Ramapriya wrote:
1. If the principles of flight are universally applicable, why is it
that birds don't stall or spin? I wonder especially because I'm sure
I've seen some gulls hover in mid-air doing pretty much nothing except
having their wings spread!


The direct answer is that they do, in fact stall, but as others have
said, instantly recover by changing the shape of their wings. One of
the funniest things I've ever seen was a flock of geese gracefully
approaching a glassy smooth lake in Iowa for landing. Having misjudged
their distance above the water, they flared a few feet too high, and
promptly entered what was indeed a deep stall. Wings flapping, panic in
their eyes, they dunked into the water. It was hilarious!

As far as spins, birds have mechanisms to avoid asymmetric lift (which
is the root cause of spins) that airplanes do not, like simply folding
the unstalled wing. To sum up, generally speaking birds have excellent
situational awareness that keeps them out of trouble most of the time,
but they do make mistakes at which time they are better prepared to
recover then man made flying machines.

The gulls that "hover" are only hovering with respect to the ground.
They cannot remain aloft without a relative wind providing their wings
with lift, and they cannot maintain altitude without flapping unless
they are in rising air. They simply use the rising air as their power
source, and gliders do the same thing to achieve amazing durations of
time aloft completely motorless.

2. We know it's possible for an aircraft to fly at angle to the
direction in which it's pointing. Does anyone know why does such a
thing not happen in boats or ships? After all, both ships and
aircrafts use media to float on... any sailors here could answer that,
I guess

Ramapriya


Crabbing does happen with watercraft - it's the same exact vector
problem as in flying, only at a slower pace.

Hope this helps.

-Aviv