Why does the engine cowling open up during thrust reverse during landing?

[coke]

Hey guys,

Was really interested in this. I think that the moment after the touchdown and the thrust reversers kick in, the cowling opening up is just the best moment of flight.

But, why does the cowling open up in the first place?

Jerry

[Moose135]

Depending on the engine design, that's how the thrust is redirected forward to slow the aircraft.

[Foxtrot]

^Yeah, basically to slow down the aircraft. The Pilot In Command (PIC) does that, usually these days if conditions allow, the PIC will just pull back on the thrust reverser throttles, just enough to open the thrust reverser doors (whatever they maybe). This is called "idle reverse", where the engines are on idle power but with the thrust reversers engaged. Of course if the weather's not that cooperative or if the runway's short than idle reverse is not used. I'm sure the more technically savvy members of the forum will probably nit-pick on my explanation but that's what I gather from what I know so far.

[MCM]

Moose's explaination is the correct one -

During reverse thrust operation, the engine itself continues to work in exactly the same way as before - air in the front and out the back.

Reverse thrust simply redirects the outflow air so that instead of going straight out the back (with the thrust pushing the aircraft forward) the air is redirected out the side of the engine and slightly forward, slowing the aircraft down.

The cowl opens and the requisite internal parts move into position to direct the airflow out sideways/forwards.

There are other sorts of reverse thrust systems as well... this is just one such system.

[coke]

thanks MCM, so I suppose then it's the same as the system on the MD-80s? Like how the trailing segment will fold and deflect the air?

[Moose135]

I've seen three major types of thrust reversers. The "bucket" type, like you see on the MD-80 and other lower-bypass engines, has the reverser sections pivot into the exhaust airflow to redirect it:

[photoid=7195636]

You also have the type where reverser "petals" open from the cowling to redirect airflow:

[photoid=7103087]

Finally, there is the type where a section of the cowling slides to expose a set of vanes that redirect thrust:

[photoid=7040123]

You may see some variations on these, but this should give you a good idea of what they look like.

[coke]

Thanks to everyone :P