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Re: The aerodynamics of birds
> I'm not sure that I'd expect a bird to routinely experience positive g-forces
> 10-14 G either. Mike Habib, what sorts of load and safety factors are you
> seeing on avian long bones?
Short version: the experienced loads apparently vary tremendously across birds
(not a surprise), and I would roughly say that albatrosses (low end) probably
routinely experience 2 g's of force, while peregrine falcons and osprey (upper
end) probably experience over 10 g's fairly regularly. So, the statement could
be accurate, if limited to very specific groups (mostly birds of prey).
Longer version/qualifiers: my dataset is a comparative one based on skeletal
morphology, and so I can only give exact values of loads experienced and in
vivo safety factor with certain assumptions. I rely on literature values for
the expected maximum failure stresses for avian bones (which appears to be
quite consistent, see work by Cubo&Casinos, or the recent overview by Garcia
and Silva) and the expected safety factors. The latter is also reasonably
consistent within clades, at a broad scale, but I have reason to suspect that
the safety factors of birds of prey are unusually high (and those of
soaring-based, marine taxa are likely rather low).
So, to put some numbers on this babbling: A 93.79 Newton Diomeda exulans
(wandering albatross) has a length-corrected humeral section modulus of 1.18
mm^2. A 7.16 Newton Falco peregrinus falls out around 0.54 mm^2 for the same
measurement. This means that the relative strength of the albatross humerus is
0.012 mm^2/N while the falcon's hs strength is a whopping 0.076 mm^2/N: over
six times that of the D. exulans individual. Using a failure stress of 275 MPa
for avian bone (reasonable number, based on the literature), the albatross
should see wing structure failure somewhere near 3.5 g's while the falcon's
wing spar won't fail until a somewhat ridiculous 20 g's.
The thing is, of course, that I don't think a falcon actually sees seven times
the acceleration of an albatross on a regular basis, even coming out of stoops.
In reality, I expect that seabirds fly at a modest safety factor, while
falcons fly with a robust one. I have good qualitative evidence for this, but
no hard numbers...yet. In any case, taking that into account, I get the rough
2 g and 10 g values mentioned at the outset, but these are rough estimates. In
reality, the one concrete thing I can say is that the resistance to load in
avian long bones varies about seven-fold, just among humeri in volant taxa.
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205