Weathervaning

[Koopas Ly]

Just a quick question...

During a crosswind landing, for instance a left crosswind, you'd lower
the left, upwind wing to counter the right drift induced by the
crosswind.

You'd also use some right rudder to keep the nose straight and prevent
it from "weathervaning".

Is this "weathervaning effect" caused by your leftward relative motion
due to the left bank OR by the rightward crosswind ITSELF?
Personally, I think that the former applies. The rightward crosswind
only displaces the airplane to the right. Only the relative motion of
the airplane with respect to that airmass would induce the
weathervaning effect. I presume that the airplane does not know,
aerodynamically, of the left crosswind.

Next thing I was wondering, which is related to the above: say you're
dead on centerline on landing, and all of a sudden a crosswind from
the left starts blowing. The effect would be that you should only be
displaced to the right of runway centerline. Your airplane nose would
still be parallel to the centerline. Do you agree?

Alex

[Roger Long]

As far as the plane is concerned, there is no such thing as wind until the
wheels touch the ground. When you are in a hot air balloon, you don't feel
any wind at all because you are being carried along with it. Same with the
plane in a steady wind. If the wind is changing speed or direction quickly,
the plane will feel it momentarily because its inertial prevents it from
moving instantaneously. In the simple case of a steady crosswind, there is
no wind aerodynamically. The fact that the ground is sailing along sideways
is irrelevant, until the wheels touch that is.

You don't want the wheels to touch while you are moving sideways so you
bank. This angles the lift to the side which pulls the plane in that
direction. You bank until the plane is being pulled sideways by its wings
at the same speed as the wind. There is now a wind blowing sideways on the
rest of the plane, including the rudder. The rudder, having lots of
leverage way back there, tries to turn the plane into the wind. The effect
you are counteracting with the pedals is caused by the bank; not by the
wind.

--
Roger Long

[Kobra]

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

Kobra


"Koopas Ly" > wrote in message
m...
> Just a quick question...
>
> During a crosswind landing, for instance a left crosswind, you'd lower
> the left, upwind wing to counter the right drift induced by the
> crosswind.
>
> You'd also use some right rudder to keep the nose straight and prevent
> it from "weathervaning".
>
> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?
> Personally, I think that the former applies. The rightward crosswind
> only displaces the airplane to the right. Only the relative motion of
> the airplane with respect to that airmass would induce the
> weathervaning effect. I presume that the airplane does not know,
> aerodynamically, of the left crosswind.
>
> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?
>
> Alex

[Kobra]

>Your airplane nose would
>still be parallel to the centerline. Do you agree?

No, the airplane is pushed harder from the tail because of the vertical
stabilizer. Therefore the nose turns into the wind.

Kobra

"Koopas Ly" > wrote in message
m...
> Just a quick question...
>
> During a crosswind landing, for instance a left crosswind, you'd lower
> the left, upwind wing to counter the right drift induced by the
> crosswind.
>
> You'd also use some right rudder to keep the nose straight and prevent
> it from "weathervaning".
>
> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?
> Personally, I think that the former applies. The rightward crosswind
> only displaces the airplane to the right. Only the relative motion of
> the airplane with respect to that airmass would induce the
> weathervaning effect. I presume that the airplane does not know,
> aerodynamically, of the left crosswind.
>
> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?
>
> Alex

[CASK829]

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

[Peter Duniho]

"Koopas Ly" > wrote in message
m...
> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?
> Personally, I think that the former applies.

Yes, the former applies. It doesn't have anything to do with the crosswind,
and has everything to do with the airplane's sideways motion through the
airmass. The vertical stabilizer tends to orient the airplane into the
relative wind, and in a slip, you are trying to maintain an orientation at
an angle to the relative wind. Rudder is necessary to counteract the
vertical stabilizer's normal force.

(Oversimplifying, of course, since there are other forces involved that act
in a variety of directions, including both with and against your rudder
input).

> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?

