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!

I'm not convinced that birds don't stall. However, they seem to know what to do about it, and do it instinctively. Also, birds can change their wings' shape and angle of incidence, and they can flap their wings if necessary, increasing the airflow
over the wing (and doing other things). Most airplanes can't do that. Betcha their feathers act as excellent stall warning indicators too. Closely watch gulls hovering in a gusty wind... they are not frozen (watch their wings closely) and I bet
every now and then they dip their nose. Near a stall? Maybe. Otoh maybe they just are good enough pilots that they stay away from the stall regeme.



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

IT does happen with boats and ships, all the time. The standard vector problem in high school is a boat travelling across the river in a current. For more fun, consider sailing, where you travel at an angle to the wind and the ground, and the
course you track is different from the direction from where you are to where you want to be (due to tacking).

Planes can "skid" in the air to some degree, moving sideways through the air. Boats can also move sideways through the water, but it's harder since water is much more massive (though there is less weathercocking tendency (with respect to the water),
in fact maybe even none, on a boat, and there is some in an airplane (which helps keep the mose pointed forward).

Jose
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