cancidas
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Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 11:52 am

a flight instructor i was talking to today told me that he read somewhere that thurst reversers really do not have a large effect on the landing performance of the aircraft. perhaps that may be true on older aircraft engines but i doubt that fact when it comes to newer planes. take a 757-200 for example. i have seen them stop in 2000 to 3000 feet of runway at LGA. now before you all tell me how im probably wrong i know that they are not carrying much fuel at that point. i have also seen some 75s take much more than that 3000 ft to come to a stop.

so here is my question, what is the effectivness of thrust reversers on modern day aircraft?
"...cannot the kingdom of salvation take me home."
 
USAFHummer
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 11:55 am

Thrust reversers are only considered an aid to landing operations, and when an aircraft's landing performance is calculated, it is assumed that the thrust reversers will not be used...just an aid...

Greg
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QantasA332
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 11:58 am

Obviously quite effective -- why would aircraft manufacturers sacrifice the extra weight brought by T/R assemblies for a not-very-effective system?

qantasA332
 
MITaero
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 12:18 pm

>Obviously quite effective -- why would aircraft manufacturers sacrifice the extra weight brought by T/R assemblies for a not-very-effective system?

Exactly. You really feel the (reverse) acceleration when they turn on, don't you?
 
Inbound
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 12:19 pm

reading from the performance charts of the L1011, I came across this in the Appendix section

"Landing with Reverse Thrust Inoperative - The effect of inoperative reverse thrust is to increase the landing distance required, LDR, by 10%"

however, for the sake of comparison...

"Landing with Anti-Skid Inoperative - When the Anti-Skid system is inoperative, the LDR required should be increased by 50%"
Maintain own separation with terrain!
 
liamksa
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 12:27 pm

Reverse thrust is most useful on contaminated runways when braking ability is reduced, and most effective at higher speeds. At lower speeds there is the danger of foreign object damage and compressor stall/surge due to the ingestion of the engine's own exhaust gases.

Even cascade type reversers which reverse bypass air only as opposed to buckets which reverse both streams are effective aids to decelleration, and will take a good slice out of your landing distance.

As a piston driver i'm no authority but would question your instructor's source.  Big grin
 
Guest

RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 3:07 pm

It must be remembered that thrust reversers don't stop airplanes, brakes stop airplanes. If I'm flying with a pilot who hasn't learned that basic lesson I won't let him touch the T/Rs at all.

In fact, if for whatever reason, I have to make a "maximum effort" stop I usually won't even bother with the T/Rs - by the time they've gone through the deployment cycle I can usually have the airplane pretty much stopped. (Remember, they loose their effectiveness as the aircraft's speed decreases.) Also, many aircraft have minimum speeds at which the reversers can be deployed with anything more than idle reverse power.

As has been mentioned previously, for most airplanes, the braking effect of thrust reversers isn't normally included in calculating the landing distance. Their effect is only used as a "pad" or "cushion". In the case of the L-1011, you have to remember that, in the case of a 3000' landing roll, they only "save" 10% or 300' - hardly significant under most scenarios.

Proper technique is to get the airplane on the ground and get on the brakes while deploying the T/Rs. However, under certain conditions, T/Rs can be very destabilizing and should not be deployed - they can seriously aggravate weather vaning tendencies. I've seen pilots nearly run off the runway while piddling around with the reversers.

Jetguy

[Edited 2004-01-25 07:25:32]
 
POSITIVE RATE
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 6:30 pm

I think the bucket/clamshell type reversers found on the older jets like the 737-200/DC-9/727 were more effective than the newer cascade types. This is because the clamshell reversers actually fully reverse the exhuast thrust the other way, whereas the newer cascade reversers don't fully reverse the flow, rather they direct it more upwards. I'd say that's why you see aircraft with clamshell reversers doing powered pushbacks but you never see aircraft like the newer 737's doing powered pushbacks.
 
liamksa
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RE: Effectiveness Of Thrust Reversers

Sun Jan 25, 2004 7:26 pm

Another factor in powerbacks being more common on aircraft with aft-mounted engines is they are less succeptible to FOD.

PS Happy Australia Day +ve Rate  Big grin A few cleansing ales on the cards I think!
 
cancidas
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RE: Effectiveness Of Thrust Reversers

Mon Jan 26, 2004 12:21 am

i actually have seen a privately owned 737-500 do a powerback, thats why i know they can do it. they did it because there was no tug powerful enough on the small airport to puch it back fully fueld and loaded.
"...cannot the kingdom of salvation take me home."
 
bio15
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RE: Effectiveness Of Thrust Reversers

Mon Jan 26, 2004 1:20 am

MITaero: Exactly. You really feel the (reverse) acceleration when they turn on, don't you?