I disagree. Because of inertia, a change in the air mass's momentum will
momentarily not be compensated for by the airplane's configuration. Until
the airplane "catches up" with the air mass, the relative wind is from the
left, and will cause a temporary yaw force turning the airplane to the left.
The force will gradually diminish as the airplane accelerates in the
direction of the new movement of the airmass. The airplane will remain in
this orientation unless the pilot adjust for it (and of course, the pilot
most likely will).

Pete

[Robert Moore]

(Koopas Ly) wrote

> You'd also use some right rudder to keep the nose straight and
> prevent it from "weathervaning".
> Is this "weathervaning effect" caused by your leftward relative
> motion due to the left bank OR by the rightward crosswind
> ITSELF?

There is NO weathervaning effect until the wheels touch the ground.
Banking an airplane (putting a wing down) causes it to turn. You
use opposite rudder simply to keep it from turning due to the bank.

Bob Moore

[Maule Driver]

Great question!!
"Koopas Ly" > >
> During a crosswind landing, for instance a left crosswind, you'd lower
> the left, upwind wing to counter the right drift induced by the
> crosswind.
>
> You'd also use some right rudder to keep the nose straight and prevent
> it from "weathervaning".

This is a proper xwind landing conditon and is a slip (forward vs side is
debatable). You are banking left into the xwind to counteract the drift and
get the a/c to track down the centerline. You apply enough righ rudder to
align the nose with your direction of movement (i.e. the centerline). I
would not call this weathervaning.

> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?
> Personally, I think that the former applies. The rightward crosswind
> only displaces the airplane to the right. Only the relative motion of
> the airplane with respect to that airmass would induce the
> weathervaning effect. I presume that the airplane does not know,
> aerodynamically, of the left crosswind.

I'll leave weathervaning undefined but the last line is correct - as long as
the a/c is airborne, it does not know of the left crosswind. The xwind is
relative to the ground and if you are not touching the ground, then you
might as well be at 30,000feet
>
> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?
>
Yes & No. The the a/c will weathervane into wind. That is, the nose will
tend to point into the wind. The wheels represent the vertical axle of the
weather vane. Assume 1 wheel (glider) on the CG. You could grab a wingtip
and rotate with little effort. The wind hits the side of the aircraft. End
of of the a/c with the most side area will be rotated downwind. The tail,
like feathers on an arrow, will always be that end.

Now the whole a/c will also tend to be displaced left to right. But imagine
same single wheel and try to grab a wingtip and drag the a/c against the
friction of the wheel. You won't be able to budge it.

So, the net effect is that the weathervane effect will be immediate and
dominant with weight on the wheel and low speed. At higher speeds and
lighter weights, the drift will take over. What happened to weather vaning?
It is still their but the forward motion of the a/c changes the net wind
vector and the a/c ends up 'weathervaning' closer to straight ahead than
towards the xwind.

A gllider tow from a dead stop in a xwind demonstrates all that very nicely.
As soon as you start rolling, balanced on one wheel, the glider will rotate
(vane) right into the wind. So downwind rudder is required at first. As
speed picks up, both the rudder becomes more effective and the relative wind
vector shifts towards your intended direction of flight so less downwind
rudder is required. As the wing starts lifting the weight off the wheel,
the glider starts drifting downwind with the tire scrubbing sideways.
Dipping the upwind wing fixes that and you leave the ground in a slip. As
soon as the wheel is off the ground, you can basically let go of the
controls for a second and the plane will assume an immediate wings level
crab relative to the ground and off you go. This is less complicated than
it sounds but you have to do it a few times before you can do it right.
Until that time, the pull of the towrope on a quickly accelerating tow plane
will keep you out of the weeds while you regain your dignity. (BTW, all that
happens in reverse on landing but again, the stars are favorable and
decreasing energy tends to mask problems at the end. Alight gracefully and
complain about sudden 'turbulence' on rollout.

On a powered trike (C150), the friction of 3 wheels and a favorable
configuration of CG and CP will make all of that invisible in light xwinds.
Conversely, a J3 will amplify all of the above and the weeds await the
unwary.

[Peter Duniho]

"Robert Moore" > wrote in message
. 7...
> There is NO weathervaning effect until the wheels touch the ground.

