Re: large fossil birds

[jrc <jrccea@bellsouth.net>]

----- Original Message ----- 
From: "Michael Habib" <mhabib5@jhmi.edu>
To: "jrc" <jrccea@bellsouth.net>; <dinosaur@usc.edu>
Sent: Thursday, December 01, 2005 1:51 AM
Subject: Re: large fossil birds


>  For example, those with high aspect ratios tend to hold the tips of the 
> wings angled slightly downward and back

This is a multi-fold response, partly related to optimum orientation of the 
skeletal support system,  partly a response to the spanwise component of 
flow over the wing, and partly related to reducing span to minimize profile 
drag at the higher speeds associated with increased loading.

> (especially by marine birds during rapid soaring).

Reduction of span to reduce profile drag at high speed.  Note that falcons 
in a maximum dive will actually retract the wings so much that they form a 
double delta, with the tips brought in almost to the midline, and the 
effective tip being formed by the wrist.  Peregrines do this a lot.

>  If I remember correctly, this serves to form more favorable vortex 
> patterns at the distal portions of the wing and thus reduce drag near the 
> tips, but my memory may not be serving me correctly on that one.

This is correct for the mild aftward direction (it also greatly increases 
stability).  The extreme aftward position with retracted elbows and wrists 
forward tends to be associated with profile drag.  Also keep in mind that an 
elliptical planform doesn't result in minimum induced drag for a cranked 
wing.  Minimum induced drag for a cranked wing occurs with a more triangular 
tip.

> In fact, wing shape changes during different flight modes are often 
> ignored (as far as I can tell) in discussions of flight.

Depends upon whom you're talking to.  For example, it'll come up a lot in 
discussions with Pennycuick, Rayner, and MacCready.  And some others too.

> For example, the 'classic' picture of vultures (which is the image 
> generally illustrated) is one with the wings held out at maximum span and 
> the tipslots fully open.  However, this is mostly the case during 
> loitering and landing.

It's used when the wing is at high lift coefficients (low speed) in order to 
increase the effective aspect ratio of the wing and reduce the induced drag.

>  If you can catch vultures moving between updrafts you'll notice that they 
> bend the wings and close their tipslots.

Induced drag decreases with the square of the airspeed, so at high speed the 
high effective aspect ratio is unnecessary and is reduced by closing the 
tipslots (thereby also reducing the profile drag).  Essentially, they are 
morphing the wing for minimum total drag in response to the speed that they 
wish to fly.

>  So, in that case, wing loading is behaviorally increased during travel 
> between updrafts.

Increasing the wingloading increases the airspeed, but (depending upon the 
specifics of the morphed shape) does not necessarily reduce the lift/drag 
ratio and the distance that can be glided.  They are minimizing the time 
spent in downdrafts.  As an aside, for a given wing, changing the load has 
no effect whatever on the slope of the glide, or the distance that can be 
glided.  The heavier the bird (fat load), the faster it flies, increasing in 
exactly the same proportion as the sink rate increases.  The additional 
weight is only a factor on launch, landing, and when the average updraft 
speed is less than the bird's minimum sink rate (which does increase with 
fat load).

>  I'm not sure how the mechanics of wing shape changes during flight would 
> work for pterosaurs with their unique wing membrane.  Would they be 
> limited in their ability to fly with shortened wingspans due to disruption 
> of the wing surface by membrane folding?

They are more limited in that regard than birds, but less limited than bats.

> anecdote........  a very large lake with a distinct treeline I noticed not 
> only a very strong wind (even at ground level), but had the enjoyable 
> experience of watching about 6 or 7 vultures riding the wind shears. 
> ....... I got a rough estimation of 40-60 mph for a couple of them. 
> Considering how one usually catches turkey vultures moving, that's really 
> cookin').

That means that they were in substantial lift due to the interaction of the 
wind with the trees and shoreline.  You'll also see falcons and vultures 
making use of this effect along treelines between adjacent fields.  Not to 
mention highway embankments.

I would expect flight speed in the mid to large size azhdarchidae to 
approach 45-70 mph with the larger ones being faster, but also note that 
membrane wings have an upper speed limit that is a function of aeroelastic 
number, which is in turn partly an inverse function of lift coefficient.  In 
short, the wing flutters if the speed gets too high.