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Reverse Thrust Capability

Posted: Sat Feb 15, 2003 8:27 am
by QANTAS747-438
When an aircraft is using its reverse thrust...


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What percentage of reverse thrust is shooting forward in relation to the total thrust that was normally exiting before reversal?

How many pounds of thrust is exited on reverse thrust?

Also, how much assistance does the reverse thrust give? Is it the majority of slowing down, or just help out?

Thanks in advance!

RE: Reverse Thrust Capability

Posted: Sun Feb 16, 2003 1:51 pm
by cdfmxtech
What percentage of reverse thrust is shooting forward in relation to the total thrust that was normally exiting before reversal?
Depends on the type of thrust reverser. The fan reverser type used on that B777-300 deflects fan air only. So if 70% of fan air is used for thrust, then let's just say that you have 70% less useable air for thrust. On a JT8 (used on Md80s), those clamshell reversers deflect all reverse thrust.

RE: Reverse Thrust Capability

Posted: Sun Feb 16, 2003 3:03 pm
by doug_or
A guy at GE told me that full reverse gave about 20-30% as much useful power as full (foward) power

RE: Reverse Thrust Capability

Posted: Sun Feb 16, 2003 4:16 pm
by USAFHummer
I was told by a US pilot (I forget the type he flew, it was either a 737 or a 757) that the reversers only shortened landing distance by about 2000 feet or so...

Greg

RE: Reverse Thrust Capability

Posted: Sun Feb 16, 2003 9:24 pm
by David L
Although 70% of the fan air may be deflected, it's not deflected fully forward but outward and slightly forward. You need to extract the forward component which is why you get the effective forward thrust quoted above of about 20-30% (not my figures but it sounds about right).

RE: Reverse Thrust Capability

Posted: Sun Feb 16, 2003 11:42 pm
by aeroguy
This may be a minor detail but the flow does not necessarily need to be "reversed" by a thrust reverser in order to create reverse thrust. If you design a machine that takes flow in from the front and exhausts it out the side, the net thrust this machine produces would be in the backward direction. You can see from observation or pictures that a jet engine with a cascade or bucket type thrust reverser acts in this manner, with "forward" deflection of the flow not necessarily required for it to still provide backward thrust. It may seem counter-intuitive but the momentum equation to my knowledge does not lie...

RE: Reverse Thrust Capability

Posted: Mon Feb 17, 2003 6:01 pm
by buckfifty
You don't want to direct the thrust totally forward anyway. Don't want the engine to ingest the gas it just expelled, as it totally defeats the purpose.

Not to mention the fact that FOD damage limits the speed at which reverse thrust can be deployed. For every degree the duct increases towards a forward angle (i.e. on a cascade vane type), the minimum speed at which the reverse thrust can be deployed is bound to go up because of this reason.

RE: Reverse Thrust Capability

Posted: Mon Feb 17, 2003 6:37 pm
by CX Flyboy
In Cathay we only use idle reverse normally, because reverse is relatively useless. We rely on the brakes more, as they are cheaper to maintain than going to full reverse everytime we land. However, we do select idle reverse so it is there in case we need it.

RE: Reverse Thrust Capability

Posted: Mon Feb 17, 2003 9:03 pm
by Rick767
The effectiveness of Reverse Thrust is less than that perceived in general. They sound as though they are slowing you down a lot, and they feel as though they are too but in reality that is a combination of moderate to heavy braking combined with full reverse (since that is the only time most airlines use it).

In our airline we use full reverse as SOP on all landings (except where use is discouraged for noise reasons and/or we can roll to the end). In those circumstances we go reverse idle and therefore have full reverse thrust available to us in a short space of time if we need it (as Cx Flyboy stated).

RE: Reverse Thrust Capability

Posted: Sat Feb 22, 2003 12:49 pm
by dw747400
I've heard that to save weight, Airbus is planning to equip only two engines on the A380 with reversers. Makes much more sense after reading the above.

RE: Reverse Thrust Capability

Posted: Sat Feb 22, 2003 7:39 pm
by POSITIVE RATE
Yeah reverse thrust sure does feel like it really is slowing you down from a pax point of view, but as stated above that may be due to a combination of reverse and hard braking. The 737NG reverse really gives you a push in the backside when deployed. The best i remember was on an AA DC-10 , when reverse thrust was applied everything in the cabin was literally thrown forwards and it must have been full reverse cos damn those engines were revving hard!!

RE: Reverse Thrust Capability

Posted: Sat Feb 22, 2003 9:18 pm
by Guest
Talking about operation in L4 we use full reverse on all landings.
We use full reverse also where the reverse use is discouraged for noise reason if we are making a CAT 2 o CAT 3 approach.

RE: Reverse Thrust Capability

Posted: Sun Feb 23, 2003 1:07 pm
by buckfifty
In fact, the carbon brakes on most modern airliners actually work better at higher temperatures. So the harder you brake, the more efficient they are at doing their job, and suprisingly, they'll also suffer less wear too.

RE: Reverse Thrust Capability

Posted: Mon Feb 24, 2003 12:49 pm
by ConcordeBoy
I've heard that to save weight, Airbus is planning to equip only two engines on the A380 with reversers. Makes much more sense after reading the above.

Um, wouldnt that be GE/PW's & RR's decision... not Airbus'??

RE: Reverse Thrust Capability

Posted: Tue Feb 25, 2003 1:08 am
by Rick767
"Um, wouldnt that be GE/PW's & RR's decision... not Airbus'??"

Not really, the engine manufacturer will build the engine options to specification at the request of the aircraft manufacturer. Airbus is the client here, remember?

Embrear is able to offer the ERJ-145 with or without Thrust Reversers, for example. The benefit for those who choose not to have reversers (like British Airways) is an aircraft weight saving of about 150-200kg off the top of my head. Saves plenty fuel in the long term, and reduces maintenance costs.

RE: Reverse Thrust Capability

Posted: Wed Feb 26, 2003 8:33 am
by dw747400
Also, I believe the Reversers are considered part of the nacelles, which is primarily Airbus's responsibility. I would assume that some minor modifications would need to be done on the engine, but the actual reverser would be installed by Airbus during final assembly, not by the Engine Alliance or RR.

RE: Reverse Thrust Capability

Posted: Wed Feb 26, 2003 4:17 pm
by POSITIVE RATE
They must be pretty effective is POWERED pushbacks are possible.