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In this video when the aircraft is turning left(2:25) the spoilers on the wing move. What causes them to move?

a320 tunisair

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Two reasons that are really one reason. In short: Because lift.

In this case though, it's "to get rid of lift". The spoiler's job is to reduce the amount of lift the wing experiences. This can be done symmetrically, on both wings at the same time, to increase the rate of descent or keep an aircraft from lifting back off the ground when in ground effect, or in this case, it can be done asymmetrically, or only on one wing.

In order to turn, an aircraft has to create an imbalance of force, more on the side outside the turn, less on the inside of the turn. The wing to the inside of the turn needs to drop, but it doesn't really want to...it has lots of air pushing it up (when it doesn't, you will have a very bad day)!

The spoiler in this case is just acting as an extension of the aileron. By moving upward with the aileron itself, it spoils the airflow over the top of the wing a bit, reducing the lift, and causing the wing to drop a bit, raising the other wing. It also acts as a larger surface area for the incident airflow to impart force on, which also pushes the wing down.

On the other side, the aileron is doing the opposite, increasing the wing's effective camber, at least near the tip, and increasing lift. Flaps do the same, but also are usually used symmetrically to increase lift on both wings. In some airplanes, you can have a 'flaperon', which can also be used as an additional control surface while also doing it's flap-thing.

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  • $\begingroup$ this sort of answers the question, but not really. Most aircraft use opposite aileron on the inside wing, but as you can see in the picture, aileron is in its normal position. The question in this case is: why is this plane using the spoiler to reduce lift on the inside wing instead of the aileron? the answer is in this question: aviation.stackexchange.com/questions/140/… $\endgroup$
    – rbp
    Feb 15, 2016 at 18:48
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That is controlled by the pilot (indirectly via the computer system) who uses the spoiler to control the turn of the airplane instead of ailerons. Notice that the airplane turns when the spoilers move. It does so by killing the lift on one side by "spoiling" the airflow while the lift on the other wing creates the roll and bank moment with the "unspoiled" lift.

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  • $\begingroup$ no, its done automatically $\endgroup$
    – rbp
    Feb 15, 2016 at 18:45
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    $\begingroup$ The pilot does not directly directly control asymmetrical spoiler deployment. In the case of the aircraft in the question, the flight control computer commands asymmetrical spoiler deployment based on it's interpretations of the autopilot or pilot control commands combined with other flight data inputs. $\endgroup$
    – J W
    Feb 15, 2016 at 18:47
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    $\begingroup$ that is a very literal answer to the question! $\endgroup$
    – FreeMan
    Feb 16, 2016 at 21:53

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