March 3, 2010 | David F. Coppedge

Flight Design: Flies and Birds Get it Wright

Parse the following sentence for logical consistency: “Just as the Wright brothers implemented controls to achieve stable airplane flight, flying insects have evolved behavioral strategies that ensure recovery from flight disturbances.”  That is the first sentence from a paper in PNAS yesterday about the stabilizers in fly wings.1  Ristroph et al just compared design principles employed purposefully by inventors to the trial-and-error process of evolution.
    The authors studied how fruit flies recover from disturbances.  They made them stumble while flying, and watched how they responded.  Their abstract continued the invention motif all the way up to modern times: “Thus, like early man-made aircraft and modern fighter jets, the fruit fly employs an automatic stabilization scheme that reacts to short time-scale disturbances.”  It only takes them 60 milliseconds to recover to within 2 degrees of their original heading.  They do this because they are equipped with “a pair of small vibrating organs called halteres that act as gyroscopic sensors.”  More aerodynamic engineering lingo ensues forthwith: “These findings suggest that these insects drive their corrective response using an autostabilizing feedback loop in which the sensed angular velocity serves as the input to the flight controller.”  The word “control” was one of the most prominent in the paper, used 27 times.  Later, their transition from biology to human engineering was seamless:

Flight control principles uncovered in this model organism may also apply more broadly, and this work provides a template for future studies aimed at determining if other animals employ flight autostabilization.  The control strategies across different animals are likely to share common features, because the physics of body rotation is similar across many animals during flapping-wing flight.  Additionally, animals that lack halteres may use functionally equivalent mechanosensory structures such as antennae.  Finally, the control architecture of the fruit fly offers a blueprint for stabilization of highly maneuverable flapping-wing flying machines.
    For fixed-wing machines, the need to overcome instabilities spurred the invention of autostabilizing systems by 1912, only 9 years after the Wright brothers first manually controlled airplane flight.  The development of such automatic steering systems also led to the first formal description of proportional� integral�derivative control schemes and advanced gyroscopic sensor technology.  The fruit fly’s autostabilization response is well-modeled by a simple PD scheme that receives input from gyroscopic halteres, and, like airplanes, uses fine adjustment of wing orientation to generate corrective torques.  Roughly 350 million years after insects took flight, man converged to this solution for the problem of flight control and joined animals in the skies.

Want to see what animal flight technology has achieved?  Look no further than the aptly-named swift.  The common swift (Apus apus) is the speed champ in the category of sustained level flight.  The BBC News reported that swifts have been measured faster than peregrine falcons in level flight, though the falcon, employing gravity, sets the record in freefall dives.  A swift was recently measured going 69.3 mph, “the highest confirmed speed achieved by a bird in level flight,” said Swedish researchers publishing in the Journal of Avian Biology.  This is nearly triple their normal fast flying rate of 22-26 mph.  Apparently males do it to show off in “screaming parties” when flocks of swifts come together in jubilant displays of prowess.
    Dr. Per Henningsson said, “It is remarkable that a bird that otherwise appears to be ‘finely tuned’ to perform at a narrow range of flight speeds at the same time is able to fly more than twice as fast when it needs to.”  The reporter added, “That means the birds need to be able to radically alter their aerodynamic performance, by altering their wing profile and physiology, depending on whether they are flying normally or in a screaming party.”  The article includes a short video of swifts in flight.  They go by in a blink of an eye, so a slow-motion sequence follows the real-time blip.  Reporter Jody Bourton called them “supercharged swifts”.

1.  Ristroph et al, “Discovering the flight autostabilizer of fruit flies by inducing aerial stumbles,” Proceedings of the National Academy of Sciences, online March 1, 2010, doi: 10.1073/pnas.1000615107.

The fruit fly experimenters only slipped on the E-word banana once, but then they got back up and talked design engineering the rest of the time.  But the cognitive dissonance of hearing them use evolution in the same sentence as the Wright brothers, engineering and flight control principles was jarring.  Maybe they did it on purpose.  It could have been to raise awareness of the logical inconsistency.  Or it could have been to ensure their intelligent-design paper got past the censors.  Hopefully that was the case; otherwise, it betrays endemic mental illness in the halls of academia.
    Next time you see a fruit fly or gnat, watch it for a while.  Think about how much technology is built into that tiny, tiny body.  It does things that our best aerospace engineers would like to imitate.  Become aware, also, of the birds in your area.  Watch some swifts in flight if you can.  You might just want to join their screaming party.  Flap your arms long enough, and you might be able to join them in a few million years.  Actually, probably not.  For more on swifts, see 12/09/2004, the 04/29/2007, and 07/18/2007.

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