As it has been previously stated, most of the stopping action is done by the brakes which come into action before the T/R most of the times. The loud rumble along with high deacceleration may lead to thinking it is the thrust reversers doing the job, but it is mostly done by the brakes.


---
Some airlines have as SOP no thrust reverser deployment. When no thrust reversers are used, brakes are used to a higher extent and reach higher temperatures, which is a desired condition with carbon brakes. Carbon brakes are more effective at high temperatures and wear less. If I am not wrong, WN has this as a standard procedure, can anyone confirm this?

Good Luck
-Alfredo
 
MITaero
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RE: Effectiveness Of Thrust Reversers

Mon Jan 26, 2004 5:04 am

I know the difference between the brakes and T/Rs. My point is that you can feel the T/Rs when they're activated - they must be exerting force on the plane. That's all.
 
Okie
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RE: Effectiveness Of Thrust Reversers

Mon Jan 26, 2004 6:52 am

Well this topic got my curiosity up. So I thought I would make a pile of assumtions and see what I came up with, so please feel free to shoot holes in it.
So if you take a CFM56 which stands for a 5.6 to 1 bypass ratio or 82% thrust from the bypass
and assuming that the petal doors divert all of the bypass air at 45 degrees to the direction of the airplane netting 50% effectiveness or 41% of the thrust.
and then you deduct the 18% still going out the back of the engine leaves you with 23% net.
If you take those numbers against a 737-300 would be only 4,600 lbs of reverse thrust per engine or 9,200 lbs of reverse thrust total. Not a massive number for sure.
So, one would have to assume that a 10 to 15 percent less stopping distance would be a good SWAG (SWAG=scientific wild a$$ guess)
Maybe someone can give us some real numbers.

Okie
 
bio15
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RE: Effectiveness Of Thrust Reversers

Tue Jan 27, 2004 8:20 am

Interesting-to-read SWAG  Smile

I understand your reasoning, and it seems correct for the most part, except for the 50% effectiveness part:

If you use vectors, and assume the reverse flow is angled at 45 degrees, the actual component of reverse thrust would be the horizontal thrust times the cosine of 45 which is about 0.7. Multiplying this by .82, you'd get .57 effectiveness of the thrust in reverse for the first part.

---
Yet, as you stated, true figures are different, but there's nothing like a healthy wild-guessing to excercise the basic maths  Smile

-Alfredo
 
Okie
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RE: Effectiveness Of Thrust Reversers

Tue Jan 27, 2004 1:53 pm

Thanks for the help there Bio15
I guess I am starting to see why some of the operators are just opening the buckets/petals and not thrusting up unless they have to when landing on long runways.
Thanks again

Okie

 
darkblue
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RE: Effectiveness Of Thrust Reversers

Wed Jan 28, 2004 3:18 am

While estimating the effectivity using a 45deg angle is not a bad approach at finding a ballpark number of 57%, remember that there are some big pressure losses associated with turning the flow around 135degrees. Off the top of my head, I believe the effectivity for a cascade type of reverser is around 40%-45%.

DB
 
FDXmech
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RE: Effectiveness Of Thrust Reversers

Wed Jan 28, 2004 3:49 am

>>>In fact, if for whatever reason, I have to make a "maximum effort" stop I usually won't even bother with the T/Rs - by the time they've gone through the deployment cycle I can usually have the airplane pretty much stopped.<<<

That's doesn't speak much for the design of the t/r's on your aircraft.
You're only as good as your last departure.
 
FDXmech
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RE: Effectiveness Of Thrust Reversers

Wed Jan 28, 2004 4:59 am

>>>Thrust reversers are only considered an aid to landing operations, and when an aircraft's landing performance is calculated, it is assumed that the thrust reversers will not be used...just an aid...<<<

I think the, "it's just an aid" argument is taken out of context.

The thrust reversers are omitted from rejected take-off calculations not because of the minimal impact they have in shortening stopping distance, but rather the opposite.

They are basing a rejected T/O on an "almost" worst case scenario. No T/R's, brakes worn to minimum (less able to absorb kinectic energy), etc.

This is because they want to base stopping distance on the bare bones equipment to stop the airplane. You can bet that if T/R's were thrown into the equation, stopping distance would be further reduced. The exact amount or percent I don't know.

I might point out that regulations require airplanes reach V2 speed no more than 35' above the runway with one engine out. T/O calculations are based on having lost an engine at V1.
This doesn't mean the extra engine is merely an aid after V1.
It's basing the numbers on conservative worst case situations, not optomistic projections.
You're only as good as your last departure.

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