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I saw an image of an F-15 landing. What is that flap-like thing in the top of the plane in this picture? I couldn't find it on any other pictures or drawings of this aircraft.

f15 flap thing

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    $\begingroup$ That's an air brake. The f-35 has one also and a few other jets have them too. Designed to slow the aircraft down even more with the addition of flaps. $\endgroup$
    – Ethan
    Dec 2, 2015 at 23:39

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It is a speed brake (air brake). Here is a closeup of it.

Speed brake

Image from aero-cafe.com

It is usually used reducing speed landing (approach). It is also used for other functions- to slow down quickly at high speeds and to prevent too much speed buildup in a dive.

This image shows it being used during approach.

F-15 speed brake

"F-15A 21st TFW landing with extended speed brake 1984" by SSgt. K. Vail, USAF - U.S. DefenseImagery photo VIRIN: DF-ST-85-11671. Licensed under Public Domain via Wikimedia Commons.

Similar airbrakes are also found in other aircraft, notably the Su 27 and its derivatives.

Su 27 airbrakes

Image from aerospaceweb.org

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    $\begingroup$ The F/A-18 and the F-35 have speedbrakes as well, to add a few more aircraft with those to the list. $\endgroup$ Nov 30, 2015 at 13:49
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    $\begingroup$ @SMSvonderTann only the F/A-18 A-D has the speed brake, it was deleted for the E, F and G. $\endgroup$
    – Moo
    Nov 30, 2015 at 13:51
  • $\begingroup$ @SMSvonderTann I'm not sure about speedbrakes in F35. You're probably talking about the covers of vertical fan? $\endgroup$
    – aeroalias
    Nov 30, 2015 at 13:54
  • $\begingroup$ @aeroalias The A and C versions of the F-35 don't have a lift fan but the brake is still there, the B version does have a lift fan and the cover is used as a brake as well. $\endgroup$ Nov 30, 2015 at 14:12
  • $\begingroup$ @SMSvonderTann hmmm... My understanding is that the speed brakes of F35 operate by deflection of control surfaces. $\endgroup$
    – aeroalias
    Nov 30, 2015 at 14:53
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It's an air brake, also called a speedbrake.

https://en.wikipedia.org/wiki/Air_brake_(aeronautics)

Another view of an F-15 landing with the speed brake open:
Eagle landing
Source: Wikipedia, click to embiggen

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