# Re: Fw: Dinosaurs and birds

Note: I assume "stride rate" = stride length?
`Yes, sorry for the typo.`

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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. 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). 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. 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. 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. 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 too for running rapidly. 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.`