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This related question on Aviation SE describes how in most aircraft spoilers (flight spoiler mode not ground spoilers) are deploy-able at any speed But flaps on the other hand mostly have a V_FE above which they cannot be deployed.

I assume the speed limit imposed on using flaps is due to structural reasons? I am just wondering why spoilers do not have a similar limit? Looking at a spoiler photo in the deployed position it looks like the spoiler mechanism ought to be under a lot more stress than the flaps. Am I wrong here?

Or are spoilers reinforced to a degree out of need that is not felt necessary for flaps? i.e. There can be a legitimate need to deploy flight-spoilers at a speed higher than for flaps.

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Spoilers are used to shed energy when the aircraft is too fast or too high. That means they need to be used when the aircraft is flying fast and are designed accordingly.

On the other hand flaps are used to increase lift and when flying fast, there is plenty of lift, so there is no point in using flaps at high speed and therefore they are not designed for it.

Additionally flaps on large aircraft are more complicated mechanically. Spoilers only rotate around their forward edge, but flaps on large aircraft extend to the rear in addition to rotating downward.

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  • $\begingroup$ OK but this does not answer the why part of the question. Normally both of these surfaces will experience roughly the same aerodynamic forces. But what is the difference that makes spoilers more powerful? Complicated == fragile for flaps is not an argument by itself. One might argue, "it's more complicated to be more durable". $\endgroup$
    – Stelios Adamantidis
    Jul 26, 2015 at 15:39
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    $\begingroup$ @Stelios, the answer addresses why. Because the difference in what is useful. Perhaps I should delete the third paragraph; it's mostly irrelevant to the answer! $\endgroup$
    – Jan Hudec
    Jul 26, 2015 at 16:05
  • $\begingroup$ @SteliosAdamantidis Why are the forces that spoilers experience the same as flaps? Flaps have a greater surface area and should experience much higher forces for the same airspeed. $\endgroup$
    – Wayne Conrad
    Dec 17, 2016 at 16:54
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Spoilers reduce lift potential when deployed and increase drag. This makes them doubly helpful at high speed: Not only will the aircraft slow down, but the pilot has less of a chance to break something important when they are deployed.

Contrast this with flaps: They create only a moderate drag increase, but vastly increase the potential lift. If they would be deployed at high speed, the pilot (or an unexpected gust) could easily overstress not only the flap structure, but also the whole wing. Extending them at high speed is suicidal.

Therefore, some regulations demand that spoilers must be useable even beyond the maximum speed. For gliders, where the spoilers are called airbrakes, JAR part 22 says in AMC 22.143:

Compliance with 22.143(a) should include the extension of airbrakes at speeds up to 1.05 VNE. The time to extend airbrakes should not exceed 2 seconds.

For airliners the regulations are more lenient, and the manufacturer is free to limit their use as he sees fit. However, it is good practice to allow spoiler extension over the full speed range.

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    $\begingroup$ In typical modern airliners are there airbrakes independent from the spoilers? $\endgroup$
    – curious_cat
    Jul 26, 2015 at 17:42
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    $\begingroup$ @curious_cat: No. What is called airbrake in gliders is called spoiler in airliners. $\endgroup$
    – Peter Kämpf
    Jul 26, 2015 at 18:14
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    $\begingroup$ @curious_cat: BAe146/AvroRJ has an airbrake on tail that is not a spoiler (spoiler implies it is on the wing and interferes with lift), but it is an exception. $\endgroup$
    – Jan Hudec
    Jul 27, 2015 at 13:13
  • $\begingroup$ @JanHudec Very interesting. I had never heard of this config. So many different ways to design an aircraft, eh. $\endgroup$
    – curious_cat
    Jul 27, 2015 at 14:58
  • $\begingroup$ @curious_cat: This is not so unusual. The Fokker 70 and 100 uses it as well, as did older designs like the Dornier 217. $\endgroup$
    – Peter Kämpf
    Jul 27, 2015 at 16:21

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