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

Re: Tail feathers (fwd)

---------- Forwarded message ----------
Date: Sun, 21 Jun 1998 19:55:22 -0700
From: "James R. Cunningham" <jrccea@bellsouth.net>
To: John Bois <jbois@umd5.umd.edu>
Subject: Re: Tail feathers

John Bois wrote:
John, I'm replying off list because I've about used up my list quota for
the day.  You can forward this if you wish.

> Jim, I wonder if you would comment on the following idea I haven't seen
> before. The skill you are talking about applies not only to herbivores
> but also to carnivorous birds.  For example, Caracara and Chimango birds
> of prey wait patiently near a rhea and its hatchlings.  If one strays
> (this can happen especially in heavy weather) from the parent they are
> immediately snatched up.  Now, the rhea is an extremely aggressive parent
> and would kill the birds of prey if it could get near them.  But it can't
> because (I'm guessing) the predators can make a speedy getaway.  So, the
> evolution of this tail apparatus was likely a new problem for non-avian
> dinosaur hatchlings sometime in the latter half of the Cretaceous.

The key features in the near vertical takeoff are gross weight
(animal+prey),wingspan and aspect ratio, flapping frequency, wing
loading, and density altitude.  The old style tail would neither help
nor hinder significantly in this regard. Both tail types can contribute
to lift at low speed. However, the reduction in stability and consequent
increase in manuverability due to the new tail might be a big help in
other ways.  Small birds can make more use of unsteady effects than
large ones can because they have a higher wingbeat frequency, and can
achieve a higher coefficient of lift on takeoff and landing.  Pigeons
can do about 3.4 when flapping actively, 1.54 when gliding.  I don't
know of any bird that can do better than 1.63 steady state, and unsteady
lift coefficient contributions due to flapping, clap & fling, wrist
deflection,and rotation in pitch fall rapidly with increasing size.  As
with all flying machines, power required increases faster than power
available. At some point the bird of prey will become too large to make
a vertical launch. Off the top of my head, I'd expect him to wait
nearby, watching for an opportunity, then launch, gain altitude, swap it
for airspeed and excess lift, grab the prey, pitch up and swap the
airspeed back into altitude thus minimising the time near the ground
(and the adult rhea).  I'm clueless about how they actually do it, never
having seen these animals.  However I've seen eagles do the kinetic vs.
potential energy swap when catching fish. I'd need more data on the
Caracara and Chimango and the weight of the rhea hatchlings before I
could say anything intelligent (if then).  Most flying animals dispense
with longitudinal stability as they evolve, depending more on active
control, but that has more to do with manuverability than climb
performance. If you have information on wingspan, area, and flapping
frequency, the calculations required to determine launch capabilty and
rate of climb are straight forward.