pa asked in Cars & TransportationAircraft · 1 decade ago

what is the meaning of reverse thrust of airplanes when braking?

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  • 1 decade ago
    Favorite Answer

    Most thrust reversers do turn the flow slightly forward - maybe 20 degrees or so. But the velocity at that point is pretty low, so the actual thrust in the forward direction created by that flow is pretty small. The way that the reverser works is to convert the gigantic momentum induced by the fan into a sudden stop in the axial sense relative to the engine case. This works essentially the same way if you just turn the flow straight out to the side without directing it forward at all. You don't need to turn the flow back forward. You just need to pull that flow in like gangbusters with the fan - which it does very well when you run the engine up - and then stop it instead of pushing it out the back even faster. The clamshells, and the vents and the baffles do that. They put up a wall for that flow to hit as it is pulled in by the fan. Sometimes they turn it slightly forward in a pretty inefficient manner, but that is secondary (ADDED: This is why reversers do not really work very well when you are sitting static at the gate. They are not really designed for that).

    ADDED: Think of it as moving much faster down the runway (at the speed of the bypass flow) - and suddenly putting up a big flat wall on each side of the aircraft - like 10 times the size of the engine inlet. That would slow you down pretty quickly.

    Source(s): 32 year career in turbine engine design and test.
  • 1 decade ago

    Just as the name implies, commercial and jet airplanes have a feature that reverses the reward thrust of the engine forward. This reversal of the engine thrust forward, aids the aircraft in braking thus allowing the plane to stop in a shorter distance. Next time you are at an airport, look at the engines of the planes when landing. You will see the covering open on some planes and others will have plates that come down over the rear of the exhaust forcing the exhaust forward. Now military planes, and commercial planes, with propeller's actually rotate the propeller's in a way that allows the air flow pushed forward again allowing the aircraft to stop in a shorter distance. Propeller driven aircraft enjoy this feature because by using the propeller's to slow the aircraft the pilot need not apply a large amount of the aircraft breaks to stop the aircraft.

    Source(s): Airman Information Manual (AIM)
  • mjk
    Lv 4
    1 decade ago

    Larry454 gives the most correct answer, but I want to add. Older jet engine technology used the 'clamshell doors' to redirect the engine exhaust thrust towards the front of the aircraft, to 'push' against the direction the aircraft is moving to slow it down. Later, engines with higher bypass air ratios, stopped the practice of using clamshell doors and don't bother with the core engine (hot stream) exhaust at all. What the later engines do is slide back some sleeves (trans-cowls) to expose the reverser louvers, which are angled vanes, then put 'blocker doors' up in the engine bypass duct, to prevent the bypass (fan) air from continuing out the back of the engine, and redirect it at a forward angle out the reverser louvers, against the direction of the aircraft's travel.

    Source(s): 36 years in industry.
  • 1 decade ago

    All that "Hello" said is spot on correct.If you compare the thrust of the jet engine to a garden hose , the closed clam shells would have the same affect as putting a coffee cup over the end of the hose, it comes right back at ya

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  • hello
    Lv 6
    1 decade ago

    a shield drops over the back of the engine forcing the thrust forward and allowing the engine to be used to slow the aircraft down. if you ever sit in a window seat right on or slightly aft of the wing you can watch this, it looks like a half clam shell on most birds.

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