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Re: Fw: Dinosaurs and birds
On Tuesday, April 10, 2007, at 03:32 PM, don ohmes wrote:
> A). A running bipedal animal can use an incremental increase in
> forward thrust to increase stride length, w/out decreasing stride
> frequency, therefore increasing forward velocity.
This seems reasonable, but it doesn't work out, even if the thrust is
increased incrementally. If the hind limbs are running at their limit
for a given gait, then adding stride length will by definition reduce
the stride frequency or the force imparted per stride. In either case,
the animal cannot exceed its previous velocity, and will have to push
against drag on top of it. I can post the computations, if you would
like. However, it is actually reasonably intuitive if you look at the
constraints. In order to increase stride length without losing
frequency or imparted force per stride the animal must move faster
through the air, which means either pushing harder with the feet
(already disallowed) or reaching a speed during the airborne phase via
forelimbs that is above the animal's normal running speed, in which
case it falls over when it comes back down.
1). Why in the world would it immediately fall over? The implication, as
pointed out in my last post, but edited out of this one, is that a biped
running in place (ie, relieved of the necessity of imparting thrust w/ the hind
feet) cannot exceed the stride frequency attained at full unassisted sprint.
Maximum (unassisted) running speed is not constrained by simple stride
frequency in humans. A man running w/ a 20 mph tail wind is going faster than
his "normal running speed". Will he fall over? No! A man running in still air
at top speed that catches a wind gust strong enough to increase his speed an
increment above "normal running speed" doesn't "fall over"!
Some implications of your contention if
applied across the board to humans: a man
running 'in place' cannot exceed the stride frequency attained when
sprinting. Olympic sprinters in the upcoming games should be allowed to wear
rocket packs that
generate a pulse of thrust at the top of the stride. Track records set on
a day when the wind is over 12 mph (I think that used to be the limit
of wind-assistance) should be allowed in the record books. Football
(American style) fields should include a slight downhill grade toward
the visitors goal line in stadiums; and no more swapping goal lines at
halftime. If anyone complains, it can be demonstrated by your computations that
in speed garnered from use of these devices will cause the hind limbs
to fail... or they will "fall over".
I am entirely serious on this one. Think of the money you can win on
bar bets w/ professional athletes if you are correct!
"Hey, Mr. (insert name of favorite wide receiver), bet you a million dollars
you can wear any rocket pack,
designed and built by any team of scientists you chose, and you can't
increase your maximum speed." You'd have every skill guy in the league
pounding on your door, and you'd be rich beyond your wildest dreams. Actually,
ANY athlete. If
you are right, which of course, you are not.
But a word of warning; if you lose, don't try to
wiggle out by saying, 'Oh, the increase in speed wasn't significant."
Those guys understand that margin of victory is _meaningless_. They
_don't_ understand "small" windows of advantage. You either win, or you
_don't_. Stipulate that from the beginning. But if you are right, you are
going to be a VERY rich man.
There is no solution
possible where stride length increases without reducing stride
frequency or imparted force, if you don't let the hind limbs work any
harder and the animal cannot actually fly.
2). Top running speed has no intrinsic relationship to minimum flight speed.
There is NO connection WHATSOEVER..
This solution only becomes
possible at the point where the animal is actually holding itself up
with the forelimbs, and helping to push with the hind limbs, which is
3). In animals that can ALREADY fly it is called 'launching'. In flight
wanna-bes, and mallards w/ clipped wings, it is called "wing-assisted" running.
> I take the gist of your comments on "friction coefficient" to indicate
> you calculate that an upward thrust vector (or any reduction of the
> gravitational vector) reduces the ability of the feet to impart thrust
> to the substrate, and therefore reduces speed.
> 1). Consider a running bipedal animal w/ clawed hind feet on hard flat
> ground. At some point, imparting more thrust w/ the feet becomes
> counter-productive w/ respect to speed because traction is broken, and
> energy is wasted. If forward thrust from the forelimbs is applied at
> this point, the foot thrust required to maintain speed is reduced,
> reducing slippage and reducing energy waste. In other words, the hind
> leg is returned to it's optimal state relative to thrust vs slippage,
> which obviously varies by substrate.
