mechanosensory-based haltere system (Sherman and Dickinson, 2003). Thus, the halteres are the most likely source for the sensory signal that initiates the counter-turn to terminate each saccade. This notion is supported by the observation that the angular magnitude of free flight saccades are not substantially increased by reducing the contrast of a surrounding visual panorama (Tammero and Dickinson, 2002b).