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Re: Adaptive Advantage



Tim Williams wrote:

> The webbed surfaces, which are positioned far from the body,
> promote drag,

Why do membrane surfaces, when positioned far from the body, promote
drag?  What
is the specific fluid process that increases drag as the membrane
surface moves
further from the body?

> Also its tail is long and covered in a pairwise
> arrangement of retrices.  Although the tail feathers are asymmetrical,
the
> long wide tail is not much good for generating lift.

Uhm, it can function as a low aspect ratio wing, generating lift by
means of the
paired longitudinal counter-rotating vortices that are shed near the
front and
then move slightly inboard as they flow parallel over and along the top
of the
long axis of the tail.  It's a process that can create a substantially
higher
maximum lift coefficient than a high aspect ratio wing can, and it can
also
achieve a higher deck angle.  Though it IS a rather draggy process.
Dirigibles
and blimps get quite a bit of lift through this process, and several
aircraft
use it.  In Archaeopteryx, the tail appears likely to fly at about half
the lift
coefficient of the wings, thereby carrying most of the weight of the
hind limbs
(plus the tail shaft).  Carrying part, but not all, of the weight of the
hind
limbs would have the dual advantage of reducing the wing loading plus
providing
a net tail/hindlimb downforce to offset the nose-down pitching moment
produced
by the wings.  This would also allow the wings to be swept further
forward
during flight.  At the Ostrom symposium during one of the question and
answer
sessions, the possibility was discussed that if Archie were able to
spread and
separate his tail feathers, then the tail could have flown as a cascade,
thereby
greatly reducing tail drag, but at the penalty of also reducing maximum
possible
tail lift if the tail were cascaded.  I personally have no opinion on
the
likelihood that the tail was cascaded -- though I was the one who
initiated the
discussion.  Since the tail would normally fly at a CL lower than that
of the
wings in either case, the maximum achievable tail CL isn't really an
issue.

>  Excellent for creating
> drag though.

Yup.  Unless it's cascaded.

Cheers,

Jim