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Re: was: Pterosaur.net now: pteroid
On Jan 15, 2010, at 8:27 AM, David Peters wrote:
What tells you the pteroid can be depressed? Which muscle depresses
the pteroid? What bone anchors that muscle? There's nothing out at
the tip to pull it up and down. If you're attempting to manipulate
the pteroid near its articulation, then you've got a very short
lever to work with. And finally, are you articulating the pteroid
principally on the radiale?
Mark and Jim have already taken care of these issues, so I will not
belabor the point.
On Jan 15, 2010, at 10:07 AM, David Peters wrote:
So you advocate the depression of the pteroid even though it creates
an anterior V in the leading edge of the wing?
Even though the leading edge of the propatagium is maintained by
Jim already indicated why this V shape does not occur, but I would
like to note that you actually answered this question with your own
second question: part of what prohibits the "kink" you imagined is the
tension in the leading edge. That tension does not prohibit pteroid
depression, but it means that the pteroid takes most of the
propatagium with it during depression. Again, remember that this is a
very small about of depression. It does not take much.
Remember the rest of the wing is creating drag that is trying to
overextend the elbow. That is a constant force while flying. The
propatagium is the only thing that prevents that from happening, as
in birds and bats. It's a passive organ.
While the propatagium helps, birds and bats also have perfectly
reasonable elbow flexors which can engage to resist elbow extension.
Also, their propatagia are not passive organs, either (albeit probably
not as finely controlled as the pterosaur propatagium).
1. That fragile tip of the pteroid ain't gonna hold up if you put
lateral stresses on it. It can only survive longitudinal stresses.
I presume you mean bending, when you say lateral? It is indeed weaker
in bending than in tension and compression, but Palmer and Dyke (2009)
already did the calculations on this, and the bending strength is
sufficient for a minor depression.
2. It's extremely difficult to put a kink in the middle of an object
being pulled from both ends. It keeps rebounding to a straight line.
That kink would be right at the tip of the pteroid.
See above. Tension prevents "kinking", but it does not prevent
3. There's no muscle strong enough to pull the pteroid ventrally
with such a short lever arm originating at the articulation no
matter what bone you'd like to articulate it on.
You have your lever arms backwards - the short base of the pteroid is
the out-lever, not the in-lever. Most muscles work at a serious force
disadvantage because they work on a long bone element - a long out-
lever. The short one at the base of the pteroid actually reduces the
disadvantage. In other words, less force is required. Same reason
that digging animals have short, stout humeri - less excursion, more
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