Of course there is. There is "weathervaning" any time the relative wind is
not parallel to the longitudinal axis.

> Banking an airplane (putting a wing down) causes it to turn.

Actually, in some aircraft, banking an airplane causes it to turn in a
direction *opposite* of the bank. Rudder is necessary in a turn, to make
sure the turn is coordinated and is as efficient as possible. Many
airplanes, without the use of rudder, will simply slip sideways without any
change in heading at all.

> You
> use opposite rudder simply to keep it from turning due to the bank.

That's one way to look at it. But think about why the airplane is turning
due to the bank. The yaw occurs because bank alone causes a slip, which
causes the relative wind to come from an angle to the vertical stabilizer,
which causes yaw. That yaw can be described as "weathervaning" and the
rudder is used to counteract it.

Pete

[G.R. Patterson III]

Koopas Ly wrote:
>
> During a crosswind landing, for instance a left crosswind, you'd lower
> the left, upwind wing to counter the right drift induced by the
> crosswind.
>
> You'd also use some right rudder to keep the nose straight and prevent
> it from "weathervaning".

No, I would use right rudder to keep the nose straight.

> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?

Weathervaning is *always* caused by the wind. That's the source of the name of
the effect.

> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?

Depends on my airspeed. If I'm close to touchdown, my aircraft will turn into
the wind. Regardless of speed, it will also drift.

George Patterson
If you're not part of the solution, you can make a lot of money prolonging
the problem.

[Kobra]

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
>
>

[Ron Natalie]

"Robert Moore" > wrote in message . 7...
> (Koopas Ly) wrote
>
> > You'd also use some right rudder to keep the nose straight and
> > prevent it from "weathervaning".
> > Is this "weathervaning effect" caused by your leftward relative
> > motion due to the left bank OR by the rightward crosswind
> > ITSELF?
>
> There is NO weathervaning effect until the wheels touch the ground.
> Banking an airplane (putting a wing down) causes it to turn. You
> use opposite rudder simply to keep it from turning due to the bank.
>
It's sort of the opposite of weathervaning. The vertical stab will normally keep the
tail in line with the aircraft (coordinated flight). Using the rudder will
force it out of line (and in alignment with the direction of travel on the ground).

[David Megginson]

"Roger Long" m> writes:

> As far as the plane is concerned, there is no such thing as wind until the
> wheels touch the ground.

Unless the wind changes too fast (i.e. sudden gusts, wind shear, etc.).


All the best,


David

[David Megginson]

Robert Moore > writes:

> There is NO weathervaning effect until the wheels touch the ground.
> Banking an airplane (putting a wing down) causes it to turn. You
> use opposite rudder simply to keep it from turning due to the bank.

In flight and on the ground (even tied down), the aircraft tries to
weathervane into its relative wind, which is aircraft-referenced
rather than ground-referenced. That's why the plane turns when you
bank the wings (the vstab makes the plane weathervane towards the
sideslip).

Only when the plane is not moving or moving very slowly on the ground
is the relative wind from roughly the same direction as the (static)
wind that the windsock sees.


All the best,


David

[David Megginson]

"Peter Duniho" > writes:

> Actually, in some aircraft, banking an airplane causes it to turn in a
> direction *opposite* of the bank. Rudder is necessary in a turn, to make
> sure the turn is coordinated and is as efficient as possible. Many
> airplanes, without the use of rudder, will simply slip sideways without any
> change in heading at all.

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?


All the best,


David

[G.R. Patterson III]

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

Maules, for one. Give mine aileron and no rudder, and she will quite happily
fly sideways on the same heading. Something about "adverse yaw"?

George Patterson
If you're not part of the solution, you can make a lot of money prolonging
the problem.

[Roger Long]

That's what I said. Did you read the whole thing?
--
Roger Long

David Megginson > wrote in message
...
> "Roger Long" m> writes:
>
> > As far as the plane is concerned, there is no such thing as wind until
the
> > wheels touch the ground.
>
> Unless the wind changes too fast (i.e. sudden gusts, wind shear, etc.).
>
>
> All the best,
>
>
> David

[David Megginson]

"G.R. Patterson III" > writes:

> Maules, for one. Give mine aileron and no rudder, and she will quite
> happily fly sideways on the same heading. Something about "adverse
> yaw"?

