Weathervaning

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

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

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

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

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

"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

(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

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.

"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

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.