ground spoilers question

[newmei]

Ok why after landing when the ground spoilers are deployed do the spoilers extend at different degrees why are'nt they equal in extension?

Thanks

 

[avbug]

What aircraft, and are you asking about the difference between spoiler deflection in ground mode vs. flight mode, or the difference between spoilers (assymetry)?

 

[newmei]

I think it was a B-757 I last saw it on. I'm talking about ground spoliers at touchdown, the middle two spoilers (or is it the end two) don't deflect upward as much as the others. Why?

 

[Snoopy58]

Why? No reason they need to.

On touchdown, everything extends as much as possible to dump life & put all the jet's weight on the wheels so the brakes can be maximally effective. Obviously you want symmetric spoilers on the two wings, but beyond that, everything drives to the fully deployed position, whatever that happens to be for that spoiler. Based on the design of the wing, where the hinge is in relation to other hardware, what sort of actuator (size & speed) is used, how much room you have on the wing for the spoiler panel itself, etc etc etc, some spoilers will be bigger than others, and some will have different actuator attachments, so some come up farther than others. Flight spoilers (ie those that are used in combination with the ailerons to control roll) will have different characteristics than spoilers that only come up on the ground (i.e. all-or-nothing). It might be advantageous to have less of a "barn door" with more precise control for a flight spoiler, whereas a ground spoiler might be as big as possible.

At the end of the day, though, they all are dumping lift & adding drag, and as long as both wings do so evenly, that's all that really matters. The engineers do what they need to in order to get maximum drag & lift-dumping out of the wing, and even spoiler extention isn't much of a priority.

At least that's how I understand it, not being an Aero geek.