"Weathervaning" AFTER takeoff

[FastEddieB]

As background, over on the Pilots of America site there was recently a discussion about crosswinds.

One person suggested they could feel their plane "weathervane" into a crosswind right after takeoff. Another poster concurred.

I suspected we had a "Stick and Rudder" teaching moment. I'll copy and paste a couple of my comments:

Post #1:

A weathervane and a plane on the ground have something in common: a pivot point that the wind can work against. In each case the larger vertical surface aft of the pivot point causes the weathervane or the plane to point into the wind.

But...

At the very instant the plane's tires leave the surface, there's no longer a pivot point for the wind to work against. From that point on, barring gusts, the plane will simply move sideways with the moving air mass with no tendency to point into the wind.

So, where does this illusion come from? I think you hinted at it with "holding correct crosswind control inputs". As the plane leaves the ground, some aileron should still be held into the wind. So the plane begins a turn into the wind, not due to "weathervaning" but simply due to the control inputs.

I'm pretty sure I'm right on this one, but am open to persuasion.

Post #2:

Let me restate that I am perfectly willing to accept that there may be a brief period right after takeoff when a crosswind can weathervane a plane.

But let me make one of two observations...

On zero-zero practice instrument takeoffs and departures, I never recall anyone telling me to look out for weathervaning right after takeoff that would need to be corrected. And I don't think I ever felt a need to warn my students about that.

For instance, taking off on RWY5 with an easterly wind, we would set the heading indicator to the runway heading and apply gradually decreasing right aileron into the wind, maintaining 050° with rudder. By takeoff, if no WCA was called for ("Maintain runway heading") a firm rotation would have the plane climbing wings level on a heading of 050°. I don't ever recall the plane "weathervaning" to 060° or 070° and needing to be steered back to the left to maintain runway heading.

But maybe it's subtle and I missed it. Do any CFII's recall having to teach that weathervaning after takeoff was something that one needed to routinely correct for?

Second observation to follow...

Post #3:

Observation(s) the second...

I have logged about 4,500 hours of dual given over the years, including lots of crosswind takeoffs.

And so I've seen my fair share of takeoffs with inadequate crosswind technique applied.

It sure seems to me that when that happens, the plane just gets "pushed" downwind on whatever heading it's on. Absent any turn to a wind correction angle to compensate for drift, it has not seemed to me that the crosswind "assists" the student in any way by making his WCA for him, via weathervaning or whatever.

Curious as to what other instructors have found.

One fellow, who was an engineer, suggested that for a brief period right after takeoff there would be a "weathervaning" tendency, the "pivot point" being the inertia of the CG, whereby the wind would tend to yaw the plane into it.

Anyway, I channeled Mythbusters and came up with this:



Here's the result:

http://youtu.be/NhCFAJybVyw?list=UUIRbX ... AUSLfkBO7w

Probably not a perfect experiment, but to scale the leaf blower was probably producing at least gale force winds at 90º. I would have thought if "weathervaning" was a real thing, we should have seen at least a tiny amount of yaw into it.

Interested in everyone's take, but especially the Professor's!

[CTLSi]

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[SportPilot]

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[FastEddieB]

CTLSi,

With all due respect, there's a reason I twice asked specifically for instructors to opine on their experience in this matter.

You managed, in just one post, to reinforce the fact that you have yet to internalize the effect of wind on an airplane in flight. I already knew that from at least a half dozen posts of yours that reveal the same general misconceptions. Even your "boat in a stream" attempt at analogy is flawed.

Anyway, your post, as wrong as it is, can only drag this discussion down at least one full level. This thread was not meant to debate basics.

As such, I'm going to go ahead and put you on ignore for at least the duration of interest in this thread. I would ask that others also ignore your posts to this thread, and to refrain quoting them, so we can tease out the actual mechanics of what happens in the split-seconds after an airplane leaves the ground in a crosswind. And not get bogged down in correcting a newcomer's very basic misconceptions.

Thanks!

[FastEddieB]

I would also ask if someone is going flying anyway, and can combine a crosswind takeoff with a GoPro or equivalent...

...to hold the plane on a tad longer than normal, then pull the airplane cleanly into the air holding the wings level and just enough right rudder to correct for the typical left-turning tendencies, starting out on runway heading. This will be to observe if, absent any control input, the plane will - on its own, yaw into the wind once the wheels are clear of the surface.

I will attempt the same in my Sky Arrow next time I'm up and the conditions are right.

[drseti]

Eddie,
I generally have my students hold steady elevator (stick very slightly aft of neutral) during the takeoff roll, and let the plane fly itself off. In a crosswind, when the nosewheel is off the ground and the mains are still in runway contact, there's a brief instant when the plane will indeed pivot on the mains and weathervane into the wind. I have my students note where the nose is pointing, and then manually keep it there once the mains clear the ground. By lucky coincidence (or perhaps by design), in my plane the resulting crab is just right for tracking runway centerline in the climbout!

[FastEddieB]

Exactly.

With aileron held into the wind, the plane is primed to bank into the wind as soon as the wheels leave the ground. As I said, maybe that masks any brief "weathervaning" period for me. And you.

