"Weathervaning" AFTER takeoff

[FastEddieB]

Airplane Flying Handbook:

As both main wheels leave the runway and ground friction no longer resists drifting, the airplane will be slowly carried sideways with the wind unless adequate drift correction is maintained by the pilot. Therefore, it is important to establish and maintain the proper amount of crosswind correction prior to lift-off by applying aileron pressure toward the wind to keep the upwind wing from rising and applying rudder pressure as needed to prevent weathervaning.

INITIAL CLIMB
If proper crosswind correction is being applied, as soon as the airplane is airborne, it will be sideslipping into the wind sufficiently to counteract the drifting effect of the wind. [Figure 5-5] This sideslipping should be continued until the airplane has a positive rate of climb. At that time, the airplane should be turned into the wind to establish just enough wind correction angle to counteract the wind and then the wings rolled level. Firm and aggressive use of the rudders will be required to keep the airplane headed straight down the runway. The climb with a wind correction angle should be continued to follow a ground track aligned with the runway direction. However, because the force of a crosswind may vary markedly within a few hundred feet of the ground, frequent checks of actual ground track should be made, and the wind correction adjusted as necessary. The remainder of the climb technique is the same used for normal takeoffs and climbs.

[drseti]

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

That pretty well summarizes my (admittedly unconventional) approach, Eddie.

[Wm.Ince]

Airplane Flying Handbook:

As both main wheels leave the runway and ground friction no longer resists drifting, the airplane will be slowly carried sideways with the wind unless adequate drift correction is maintained by the pilot. Therefore, it is important to establish and maintain the proper amount of crosswind correction prior to lift-off by applying aileron pressure toward the wind to keep the upwind wing from rising and applying rudder pressure as needed to prevent weathervaning.

INITIAL CLIMB
If proper crosswind correction is being applied, as soon as the airplane is airborne, it will be sideslipping into the wind sufficiently to counteract the drifting effect of the wind. [Figure 5-5] This sideslipping should be continued until the airplane has a positive rate of climb. At that time, the airplane should be turned into the wind to establish just enough wind correction angle to counteract the wind and then the wings rolled level. Firm and aggressive use of the rudders will be required to keep the airplane headed straight down the runway. The climb with a wind correction angle should be continued to follow a ground track aligned with the runway direction. However, because the force of a crosswind may vary markedly within a few hundred feet of the ground, frequent checks of actual ground track should be made, and the wind correction adjusted as necessary. The remainder of the climb tech- nique is the same used for normal takeoffs and climbs.

Excellent Eddie!
Just what I expect from a professional.
Thank you.

[3Dreaming]

I was working out of town today, and missed all of this. Eddie, I don't agree that the airplane will start a turn in the direction of crosswind correction, unless you are holding in to much correction. I think the weathervane that some are experiencing is just the simple lack of keeping the aileron into the wind and rudder tracking the runway correction in place and letting the airplane switch to a crab for the crosswind.

[SportPilot]

.......

[FastEddieB]

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

That pretty well summarizes my (admittedly unconventional) approach, Eddie.

So, to be clear, you're in the camp that feels there IS a yawing moment caused by the wind that occurs after the wheels leave the ground?

I was thinking about this further last night, and this video came to mind:

http://youtu.be/b_WmjWAGkLI

Go to about 1:55. Even a 55k nearly direct crosswind does not seem to "weathervane" that third Cub tow plane. It behaved kind of like Andy alluded to: wind gets under the upwind wing and the plane just moves downwind with no weathervaning apparent. Admittedly, the pilot could have been counteracting this yawing moment, but if anything it seems the pilots were all scrambling to come around into the wind as soon as possible, rather than remain in the crosswind and being forced downwind. The Cub in question does not ever come around into the wind until the pilot eventually banks and turns it into the wind.

[drseti]

So, to be clear, you're in the camp that feels there IS a yawing moment caused by the wind that occurs after the wheels leave the ground?

Well, after the nosewheel (or tailwheel, as the case might be) has left the ground, anyway.

