Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
virginA346
Topic Author
Posts: 24
Joined: Wed Dec 31, 2003 10:08 am

Reverse Thrusters

Mon May 31, 2004 5:15 pm

Pardon my ignorance, but how exactly do reverse thrusters work??
Thanks in advance.
Mayotee- sounds like payotee but isnt.
 
dl757md
Posts: 1483
Joined: Mon May 24, 2004 9:32 am

RE: Reverse Thrusters

Mon May 31, 2004 5:37 pm

Don't worry about it. We all had to learn sometime. It's great to have a place to come and find out what you want to know.

There are several great threads on htis in the tech/ops section of this forum.
Please go there and search for "thrust reverser".
757 Most beautiful airliner in the sky!
 
Jetfixer75
Posts: 29
Joined: Sun May 16, 2004 2:21 pm

RE: Reverse Thrusters

Mon May 31, 2004 5:38 pm

The way thrust reversers work is they redirect the fan air on the turbofan engines by blocking off the air with blocker doors. That air is redirected forward through cascade vanes to help slow the aircraft. On turbojet engines, they use clamshells to block off all the air as it is mixed as it leaves the exhaust.
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Reverse Thrusters

Tue Jun 01, 2004 4:56 am

First you might need to get a brief insight into how an engine works.

Engines are separated into three main parts. The first part is the compressor, where air is sucked in and compressed. It is then brought into the combustion chamber which is the second part. Here fuel is injected, it mixes with the air and it detonates expading rapidly into the third part (rearmost) of the engine: the turbine. The flow passes by the turbine blades and makes it spin rapidly.

Here is the deal: The turbine is attached to the same shaft as some part of the compressor. In that way power is fed back to the compressor by making it rotate faster. Turbojet engines have the compressor, combustion chamber and turbine section in the same case, and all the thrust is generated by the air that exhausts through the turbine. Turbofan engines, on the other hand, have a large fan before the compressor. This fan blows some air into the compressor, and some air into an empty bypass duct around the compressor, chamber, and turbine. Most of the thrust is generated by this bypass air, and only a small amount by the exhaust air that passed through the turbine. The fan is at the opposite end of the turbine, but it is attached to the same shaft so the turbine feeds power to the fan. Turbofan engines are also called bypass engines.

Here are some pictures so you get an idea:

Turbofan engines without their cases: You can clearly see the larger fan section at the front. The remaining parts are the ones you see surrounded by pipes and cables. Inside there you would find the compressor stages, combustion chamber, and turbine stages at the rear end.

View Large View Medium
Click here for bigger photo!

Photo © Bruce Leibowitz
View Large View Medium
Click here for bigger photo!

Photo © Julio Castillo Jr.



Turbofan engine with the case. The larger ring-hole at the back is where the fan bypass air exits, and the smaller ring-hole with the cone is where the hot combustion exhaust air exits.

View Large View Medium
Click here for bigger photo!

Photo © Damiano GUALDONI



Turbojet engine. Much smaller in diameter because there is no large diameter fan at the first stage.

View Large View Medium
Click here for bigger photo!

Photo © Joan Martorell



------
Ok, done with the briefings. There are different kind of reversers for each engine type. The first type of reversers are clamshell and bucket type reversers. These consist of two panels that block the exit flow and re-direct it forward. Since the panels are located at the rear end of the engine, all the flow is used for reverse thrust. Most turbojet engines use these types, and some turbofan engines such as the ones in the 717.


View Large View Medium
Click here for bigger photo!

Photo © Ben Wang
View Large View Medium
Click here for bigger photo!

Photo © Bruce Leibowitz



View Large View Medium
Click here for bigger photo!

Photo © Elliott Kefalas



------
The other types are found on turbofan engines mostly which are the cascade and "petal" type reversers. They redirect the bypass fan airflow forward. The cowl displaces to the back and the flow passes through some vanes that redirect the flow forward. The reverse flow comes out through the opening generated by the displaced cowl (the green thing on the pictures). The petal type reversers work much in a similar way, but you can clearly see how the air is redirected through the petals.

