Re: Origin of feathers

[Bill Adlam <wa105@mead.anglia.ac.uk>]

Patrick Norton wrote:

>Finally, it appears that those who accept the   
>"parachuting-gliding-flapping" origins of flight need to also explain how   
>the ancestors of Archaeopteryx evolved to the flapping stage (however   
>limited) with such an inadequate gliding design. Based on measurements   
>from a copy of the Berlin specimen of Archeopteryx, I estimate the aspect   
>ratio of A's wings at between 2 to 2.5. This is consistent with the   
>aspect ratio you get if you measure Paul's silhouette of Archaeopteryx in   
>PDW. From an aerodynamic perspective, aspect ratios in that range produce   
>a very low lift/drag ratio and therefore a fast sinking rate when   
>gliding. 
>Volant birds in that range today compensate for a fast sinking   
>rate with powerful flapping.  IMHO, with a fast sinking rate and without   
>a powerful compensatory flapping ability, Archeopteryx must have perched   
>somewhat precariously.

There are two important aspects of gliding performance: loss of altitude per 
unit distance and loss of altitude per unit time.  I can't remember which of 
them is sinking rate, so I'll assume you're right.  

Increasing wingspan and decreasing chord (distance from leading to trailing 
edge) increase the distance one can glide from a given height.  However, 
increased wingspan decreases manouevrability, and decreased chord increases 
stalling speed.  Albatrosses and many glider aircraft are built for this fast, 
efficient, but unmanouevrable gliding.  

However, high aspect-ratio gliders lose height rapidly.  Extending the wing 
backwards increases both lift and drag.  The overall effect is to allow the 
glider to remaion in the air longer, though covering a shorter distance.  Many 
gliding birds, such as vultures, have aspect ratios lower than Archie's.  They 
need relatively short wings to stay within thermals, which I admit is not a 
likely strategy for Archaeopteryx.  Archie must have gained height by climbing, 
perhaps also by flapping flight.

As for parachuting, here it is important to maximise drag.  That's the primary 
function of a parachute.  (Secondary function is to control direction & rate of 
descent.)