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What is the specific aerodynamic effect which requires one to hold into-wind aileron to keep the wings level during takeoff in a B737? (i.e. dihedral or sweep)

Subsequently, what is the aerodynamic reason for having to increase that into- wind aileron input immediately after Takeoff.

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  • $\begingroup$ I am not sure I have any idea about what you are talking about. $\endgroup$ – SMS von der Tann May 23 '16 at 21:04
  • $\begingroup$ I am referring to a crosswind takeoff situation where one has to hold the aileron down into the wind during the takeoff roll. Then after takeoff one has to increase the aileron input or the aircraft will roll rapidly downwind? $\endgroup$ – Minimax1 May 23 '16 at 21:16
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    $\begingroup$ @Minimax1 Typically you decrease the aileron input as airspeed increases, perhaps that's causing some confusion? Although I know nothing about B737s, so I have no idea what crosswind techniques to use in one. $\endgroup$ – Pondlife May 23 '16 at 21:32
  • $\begingroup$ I second Pondlife. I have never heard of increasing aileron input for crosswind correction after liftoff. May be a point of confusion. $\endgroup$ – Ryan Mortensen May 23 '16 at 23:46
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The upwind aileron is held down in order to keep the crosswind from pushing the airplane downwind. This is called crosswind drift. Usually the aileron begins fully deflected and reduced as it becomes more effective with increased airspeed. Upwind elevator is coupled with downwind rudder, which keeps the airplane in the centerline.

When I have taken off in a glider with a very strong crosswind, I have started the roll all the way on the downwind side of the runway and taken off diagonally upwind in order to to reduce the crosswind component.

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When a crosswind hits an aircraft it affects the windward side more because the fuselage shadows the leeward wing. Therefore, what happens is that the wind tends to lift up the wing on the windward side. This only happens when the plane is on the ground because what happens is that the wind fills the space between the ground and the wing and pressurizes this area. This pressurization is called ground effect. If the wind is a headwind (such as when landing), then the area under both wings is equally pressurized, but if there is a crosswind, then the windward side will be pressurized harder and tend to lift up the wing. In such cases the pilot needs to use the aelirons to counteract that effect.

Note that any upgust can have a similar effect. For example, when landing on a windy day, the wind will hit the ground and move upwards creating upgusts. These can hit your wing and flip you over, if the aircraft is light. Therefore, when landing a pilot will always be wing down to the wind.

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  • $\begingroup$ Thanks Tyler. What about the necessity to increase that aileron input to prevent aircraft roll after rotation? $\endgroup$ – Minimax1 May 23 '16 at 21:31
  • $\begingroup$ @Minimax1 Even in a small plane, crosswind has less effect when taking off because the aircraft is moving faster. $\endgroup$ – Tyler Durden May 24 '16 at 12:07

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