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Re: feather asymmetry

Putting aside full powered flight for a moment, does this also mean that other 
aerodynamic advantages of feathers, such as running up near vertical slopes, is 
impeded until 4:1?


Sent from my iPhone

On 7 Jul 2014, at 06:14, Mike Habib <biologyinmotion@gmail.com> wrote:

> 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
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