Cascade type reversers:

View Large View Medium
Click here for bigger photo!

Photo © Europix
View Large View Medium
Click here for bigger photo!

Photo © Europix



Petal type reversers:

View Large View Medium
Click here for bigger photo!

Photo © Eduard Brantjes
View Large View Medium
Click here for bigger photo!

Photo © Airsnaps



View Large View Medium
Click here for bigger photo!

Photo © Tommy Mogren / Viking Slides



Remember that these former two types only redirect bypass air. This means that while the reverser is working, there is a small amount of core thrust still being generated. On the other hand the first two types redirect the entire airflow, and for this reason they are a bit more efficient. However, reversers are only a 'comfort gadget' since they are not a requirement for airworthyness. Reverse thrust in not even used in runway length calculations for stopping distance. Some airlines choose not to use them.

I hope it clears it up for you. If you want to dig deeper into the subject, then try a search on the forum. There's a lot more valuable information in the airliners.net database. Good Luck!


Regards,
-Alfredo
 
greasespot
Posts: 2968
Joined: Sat Apr 24, 2004 10:48 am

RE: Reverse Thrusters

Tue Jun 01, 2004 10:15 am

Ummm actually the JT8-XX which is on the727. DC-9 and 737-200 is a turbofan and not a turbo jet. The last couple A/C with turbo jets are the VC-10 (I think) and the 707 with the RR conways.


Going from memory here I think I had it explained to me this way. IF It has a bypass duct and therefore a bypass ratio then it is is a turbo fan. If all the air goes through the engine core with no bypass then it is a turbo fan.

GS...
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
 
liamksa
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

RE: Reverse Thrusters

Tue Jun 01, 2004 12:39 pm

Going from memory here I think I had it explained to me this way. IF It has a bypass duct and therefore a bypass ratio then it is is a turbo fan. If all the air goes through the engine core with no bypass then it is a turbo fan.

We know what you mean  Big grin
 
Arcano
Posts: 2301
Joined: Mon Mar 22, 2004 4:34 am

RE: Reverse Thrusters

Tue Jun 01, 2004 2:14 pm

Alfredo, GOOD REPLY!

I couple of questions, I'll try to express myself clear:
1. The engines as the one you posted for the EK330 (the same of the 343s) are then a turbofan? there is no "ring" where the air is bypassed, then is the petal any sort of clamshell thruster?

2. This is hard to clearly ask. I think the jets are based on a physical principle: Action and Reaction. In two words: everytime you push on something, that you give energy to something, an opponent vector of the same characteristics of that energy (strength and direction) is generated. On an aircraft: the fan pushes air back, the aircraft advance front. When you walk: your legs "pushes the ground" back, you advance front. Got me?

Then, before the air is redirected forward by the reverser system, it passed through the fan, right? then before every molecule of air reaches the reverser that redirects it, it was pushed back by the fan, so the fan is still "generating vectors that make the aircraft advance" by pushing air back while the reverser is working. This is because the impulse that propels the aircraft forward is generated by the fan, which is working while the reversers are deployed.

Virgin: funny, I had the same doubt yesterday. I made a search and found very good information, with drawings and everything. I suggest you the same. So far, every doubt I had with aviation was already posted in this forum, thanks you all for the patience and for share your knowledge with us, because some of us love aviation but are not in business, so it's very hard to find this kind of answers outside A.net.

Arcano )(
in order: 721,146,732,763,722,343,733,320,772,319,752,321,88,83,744,332,100,738, 333, 318, 77W, 78, 773, 380, 73G, 788, 789, 346
 
QantasA332
Posts: 1473
Joined: Sat Dec 13, 2003 5:47 pm

RE: Reverse Thrusters

Tue Jun 01, 2004 4:09 pm

The engines as the one you posted for the EK330 (the same of the 343s) are then a turbofan? there is no "ring" where the air is bypassed, then is the petal any sort of clamshell thruster?

