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Specific application of spoilers was discussed in What is the benefit of spoilerons compared to ailerons? As I remember it, 40yrs ago gliders had spoilers close to the wing tips - perhaps slightly inboard of the normal aileron position.

Current aircraft design places them mid-wing or even next to the wing root. What design principles favor a particular span-wise location.

To be specific; I am interested in roll control not drag.

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    $\begingroup$ But are those spoilers near wing root actually being used for roll control? I don't think so. The one on your picture certainly isn't—the spoilers are deployed symmetrically. And I don't think they use them that way at all. Airliners do, but that use always deploys the outboard panels only. $\endgroup$
    – Jan Hudec
    Commented Jan 6, 2018 at 19:44

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On airliners, spoilers serve three purposes, roll enhancement (assist the ailerons when greater roll rate is commanded), speed brakes (help slow down and descend faster), and ground spoilers (kill lift in order to transfer weight from the wings to the wheels for more effective braking). Spoiler span-wise location is a compromise to accomplish all three goals. Also, the outboard spoilers are primary for roll control and the most inboard spoilers may only be used on the ground. Spoilers are not placed in front of the ailerons because they would interfere with roll control. The chord-wise location leaves the wing box clear for fuel tanks.

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It has to do with the aerodynamic and the coefficient of the drag and lift. When the spoilers are close to the wingtips (end of the wing) the coefficient of the drag is less than when they are at the middle of the wing. The same happens when they are at the beginning of the wing. The drag coefficient is less. So at the middle of the wing, the drag coefficient is the highest, which fulfills the role of the spoilers. This is most likely the reason why people changed the position from the end of the wing to the middle, as this apparently is better.

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  • $\begingroup$ This is most likely because the length of a glider wing is smaller compared to an airliner's, and because of this, the position of the spoilers don't matter as much. $\endgroup$
    – Stallmp
    Commented Jan 5, 2018 at 10:41
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    $\begingroup$ Thanks for the comment, I saw that I had left ambiguity as to the specific purpose of the spoilers I was interested in - I am specifically interested in roll control and edited my question to reflect this (my fault not yours :). I certainly understand placement for DRAG only purposes. But mid-wing or inboard seems to defeat effective roll control. $\endgroup$
    – jwzumwalt
    Commented Jan 5, 2018 at 11:21

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