At what bank angle, and for how long? With very draggy ailerons, I
can imagine that there might be one bank angle where drag from the
aileron on the high wing exactly counteracted the weathervaning
tendency of the plane, but it would be amazing if that were true of
every bank angle up to 30 deg (for example).


All the best,


David

[David Megginson]

"Roger Long" m> writes:

> That's what I said. Did you read the whole thing?

I thought I had, but I missed that part. Apologies.


All the best,


David

[Robert Moore]

"Peter Duniho" > wrote

> Of course there is. There is "weathervaning" any time the
> relative wind is not parallel to the longitudinal axis.

But I think that the discussion was weathervaning into the
actual wind, not the relative wind. If I bank away from the
wind, does the airplane then weathervane into or away from
the 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. Without the pivot, a weathervane
doesn't weathervane.

Bob

[Roger Long]

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
> >
> >
>
>

[Teacherjh]

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)

[Gary Mishler]

"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

[Gary Mishler]

"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.

[Peter Duniho]

"Roger Long" m> 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

[Peter Duniho]

"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

[David Megginson]

(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

[Peter Duniho]

"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

[Philip Sondericker]

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.

[Peter Duniho]

"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

[Kobra]

I think we are in agreement, but talking about it differently. Here is the
original poster's question:

"say you're dead on centerline on landing, *and all of a sudden* a crosswind
from the left starts blowing."

And my answer to the *all of a sudden* we have a crosswind question and he
wanted to know what is it that would make the plane "weathervane" in an *all
of a sudden* wind...my answer was:

"It's from the wind striking the vertical stabilizer and pushing the nose
into the wind." Which is correct as an *all of a sudden wind* = GUST!

Kobra





"Roger Long" m> wrote in
message ...
> 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
> > >
> > >
> >
> >
>
>

[Peter]

Gary Mishler wrote:

> "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".

The term "weathervane" is also frequently used to describe the action of a
plane in flight and its orientation compared to the local relative wind.
For example, the site
http://www.faatest.com/books/FLT/Chapter17/VerticalStability.htm says:
"Yawing or directional stability is the more easily achieved stability in
airplane design. The area of the vertical fin and the sides of the fuselage
aft of the center of gravity are the prime contributors which make the
airplane act like the well known *weathervane* or arrow, pointing its nose
into the relative wind.

In examining a weathervane it can be seen that if exactly the same
amount of surface were exposed to the wind in front of the pivot point as
behind it, the forces fore and aft would be in balance and little or no
directional movement would result. Consequently, it is necessary to have a
greater surface aft of the pivot point that forward of it.

Similarly in an airplane, the designer must ensure positive directional
stability by making the side surface greater aft than ahead of the center
of gravity (Fig. 17-31). To provide more positive stability aside from that
provided by the fuselage, a vertical fin is added. The fin acts similar to
the feather on an arrow in maintaining straight flight."

So the regular weathervane has a mechanical pivot point and the plane (or
arrow) has its center-of-gravity about which it pivots, but both act in the
same way to turn into the direction of the relative wind.

> 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.

With the rare exception of a few planes doing acrobatic maneuvers, all the
ones I've seen flying are in fact pointing into their relative wind (or at
least within a few degrees of it). You can still turn since your relative
wind will also change direction as you change course, but you'll always
have the nose of the plane facing into the relative wind.

> 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.

Once she's airborne there's essentially no crosswind since the nose will be
kept pointing into the relative wind; largely thanks to that large vertical
stabilizer which keeps the center-of-pressure behind the center-of-gravity
and makes the plane act like a windvane.

[G.R. Patterson III]

David Megginson wrote:
>
> "G.R. Patterson III" > writes:
>
> > Maules, for one. Give mine aileron and no rudder, and she will quite
> > happily fly sideways on the same heading. Something about "adverse
> > yaw"?
>
> At what bank angle, and for how long?

