Re: vaulting pterosaur launch, questions

[jrc <jrccea@bellsouth.net>]

Comments inserted
JimC

----- Original Message ----- 
From: "Erik Boehm" <erikboehm07@yahoo.com>
To: "Tim Williams" <twilliams_alpha@hotmail.com>; <mhabib5@jhmi.edu>
Cc: <dinosaur@usc.edu>
Sent: Monday, June 16, 2008 1:46 AM
Subject: Re: vaulting pterosaur launch, questions


> I would be very interested in knowing what sort of environment pterosaurs 
> and archaeopteryx's ancestors evolved in.

As would we all....  :-)

> I don't know how the later ones started flying (birds today will both do a 
> running takeoff, or just leap into the air), but I highly doubt the first 
> ones to begin to fly just leapt into the air.

It seems to me to be an easier mode than running, but the two are not 
mutually exclusive.

> Going away from paleontology, and into aviation:
> Birds always take off into the wind,

Like pilots, usually, but not always.  Other constraints can override the 
desire to takeoff into the wind.  As an aviation-related example, when 
taking off from and landing on the sandbars on the Mississippi and White 
Rivers, you usually have to do it downriver no matter the direction of the 
wind, due to the saw-tooth shape of the ripples and dunes (they will take 
your landing gear off if you hit them going upstream, while you jump them 
like jumping off a ski jump when going downstream)

> a 9 mph wind would be enough to make up the difference between the 2 m/s 
> running speed, and 6 m/s stall speed.

Only until the wind decelerates the animal so that it's 'rest' speed matched 
that of the wind.  Steady wind can help with launch, but not with sustained 
flight.  Orographic lift would do more good in the long run.

It can also make up the difference between a 2 m/s leaping speed and a 6 m/s 
stall speed (diminishing as the animal decelerates due to drag).

> The local weather and terrain can dramatically effect how easy it is to 
> fly. Perhaps this Tree vs Ground debate is overestimating the requirements 
> for flight, and ignoring the situations where I personally find it to be 
> easiest to fly.

Trees and ground are not mutually exclusive.  Neither are convective and 
orographic lift processes.

> Certainly, windy coastal dunes or cliffs would allow very easy flight 
> without the need to run anywhere near flying speed, or flap.

There also has to be a purpose for the flight, and though shear can provide 
energy for flight, it can also make control more difficult (I can takeoff 
and land a Piper J3 in direct crosswinds gusting to more than 30 knots, but 
I sure don't enjoy it or search out the experience.

> It is at coastal dunes that humans were first able to reliable practice 
> flight- first at Kitty Hawk with the Wright brothers, and then southern 
> california beaches with hang-gliding.

But, they were deliberately lookng for orographic lift.  Birds can can and 
could do that as well, but it is not a requirement.

> Early hang-gliders had horrendous glide ratios (like 4:1), and pretty bad 
> sink rates- but could still soar coastal dunes with sufficient wind. If I 
> recall, that was about the estimated glide ratio of Microraptor (which 
> occurs after Archaeopteryx, so I'm assuming the Dromeosaur linage is 
> secondarily flightless).

It's also about the same as the glide ratio for Archaeopteryx with 
non-cascaded tail.  If the tail could cascade, Archie's gliding performance 
would improve substantially, but an ability to cascade the tail has never 
been studied or demonstrated.

> I don't know how bats evolved, and the numerous arboreal gliding animals 
> do seem to strongly hint that perhaps an arboreal origin is likely. How 
> would an animal transition from gliding flight between trees, to powered 
> flight?

One way could be by intermittent flapping, initially for control.

> I wonder if there were any forests that went right up to the beach with 
> coastal cliffs/dunes when and where pterosaurs/bats/birds evolved.

Most likely were.

> Such an arboreal glider should be able to fly in the ridge lift, and could 
> then perhaps develop further flight adaptations.

That's true, as are a number of other scenarios.

> I learned to fly hanggliders at coastal dunes, flying there is super 
> easy - I can run nowhere near fast enough to get off the ground with my 
> glider, doesn't stop me from flying.

That's true of leaping as well.  And leaping doesn't require flapping as a 
precursor either.

All the best,
JimC