I think you're confusing turbofans with turbojets. Turbofans have bypass, turbojets don't - not the other way around - and almost all modern commercial aircraft use the former (for obvious reasons regarding efficiency). Accordingly, petal reversers are simply a variation on the cascade reverser, and they're not really similar to clamshell types. However, the 'petals' themselves are basically performing the same sort of redirection job of clamshells, though only to bypass air (as pointed out before).

As for your second question, I'm pretty sure I get what you mean but not 100%...I think I'll leave answering it to someone else just in case I'm misinterpreting...

Cheers,
QantasA332
 
greasespot
Posts: 2968
Joined: Sat Apr 24, 2004 10:48 am

RE: Reverse Thrusters

Tue Jun 01, 2004 9:10 pm

OOPs.....Glad you Know what I meant. For those that do not I mean if all the air goes through the core it is a turbo jet. If there is bypass it is a turbofan.

I have to somehow teach this thing to post what I mean and not what I type.  Smile/happy/getting dizzy

GS

[Edited 2004-06-01 14:10:30]
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Reverse Thrusters

Wed Jun 02, 2004 12:17 am

Hi greaser, you're correct about turbojets and turbofans. If it has a very small bypass duct it will still be a turbofan. I should have pointed this out.

-----

Arcano: The petal reverser engine is a turbofan engine. Just as QantasA332 mentioned the petal reverser re-directs the flow in a similar way as the clamshell, but it only works with the bypass duct. This bypass air is flowing inside the engine but around the center 'cylinder' of the engine. That's the 'ring' I am talking about. The reason you can't see the ring in the engine you mention, is that the case is elongated to the rear and it covers the hot exhaust pipe. If you look at it from behind you should see both 'rings' on the inside of the longer case.

I do understand your second question. The analysis you do in such case is called a "Control Volume". You would learn that in any fluid dynamics class. What you do is you take the entire engine and enclose it in an imaginary rectangle. This is the control volume. Whatever goes inside the control volume must exit right? The force generated by a flow in a control volume analysis is also in vectorial notation and it equals m(dotted) multiplied by v

m(dotted) is mass flow and v is fluid velocity, in this case air. I will not get too deep into the process of calculating the resultant force, but with a continuity equation you can easily find the net resultant force acting on the engine. This is the mathematical-physical process and it will show you that there will be a net resultant 'reverse' force.

-----

The simpler explanation: The air thrusted by the engine generates forward thrust, just as you mention. But as soon as it impacts the reverser walls it generates a backward force that counteracts the forward force being generated by the engine - this force is generated because the reverser system is physically attached to the engine. Now, if not only you strike the flow against a wall but also redirect it a bit forward, then you will do two things: one, counteract the forward thrust by stopping the flow, and two blow it forward, generating a new net thrust in the opposite dorection.

Hope it's clear now!


regards
-Alfredo
 
Arcano
Posts: 2301
Joined: Mon Mar 22, 2004 4:34 am

RE: Reverse Thrusters

Wed Jun 02, 2004 3:08 pm

Now I got it, perfecto, Gracias!

Qantas: Yes, sorry, I mistyped. This is what I meant:

The engines of the 343 does not have this bypass, then are "turbojets"

View Large View Medium
Click here for bigger photo!

Photo © Oliver Brunke
View Large View Medium
Click here for bigger photo!

Photo © Oliver Brunke


There is just one "air exit", so no bypass.
Then, since clamshells are associated to turbojets, why petals are not clamshells? or my mistake is to think this engine as a "turbojet"?

BTW, when I mentioned some other posts about reversers, I meant yours, the one with the draws.
Here is a link:
https://www.airliners.net/discussions/tech_ops/read.main/90134/6/

Thank you all. You got my point and perfectly explained. Do you help me in this last issue?

