[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Fw: Dinosaurs and birds



As usual, mine are the numbered and somewhat cranky comments.

> ================================================
> Don sez-- 1). Thrust from a source other than the hind legs reduces "the 
> force 
> imparted per (hind limb) stride" required to maintain a given rate of 
> forward motion. Therefore, the hind limbs are free in that case to 
> increase stride frequency; because, as you say, increasing stride 
> frequency requires reducing "...requirements of imparting force to the 
> substrate."

Good argument.  Unfortunately, as best I can tell, the animal loses 
force too fast to make the gain in force pulse rate worth it.  

========================================
1). Huh? Can you clarify? Because the statement above sounds like good 
old-fashioned double-talk, in the context of my "good argument".

To recap-- You stated that forward aero-thrust will cause the biped to 
immediately fall over; when I pointed out that if stride frenquency increases 
the animal doesn't have to fall over, you stated that stride frequency can't be 
increased when the hind legs are at maximum thrust. When I pointed out that 
obviously the legs are no longer REQUIRED to produce thrust to maintain forward 
motion, and therefore can cycle faster, (thrust production now taken over by 
the wings/rocket/wind/car/UFO/whatever, and the hind limbs now needed only to 
hold the body up), you replied-- "...the animal loses force too fast to make 
the gain in force pulse rate worth it." 

Note: for those who have lost the thread, this is about whether a bipedal 
animal can increase its speed of forward motion through use of aerodynamic 
thrust while still in the running mode; in other words, a condition where 
periodic contact with the ground is still necessary to counter-act 
gravitational forces. This situation can occur due to control issues, or 
'grey-zone' aerodynamic power generation issues.
, 
Mike claims this is physically impossible, even for humans equipped with 
high-tech rocket packs, due to something called the "limiting hindlimb 
mechanic". To quote; "The 
animal can point the thrust in any direction it wants, and apply it in any 
series of pulses or continuous push that it wishes.  If the hind limbs are the 
limiting factor in velocity, then aerodynamic thrust still does not speed them 
up...." --MH.

"If", indeed. Surely you did not mean to posit your conclusion as a 
pre-condition to your argument. In any case, the hind limbs are NOT the 
limiting factor, at the rate of forward motion that can be achieved by running; 
THRUST is the limiting factor. This conclusion requires only 2 assumptions; 1) 
the stresses on the leg are less at the front of the stride cycle than at back, 
and 2) it is possible for the hind limbs to increase cycle frequency when not 
generating the primary thrust necessary to maintain forward motion. Both 
assumptions appear to me to be manifestly true.

To sum: in theory, a biped that can sprint at a given velocity can exceed that 
velocity, w/out falling down, by applying appropriate thrust to the upper body. 
This is because the stride frequency at maximum unassisted running speed is NOT 
the maximum stride frequency attainable by the hind legs. The maximum speed a 
biped can generate through unassisted use of the hind limbs does NOT constitute 
some magical theoretical barrier that cannot be exceeded. Whether a given 
anatomy can generate the necessary thrust using the
forelimbs is open to debate per iteration. Ditto the question of
conveyance of advantage through the accomplishment of such exploits, or the 
relative increase in speed. 

As to what degree of speed increase is advantageous, note that National 
Football League scouts consider .1 sec to be a significant margin of victory in 
the 40 yd dash (or ~ 2%), an event that emphasizes acceleration, but includes 
maximum velocity. As the relative speed increase attainable before hind limb 
failure occurs is logically inverse to body mass, significant advantage can be 
reasonably assumed at some mass.
========================================

I can't find a quantitative solution wherein an animal whose hind limbs are at 
maximum capacity gains top speed from producing aerodynamic thrust.  
Every time I try, one of the variables has to exceed the maxima for the 
hind limb capacity.  The problem appears to be that to actually cash in 
on that stride rate increase, the animal has to be able to generate the 
same force as running unassisted at a lower duty factor, which requires 
higher hind limb power (that we've already limited). 

=======================================
2). Total power relative to forward motion is not coming from the hind limbs in 
this situation, yet evidently there is a term in your equation  that 
"...requires higher hind limb power (that we've already limited)". [???] Why is 
that? In fact, relative to the theoretical situation as posited, NO forward 
thrust is required from the hind limbs... nor is any aerodynamic lift required, 
as the legs serve to counteract gravity. Note, that can be done mechanically, 
w/out thrust generation other than that required to pull a given leg forward 
into the front of the stride cycle...
=======================================

 That means it 
breaks even at best, but has to pay drag to do it.  If you can 
calculate a solution, please let me know.  I cannot, thus far.  --MH

> Further: if "running" in bipedal animals is strictly defined as 
> locomotion in which stride length (and thus airborne phase length) is 
> determined by thrust from the hind legs only, and if locomotion in 
> which the resultant stride length includes the effects of aerodynamic 
> thrust from the fore-limbs is defined as "flying", then I suppose one 
> can say that increasing running speed w/ fore-limb assistance is 
> "impossible", because then "they're flying".

Not quite what I meant.  If the animal is supporting its full weight 
with lift, and doing so by overcoming drag with thrust (from the 
wings), then it is flying.  

============================================
3). Flight requires a flight-control phenotype. Simple thresholds of lift and 
thrust production do NOT mean FLIGHT-CAPABILITY in any relevant ecological or 
evolutionary sense. And lift is not relevant to my argument.
============================================

Just increasing a leap or stride with a 
wing pulse isn't flying, and I didn't mean to imply that it would be. 
--MH

> Most people consider flying to be a state in which touching the ground 
> is not necessary to maintain the state, even "... the legs just giving 
> a few extra pushes." (see a).

