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Re: feather asymmetry
On Jul 7, 2014, at 12:39 AM, Tim Williams <firstname.lastname@example.org> wrote:
> Alan Brush <email@example.com> wrote:
>> Regarding the evolution of feather asymmetry see:
>> Feo, TJ and RO Prum 2014 Theoretical Morphology and Development of Flight
>> Feather Vane Asymmetry with Experimental Tests in Parrots. JEZ DOI:
>> Discussion includes steps that must have occurred in the evolution of
>> vane asymmetry.
> It certainly highlights the developmental complexity required to
> produce an asymmetric vane. Very interesting. Thanks for the ref.
Great reference - thanks to Alan for sharing, and to Tim for including the
quoted text (the original message showed up truncated for me).
> From this I infer that an asymmetrical feather is useful for
> powered/flapping flight, but doesn't necessarily mean powered/flapping
Indeed. More specifically, however, it means that the advantageous effects of
asymmetry do not “kick in” at asymmetry values much below 4:1, because the
center of lift sits near 1/4 chord. That’s why anatomical asymmetry and
functional asymmetry are not the same thing, and why folks as far back as
Speakman and Thomson (1994) have made a point of not describing the slightly
asymmetric feathers of animals like Archaeopteryx as being functional
asymmetric. This same issue has been noted by folks like Colin Palmer, Colin
Pennycuick, and myself more recently, but it’s not a new observation.
> Asymmetry reduces torsion of the feather in response to
Only right at a 4:1 vane ratio on a relatively flat feather . If the ratio of
trailing to leading vane is much less than 4:1 then the feather will tend to
twist towards a higher angle of attack, which is a problem at high lift
coefficients. If the ratio is much greater than 4:1 (typical for living flying
birds) then torsion is also promoted, but in the washout direction (angle of
attack is mediated) - that yields a “leading edge down” during level flight or
downstrokes with a high angle of attack, which reduces stall risk, and a sharp
“leading edge up” effect during the upstroke, which cuts circulation. Both of
these are advantageous. So in most cases, asymmetry promotes torsion.
> So asymmetry would be potentially useful for aerodynamic
> behaviors that might not entail a wingbeat: such as gliding or
> controlled descents, or simply maintaining balance or maneuvrability
> on the ground. Also, the aerodynamic remiges and rectrices were first
> developed distally on the limbs and tail, which is consistent with a
> function in control more so than lift, IMHO.
Control using the tail fan and limb foils would still be accomplished with lift
- it just might not be oriented for supporting weight. The twisting advantage
will apply to gliding, too, if the angle of attack is large. Regardless, you
make a good point that the advantage is greater for flapping flight because of
the “venetian blind” effect during the upstroke and the higher speed of the
What might make controlled descents and other transient behaviors less
sensitive to low levels of feather asymmetry is simply the timing - delaying
stall is less important if the maneuver is very brief. In fact, a dynamic stall
can be useful during some forms of turning and descent, as well as landing.
Assistant Professor of Cell and Neurobiology
Keck School of Medicine of USC
University of Southern California
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