Something like 15-25 degrees for about as long as you want to do it.

George Patterson
If you're not part of the solution, you can make a lot of money prolonging
the problem.

[Koopas Ly]

Whoa! Lots of replies. From reading all of them, I can't see a
concensus. However, I've amassed all the information and have
concluded the following:

In my first question, I believe the airplane is indeed weathervaning
during the left bank to counter the left crosswind. At the moment the
airplane is banked to the left, it is slipping. The relative wind
momentarily comes from the left, and thus the airplane weathervanes to
the left to align with the relative wind. Hence, right rudder is used
to keep the nose in line with the runway.

Regarding my second question, I can understand the variety of answers.
I believe I used the terminology "all of a sudden, a left crosswind
starts blowing". I guess it's a rather improbable scenario. From
what I gather, if the crosswind comes in the form of a sudden *gust*,
then, the relative wind would be somehow vectorially changed to now
include a slight slip component. Thus, a small weathervaning effect
to the left would be noticed in addition to the physical rightward
crosswind drift.

However, if the left crosswind comes *gradually* (in a perfect world),
I presume the airplane would only drift rightward with no directional
change.

Am I way off base here?

Thanks for your replies,
Alex (who just finished typing up his NASA form)

[Peter Duniho]

"Koopas Ly" > wrote in message
om...
> Am I way off base here?

No, I think you understand it reasonably well.

As far as the plausibility of your second scenario, I don't think it's as
implausible as you imply. Wind shear is a funny thing (funny strange, not
funny ha ha) and you could find yourself descending through or flying past
different air masses, resulting in just such a gust on final approach.

Fortunately, light planes have relatively little inertia and it doesn't take
much extra airspeed to insure against gusts. But I wouldn't say that the
scenario you proposed is all that improbable.

Pete

[Cub Driver]

>During a crosswind landing, for instance a left crosswind, you'd lower
>the left, upwind wing to counter the right drift induced by the
>crosswind.

You also want to prevent the wind getting under the wing and flipping
the plane.

all the best -- Dan Ford
email: (put CUB in subject line)

see the Warbird's Forum at www.warbirdforum.com
and the Piper Cub Forum at www.pipercubforum.com

[Cub Driver]

>Is this "weathervaning effect" caused by your leftward relative motion
>due to the left bank OR by the rightward crosswind ITSELF?

I vote for the crosswind. On a taildragger, where the CG is behind the
mainwheels, you are always concerned about the effect of a crosswind
on the fuse and tail. This is as true on takeoff as it is on landing,
and it is true when there is no relative motion (because the tires are
still on the ground).

all the best -- Dan Ford
email: (put CUB in subject line)

see the Warbird's Forum at www.warbirdforum.com
and the Piper Cub Forum at www.pipercubforum.com

[Cub Driver]

>This is a proper xwind landing conditon and is a slip (forward vs side is
>debatable). You are banking left into the xwind to counteract the drift and
>get the a/c to track down the centerline. You apply enough righ rudder to
>align the nose with your direction of movement (i.e. the centerline). I
>would not call this weathervaning.

Neither would I. Perhaps taildraggers look at these things
differently?

When I read the post, I just assumed that the poster was worried about
weathervaning once he was on the ground. Isn't that why everyone
forward-slips in a crosswind? You can solve the drift problem much
easier by crabbing.

all the best -- Dan Ford
email: (put CUB in subject line)

see the Warbird's Forum at www.warbirdforum.com
and the Piper Cub Forum at www.pipercubforum.com

[Cub Driver]

>concensus.

Note that the word is consensus.

The answer should make SENSE.

It does not suffice to ake a vote (i.e., a CENSUS).

This is a serious matter and could lead to the death of democracy.

all the best -- Dan Ford
email: (put CUB in subject line)

see the Warbird's Forum at www.warbirdforum.com
and the Piper Cub Forum at www.pipercubforum.com

[David Megginson]

"G.R. Patterson III" > writes:

> Something like 15-25 degrees for about as long as you want to do it.

That's interesting. Have you ever tested it formally and recorded the
results, i.e. write down your starting heading, hold a 15 degree bank,
set a timer for two minutes, write down your ending heading, and then
level out again?