)(
in order: 721,146,732,763,722,343,733,320,772,319,752,321,88,83,744,332,100,738, 333, 318, 77W, 78, 773, 380, 73G, 788, 789, 346
 
liamksa
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

RE: Reverse Thrusters

Wed Jun 02, 2004 4:51 pm

There is just one "air exit", so no bypass.

The CFM56s are turbofans but the bypass air and flow through the core are mixed before being exhausted out the back. I'd imagine if you removed the cowlings you could see it quite easily.

The bypass ratio is around 6.5 : 1 meaning there is 6.5 times the mass flow of air flowing through the bypass duct compared to through the core.
 
BA
Posts: 10515
Joined: Fri May 19, 2000 11:06 am

RE: Reverse Thrusters

Wed Jun 02, 2004 6:07 pm

Just some corrections.

Bio15,

Turbojet engine. Much smaller in diameter because there is no large diameter fan at the first stage.

That is a 737-200 that you posted which is powered by Pratt & Whitney JT8Ds which is a low-bypass turbofan.

These consist of two panels that block the exit flow and re-direct it forward. Since the panels are located at the rear end of the engine, all the flow is used for reverse thrust. Most turbojet engines use these types, and some turbofan engines such as the ones in the 717.

The air actually is not really directed forward. On clamshells, the air is directed at a relatively steep vertical angle slanting slightly forward. All the engines you showed in those pictures are turbofans. The DC-9/MD-80, 727-200, and 737-200 are all powered by the venerable Pratt & Whitney JT8D turbofan series which is a low-bypass turbofan.

Arcano,

The engines of the 343 does not have this bypass, then are "turbojets"

Actually they do have bypass. They are indeed turbofans.

The A340-200/300 is powered by CFM56-5Cs. The entire CFM56 family are turbofans.

Then, since clamshells are associated to turbojets, why petals are not clamshells? or my mistake is to think this engine as a "turbojet"?

As I stated, clamshell reversers are not only associated to turbojets. Infact, a relatively few number of turbojets were ever given thrust reversers. Thrust reversers were added at around the time turbofans were developped. The turbojet was quickly rendered obsolete when turbofans came out because turbofans were much more fuel efficient, much quieter, more environment friendly, and produced much more power because most of the thrust is delivered from the bypass area rather than the core. On turbojets, the thrust generated comes only from the core as a turbojet does not have a bypass area.

Petal and cascade reversers came later on, on larger engines well after the turbojet era had come to an end and as a result were never applied to turbojet technology at least from what I know. It would be a lot more complex and difficult, however I believe it would be possible to apply petal and cascade reversers to turbojets.

Regards
"Generosity is giving more than you can, and pride is taking less than you need." - Khalil Gibran
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Reverse Thrusters

Thu Jun 03, 2004 1:46 am

BA, thank you for pointing that out. As much as I was aware those were low bypass turbofans I messed up the explanation by pointing them out as turbojets due to their similar shape. The B727 does have turbofan engines with a very low bypass ratio, the JT8D.

---
The re-direction of the flow when reverse is active, results in a net forward thrust. Yes, the flow is deflected at nearly 20 degrees from the vertical. In a way this is a forward re-direction of the flow if you take into account the net force generated. That is, the vertical components cancel out and the forward components add up. Thank you for clarifying anyway!

Good luck
Alfredo
 
Arcano
Posts: 2301
Joined: Mon Mar 22, 2004 4:34 am

RE: Reverse Thrusters

Thu Jun 03, 2004 10:48 am

Thank you all. Airliners.net college is waiting for you, guys!
in order: 721,146,732,763,722,343,733,320,772,319,752,321,88,83,744,332,100,738, 333, 318, 77W, 78, 773, 380, 73G, 788, 789, 346

Who is online

Users browsing this forum: 744SPX, Gillbilly and 24 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos