j0rdan
Topic Author
Posts: 63
Joined: Fri Feb 05, 2010 10:04 pm

Reverse Thrust

Thu Feb 11, 2010 5:38 am

Hi,

Forgive me if this has been discussed before but i cannot seem to find it anywheres, this is also my first post so feedback is appreciated.

OK, to the question(s),

I was wondering if anyone could explain generally how reverse thrust works, concepts of it etc..
I know there is a few different type such as 'clamshell' on 732, DC9; One where 4 'flaps' pop out on some A32X aircraft; and one where the engine 'slides' open such as on the 767, 777 etc. But i was wondering more specifically what is happening inside the engine when reverse thrust is deployed more particularly the 'sliding' style I mentioned. Also if there are proper names for these different styles.

What kind of reverse thrust do the 748 and 787 use?

Thanks in advance,

- j0rdan
 
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jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

RE: Reverse Thrust

Thu Feb 11, 2010 7:10 am

Quoting j0rdan (Thread starter):
particularly the 'sliding' style I mentioned

These types of TR's are usually known as the translating cowl type. The operation is detailed in the picture below.

http://www.freepatentsonline.com/6568172-0-large.jpg

The translating part of the cowl is mounted on slider tracks or rails, and is pushed back either by jackscrews or pneumatic / hydraulic jacks. Inside the fan duct are blocker doors that are attached to pull rods on their aft edge, and to the translating cowl on their forward edge. As the translating cowl moves back, the geometry of the pull rod and blocker door is such, that the blocker door moves to block off the fan duct.

Simultaneously, as the translating cowl moves back, it uncover a series of cascade assemblies. The cascade assemblies have angled vanes to redirect the fan air forward. Thus, the blocker doors block the normal, rearward path of the fan discharge air, which is then redirected partially forward by the cascades to achieve reverse thrust.

These videos show the cascade vanes and blocker doors.

http://www.youtube.com/watch?v=liETcvkzrwk
http://www.youtube.com/watch?v=BktHAM1zOw0&feature=related
http://www.youtube.com/watch?v=fGnrvHEvf4M&feature=related

Regards, JetMech

[Edited 2010-02-10 23:26:56 by jetmech]
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vikkyvik
Posts: 11799
Joined: Thu Jul 31, 2003 1:58 pm

RE: Reverse Thrust

Thu Feb 11, 2010 7:10 am

Well, to answer some of your questions...

The types of thrust reversers that I'm familiar with:

Clamshell (found on the 737-100/200):


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Photo © Wolodymir Nelowkin



Petal (found on the A340-200/300):


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Photo © Fabricio Jiménez



Cascade (found on the 767)...as an aside, why was it so much harder to find a photo of deployed cascades???:


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Click here for bigger photo!

Photo © Youri Thonon - Contrails Aviation Photography



Now, I can't speak knowledgeably about how exactly the thrust reversers work (i.e. the actual components that get shifted around) as I'm not familiar with aircraft engines. But here's the concept(s):

On clamshell reversers, the whole airflow (core and bypass) gets redirected forward and outward. So you basically have zero thrust pushing the airplane forward, and some percentage of your total thrust pushing the airplane backward (i.e. braking).

On petal and cascade reversers, only the bypass airflow is reversed. However, on modern turbofan engines, the vast majority of the thrust on the ground comes from the bypass air. So most of your thrust has been redirected forward and outward, and is braking the airplane. The core flow is still going out the back end of the engine, and is therefore still pushing the airplane forward.

Note that I said "forward and outward" for the reversed thrust. That means that you can never get 100% of your max thrust directed solely forward. Also, engines typically can't be used up to max thrust when you're in reverse. So even at max reverse, you'll only be getting some percentage of your max thrust, now directed forward.

Also worth noting that reverse is typically only used from touchdown down to some specified speed (i.e. 60 or 80 knots). Below that, you increase the danger of FOD (foreign object damage - the engine sucking in a rock or something), and re-ingestion of exhaust (the engine re-inhaling the exhaust from the reversers).

I'm sure I missed some stuff, so anyone please feel free to correct me!
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bri2k1
Posts: 952
Joined: Sun Dec 19, 2004 4:13 am

RE: Reverse Thrust

Thu Feb 11, 2010 4:19 pm

Quoting j0rdan (Thread starter):

Welcome! There is a lot of knowledge available here -- that's why you joined, right? Be sure to try out the search feature. It can take a few tries to get the results you're looking for, but many questions like this one have been answered several times over. Here are links to a few more threads on reverse thrust:

Which T/R Is The Most Efficient? (by C5LOAD Jan 19 2010 in Tech Ops)

Question About Thrust Reverser Types (by YULspotter Aug 15 2006 in Tech Ops)

The Best Thrust Reverser? (by Lockheed Dec 22 2005 in Tech Ops)

Thrust Reverser Styles On Certain Engines...why? (by GotAirbus Dec 30 2002 in Tech Ops)

Thank you for not calling them "reverse thrusters!" They reverse the thrust, making them thrust reversers; they do not thrust the reverse (which would presumably make a reverse thruster?)
Position and hold
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Reverse Thrust

Fri Feb 12, 2010 1:56 am

Quoting vikkyvik (Reply 2):
On petal and cascade reversers, only the bypass airflow is reversed.

Although this is common in practice, there's no inherent reason you can't reverse core flow with a petal or cascade reverser. The C-17 has full (bypass and core) cascade reversers. I believe early L-1011's did as well.

Tom.
 
Western727
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Joined: Wed Jan 03, 2007 12:38 pm

RE: Reverse Thrust

Fri Feb 12, 2010 3:39 am

Quoting tdscanuck (Reply 4):
I believe early L-1011's did as well.

They did...however, the core T/Rs were clamshell, IIRC. I remember seeing a picture of it on the #2 engine of an early L-1011 (TWA?) before they were deemed too heavy for their own good and removed from existing airframes.
Jack @ AUS
 
vikkyvik
Posts: 11799
Joined: Thu Jul 31, 2003 1:58 pm

RE: Reverse Thrust

Fri Feb 12, 2010 7:09 am

Quoting tdscanuck (Reply 4):
Although this is common in practice, there's no inherent reason you can't reverse core flow with a petal or cascade reverser. The C-17 has full (bypass and core) cascade reversers. I believe early L-1011's did as well.

You know, I knew that and I remember intending to include it in my post....but I didn't.

Thanks Tom!

~Vik
I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 
j0rdan
Topic Author
Posts: 63
Joined: Fri Feb 05, 2010 10:04 pm

RE: Reverse Thrust

Mon Feb 15, 2010 8:45 pm

Thanks everyone for the info!

- j0rdan
 
AirframeAS
Posts: 9811
Joined: Thu Feb 05, 2004 3:56 pm

RE: Reverse Thrust

Tue Feb 16, 2010 1:52 am

Quoting j0rdan (Thread starter):
One where 4 'flaps' pop out on some A32X aircraft

Those are called "blocker doors".
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