All the best,


David

[Gary Mishler]

"Peter" > wrote in message
news:dAYrb.121381$9E1.602249@attbi_s52...
> Gary Mishler wrote:

> > 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.

When I wrote the above I was referring to actual wind because I thought
(right or wrong) that was what the post I was replying to was referring to.

> With the rare exception of a few planes doing acrobatic maneuvers, all the
> ones I've seen flying are in fact pointing into their relative wind (or at
> least within a few degrees of it).

Absolutly correct when you are talking about relative wind rather than
actual winds.
Again, I was talking actual winds and you were talking relative wind.

This thread got very confusing because posts and replies were mixing
relative vs. actual wind and some were confusing coordinated turns vs side
slips.

I think if everyone goes back through the thread we would find that we all
would agree *IF* we were all on the same page using the same assumptions of
actual/relative wind and turn/slip.

[G.R. Patterson III]

David Megginson wrote:
>
> That's interesting. Have you ever tested it formally and recorded the
> results, i.e. write down your starting heading, hold a 15 degree bank,
> set a timer for two minutes, write down your ending heading, and then
> level out again?

Nope. I've just regarded it as a "circus trick". If I'm taking another pilot
up for his first flight in a Maule, I may demonstrate the phenomenum just for
laughs.

George Patterson
If you're not part of the solution, you can make a lot of money prolonging
the problem.

[Verbs Under My Gel]

"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
>

Yes, but a weather vane's COG, if you will, is affixed to terra firma,
unlike an airplane in flight.

[Verbs Under My Gel]

"Gary Mishler" > wrote in message news:<fBXrb.164593$HS4.1334282@attbi_s01>...
> "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

....hence the difference between "course over ground" and heading.

[Julian Scarfe]

"Koopas Ly" > wrote in message
m...
> Just a quick question...
>
> During a crosswind landing, for instance a left crosswind, you'd lower
> the left, upwind wing to counter the right drift induced by the
> crosswind.
>
> You'd also use some right rudder to keep the nose straight and prevent
> it from "weathervaning".
>
> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?
> Personally, I think that the former applies. The rightward crosswind
> only displaces the airplane to the right. Only the relative motion of
> the airplane with respect to that airmass would induce the
> weathervaning effect. I presume that the airplane does not know,
> aerodynamically, of the left crosswind.

From the point of view of the aerodynamics, the only thing the airplane
"knows" is the sideslip angle. It doesn't matter if you perceive this as a
forward slip (nil wind, tracking 270, heading 280) or a side slip (S'ly wind
causing 10 knots right drift, tracking 270, heading 270). In both cases
there's a sideslip angle of 10 degrees.

Is the sideslip angle because of the left bank or the rightward (from the
left?) crosswind? Is my mousemat compressed because the mouse presses down
on it or because the table presses up on it? :-)

If there is a sideslip angle there is a yawing moment, because of the
lateral lift from the fin. You can counter this with rudder, or let the
airplane yaw at a rate that increases until the natural yaw damping moment
balances it, as it does in a turn if you don't use rudder.

> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?

No, not if you make no change in control inputs. You've introduced a
sideslip angle, so the airplane yaws.

Julian Scarfe

[Kobra]

Read the original poster's question. He asked, "...say you're dead on
centerline on landing, *and all of a sudden* a crosswind
from the left starts blowing..." That's a gust and he wanted to know what
force was turning the plane into the wind. An "all of a sudden wind" will
"weathervane" the plane. That was the question, that is the answer...we've
been splitting hairs for 50 posts now.

Kobra




"Verbs Under My Gel" > wrote in message
om...
> "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
> >
>
> Yes, but a weather vane's COG, if you will, is affixed to terra firma,
> unlike an airplane in flight.

[David CL Francis]

On Mon, 10 Nov 2003 at 13:40:27 in message
>, Koopas Ly
> wrote:

>Next thing I was wondering, which is related to the above: say you're
>dead on centerline on landing, and all of a sudden a crosswind from
>the left starts blowing. The effect would be that you should only be
>displaced to the right of runway centerline. Your airplane nose would
>still be parallel to the centerline. Do you agree?

