Re: feather asymmetry

[Mike Habib <biologyinmotion@gmail.com>]

On Jul 7, 2014, at 12:39 AM, Tim Williams <tijawi@gmail.com> wrote:

> Alan Brush <brushes2@juno.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:
>> 10.1002/jez.b22573.
>> 
>> 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
> flight.  

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
> airflow.

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 
wing.

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.

Cheers,

—Mike


Michael Habib
Assistant Professor of Cell and Neurobiology
Keck School of Medicine of USC
University of Southern California
Bishop Research Building; Room 403
1333 San Pablo Street, Los Angeles 90089-9112

Research Associate, Dinosaur Institute
Natural History Museum of Los Angeles County
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