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

Re: New Feathered Dinosaur

Gordon Martin Human wrote:

> Now, what do you define as commonly used soaring altitude"?

Near or below the cloud bottoms, for the same reason that human pilots usually
stay below the cloud bottoms.

> Those birds requiring thermals before they can rise (i.e. the larger ones)
> presumably have to make like a glider: ie fly from thermal to thermal, which
> requires quite a lot of height to be sure of getting to the next one.

True.  in today's atmosphere, as a rough approximation about 20% of the air mass
is rising while about 80% is falling (but falling at a lower average velocity
than that of the rising air).  Now, assuming that the cloud bottoms are at 5000
feet and the terrain is at 1000 feet (both are approximations -- what I'm really
saying is that the beast has to find another thermal or some other form of lift
before he descends more than 4000 feet).  Then assuming a glide ratio of 25:1 or
a bit more, the animal has to find another source of lift before he travels more
than 4000*25=100,000 feet, about 19 airmiles if in zero lift conditions.
There's a good chance he would find a source of microlift long before it bacame
a problem for him.  I can't really speak for other pilots, but when in an open
cockpit during average spring, summer, and fall conditions, I consider that to
be shirt sleeve weather, more for reasons of modesty than because I actually
need the shirt.  This is while I'm traveling faster than the animal, so wind
chill is more of a problem for me, and if I'm warm blooded I have more need to
keep my temperature up than a hypothetical exothermic soaring animal would. It
is often postulated that it was warmer during the Cretaceous, which would
further reduce the problem.  Let me repeat though -- I am not saying that large
pterosaurs were exothermic, only that endothermy isn't required for soaring
flight, then or now.

> We may
> say thermals are by definition warmer than surroundng air, but they are
> still subject to cooling and at 100 m we are talking about 6.5*C drop...and
> 1000 m is not that high......

No it isn't very high.  Just enough to travel 15-18 airmiles before needing
another thermal, orographic, or microlift boost.

> Immerse a nude (poikilothermic?)soaring (not rapid wingbeat flight)
> pterosaur for hours at that height or above and what would happen?

Probably less than would happen to me, and that wouldn't be much -- till I got
back down and my wife got hold of me with a brickbat.

> I'm only reminded of the need for a wet suit for scuba, due to the heat loss
> due to immersion in water. Much greater than that of air (~25 x), but stay
> high long enough and I don't really see how you can escape the chill....

The key is 'stay high long enough'.  That'll  freeze your rear off.  But even
swans don't fly that high.  To digress, Whoopers are the largest birds known to
fly with continuous flapping and though they've been reported at 17,000 feet by
airline pilots, all proven altitudes have been much lower, ranging from a few
feet to a few thousand feet.

Again, and I can't repeat this often enough, I am not saying that large
pterosaurs were exothermic.  This is only a discussion of the mechanics of
soaring flight.

All the best,