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Re: Gliding/soaring requirements
Erik Boehm wrote:
Ahh... the danger of assumptions. A loon will not be thermal soaring
anytime soon, because of behavioral reasons.
Yes, that's the precisely the point. I never said it couldn't ride a
thermal - loons are not well adapted to thermal soaring, compared to
other birds, and so we predict that they won't use them (more on this
below). For a bird, the min sink of a loon is quite high (but that's
not the primary constraint here, necessarily).
The performance specs you just described are very close to a "high
performace" hang glider, which get sink rates of 1.0-0.9 m/s and max
glide ratio's of 14-17:1.
Sounds about what I'd expect - good to have hard numbers from someone
in the know, though.
Such gliders have flown 438 miles on thermals alone(at the time, the
highest manufacturer claims were 15:1), and more than a few 400+
"recreational" hang gliders may only get 1.2 m/s sink rates, and
glides of 12:1, they still regularly thermal for hours, and such
gliders were setting the records back in the 90's and 80's with
200-300 mile flights.
I don't know the circle diameter of a loon, but I'm certain its much
smaller than that of a hangglider.
At a 20 degree bank, it'd be about 35-45 meters. They have a
tremendously high wing loading for a bird.
Loons seek out water, not soaring over just any open space that
happens to be a thermal generator.
But they do fly over land between water sources, are occur over
coastal ocean environments that produce marine thermals. Lack of
access is probably not the constraint. Of greater importance is their
aerobic capacity and flight speed - loons are highly adapted for
rapid, straight flight over long distances. They can sustain constant
flapping for very long periods, cruising at speeds comparable to some
freeways. Given their very wide circle radius, and high min sink
(relative to other birds), loons are looking at rather poor cross-
country travel rates using thermal soaring as compared to continuous
flapping. Note that many other birds with similar planforms and
muscle compositions also rarely (if ever) utilize thermal climbing,
including many that travel overland and certainly come in contact with
thermals. Such taxa don't completely avoid external sources of lift,
of course - cloud streets and ridge lifts may be used, for example,
but thermal circling is rarely utilized by animals like ducks, geese,
loons, or grebes.
So their behavior is why - they don't go to geographies likely to
generate thermals, and probably would be very rarely interested in
thermalling if they happen to come across one.
The second part is correct - a loon will generally be disinterested in
loitering, and thus will not use thermals if encountered. The data I
supplied predicts this trend, when compared to the same parameters for
Seaguls on the other hand, thermal quite often, and apparently so do
some types flying foxes.
Yes, seagulls are well adapted to thermal soaring, gain significant
benefits from doing so, and loiter often. The literature on soaring
flying foxes indicates that only a few taxa have been seen to soar on
thermal columns. I don't recall the frequency off-hand, but I have a
vague recollection that it was not especially common.
Some birds extensively make use of thermals, and are optimised to do
You could compare a loon to them, and conclude they *can't* thermal,
but you are not comparing them to the minimum threshold.
True - and I never said they can't thermal - I said that they aren't
expected to do so.
I am quite sure a loon could thermal based on your numbers.
Me too. I'm also quite sure that they will almost never bother.
Out in the some of the hotter areas of the west US coast, come
midday, thermals giving 8+ m/s climb rates are not particularly rare
- practically anything can thermal inside one of those. A guy on
this list sent me a message saying there were some days where he
was, they could soar a J-3 cub single engine aircraft.
Yup - it's pretty intense. It also makes the point that many birds
commonly cited as being specifically optimized for thermal soaring
(like vultures) actually have planforms far broader than needed (or
optimal) for such flight, if one assumes purely travel-based
selection. Such taxa must have additional requirements - these appear
to be related to a loiter-based feeding ecology and launch in
cluttered environments. The need to carry very heavy gut loads after
feeding may also factor.
One thing I would be very interested in, is the relationship between
the much warmer climate, and thermal frequency and intensity.
It could be very relevant to the large pterosaur discussions, if not
Definitely worth considering - any thoughts out there?
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205