In a sense, but if the animal pushes itself forward with the forelimbs
while the hind limbs are slipping, it will probably fall over.
4). Why? The thrust foot slips, the front foot is forward, in position to begin
the next stride...
A more efficient solution, most of the time, will be the WAIR type solution:
impart momentum to the air such that your resultant force vector is
mostly down (ie. negative lift force) and thus increase friction on the
> In fact, reduction of the gravity vector on the hind foot can
> _increase_ the thrust potential of same, rather than reducing it.
> (Anyone who doubts this should run a zip-line (or 'zip-rail", to coin
> a term) across a plowed field such that some body weight can be
> supported by arms, and compare the time of crossing the field to an
> un-assisted crossing.)
The reason that the zip line passenger moves faster is that they trade
in gravitational potential more effectively and/or reduce loss of
momentum at the end of strides. Even if the zip rail is horizontal,
the individual is gaining speed by pushing and gliding: it's a lot like
inline skating at that point (just on the hands instead). If the zip
rail is positioned such that the person on it is actually suspended
partially, and thus cannot put their full weight against the ground,
then they will actually be slowed down (cannot push as hard). They
might be able to get up to a higher speed if they have enough distance,
but pushing little by little and gliding to build up momentum. This is
not analogous to the situation at hand, however. (It is analogous to
an animal that is fulling gliding and pushing off the substrate at
intervals, such as a flying fish). --MH
> C). We agreed that adult living birds are not good models. They can
> fly, and the behavioral phenotype reflects that. They don't _need_
> wing-assisted running, why would they seek to use it? When they need
> more speed, they launch.
Some volant birds hunt on the ground, and would presumably utilize the
wings to increase running speed after prey if they could. I can think
of several other cases where adult birds would use wing assisted
running to reach higher velocities if it worked. Besides, we can show
quantitatively that the birds cannot increase their top running speed
with the wings. --MH
> BTW-- the tails are important to the traction/sub-optimal substrate
> debate (see comment 1), and also (it seems to me) allow (when used in
> conjuncture w/ the fore-limb "wings') the hind limbs to be more
> free/effective/important in (prey) capture/fighting...
Tails can be very important in some situations, and very unimportant in
others. I think you're correct though; tail evolution was probably
important in early avian flight evolution, even though some of the
earliest birds heavily reduced the aerodynamic importance of the tail
(ie. Confuciusornis). --MH
> It will clear higher obstacles, however.
> E). Which in many environments increases its rate of forward motion,
> in the non-theoretical sense.
Absolutely. I think that would be a great use of incipient wings.
Especially for a dromeosaurid. Think of all of the advantages to
hunting in tall undergrowth for a good leaper. Modern analogs are
numerous and include both mammals (foxes, felids) and birds (some
raptors, gruiforms, and others). --MH
> The only thing I assumed was that the animal isn't slipping heavily.
> That is hardly
> "garbage in"; it is pretty realistic since most running animals don't
> slip constantly. It was not a narrow computer simulation, it was just
> F). Sure they do. Every step they take they slip, their entire lives.
Not enough to make the incipient wings worth it, most of the time. If
the animal is using the forelimb forces to increase traction, then it
is pushing against drag to do so. The increase in traction has to be
worth the drag costs. Thus, the slipping has to be heavy for it to be
helpful. This might be the case during fast starts and unusual
substrates. I think the fast starts are probably the more important of
the two, statistically.
In addition, we are assuming that the upstroke is powerful for the
traction increase model; that's probably not a good assumption for
basal birds. --MH
5). Huh? Mike, what happened to the claws? I have to remind you, I am really
skeptical of the benefits of increasing foot friction through down \ward force
vectors unless you are wearing running shoes and are on pavement. I am a big
fan of claws, however. Roots and such aren't unusual at all. No need for
increased downward force vectors.