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In this video (minutes from 10:46 to 11:00), the spoilers seem to be slightly deployed during take off. What's the purpose of this? I've already seen this in other videos too.

I've also read the answers to the question When can spoilers be used on airliners, but they seem to be related to slowing down the aircraft or for descending faster. But what is the point during take off, when it's supposed the pilots want to climb and go faster?

Community
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Claudi
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    possible duplicate of When can spoilers be used on airliners? – fooot Aug 13 '15 at 16:18
  • @reirab I agree it's a bit different but casey addresses this use of spoilers multiple times in his answer. – fooot Aug 13 '15 at 16:21
  • @fooot Oh, you're right. I somehow missed that part when I read over his answer. – reirab Aug 13 '15 at 16:23
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    Claudix, the slight deployment here is being done automatically as part of roll control (in conjunction with the ailerons.) See the end of casey's answer on the linked question for more detail. – reirab Aug 13 '15 at 16:24
  • That was exactly my doubt, @reirab! I also read about the roll control in the possible duplicated question, but what shocked me was the fact to activate the spoilers in a such slightly way. Thank you! – Claudi Aug 13 '15 at 16:28
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    Yeah, when using them for roll control, the deflections are usually very slight. Spoilers are extremely effective at reducing the lift of a wing, so you don't want to deflect them too much in flight. – reirab Aug 13 '15 at 16:33
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    I'm voting to leave the question open, as I think if deserves its own answer, the duplicate being not very talkative about the use for roll control (e.g. why is it preferred to other surfaces). It seems the main reason is to increase the roll effect of the ailerons when flaps are extended (and more lift needs to be generated / countered to get the same amount of roll). – mins Aug 13 '15 at 17:55
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    Funny that none of the three answers mentions yaw control. Ailerons give plenty of roll control already, but create adverse yaw. Spoilers should help to counteract this undesired yaw, which is worst at high lift coefficients. – Peter Kämpf Aug 14 '15 at 04:48
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    Slightly off-topic, but pilots of some high-performance gliders take off with spoilers partially extended. The reason is to direct more air outboard across the ailerons, increasing control authority at low speeds and decreasing the change of dragging a wingtip along the ground. – KJP Aug 14 '15 at 13:54

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Spoilers lower lift and increase drag on the side where they are deployed. Symmetric spoiler deployment, therefore, is for slowing down. Asymmetric deployment not only creates a (small) rolling moment, but primarily a yawing moment which helps to counteract the adverse yaw created by ailerons, especially at the high lift coefficient at takeoff and early climb.

Spoiler deployment increases pressure ahead of the spoiler due to the slowing of local flow ahead of the obstruction caused by the spoiler, and it creates a local flow separation at and behind the spoiler, which in turn lowers the local pressure. Only the first effect lowers lift, but both create drag. The main effect of a limited spoiler deflection, therefore, is to increase drag.

Peter Kämpf
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  • Wouldn't the outboard spoilers be used for roll control? Because in the video two spoilers in the middle are slightly raised and the outer ones are not. – Jan Hudec Aug 14 '15 at 07:50
  • @JanHudec On this document (p.6) a scheme state only one spoiler per wing (out of5) is not used for roll control. – Manu H Aug 14 '15 at 09:32
  • @ManuH: Yes, but it includes the outboard ones and in the video the outboard ones don't move. The video is 737 though and I don't have a reference for what that uses for roll control. – Jan Hudec Aug 14 '15 at 12:46
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    Just to add to (not disagree with) this answer: spoilers will occasionally be used in strongly gusting wind conditions to ensure the aircraft does not accidentally lift off the runway before reaching V1. – Jon Story Aug 01 '16 at 14:29
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In this case, the flight spoilers are being used for roll control. Usually, the spoilers are used for roll control in concert with the ailerons, and are used during descent in order to slow down.

However spoilers are also used during take off in case of a cross wind.In that case, the pilot may want to reduce some lift in the upwind wing and employ spoilers instead of/along with the ailerons. A very small deflection in spoilers helps in increasing the drag over the wing and thus aid in roll control.

However, it is to be noted that the operation of spoilers during takeoff is very restricted in order to prevent flow separation.

In modern aircraft, the spoilers are used automatically with the ailerons. So, when the pilot operates the yoke, he is not consciously deciding to employ them, but rather the control system does it for him.

aeroalias
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Large jet aircraft have multi function spoilers. They aide in turning the aircraft at low speed. When the aircraft needs to be slowed down in the air and on landing all of the spoilers are activated.

Eric
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I agree with Eric. Many business jets gave a spoiler on feature which aids the pilot with roll control. Spoilerons are typically activated with a flaps below a preset level and aileron movement.

Spoilerons usually rise up when the aileron on the side rises. Thus, they aid in rolling the airplane.

Most likely on takeoff the pilots are correcting for winds and the spoileron system is active.

wbeard52
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