Two non-aviation questions

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!

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

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
--
Freedom. It seemed like a good idea at the time.
for Email, make the obvious change in the address.

I'm not convinced that birds don't stall.

They're experts at stalls, spins and recovery. (Ospreys, kestrels...)

Last night I was reading a commercial rating textbook and one section
addressed the question of whether, like humans, birds ever forget to lower
their landing gear. Below the paragraph is a series of three photos
showing a seagull landing feet up, sliding his ass across the ground.

It would appear he forgot to do his GUMPS check.

-c

"Ramapriya" wrote in message
m...
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!

Some kinds of birds stall all the time, they just don't spin since they
recover immediately. Watch swifts or swallows feeding, or harriers hunting.
They continuously stall, then recover.

Gulls can't hover, they just fly forward the same speed as the approaching
wind. Kestrels can hover, they do it by adjusting the angle of attack and
thrust. Hummingbirds can, of course, hover. In fact, they can fly backwards.

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

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

In addition to the issue of boats traveling through water with a current,
there is actually the issue of sailboats traveling through the water at a
different angle from which they are pointed due to the push on the sail.
This slip angle creates an angle of attack on the centerboard, daggerboard,
or keel to provide an opposing force.

John Bell
www.cockpitgps.com

On Sun, 7 Nov 2004 at 20:11:39 in message
, John Bell
wrote:
In addition to the issue of boats traveling through water with a current,
there is actually the issue of sailboats traveling through the water at a
different angle from which they are pointed due to the push on the sail.
This slip angle creates an angle of attack on the centerboard, daggerboard,
or keel to provide an opposing force.

An additional way of looking at this is to visualise that a boat sails
because it has one lifting part (the hull, keel and rudder) in the water
and another lifting system (the sails) in the air. The boat can move
because there is a relative velocity between the two fluids. It is
obvious you can sail if there is no wind at all provided you are in a
tidal flow.

Every one knows a sailing boat can tack up wind but can it tack down
wind? The answer is not much, mainly I believe because of the much
higher drag of the water. Sand yachts and ice yachts can, and they can
sail on a reach, downwind faster than the wind is blowing.
--
David CL Francis

"David CL Francis" wrote

The boat can move
because there is a relative velocity between the two fluids. It is
obvious you can sail if there is no wind at all provided you are in a
tidal flow.
********************

Not really. Relative velocity between the wind and water does not enter in,
much. In the case of a tidal flow you cite, it would have to be one hell of
a tidal flow; enough to make apparent wind, and a LOT, at that.

With very slow wind speeds, there is not enough wind to keep the sail in in
an airfoil shape, then you go nowhere, other than the speed of the water.
Most flows do not reach a minimum speed.


Every one knows a sailing boat can tack up wind but can it tack down
wind? The answer is not much, mainly I believe because of the much
higher drag of the water. Sand yachts and ice yachts can, and they can
sail on a reach, downwind faster than the wind is blowing.
--
David CL Francis

More problems. Sailboats *can* tack down the wind. They often do, if they
do not have a spinnaker. (Large parachute shaped sail, added while going
down wind) Problem is, you have to go a lot faster to justify the extra
distance. Usually, you just go down wind slower, and don't worry about it.

The drag of the water *is* an issue, but not as you state. The boat, until
you ride on top of the water, can go only so fast, almost irrespective of
how much extra power you add. It is called hull speed, and generally
speaking, it is higher, the longer the boat is.

When you are going into the wind, the hull speed is not tough to get to, if
the wind is strong enough. The maximum angle needs to be about 15 degrees
or more away from straight into the wind. (again, generalities) The sail is
acting like a wing, and airflow is accelerated around it to pull the boat
into the wind, just like an airplane wing provides lift. The boat slips
sideways, due to the wind pushing against the sail, which pushes sideways on
the water, through the keel and rudder. That is what gives the drift.

What kills downwind speed, is the fact that the sail is only acting as a
barn door, or flat plat, using the wind pressure, and does not have lift in
addition. Those are the main factors limiting down wind speed.

In the case of ice boats and land boats, the lower friction removes the hull
speed limitation. The craft continues to accelerate, until a wind that
*was* coming from the side or partway from behind, now with increased hull
speeds, will appear to come from the front of the craft, now called apparent
wind. The sail can then act like a wing again, and use the lift to continue
to go faster and faster. Problem is now, you can only go around 45 degrees
into the wind, without slowing way down again.