[dwayne]

Wouldn't different surface areas of the fuselage be a factor? I would think since the vertical stabilizer/rudder have more area exposed to a crosswind, they would be affected more. If not, then let's double their size (area) on the same fuselage. Still won't weathervane?

[MrMorden]

So, to be clear, you're in the camp that feels there IS a yawing moment caused by the wind that occurs after the wheels leave the ground?

Well, after the nosewheel (or tailwheel, as the case might be) has left the ground, anyway.

But, you don't want to allow the wind to yaw/weathervane you at THAT point...that would leave the mains on the ground and you turning to the edge of the runway! Much better to use rudder and upwind aileron and keep traveling straight, wouldn't you say?

[CTLSi]

......

[FastEddieB]

Wouldn't different surface areas of the fuselage be a factor? I would think since the vertical stabilizer/rudder have more area exposed to a crosswind, they would be affected more. If not, then let's double their size (area) on the same fuselage. Still won't weathervane?

I don't know.

Tailwheel aircraft like the Cub in my video have vastly more vertical surface aft. Yet that plane still just seemed to be pushed laterally downwind, not weathervane observably - even on the ground.

Reducto ad absurdum, I imagine a bowling ball with a pipe coming out of it and then a large vertical surface. We know that once in the air mass, it will have zero weathervaning tendency regardless of its configuration. It's the brief and transitory moment right after leaving the surface that's an issue. It seems as if the mass/inertia of the bowling ball might serve momentarily as a pivot point once launched into a wind. If that's the case, the same thing would logically be true to a lesser extent to an airplane with more vertical surface aft of its CG. I just don't think I observe the effect in any meaningful fashion.

[CharlieTango]

If I may talk in terms of what the plane "knows" in a laminar flow wind.

In the take off roll it knows there is a crosswind.
In the air it doesn't know there is a crosswind.

Assuming a rapid rotation with all 3 gear simultaneously leap into the air. In the first moment while the plane still knows there is a crosswind it will yaw into the wind due to its yaw stability, that's why it feels automatic.

Very soon the plane is flying into the relative wind mostly on its nose and it will drift if you didn't let it crab sufficiently into the wind.

[FastEddieB]

Assuming a rapid rotation with all 3 gear simultaneously leap into the air. In the first moment while the plane still knows there is a crosswind it will yaw into the wind due to its yaw stability, that's why it feels automatic.

Sounds logical*.

Can anyone demonstrate it for me? I'd just like to see a video showing how much yaw actually occurs. I'll do it myself when I have the right conditions.

If it's there, either I missed it all these years or it's negligible enough that I've gotten away with ignoring it.


*But then why did the Cub in the video not point into the wind on its own? Nor my model?

[zaitcev]

So, to be clear, you're in the camp that feels there IS a yawing moment caused by the wind that occurs after the wheels leave the ground?

Surprisingly enough, there is. That is because airplane is slipping against the wind while in contact in the ground. If you were in the air, you would not be able to maintain this slip without rudder input. If you center the rudder at the moment of takeoff, the wind will exert yaw on you until the airplane weathervanes into the wind and becomes "embedded into the air soup". Note that the amount of this weathervaning is not the angle at which the wind blows, even relatively to the speeding airplane. It's less, because simultaneously with this weathervaning the airplane accelerates sideways and its slip angle decreases quicker than it turns. This is a purely transitional effect due to airplane's inertia. Certainly airplane does not know the direction of the wind while in the air.

[FastEddieB]

Thanks.

Very similar to the explanation an engineer gave over on Pilots of America, which I find pretty convincing.

I'm going to be looking carefully for the effect going forward. I think it must be very brief and small to have missed it all these years, and to never remember it being taught. The Airplane Flying Handbook which I quoted also doesn't seem to think its worth mentioning.

I still find what happened to both my model and the Cub in the video as somewhat disconfirming to the theory, but I care more about what's true than my (perhaps) wrongly held beliefs.

"I stand corrected" are still words I long to type. They mean I learned something new!