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Re: avian flight



> > > <<Remember the tail of *Archaeopteryx* that prevented it from gliding?
>>
> > > Mr Cunningham answer to this...
> >
> > In his answer (thanks for the explanation of glide ratio) HP James R.
> > Cunningham writes:
> >
> > > However, it will arrive at the
> > > touchdown point in only 70.7% of the time it took at the lighter
weight.
>
> This applies only to the effect of a doubling of weight on airspeed and
sink
> rate when gliding.  Note that it has no effect whatever on gliding range.

Indeed. And airspeed may become injurious or lethal...

> > That's it.
>
> That's it, in what way?  Based on the description of Ebel's work below, my
> comment wasn't at all what Ebel addressed mathematically.  He seems to
have been
> talking mostly about parachuting which is not at all the same thing as
gliding.
> As sort of an overgeneralization -- for the most part, parachuters don't
glide,
> and gliders don't parachute.

He first addresses parachuting, then gliding, and emphasizes the differences
all over the paper. I really recommend that you get it; if you don't have a
library around which has Neues Jahrbuch für Geologie und Paläontologie, tell
me, and I'll send you a scan.

> > When assuming
> > present air density (small changes there would hardly alter the
outcome),
>
> Those changes can alter the outcome.  For a given flight speed, lift is
directly
> proportional to air density.  If Archie flew in an atmosphere that was 10%
more
> dense than now (I'm not saying it was), then his flight speed would have
been
> reduced correspondingly.

According to the formula Ebel gives, airspeed is directly proportional to
the square root of air density...

W = D = cD * A * rho * v²/2        W = weight, D = drag, cD = drag
coefficient, A = wing area, rho = air density, rho * v²/2 = dynamic pressure

vS = square root of (W / [A * cD * rho/2]) = 9.13 m/s, with cD = 1.0, rho/2
= 1/16 kg s²/m^4 (??), W = 250 g, A = 479 cm² according to Yalden; weight
must be entered in kg and area in m² to give this result

> > a weight of 250 g and a wing area of 479 cm² (too low),
>
> Yalden's estimate doesn't appear to be any less likely than Rietchel's
estimate
> of 754 cm^2.  Yalden's estimate leads to a more efficient wing,  but
that's not
> grounds for a decision in favor of either.
[snip]
> > (754 cm² is probably too high, as it assumes a full set of tertials
instead of
> > a gap
> > between the wings and the body,
>
> It may be too high, but aerodynamic convention takes the extended wing
area all
> the way to the midline of the body anyway.


I think Riet_s_chel's reconstruction looks more like the fossils.
Aerodynamic convention really is that way, but it only makes sense if the
wings are continuous with the body, which is doubtful. I hope I can find the
ref for the probability of that gap (probably it has been discussed onlist,
it was in New Scientist).

> > but on the other hand it does not take into
> > account tail area; these two effects more or less cancel each other
out.)
>
> No -- they don't.

Statement against statement...

> > Archie's speed of descent was 9. m/s,
> > "corresponding to a free fall from a height of 4.2 m, would hardly be
> > acceptable to a bird with delicate bones";
>
> ??? Steady-state, the Yalden wing would probably be most efficient at a
lift
> coefficient on the loose order of about 0.9.  If the tail were acting as a
low
> aspect ratio wing lifting about 15% of the gross weight (part of the
weight of
> the tail and legs, to create a net 'tail' download) then the tail would
have
> been operating at a CL of about 0.54, and the overall L/D would have been
about
> 3:1.  Sink rate would have been about 2.95 m/s, which is the free fall
velocity
> >from a height of 0.444 meters (1.46 feet). This is a far cry from a fatal
sink
> rate or a fatal fall distance. I've seen squirrels fall (not leap) kerflop
out
> of a tree from a height of maybe 10 meters and run off with only a
startled
> blink or two.
>
> > more recent estimates of wing
> > area (754 cm²) still give 7.283570407 m/s. This is still very much.
>
> Rietchel's wing leads to cruise sink rate of about 2.35 m/s, the
equivilent of a
> freefall drop from a height of 0.28 meters (11 inches).  Again, hardly
enough to
> be fatal, though I don't agree with this line of reasoning, since it
doesn't
> appear to address the actual physics of the situation.

That formula is for parachuting, where there is no lift.

> > however, he writes:
> > ................................ The speed
> > of *Archaeopteryx* during gliding would be approximately v = 12.9 m/s,
with
> > an estimated lift coefficient of cL [index L] = 0.5, which is surely not
> > too small [I can't decide this].
>
> Based on the aspect ratio, I'd expect the wing to be more efficient at a
CL of
> roughly about 0.8-0.9, though this wouldn't lead directly to the optimum
point
> on the drag polar because of the effect of the low aspect ratio tail.  If
the
> animal with the Rietchel wing is flying at a CL of 0.9, then the low
aspect
> version of the tail would be flying at about 0.85 (unlikely), and the
airspeed
> would be about 7.07 m/s. A CL of 0.5 seems awfully fast.