I hope I have made this all clear enough.
--
Jim in NC


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On Mon, 8 Nov 2004 at 19:42:55 in message
, Morgans
wrote:

"David CL Francis" wrote

The boat can move
because there is a relative velocity between the two fluids. It is
obvious you can sail if there is no wind at all provided you are in a
tidal flow.
********************

Not really. Relative velocity between the wind and water does not enter in,
much. In the case of a tidal flow you cite, it would have to be one hell of
a tidal flow; enough to make apparent wind, and a LOT, at that.

I've done it, although I have sailed where the tide can reach 9 knots
and 5 knots is quite common. The whole driving force for a sail boat
_is_ the difference in velocity between the two fluids. It must be. In
an 8 knot tide and an 8 knot wind in the same direction you have no
option but to start the engine (or in my case I used to get out the
paddle).

With very slow wind speeds, there is not enough wind to keep the sail in in
an airfoil shape, then you go nowhere, other than the speed of the water.
Most flows do not reach a minimum speed.

Again I have made headway when you could scarcely detect any wind and
you lean the boat way over to make the sails nearer the right shape to
catch the tiny winds. In my boat it also tended to reduce the wetted
area and lengthen the waterline (I used to think!).

Every one knows a sailing boat can tack up wind but can it tack down
wind? The answer is not much, mainly I believe because of the much
higher drag of the water. Sand yachts and ice yachts can, and they can
sail on a reach, downwind faster than the wind is blowing.

More problems. Sailboats *can* tack down the wind. They often do, if they
do not have a spinnaker. (Large parachute shaped sail, added while going
down wind) Problem is, you have to go a lot faster to justify the extra
distance. Usually, you just go down wind slower, and don't worry about it.

I thought I said that - my words were 'not much'? My boat used to have
a spinnaker thank you. It came out of a chute on the foredeck.

The drag of the water *is* an issue, but not as you state. The boat, until
you ride on top of the water, can go only so fast, almost irrespective of
how much extra power you add. It is called hull speed, and generally
speaking, it is higher, the longer the boat is.

Can we call it drag please? Drag rise at hull shape speeds is not
totally unrelated to approaching the speed of sound. I know the speed of
sound in water is very high but the natural speed of waves is not do so
high. That's why bores occur - they are a sort of shock wave travelling
down the surface.

When you are going into the wind, the hull speed is not tough to get to, if
the wind is strong enough. The maximum angle needs to be about 15 degrees
or more away from straight into the wind. (again, generalities) The sail is
acting like a wing, and airflow is accelerated around it to pull the boat
into the wind, just like an airplane wing provides lift. The boat slips
sideways, due to the wind pushing against the sail, which pushes sideways on
the water, through the keel and rudder. That is what gives the drift.

What is the difference between your visualisation and mine? Not many
boats that I have sailed can get within 15 degrees of the true wind and
there is no chance of them getting to their maximum speed while doing
it. Maximum speed is on a reach when I could get up to 12-15 knots while
planning. In stronger winds I could have gone faster but I was not
usually brave enough. Incidentally the keel and the hull are also acting
like a wing but in the water. Hence the sideways 'slip' but worded
differently.

What kills downwind speed, is the fact that the sail is only acting as a
barn door, or flat plat, using the wind pressure, and does not have lift in
addition. Those are the main factors limiting down wind speed.


Because you cannot effectively tack downwind you are forced to do that.

In the case of ice boats and land boats, the lower friction removes the hull
speed limitation.

i.e. lowers the drag

The craft continues to accelerate, until a wind that
*was* coming from the side or partway from behind, now with increased hull
speeds, will appear to come from the front of the craft, now called apparent
wind. The sail can then act like a wing again, and use the lift to continue
to go faster and faster. Problem is now, you can only go around 45 degrees
into the wind, without slowing way down again.

I agree at last - more or less.

I hope I have made this all clear enough.

It made no difference. Just because you find somebody expresses things
in slightly different terms from yourself please don't automatically
assume you have to disagree with them. The drag law for boats is
different from aircraft but it is still drag as far as I am concerned.

--
David CL Francis

"John Bell" wrote in message m...
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

In addition to the issue of boats traveling through water with a current,
there is actually the issue of sailboats traveling through the water at a
different angle from which they are pointed due to the push on the sail.
This slip angle creates an angle of attack on the centerboard, daggerboard,
or keel to provide an opposing force.

As a sail boat buys I take issue with the comment "push on the sail".
"Pushing" on the sail is your last choice and used only when sailing
directly away from the wind. However, this is the way the old 1400's
sailer did it. Today we have sails that are airfoils so the air
"sucks" us around. In fact you will move MUCH faster when sailing up
wind than when sailing down wind.
-Robert