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Re: BBC Horizon on Ostrom's "New" idea.



 
--Original Message-- From: James R. Cunningham <jrccea@bellsouth.net>Date: 09 October 1998 19:46

John V Jackson wrote:

< Ostrom claimed: "Gliders depend on a stable/immobile airfoil to support the lifting surface. Flapping flight depends on flexibility in that airfoil surface, and that&rsquo;s what birds possess." This view unavoidably implies that John astonishingly does not appreciate that moving airfoils also give vertical lift to flapping birds in horizontal flight.

I've had both verbal and written communication with Ostrom on this subject.  He appears to be very much aware of and appreciate the function of moving airfoils.  JRC >

I'm sure he does appreciate the function of moving airfoils per se but there was no point in his saying the above *word for word* quote unless he meant it as a point in favour of "ground up". He is distinguishing between gliding and flapping flight in terms of immobility/flexibility of the wing.  Gliding doesn't in fact require an immobile airfoil.  A bird could glide while flexing its wing up and down even if its wasn't generating forward thrust. Few would dispute that flapping flight has evolved from gliding, at least in bats.
 
 

< Another argument for "Ground up" was illustrated by the difficulty hummingbirds have when flying in low-density helium-rich air. Since there was thought to have been more oxygen in the Jurassic atmosphere, and oxygen is heavier than nitrogen, this is supposed to show flying was easier.

While it's true that oxygen is heavier than nitrogen, this isn't the point here.  The oxygen didn't replace the nitrogen, it was present in addition, thereby increasing the total atmospheric mass, substantially lowering the density altitude relative to the present day standard atmosphere.  Performance variation with density altitude is well understood and practically demonstrated every time aircraft (and birds, etc.) take off.  >

Yes, either replacement or addition would increase the density, and yes, it does usually make "flying" in the straightforward sense easier. But when it comes to "evolving the ability to fly", no matter how dense the atmosphere, the problem of evolving all the prerequisites explained by "trees down" but not by "ground up" remains, irrespective of density.

Denser air which generates increased lift also gives increased drag of course. Since "trees down" requires the utilisation of drag in the parachuting stage, it would be facilitated by a denser atmosphere. However, unless feathers were on first appearance fully aerodynamic (and accompanied by the full arm enhancements required), the animals would, in the first few generations, experience increased drag particularly when they wanted maximum speed. 

Denser air doesn't solve "ground up"'s evolutionary impossibility, and only makes "trees down" easier.

I have been an Ostrom fan for the last 25 years, and that isn't going to change. There is however a difference between a person and his arguments.

 

JJ