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Re: Fw: avian flight



>>This corresponds to a free fall from a height of 8.5 m and would probably 
>>lead to an injury [rather, death] if the speed could not be reduced prior 
>>to touch-down.<
>Surprise, surprise. I bet that if you dropped an unfrozen supermarket 
>chicken from a height of 8.5m, it would have some serious injuries as well. 
>So...
>>However, the available mechanisms for speed reduction are restricted, since 
>>the maximum lift coefficient during landing could hardly be greater [what 
>>an understatement] than the maximum drag coefficient of cD-max = 1,4 of a 
>>cup-shaped parachute.<
>Not quite sure what this means (icky math. I'm working on it though). 
>However, why would it be impossible for Archie to rotate it's body (in the 
>air) to the verticle, so that the wings and tail are perpendicular to the 
>direction of travel (basically stall a moment before landing). I think some 
>modern birds do something similar, when landing on a narrow perch.

I quite agree. This is the normal way for birds with high wing-loading
(=high forward speed and/or high sink rate) to land. They increase the
angle-of-attack (alpha) just before landing, thereby converting their
forward speed into an upward force, do a controlled stall and land on their
feet. THis is exactly the same procedure as an helicopter making an
autorotative (unpowered) landing. Helicopters have very high sink rates and
low glide ratios, so just before impact they sharply increase collective
pitch and convert the kinetic energy of the rotor into lift to brake the fall.

If You want to see a beautiful example of this method watch auks landing on
their nesting ledges. Auks are "water-fliers", and have typical wings for
this, small and with rather high aspect ratio. This is the case with almost
all birds who use their wings underwater, even dippers which have notably
small wings and high wing-loadings for passerines. Another common
characteristic of "waterfliers" today is that they almost all have
strikingly short tails (auks, diving-petrels, penguins, gannets,
dippers....). 
It should be noted that Archaeopteryx has *large* wings with *low*
aspect-ratio and a **long** tail, i. e. exactly the morphology You would
not expect of a bird using its wings in water.

Tommy Tyrberg