I agree, but in the scenario I referred to with that comment, the 
animal didn't *require* the hind limb pushes.  I was making the point 
that if the animal holds itself up enough with lift to not risk falling 
(equals weight), and is producing enough thrust to overcome drag, then 
it must be flying, and thus not need the hind limbs anymore.  It might 
still be pushing off with them if it is in late stage running launch, 
but the animal is essentially airborne.  The punch-line was that I 
don't think an incipient flyer can manage the trick, that's all.  No 
catch-22 was intended. --MH

> 2). You stated earlier that upward trending fore-limb generated 
> aerodynamic thrust vectors were counter-productive when running, 
> because, in simplest terms, the feet slip. When I pointed out that 
> reducing the downward vector on the feet does not necessarily increase 
> slippage, due to claws and/or specific substrate conditions, you 
> stated that the case of non-slippage was "moot" (see b).

I meant ONLY that it was moot with regards to improving traction.  
Producing positive lift reduces the force that the hind limbs can 
impart.  Producing negative lift increases traction, but that's only 
useful if the animal needs more traction.  If the feet aren't slipping, 
then an aerodynamically actively upstroke isn't helpful (assuming it's 
not a forelimb propelled diver). --MH

> Logically, it follows that upward trending fore-limb generated 
> aerodynamic thrust vectors are NOT necessarily counter-productive

Positive lift is counter-productive for running speed.

====================================== 
3). Thrust. NOT lift.  Not necessarily counter-productive.
===========================================

It's very 
helpful for certain maneuvers, like leaps or tight turns.  Fortunately 
for our little near-avian, it doesn't have to produce much lift to make 
the turn tighter, so it loses only a small amount of speed (and 
probably none assuming it doesn't turn at top running speed).  So, lots 
of nice uses for forelimb airfoils in the near-avian, just not max 
speed increase.  Better leaps, pounces, falls, incline runs, and turns 
strikes me as plenty of selection pressure for the airfoil evolution.  
The fact that the animal probably can't gain running speed is thus not 
terribly pressing, to be honest.

> , and "active upstrokes" "unlikely to occur" in "basal birds" are not 
> relevant to evaluation of the case of fore-limb-assisted running, _in 
> the context of  potential evolutionary advantage_.

I think it's relevant, because the lack of a powerful, active upstroke 
limits the range of possible behaviors we can expect from basal birds, 
as well as calls certain popular mechanisms into question.  For 
example, the WAIR mechanic may have appeared much later in bird 
phylogeny than some have suggested.  

============================================
4). _If_ my understanding of the hypothesis is correct, WAIR, when advanced as 
a _primary_ flight evolution scenario, overemphasizes foot traction by 
oversimplifying lifestyle, substrate issues, and traction enhancing devices, 
and in so doing discounts the numerous advantages attainable in a vast number 
of situations in which those simplifying assumptions do not hold, yet forelimb 
assistance can be postulated. This may be a result of WAIR's use of extant 
animals w/ a specific life style as models, or natural academic 
competitiveness. 

A major strength of WAIR as an evolutionary scenario is that it does indeed 
provide a clear selective path for enhancing the upstroke, and enhancing 
differential up/down control. As you point out, this could logically mean 
"later" appearance, relative to phenotype. These opinions are long-held, and 
are one reason I refuse to pass on the speed enhancement issue. 
============================================

For our purposes here, it is 
relevant to questions of possible traction promoting behaviors in 
near-avians.  A negative lift force might help keep their footing, but 
only if the animal in question can produce the required forces. --MH

> Unfortunately, the context of the dialog had been so altered, your 
> statement regarding the "moot"-ness of the case of non-slippage could 
> easily have been misinterpreted as a rebuttal of my argument, or even 
> my understanding of the physics involved. Would you care to expand on 
> the
>  case wherein reducing the downward vector on the feet by the 
> generation of directional aerodynamic thrust does NOT increase 
> slippage of the hind feet? WITHOUT altering the context? --Don

Sure, I can expand.  Sorry for shifting the subject a bit last time and 
causing confusion.  If the normal force on the foot is reduced by 
producing positive lift, then the foot will both 1) experience lowered 
friction, and 2) impart reduced force against the ground.  Not a 
particularly useful tool for running rapidly.  

===============================================
5). That depends on environment, lifestyle and bodyplan. Not a universal law. 
Reduced foot force against the ground can highly advantageous to many exploits, 
_especially when it is discretionary_.
===============================================

Of course, if the animal 
leaps and then flaps, it could increase leap height or distance, which 
is pretty handy.  It could also extract energy from a wind gust to 
clear an obstacle, without the need for heavy flapping.  An active 
upstroke can be used to produce lift in the negative direction, to 
increase friction on the feet but this is only helpful if the feet are 
slipping heavily (enough that paying the drag costs are worth the 
tradeoff).  However, this requires the ability to produce a powerful, 
aerodynamically active upstroke, which the earliest birds (and winged 
dromeosaurids) may not have been able to manage.  They could almost 
certainly produce some kind of aerodynamically active upstroke, but 
perhaps not one powerful enough for traction benefits (especially on 
inclines or very slick surfaces).

The one caveat for assisted leaping in a cursorial near-avian is that 
they probably were limited to a continuous vortex gait.  This is 
reasonable enough, but it carries the price that while the downstroke 
produces positive forces of both lift and thrust, the upstroke produces 
negative thrust (and positive lift), thereby retarding the animal 
slightly.  This effect is limited by changing the span and twist of the 
wing during the upstroke, but the animal still pays a bit of a price 
for it.  The cost is worthwhile at high speeds, but the near-avian in 
question would probably be moving at relatively low speeds (from a 
fluid flow standpoint).  This does not make a wing-assisted leap 
untenable, but it does affect how a near-avian would be expected to 
execute it. For one thing, it makes utilizing gusts that much more 
useful  --MH


Cheers,

--Mike H.