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Re: feather asymmetry
Mike Habib <email@example.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
> 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.