Weathervaning

But there can't be a relative wind unless some other force is acting on the
plane. It goes along with the wind just like a raisin in a cake.

If the wind changes speed or direction suddenly, the inertia of the plane
drags the raisin through the cake briefly creating relative wind until the
plane reaches the same speed as the wind. The inertia is acting briefly
like an outside force.

Take a look at the next weathervane you see. It has a pivot attached to the
roof which is attached to the ground. Take a look at the next airplane you
see...

An airplane is not a weathervane.

Let's say you are in a open cockpit airplane in no wind. You are hit from
the side by a sudden 75 knot gust that continues as a steady wind. Because
of the inertia, the plane does not start moving sideways at 75 knots
instantly. You will experience a brief moment of 75 knots of wind on your
cheek which will rapidly diminish as the plane picks up sideways speed.
Because the vertical tail is well aft of the center of gravity, the plane
may well "weathervane" into the wind as you suggest. The inertia of the
plane, acting through the center of gravity, which is point bodies in space
tend to rotate about, is briefly the pivot of the weathervane. The plane
will quickly accelerate to 75 knots sideways motion. When it reaches the
speed of the wind, there will be not further evidence of wind (and no
further weathervane tendency) except by looking at the ground.

If you were in a hot air balloon, you would also feel a sudden strong wind
that dropped of very quickly to zero after which you would be sailing across
the landscape at 75 knots but able to light a candle in the basket and not
see if flicker a bit.

--
Roger Long

Kobra wrote in message
...
Do I fly?! Don't be a wise ass...why do you think they call it
"weathervaning"? What does a weathervane do? It turns the "weathervane"
into the relative wind. An airplane is a weathervane too and the wind can
and does turn an airplane into the relative wind. Definition:
"weathervaning".

Kobra

"CASK829" wrote in message
...
The tail does not know what direction the wind is blowing if the
airplane
is in
the air. So therefore it DOES NOT push the nose into the wind.
Do You fly?



It's from the wind striking the vertical stabilizer and pushing the
nose
into the wind.

Kobra

To weathervane, you need differential force. A steady wind while on the ground
allows such a force to develop. A steady wind in the air does not. However, a
gust in the air does (the resistance would be due to inertia, and centered at
the center of gravity).

Jose

--
(for Email, make the obvious changes in my address)

"Kobra" wrote in message
...
Do I fly?! Don't be a wise ass...why do you think they call it
"weathervaning"? What does a weathervane do? It turns the "weathervane"
into the relative wind. An airplane is a weathervane too and the wind can
and does turn an airplane into the relative wind. Definition:
"weathervaning".


As others have pointed out, it's only "weathervaning" if the aircraft is in
contact with the ground the same as a weathervane on a building. If the
airplane is airborne it is in a moving air mass and by definition can not
"weathervane". Think about it, if airplanes "weathervaned" in flight every
plane in the sky would want to turn into the wind instead of going where you
wanted it to. Relative wind in flight is airflow opposite the direction of
aircraft movement through the air mass, it is not defined as a headwind,
crosswind or tailwind.

The aircraft I primarily fly is a b*#&h on the ground in a crosswind due to
the large vertical stab area and the arm from the vertical to the center of
pressure. But, once airborne she's a beauty.

Regards,
Mish

"Peter Duniho" wrote in message
...

That yaw can be described as "weathervaning" and the
rudder is used to counteract it.

What you are describing is actually "adverse yaw". Weathervaning only
occurs when in contact with the ground.

"Roger Long" om wrote in
message ...
But there can't be a relative wind unless some other force is acting on
the
plane.

"Some other force". You mean, like the lift vector being tilted to one side
or the other, dragging the airplane sideways?

An airplane is not a weathervane.

No, you're right. An airplane is an airplane and a weathervane is a
weathervane. However, the airplane can behave a lot like a weathervane in a
variety of situations, including while in flight. All it takes is for the
relative wind to not be coming directly from the nose.

Pete

"David Megginson" wrote in message
...
That would require extremely draggy ailerons. I know that some planes
turn sloppy without rudder, but what planes don't change heading at
all? Gliders?

Gliders are the most common, most dramatic example, yes. And it's true that
even most of the "worst behaved" airplanes probably will eventually get
around to changing heading. But those that do, may still take a while to
get around to it, and even a "well behaved" airplane can demonstrate adverse
yaw. Just put in a bunch of aileron input without using the rudder and note
the heading change on the DG as the bank occurs.

