Re: large fossil birds

[Michael Habib <mhabib5@jhmi.edu>]

> 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.

Ah, okay, that all makes sense.

> 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.

This doesn't surprise me, because it matches familiar biological 
patterns (ie. species using cranked wings generally have tapered tips). 
 Why is it, though, that cranked wings have a different optimal tip 
profile?

> 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.

Interestingly, some eagles use very long, shallow stoops and will morph 
their wings gradually as they pick up speed to continually minimize 
drag (and probably also to increase wing loading to some degree).

> 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.

Makes sense to me.  In terms of maximum speeds, there is a rather fun 
anecdotal paper that describes an albatross utilizing winds from a 
strong Antarctic storm to fly at a sustained average speed of 127 km/hr 
for 9 hours (the animal had been tagged with a electronic tracker 
previous to the observation).  Apparently it's the fastest recorded 
sustained soaring flight speed (note that the speed given is the ground 
speed).  I have the reference in pdf for those curious.  It's obviously 
not a good example of the 'normal' conditions, but it's still fun to 
read.

Cheers,

--Mike