Weathervaning

"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

"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

"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

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.

"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" 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

"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

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)

(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