Again, I can't decide; maybe I can help with some data?

from Pat Shipman: Taking Wing
________________________________________________________
Parameter        Rietschel's Estimate        Difference from Yalden's
Estimate
Body Weight            250 g                                    0 %
Wingspan                58 cm                                    0 %
Individual wing        23.5 cm                                  0 %
length
Individual wing        13.0 cm                                 + 58 %
breadth
Two wing area        611 cm²                                 + 57 %
Total wing area        754 cm²                                + 57 %
Wing loading:            0.41 g/cm²                           - 36 %
two wing area
Wing loading:            0.33 g/cm²                          - 37 %
total wing area
Aspect ratio                4.5                                    - 36 %
Stalling speed        4.1 -- 5.0 m/s                         - 20 %
(approx.)
___________________________________________________

Neither estimate takes the tail into account. What is the effect of tails to
total wing loading and lift in modern birds?

> > if the speed
> > could not be reduced prior to touch-down. However, the available
mechanisms
> > for speed reduction are restricted, since the maximum lift coefficient
> > during landing could hardly be greater [what an understatement] than the
> > maximum drag coefficient of cD-max = 1,4 of a cup-shaped parachute.
>
> A pigeon's maximum steady-state CL is about 1.54.  Using unsteady
mechanisms
> during landing, they achieve an effective CL of about 3.0.  A frigate bird
can
> do about 1.63 steady-state.  I don't remember what the frigate bird can
manage
> when flapping, but wouldn't be much surprised at something around 2 or
more.
>
> > To be
> > able to achieve a lift coefficient of cL-max = 1,4 an airfoil section
must
> > be perfectly developed.
>
> Not true.  See above.  By comparison, a Selig s1223 airfoil can do 2.1 or
2.2
> steady-state, and a flat plank can manage 0.9.

Oho...

> > To be able to fly up a landing site in
> > a curve so that the speed is completely reduced at arrival, as Recent
birds
> > can do, requires completely developed flight abilities.
>
> Does this mean that flying squirrels have completely developed flight
> abilities?

Flying squirrels, AFAIK, glide to a tree trunk hands-first, grab that tree
trunk, pull themselves into an upright position while folding their patagia,
and get rid of the rest of their kinetic energy by clinging to the tree
trunk with their feet. Have I missed anything here?
        Birds, on the other hand, glide towards a tree limb, flap vigorously
to reduce their speed to zero or near, and then stretch out their legs to
perch. That's a situation which could easily lead to stalling and falling if
flight abilities and/or the center of gravity pose problems.
        *Archaeopteryx* could most probably land on the ground, probably it
used running landings not unlike a plane. Some confusion has arisen in the
discussions here, I just claim Archie couldn't glide and couldn't land in a
tree.

> I must admit though that in the case of birds, I am predjudiced
> towards flapping before gliding.  I also suspect Archie is too highly
derived to
> address the question of flight origins in birds.

I agree.

> > > <<Prey in
> > > the mouth would have pulled the center of gravity forward and thus
made
> > > flight easier. >>
> > >
> > > has someone tried to quantify this "easier"?
> >
> > No, not even Ebel. Good idea.
>
> Actually, a number of people have quantified this, though 'easier' may not
be
> the appropriate term for the effect on flight.  Within limits, moving the
cg
> forward of the center of lift increases the load that the craft has to
carry
> (because the  tail must carry a download to combat the nose-down pitching
moment
> and the wings then have to also carry the tail download in addition to the
body
> weight) and it tends to increase the pitch stability.  Minimum load occurs
when
> the cg is slightly aft of the center of lift, but it makes flight rather
> squirrelly in pitch. However, the 'prey in mouth' issue isn't really an
issue
> for birds, bats, and pterosaurs, because for the most part, they simply
sweep
> their wings fore or aft to put the center of lift where they want it to be
with
> respect to the cg.

Moving the wings back to the hips doesn't look feasible in Archie...

> There are several other mechanisms for developing high lift coefficients
and
> high drag for landing that we haven't addressed here, and they are
probably more
> important than the ones we have addressed.

Too bad we don't have Ebel onlist. For contact -- no e-mail address is
given, only snail-mail:

Dipl.-Ing. Klaus Ebel
Reußenbachstraße 30
D-88677 Markdorf
Germany

Dipl.-Ing. is Diplomingenieur.

> All the best, and my apologies for the long response.

Same.