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Crosswind takeoff technique(s)

Started by cagarini, Wed, 7 Nov 2018 07:07

cagarini

Good morning!

I was trying to find evidence, here at the forums and also in the rest of the Internet ( Youtubes mostly ) on how a 744 is controlled under near limit crosswind conditions on takeoff.

The question, I must confess, arose from a discussion on the way crosswind operations on ground are modelled in X-Plane, and in particular in the default ( but not only ) Boeing 744 that comes with that simulator install.

I believe PSX models it very plausibly, and if I set, say, a 30 knot effective x-wind from port side, even while stopped before the takeoff run and if under certain GWs, there is a notable bank downwind, but as soon as my takeoff run starts I use mostly rudder to counter the weathervane, complementing it with aileron input as I begin the rotation.

Now it happens - in that other flight simulator - that a permanent full yoke into the wind and rudder against it ( full cross-control ) is required under such circumstances ( ? )  and even so, we can't stop the bank!

I wonder how you real 744/77/67/57 drivers cope with it ?


Full yoke deflection at the very beginning of the takeoff run is mostly "psychological" / "placebo" effect since the efficiency of the control surfaces ( even more that of the downwind side control surfaces ) is negligible at such low speeds, but the ailerons, and the spoiler groups that complement their action, are certainly effective near rotation, or even after V1 ( ? ) .


Another question is the implementation of any form of "aileron droop" in the 744. Apparently modeled in the 748i, I wonder if it's also available in the -400s ?


Than you Gentlemen for your answers / hints :-)

Holger Wende

I recall the technique was described in the "Technique Monthy"
. The. Link should be here in the forum as well.
I think it was recommended to reduce aileron input with increasing speed durung takeoff roll.

Regards Holger

cagarini

#2
Thx Holger!

Unfortunately the link to "Technique Monthly" no longer works here :-/

But thanks to your post I did find yet another interesting post:

http://aerowinx.com/board/index.php?topic=4479.msg47423#msg47423

that I will explore trying to find more about the Subject in this thread.

Also your hint on aileron use being reduced with increasing dynamic pressure makes all sense too.


Hardy Heinlin

#3
Sure, also on the ground the aileron forces gradually rise with the rising airspeed. Even if there is no crosswind, you can bank the aircraft to that angle where the counterforces of the flexible gear struts are equal. At extremely high speeds (where the tires will blow), some wheels on one side will lift off and a pod strike will occur on the other side.

It doesn't matter if it's a takeoff or a rollout. The autopilot too keeps the ailerons into the wind during autoland rollout. The autopilot computer doesn't do this because it gets crosswind data. The autopilot computer does it because it senses an external bank force, and so the autopilot deflects the ailerons to get at least wings level. In other words, the ailerons are able to bank the aircraft on the ground -- the higher the airspeed, the higher the aileron forces.


Regards,

|-|ardy


P.S.: On the ground at high airspeeds, you can even steer the aircraft to a certain degree by aileron banking alone. The cause of this may be a mix of asymmetric gear wheel friction (less wheel friction under the higher wing, more under the lower wing) and the aerodynamic lateral turn momentum generated by the banked V-shaped wing pair (the higher wing is closer to vertical, the lower wing is closer to horizontal; so the higher wing provides some slight "rudder like" lateral turn or sideslip forces).

cagarini

#4
Thx Hardy for the additional details.

I leave here a link to an answer at that thread I mentioned, from an X-Plane forum, where a 744 pilot explains it from he's pov too, which matches my experience in PSX exactly.

P.S.: Yet another link to a PPrune thread on this same subject, and the answer from a 744 driver:
https://www.pprune.org/4503819-post23.html

The whole thread has a link in that post too ( at the top right )...


emerydc8

Crosswind takeoffs are pretty simple. Use aileron to keep the wings level and rudder to keep it on the centerline. Whatever it takes. It could be full cross-control if the wind is strong enough. Just keep in whatever cross-controls you have during the rotation. As soon as you are airborne the aircraft will weather-vane and at that point you slowly let out the rudder and aileron input.

brian747

@Holger

> "I recall the technique was described in the "Technique Monthy"..."

Congratulations on your memory!     A trip to my PS1 archives found Rets discussing this issue in his "Technique Monthly #1", dated March 1998. After discussing the rate of rotation he goes on to say:

"Now you have that part under control think about keeping the wings level during the takeoff roll by keeping a bit of aileron into wind (not much, and even less as the aircraft accelerates). Then as you rotate start to ease it off."

Fairly brief and to the point — and thank you, jcomm, for the link to the post on PPRuNe, which is more in-depth. (I enjoyed the bit about using the shiny windows in the terminal as a giant mirror for demonstration purposes, lovely idea!).

Cheers,

Brian
(Author of "The Big Tutorial" for PS1, and "Getting started with PSX" Parts 1, 2, and 3).

United744

Basic technique:

Wind from the LEFT: LEFT aileron, RIGHT rudder.
Wind from the RIGHT: RIGHT aileron, LEFT rudder.

How much depends on the aircraft and wind conditions, but starting with lots at the start of the takeoff roll isn't a bad thing. You'll soon feel how much you need once she starts rolling during takeoff.

Do this even before you commence the takeoff roll, and ease out the ailerons as the airspeed builds (but don't remove them completely). Rudder will be obvious as the nose will start to swing one way or the other, so just apply necessary rudder to keep it rolling straight.

Additional notes:

When taxiing with a headwind, PUSH the control column forward, and ROLL INTO THE WIND.

When taxiing with a tailwind, PULL the control column backwards, and ROLL AWAY FROM THE WIND.

Whether this is really required on a 747 is debatable, but if the winds are very strong/gusty, it's a good idea. A lifting surface is a lifting surface, regardless of the aircraft type.

Be careful not to roll so much you deploy the roll spoilers (the flight control synoptic is useful for this until you get used to the feel).

For extra fun: do this on a mid-wing glider during a strong crosswind on a winch launch. That will soon teach you the correct technique! :D