Weathervaning

"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

"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.

"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

"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).

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

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.

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.

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

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