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

Re: Pterosaur size

----- Original Message ----- From: "don ohmes" <d_ohmes@yahoo.com>
To: <dinosaur@usc.edu>
Sent: Saturday, December 16, 2006 6:06 PM
Subject: Pterosaur size

To verify our points of agreement; the big girls existed (11m or bigger

Yes, though the max estimated span for birds is on the loose order of 6.5-7 meters.

they could fly,


and the upper size limits of volancy generally are
constrained by launch and  landing capability?

Yes. launch more than landing.

So, anyhow, what are you using for wingload?

Depends upon the animal. Since pterosaurs tend to have similar aspect ratios, and somewhat differently from birds (which show more variation in planform), their wingloading tends to increase with wingspan. For Qn, it's roughly 4.3 pounds per square foot, or about 21 Kg/S.M. -- about the same as the most heavily loaded birds, but greater than the largest volant birds. However, for both birds and pterosaurs weight can vary substantially during the course of a single flight (as much as 50% for some birds).

Relative to a given process, selection occurs at the point of maximum stress,

And at points of average stress, and minimum stress, and so on, ad infinitum.

and it isn’t surprising that you don’t observe a large change in behavior or
morphology from the point of maximum stress to the point of least stress. The appropriate question is; how will they
perform in a habitat range of .5atm-.85atm, or .68-1.15atms?

It depends mostly on their launch technique. For example, I once calculated whether pterosaurs could fly on Mars (where surface pressure is about the same as at 110,000 feet here. It turns out that if you could supply them with oxygen, keep their blood from boiling due to the low pressure, and keep them from freezing -- some of them could. But they couldn't launch or land. Well, actually -- they could land, but not in one piece.

To use an analogy-- I weigh 100kg, and like to jog on the beach (boy, I wish, but that’s why I like thought experiments).

I can identify. I weigh about 97 Kg.

Once a week, I run a marathon with 15kg strapped on my back. I am
optimized to running weekly carrying 15 kg, and therefore finish my marathon,
which is good for me ‘cause I’ll die if I don’t. The beach of course, is just
the beach. You observe my performance and notice that I seem to be using approximately
the same running style 7 days a week. You reason that I can easily cope with a
15% change in my weight.

Well, no -- that's not an apt analogy for what we've been talking about.

Wrong. This observation tells you nothing about my
ability to cope with a lifestyle wherein I weigh 115kg on the beach, and 132kg
while running my mandatory Saturday marathon. Granted, a day at the beach is just a day at
the beach, but Saturdays are a bitch. Or more to the point, how much are my
chances of a long life improved if I weigh 85kg and marathon at 98… “small”
improvements in ambient conditions lead to small improvements in athletic performance
lead to very large improvement in the probability of success, and by the math,
a bird flying in +15% has lost 15% of its weight.

Ur, uhh, No. It doesn't work quite that way.

That is the equivalent of the
real me losing 36 lbs. Makes a HUGE difference.

Let me ask a flight question. Let us arbitrarily assume that you are a bird, or a winged whatever, and that from a height of 1000 feet in no-lift conditions, you can glide a distance of 20,000 feet. Now, let's add 15% to your weight. Without flapping, how far can you glide in the same conditions? Now, let's reduce your weight by 15%. How far?

and the Andes. Blackbirds that nest at 2500’ have lower
wingloads than sealevel nesters.

Not for any reason to do with flight -- it's actually to do with launch, and the variation between individuals or for one individual over time -- varies by more than the difference in density due to the 2500' change in altitude.

Seems like if you could determine minimum launch density, you could adjust to sealevel, and get a eco-perspective on max viable swan size.

That's true, for their present bauplan.

Checking is good, right?