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Is it to increase the air resistance of the aircraft?

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Source: reddit.com

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Those are airbrakes, they allow the aircraft to increase its drag substantially to bleed off excess energy. They are hydraulically operated, and need pressure for both deployment and retraction, i.e. they do not auto-stow, at least on this aircraft.

Airbrakes are a handy feature, especially for a ground attack aircraft like the Su-25 in your picture, as they help both with landing on short airstrips and with dive bombing, a tactic that requires to dive almost vertically on a target to minimize slant errors in targeting. This is made possible by the airbrake, which prevents the aircraft from exceeding its dive speed.

Other aircraft types use or have used airbrakes in the past, mainly to dissipate energy during an approach. Examples include some airliners, most fighters and nearly all gliders.

Fighters are usually optimized for high speeds which are dangerous to land at, and an airbrake enables them to quickly slow down while on final approach, remaining at a comfortable speed until the last moment. They also have some occasional uses in WVR (Within visual Range) combat, but this will be rabidly argued against by the BVR (Beyond Visual Range) and stealth cults.

Gliders also use airbrakes to control their speed on approach and when diving, but also to arrest undesired climbs, when caught in a thermal when they need to descend. Since typically gliders lack a hydraulic or even electric system, their airbrakes are actuated by the pilot's muscles and are more akin to spoilers.

The geometry of an airbrake can vary significantly, but the principle is the same: a large surface extended into the airstream that generates mostly drag. their location on the airframe is a compromise between structural, mechanical and control considerations, i.e., it needs to be strong enough to resist the loads, easy enough to route hydraulic lines or control rods to, and not significantly impact the aircraft's stability. The examples below are a BAe 146, an F-15, a Slingsby Capstan and a MiG-23:

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  • $\begingroup$ I'd love to know why the Su-25 has such unique speedbrakes though. They appear to be parallel to the direction of travel, which seems odd for a brake. $\endgroup$
    – zymhan
    Sep 13, 2019 at 17:41
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    $\begingroup$ @zymhan It´s hard to find a good high-res picture online, but if you trust Trumpeter models to be accurate, you can see the geometry here; it is not parallel to the airflow at all. If I had to wager a guess about why it is there, it is likely the designers did not find enough space around the tailcone to deploy them out of the engine exhaust plume. $\endgroup$ Sep 13, 2019 at 19:34
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    $\begingroup$ Indeed they're not parallel. $\endgroup$
    – user14897
    Sep 14, 2019 at 2:21
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Late answer but.....extending speed brakes/airbrakes during an approach allows the aeroplane to fly slower because it changes the aerodynamic characteristics (specifically the relationship between lift and drag). The drag caused by the speedbrakes increased the total drag but reduces the speed where the "drag curve" graph is at its lowest.

As a bonus, the extra total drag requires a higher power setting, so if the pilot has to break off the approach (go around) the engines are already running at higher revs and so take less time to spool up to full power. Retracting the speedbrakes at that point reduces the total drag and together with rapid engine spool up, helps the aeroplane to accelerate more quickly

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    $\begingroup$ I'm having a hard time believing these airbrakes allow for slower flight. They do not change wing profile, so they will not change lift curve. With or without the airbrakes in question, the wing will stall at the same AoA, and the same airspeed. $\endgroup$
    – Jpe61
    Jul 10, 2021 at 7:58
  • $\begingroup$ While more parasite drag may shift the Min L/D point to the left (i.e. slightly slower), an approach is typically flown at a margin above stall speed, rather than simply at min L/D. The second paragraph is good info, but I'm also doubtful about the first paragraph's answer. $\endgroup$
    – Ralph J
    Jul 10, 2021 at 16:09
  • $\begingroup$ Jpe61 Lift-induced drag remains essentially unchanged but when plotted on a graph shows a curve that Decreases as speed increases. Form/parasite drag increases with the extension of the speed brakes - and when plotted shows a curve that Increases with speed. When you plot the two lines on the drag Vs speed graph, the total amount of drag increases but the point where they intersect (minimum drag speed) occurs at a lower Indicated air speed. The net result is as I explained earlier. $\endgroup$ Jul 10, 2021 at 16:51
  • $\begingroup$ As a former instructor on RAF Harriers and Tornadoes, as well as being now an instructor training airline pilots, I can promise you that what I have written is 100% correct :) Trust me I've been there. To be frank, the final stages of flap on an airliner have a broadly similar effect - contributing little extra lift but lots of extra drag, shifting the drag curve up and left (to a higher total drag and lower speed) $\endgroup$ Jul 10, 2021 at 16:53

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