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Re: Testing competitive exclusion in birds, bats and pterosaurs



Mike, I wish I were as articulate as you....  :-)

Cloud streets define repeating boundary layer roll vortices, and are a wonderful source of energy for traveling. Temperature and pressure changes along those vortices cause the clouds that make them visible. Using them and microlift, Gary Osoba has set a number of world's distance records in small sailplanes with size and performance characteristics very similar to the larger pterosaurs. Non-stop flights of several hundred miles are quite feasible for these aircraft and for the larger pterosaurs (if appropriately configured).

In fact, it is even possible to remain aloft all day and to travel with nothing available but spots of strong SINK. Vertical wind shear is very effective as a source of energy for soaring, and horizontal shear is not far behind. You can extract energy anytime the air is pushing you in the direction you are going, whether that be up, down, or sideways. To stay up using sink, you need to be flying fairly fast with wings approximately level and push over downward as you enter the sink (experiencing reduced g loads). You want the downward moving air to push you downward so that you accelerate. The sink area needs to be small in horizontal extent so that you won't overspeed. Then as you exit the far side of the sink, you rapidly pull up at more than 1g, thereby converting the kinetic energy you gained back to the potential energy of altitude. You can also use the same technique along the boundary of any atmospheric 'edge' where shear is present. You can replace the downward sink with a transient overbank, a bank greater than 90 degrees, sliding back and forth across the 'edge' to extract energy from it. This works great, though it makes for a bumpy ride and vigorous maneuvering. There is one caveat. It needs a fairly high wingloading and a high aspect ratio. For example, an Anhanguera piscator with a narrow wing planform scales up consistently in increased weight, wing loading, and aspect ratio from the wandering albatross, implying the ability to uses similar sources of atmospheric energy (which include those described above). The broadwing version of Anhanguera piscator is not so appropriate.
JimC


----- Original Message ----- From: "Michael Habib" <mhabib5@jhmi.edu>
To: <dinosaur@usc.edu>
Sent: Monday, May 07, 2007 10:53 PM
Subject: Re: Testing competitive exclusion in birds, bats and pterosaurs



In addition, the planforms proposed for wings connected to the hindlimb in pterosaurs tend to be poor even for a terrestrial soaring form, with both the estimated loadings and aspect ratios often falling well below those seen in living dedicated thermal soarers. Living raptors and storks do not have wings nearly as short and broad (nor masses as low) as some individuals seem to expect. After all, living inland soaring birds use a number of sources of lift other than thermals, many of which are better extracted with greater spans and/or higher loadings. Such species also have to travel between lifting sources at a reasonable rate. Not to mention that the largest pterodactyloids would be seriously pushing the limit with regards to being able to stay within a narrow thermal ring, simply by virtue of total span. Cloud streets would be much more useful to an animal that size, and I expect that extracting energy from cloud streets would usually select for a narrow chord planform.

Cheers,

--Mike H.