< 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’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. >
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
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.