[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

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,
> 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
maximum lift coefficient than a high aspect ratio wing can, and it can
achieve a higher deck angle.  Though it IS a rather draggy process.
and blimps get quite a bit of lift through this process, and several
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
limbs would have the dual advantage of reducing the wing loading plus
a net tail/hindlimb downforce to offset the nose-down pitching moment
by the wings.  This would also allow the wings to be swept further
during flight.  At the Ostrom symposium during one of the question and
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,
greatly reducing tail drag, but at the penalty of also reducing maximum
tail lift if the tail were cascaded.  I personally have no opinion on
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

>  Excellent for creating
> drag though.

Yup.  Unless it's cascaded.