If you want to find an airplane that just keeps on slipping without turning,
I'd guess that airplanes like a Pitts or some fighter jets would be good
examples. I know for a fact that the Marchetti SF260 is a good example, but
haven't had a chance to fly other aircraft with similar characteristics.
Generally speaking, less stable aircraft are more likely to not bother to
yaw in the direction of the slip, by definition.

Probably for the vast majority of airplanes, saying that they simply won't
ever change heading is an overstatement, I admit. But many will turn
opposite the bank, and many will take a VERY long time to make any
significant heading change.

Pete

(Teacherjh) writes:

To weathervane, you need differential force. A steady wind while on
the ground allows such a force to develop. A steady wind in the air
does not. However, a gust in the air does (the resistance would be
due to inertia, and centered at the center of gravity).

I think that there's a lot of confusion in this thread about the terms
"relative wind" and "weathervaning". "Weathervaning" is an informal
term for the yaw moment contribution due to sideslip angle, where a
conventional aircraft naturally tends to align itself with its
relative wind; "relative wind" is the wind in relation to the
aircraft's longitudinal axis, not to the ground.


All the best,


David

"Robert Moore" wrote in message
. 7...
But I think that the discussion was weathervaning into the
actual wind, not the relative wind.

That's not how I read it. Alex was asking about flight on final approach in
a crosswind. In that situation, aileron by itself to maintain groundtrack
along the extended centerline will definitely cause the relative wind to not
be aligned with the longitudinal axis. That lack of alignment then results
in imbalanced forces on the vertical stabilizer, which then results in
weathervaning.

If I bank away from the
wind, does the airplane then weathervane into or away from
the wind?

Away from the the Earth-relative wind, but into the airplane-relative wind.

The only thing that can cause the airplane to
weathervane into the actual wind is for the wheels to be in
contact with the ground.

If you mean "the only thing that can cause the airplane to weathervane into
the actual wind without any other control inputs", then yes...I'd have to
agree with that. I don't think that's the question originally posed though.

(Nitpicking

Without the qualification I mention, banking into the actual wind causes a
slip in the direction of the actual wind, which causes the relative wind to
come from the same direction as the actual wind, which would cause
weathervaning into the actual wind. Without enough aileron input, the plane
wouldn't actually weathervane all the way around to align the longitudinal
axis with the actual wind, but the airplane certainly would have
weathervaned toward that direction.

It's all about your frame of reference and your other assumptions. Lacking
the necessary assumptions, one is free to imagine situations where the
statement is not true.

(End nitpicking)

Without the pivot, a weathervane doesn't weathervane.

True. However, even when not on the ground, there is a pivot. It just
happens to be at the CG rather than the landing gear. Also, the "wind"
that's relevant for the question of weathervaning changes from being the
wind relative to the Earth, to being the wind relative to the airplane.

Pete

in article fBXrb.164593$HS4.1334282@attbi_s01, Gary Mishler at
wrote on 11/10/03 6:13 PM:


"Kobra" wrote in message
...
Do I fly?! Don't be a wise ass...why do you think they call it
"weathervaning"? What does a weathervane do? It turns the "weathervane"
into the relative wind. An airplane is a weathervane too and the wind can
and does turn an airplane into the relative wind. Definition:
"weathervaning".


As others have pointed out, it's only "weathervaning" if the aircraft is in
contact with the ground the same as a weathervane on a building.

This begins to make more sense out of my own experience with weathervaning.
I had a rather "interesting" encounter with it while practicing crosswind
landings during my training. My aileron and rudder correction were perfect
all the way to touchdown. But after landing, I failed to maintain the same
or greater level of correction, and the airplane began to move in an
alarming direction (into the wind). In fact, I believe my exact words were
"Whoa!". Lucky for me, the runway was 150 feet wide.

"Gary Mishler" wrote in message
news:THXrb.166838$Fm2.146042@attbi_s04...
That yaw can be described as "weathervaning" and the
rudder is used to counteract it.

What you are describing is actually "adverse yaw". Weathervaning only
occurs when in contact with the ground.

No. Adverse yaw is drag caused by ailerons and acts *opposite* to the
direction of the bank. If you'd take the time to read my statements more
carefully, you'd see that in the statement you quoted, the yaw I'm talking
about acts in the *same* direction of the bank.

And is in fact "weathervaning".

Pete