If the cross wind _suddenly_ started blowing then it would be a sideways
gust. An aircraft does not respond instantly to a sharp edged gust ,so
for a short time the aircraft is now yawed. Its natural yaw stability
and its inertia would then cause it to rotate in yaw until it is left
slightly turned and heading in a slightly different direction. So in
that case unless you put in control inputs you will not be just
displaced parallel to your original course. The amount depends on the
size and sharpness of the gust and the aerodynamic and mass
characteristics of the aircraft.

Gusts and flying in a steadily moving mass of air are not the same
thing.
--
David CL Francis

[David CL Francis]

On Mon, 10 Nov 2003 at 18:05:54 in message
>, Kobra
> wrote:

>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".

The centre of a weathervane is fixed to the ground - a different
situation from a free flying aircraft. Only gusts can have that effect,
steady winds cannot. With the wheels on the ground an aircraft does
become susceptible to a 'weathervane' effect.
--
David CL Francis

[CASK829]

Keep me the hell away from your area of flight you don't know a damn thing now
do you. You probably think that down wind turns will cause you to crash.
At least have some knowledge of how and airplane works before you try and fly
the damn thing

John
CFII
ATP
PHD aeonautical engineering UT Austin.

>do I fly?! Don't be a wise ass...why do you think they call it
>"weathervaning"?

[ken]

both - you will get secondary effects from the aileron bank causing yaw and
the aircraft will "weathercock" into wind. For those sceptics who don't
believe the aircraft will weathercock fly hands off with no rudder trim/bias
(and reduced power so there is no slipstream effect) and see what happens,
it will turn into wind.

--


KB

"Koopas Ly" > wrote in message
m...
> Just a quick question...
>
> During a crosswind landing, for instance a left crosswind, you'd lower
> the left, upwind wing to counter the right drift induced by the
> crosswind.
>
> You'd also use some right rudder to keep the nose straight and prevent
> it from "weathervaning".
>
> Is this "weathervaning effect" caused by your leftward relative motion
> due to the left bank OR by the rightward crosswind ITSELF?
> Personally, I think that the former applies. The rightward crosswind
> only displaces the airplane to the right. Only the relative motion of
> the airplane with respect to that airmass would induce the
> weathervaning effect. I presume that the airplane does not know,
> aerodynamically, of the left crosswind.
>
> Next thing I was wondering, which is related to the above: say you're
> dead on centerline on landing, and all of a sudden a crosswind from
> the left starts blowing. The effect would be that you should only be
> displaced to the right of runway centerline. Your airplane nose would
> still be parallel to the centerline. Do you agree?
>
> Alex

[Hilton]

ken wrote:
> both - you will get secondary effects from the aileron bank causing yaw
and
> the aircraft will "weathercock" into wind. For those sceptics who don't
> believe the aircraft will weathercock fly hands off with no rudder
trim/bias
> (and reduced power so there is no slipstream effect) and see what happens,
> it will turn into wind.

Well, you could look at as you describe, or just say that the plane doesn't
like slips/skids (as a result of its inherent stability) which is the way I
think of it.


> Koopas Ly wrote:
> > Just a quick question...
> >
> > During a crosswind landing, for instance a left crosswind, you'd lower
> > the left, upwind wing to counter the right drift induced by the
> > crosswind.
> >
> > You'd also use some right rudder to keep the nose straight and prevent
> > it from "weathervaning".
> >
> > Is this "weathervaning effect" caused by your leftward relative motion
> > due to the left bank OR by the rightward crosswind ITSELF?

What's the difference? Remember, we always think about 'relative' motion.


> > Next thing I was wondering, which is related to the above: say you're
> > dead on centerline on landing, and all of a sudden a crosswind from
> > the left starts blowing. The effect would be that you should only be
> > displaced to the right of runway centerline. Your airplane nose would
> > still be parallel to the centerline. Do you agree?

No, due to the aircraft's stability, it would yaw to the left.

Hilton