I'm still on the fence as to whether it's a real thing or not. If it's there it must be VERY small and VERY brief. I think the pilots who saw it might have just been hit by a gust at the wrong time and presumed the yaw was a normal occurrence.

[Wm.Ince]

The concept is simple. The large tail on the plane acts like the tail on an old style windmill. The xwind simply pushes the tail in the direction of the xwind and the plane continue to power into that xwind. The velocity vector of the plane remains the same, going down the runway, but the nose of the plane is askew the runway.

The same concept applies to a powered boat in a fast moving river. As the water tries to push the boat down river, the rudder must be turned into the direction of current and the engine power set to equalize the current velocity. The boat will traverse the river from side to side as the nose is pointed at an angle upstream.

Pilots of America is filled with obtuse, often clumsy and incorrect information for new pilots.

All of it . . . . pure nonsense.

[MrMorden]

I had a relevant experience yesterday. I was taking off in a 8G14-ish crosswind at an unfamiliar (to me) airport. The runway is in a bit of a depression and has high trees on the upwind side. My buddy took off first, and warned me to be ready for a gust a few hundred feet down the runway where the hillside drops off and the trees thin out.

As anticipated, the gust hit, and it was stronger than I expected. Not as strong as Eddie's leaf blower, of course.

The airplane did NOT "weathervane". What it did do is roll about 20° to the left as the wind got under the upwind (right) wing. I corrected very quickly since I was ready for it, but it was still a good jolt. The airplane maintained the same heading, I just had to drop the right wing for a second to get back on centerline.

I'm glad I didn't weathervane, because I didn't want to spend the entire trip home in a 20° bank to hold course.

[SportPilot]

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[CharlieTango]

Some of us rarely see laminar flow wind. When you add shear into the mix the answer complicates because you do get shear induced weather vanning.

[Wm.Ince]

I had a relevant experience yesterday. I was taking off in a 8G14-ish crosswind at an unfamiliar (to me) airport. The runway is in a bit of a depression and has high trees on the upwind side. My buddy took off first, and warned me to be ready for a gust a few hundred feet down the runway where the hillside drops off and the trees thin out.

As anticipated, the gust hit, and it was stronger than I expected. Not as strong as Eddie's leaf blower, of course.

The airplane did NOT "weathervane". What it did do is roll about 20° to the left as the wind got under the upwind (right) wing. I corrected very quickly since I was ready for it, but it was still a good jolt. The airplane maintained the same heading, I just had to drop the right wing for a second to get back on centerline.

I'm glad I didn't weathervane, because I didn't want to spend the entire trip home in a 20° bank to hold course.

Andy,

Based upon all the wind information you had available before takeoff, as you began your takeoff roll, did you apply an aileron correction into the wind?

[MrMorden]

I had a relevant experience yesterday. I was taking off in a 8G14-ish crosswind at an unfamiliar (to me) airport. The runway is in a bit of a depression and has high trees on the upwind side. My buddy took off first, and warned me to be ready for a gust a few hundred feet down the runway where the hillside drops off and the trees thin out.

As anticipated, the gust hit, and it was stronger than I expected. Not as strong as Eddie's leaf blower, of course.

The airplane did NOT "weathervane". What it did do is roll about 20° to the left as the wind got under the upwind (right) wing. I corrected very quickly since I was ready for it, but it was still a good jolt. The airplane maintained the same heading, I just had to drop the right wing for a second to get back on centerline.

I'm glad I didn't weathervane, because I didn't want to spend the entire trip home in a 20° bank to hold course.

Andy,

Based upon all the wind information you had available before takeoff, as you began your takeoff roll, did you apply an aileron correction into the wind?

It was not needed on the takeoff roll, there was insignificant crosswind on the surface. The gust happened about ten feet off the ground, and literally lasted about a half second and then was gone. Something about the terrain there channeling the air across the runway in that one spot, I'd guess.

[FastEddieB]

The airplane did NOT "weathervane". What it did do is roll about 20° to the left as the wind got under the upwind (right) wing.

Thanks, Andy.

That's the kind of confirming anecdote I was looking for, though I'd also welcome disconfirming ones.

In all my years of instructing, I have never heard, nor given, the following advice:

"On a crosswind takeoff, just let the wind yaw you into it. That will take care of establishing a wind correction angle."

Or even,

"Right after takeoff, you'll feel the wind yaw you into it. Just go with it and then adjust to get just the right wind correction angle."

Has any pilot been told either of those, or any instructor so taught?

Again, I've seen my fair share of poorly executed crosswind takeoffs with little or no correction made. My sense is still that the airplane just moves downwind, with no discernible yaw induced by the crosswind. My little Mythbusters demo did nothing to dissuade me from that view.

But I may still be wrong on this - a clear in-cockpit video of the effect would tell a lot.

[FastEddieB]

Some of us rarely see laminar flow wind. When you add shear into the mix the answer complicates because you do get shear induced weather vanning.

I'll stipulate that gusts and shear can move a plane around any of it's axes, in any direction, or vertically either up or down.

This thought exercise involves steady winds.