Re: feather asymmetry

[Tim Williams <tijawi@gmail.com>]

Mike Habib <biologyinmotion@gmail.com> wrote:

> 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.


On the basis of their measurements of feather asymmetry, Speakman &
Thomson (1994) actually regarded _Archaeopteryx_ as flightless.  They
took into account the position of the remiges in their comparison of
_Archaeopteryx_ and modern birds, and found that the primaries of
_Archaeopteryx_ were only slightly symmetrical - not significantly
different from modern flightless birds, and substantially lower than
modern flying birds.  In Speakman & Thomson's estimation, this
accorded with the skeleton and muscles of _Archaeopteryx_ also being
unsuitable for powered flight.  Twenty years on, I'm inclined to
agree.


> 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.


Got it.  Thanks for clearing that up Mike.


> Control using the tail fan and limb foils would still be accomplished with 
> lift - it just might not be
> oriented for supporting weight.


If I'm understanding this correctly... if the lift is incapable of
counteracting weight, then flight cannot be sustained.


> 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.


If this low feather asymmetry had a selective advantage for transient
"pre-flight" behaviors, then it offers a transition to the highly
asymmetrical vane that's important in powered flight.





Cheers
Tim