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Re: New bird /pterosaur flight paper in PLoS ONE



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JimC

----- Original Message ----- From: "don ohmes" <d_ohmes@yahoo.com>
To: <dinosaur@usc.edu>
Sent: Wednesday, April 29, 2009 6:23 PM
Subject: Re: New bird /pterosaur flight paper in PLoS ONE




Since when is the lift found in "cloud streets" not thermally generated? Or "microlift", as I understand the term when used in ultralight flight.

You were previously using the term 'thermal lift' in its usual connotation, which is columns of rising warm air. You are now using it to describe any process of placing energy into the atmosphere. There is a disconnect there.


And since when do sailplanes engage in albatross-style dynamic soaring?

Pretty much every day for good pilots and appropriately sized aircraft. Ask Gary Osoba or Paul MacCready if they've ever used microlift and dynamic soaring. Unfortunately, Paul has passed on. But Gary is still out there and is still doing it. I've got some great film of him doing it for a Discovery News bit that we did some years ago.

The paper (as I skimmed it) simply measured and scaled (in vivo) flap frequencies of albatrosses and similarly flight-styled birds using empirical methods, and concluded that the larger ptero's could not flap fast enough hack that type of lifestyle, assuming that fluid conditions were the same in their time as present day.

Flapping calculations indicate that they are wrong. They leap happily to a bunch of empirical conclusions that have nothing much to do with pterosaur biomechanics. The big pterosaurs could launch and fly quite readily in today's atmosphere ( I wish they were still around to do so).

One can question the assumption that the birds studied represent a minimum threshold of 'flapping competence', or even the concept of size-related scaling, but they did in fact point out that they were not ruling out thermalling.

Since the wings of birds and pterosaurs are not structured similarly (pterosaur wings can achieve higher lift coefficients among other things), you can indeed question the assumptions. Why did they explicitly point out that they were not ruling out thermalling (which is different from dynamic soaring and microlift soaring), when the animals could utilize both that and other methods of soaring?

"...since neither those sailplanes nor pterosaurs are dependent upon thermal lift. Any paper that assumes that they are is deeply flawed."

That follows from their conclusion. It was NOT an a priori assumption.

No, it is an implicit assumption because it excludes the other usable sources of soaring lift.

I do not know if their conclusions are valid, but I certainly applaud the empirical approach...

Nothing wrong with empirical approaches, but you have to be careful to use appropriate input data and extrapolate in accordance with the constraints of that data.. I don't believe they did, though I do think they may possibly have reached a valid estimate for maximum size of procellariiform birds (but, have not tried to prove or disprove that).