Weathervaning

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" om 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

"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

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:

"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/Cha...lStability.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.

"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.

"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.

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.

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"?

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

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