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

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.

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.

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)

"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

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

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

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

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

"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