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When I first heard the word air brakes it came to me that maybe the engine start rotating in the opposite direction for that, but since that's not possible, why can't we have an extra engine for air braking?

Did I just invent a new braking paradigm shift idea? ;)

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Here's an extreme example of an RC plane using variable pitch props to reverse thrust: youtube.com/watch?v=o_o_HT2ZaKQ the setup of the prop is something like this: hobbyking.com/hobbyking/store/…. As mentioned in the answers below, real planes can do this too but obviously not as extreme. – slebetman Jul 12 '14 at 21:37
I would consider Google first before thinking you have just invented a paradigm shift. A simple search would have revealed that this has been around for many, many years. – p1l0t Jul 14 '14 at 11:48
actually I get the idea from despicableMe, when Dr. Nefario goes to rescue the girls and he shoots his clamp on Vector's ship, then reverses his jet engine – shabby Nov 19 '14 at 7:41
How do you cover the cases when you are short of fuel before landing, or the engine has failed? No engine = no brake capability. – mins yesterday

Planes with a variable pitch propeller can have a pitch range which twists the blades such that they provide reverse thrust. This started to become a common feature on high-performance airplanes in the 1930s.

Not all variable-pitch propellers can produce reverse thrust, however. The constant speed props on small propeller aircraft usually don't, because the energy of the plane after touchdown would be too small to justify the added complexity. On big airplanes which need short landing distances (the C-130, for example), they are a standard feature.

In 1936, Heinkel produced a sleek dive-bomber, complete with retractable landing gear and a reversible pitch propeller which could be used as a dive brake. The He-118 was test-flown by Ernst Udet, who did not pay attention at the briefing and went on to overspeed the airplane in a dive, destroying the prototype in the process. Thus, the clumsy, fixed-gear Ju-87 was chosen to be the "Stuka" and the He-118 would end as an inspiration for Japanese aircraft designers.

As usual, Wikipedia covers the topic well, so here ist the link.

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For completeness, reverse thrust on propeller aircraft is called "beta range", if anyone wants to google it. – falstro Jul 12 '14 at 12:19

Peter Kämpf's answer covers the systems that prop planes actually use for reverse thrust. The alternative presented in the question is a total non-starter for several reasons.

  • It's a complex and very heavy system.

  • Unless there was somewhere on the centreline you could mount this engine and prop, you'd need not just one of these things but two, for balance.

  • It causes drag while it isn't being used, which is all the time except immediately after landing.

  • For certification, aircraft have to be able to stop using just the wheel brakes, anyway, since the complexity of reverse thrust systems makes them likely to fail.

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Exactly, the added benefit would be negligible (especially since many variable-pitch props and almost all jet engines are already capable of producing reverse thrust) while the costs (in design, manufacturing, maintenance, and fuel costs as well as in performance) would be enormous. – reirab Sep 3 '14 at 20:38

Jet aircraft (especially large transports) are often equipped with thrust reversers. Not a requirement for landing, but takes some of the load and wear off the wheel brakes and tires. They have them because of the high landing speeds and large masses they have to bring to a stop in a limited amount of space.

Propeller aircraft don't fly as fast, so landing speeds are not as high; for the most part, braking requirements are easily met with wheel brakes.

Even if reverse thrust was beneficial, reversing the engine's rotation would be totally infeasible; engines are simply not designed to run either way round, and any system which would decouple the engine from the propeller (as would be needed if you wanted to implement a "reverse gear") would be a safety/reliability nightmare, not to mention heavy and expensive.

Engines are a significant part of the total weight and cost of an airplane. Carrying around an extra just for reverse thrust would be impractical for cost/weight reasons.

Many propeller-driven aircraft are equipped with variable-pitch propellers, including many light aircraft; in theory, any of them could have a "reverse" setting, but (as described in one of the other answers) you would only see it on larger aircraft designed for operation on short runways.

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"Combat zone landing" procedures for military transport aircraft, which are often turboprops, calls for full reverse after landing to stop in as short a distance as possible (less than 3000ft for a loaded Hercules, for example). – David Richerby Nov 16 '14 at 11:39
The use of reversers on an aircraft with Carbon brakes can have an adverse effect on brake wear. Since autobrake systems target a deceleration rate, the use of reverse can cause the system to cycle the brakes on and off, increasing wear. – vectorVictor Nov 17 '14 at 22:53
There are some engines that can run in both directions. – Johnny Dec 15 '14 at 23:07

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