dakota123
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Reverse-Thrust Force Compared To Brakes

Wed Feb 18, 2009 8:50 pm

So, in terms of percentage of braking force available, how much retardation is available by reverse thrust? For example, if braking is 100%, what is reverse thrust? 25%? Is it even that much?

Generalities are fine.

Thanks,

Dakota123
 
BMI727
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RE: Reverse-Thrust Force Compared To Brakes

Wed Feb 18, 2009 8:58 pm

I have no numbers to back this up, but I think that the brakes will provide more stopping force than thrust reversers under normal circumstances. Where thrust reversers really come in handy is when landing in wet or icy conditions when wheel brakes are not as effective.
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lowrider
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RE: Reverse-Thrust Force Compared To Brakes

Wed Feb 18, 2009 9:34 pm

Depending on type of reversers and your forward speed. Generally speaking, reversers are most effective at higher speeds. With lower autobrake settings, the brakes may release entirely when reversers are at their peak effectiveness. However, in terms of maximum stopping power, brakes have the ability to generate more than reversers. Reversers are supplementary.

[Edited 2009-02-18 13:58:38]
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vikkyvik
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RE: Reverse-Thrust Force Compared To Brakes

Wed Feb 18, 2009 10:47 pm

Far as I know, for uncontaminated runway conditions, aircraft landing distances required are calculated with thrust reverse neglected.

Quoting Lowrider (Reply 2):
Depending on type of reversers and your forward speed. Generally speaking, reversers are most effective at higher speeds.

In addition, thrust reverse is only used at higher speeds to decrease the chance of FOD damage to the engine, and to prevent the engine from re-ingesting its own exhaust.

It's probably not a perfectly apt comparison, but I think of thrust reversers as being like engine compression braking in my car. It helps reduce a bit of wear on the brakes, but doesn't really add a whole lot of braking power on a dry road. On a wet road, however, I'll utilize the engine compression braking a lot more (obviously I can't control the amount, really, but I can start slowing down sooner).

Either way, slamming on my brakes produces a much quicker stop.
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pilotpip
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 12:02 am

It helps, but it's not a lot. Consider that some jets like the 145 are certified without them and performance is taken without them. You can also dispatch with them inop.

Max reverse won't throw you against the seatbelt. Max braking will.
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lowrider
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 12:12 am



Quoting Pilotpip (Reply 4):
It helps, but it's not a lot.

I disagree, it may help quite a bit, depending on the circumstances. For aircraft using steel brakes the ability to delay heat build up allows for more effective braking later in roll, be it landing or RTO. They may be deferable, but I certainly notice a substantial difference when one is.
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roseflyer
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 1:32 am

While reverse thrust is useful, it is not used in determining landing length. Calculations are usually based on braking only. Reverse thrust does however increase tire and brake life. For airlines that pay for their own tires when they need to replace them will benefit from reverse thrust and having lower tire and brake wear.
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DeltaGuy
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 1:41 am

Reverse thrust isn't taken into account on the landing performance data charts for (most) jet aircraft, that I'm aware of at least. However, alot of manufacturers will put a chart in place showing reverse-thrust only calculations for worst case situations (brakes, spoilers, and antiskid inop, for example)

Reverse thrust is cheap, brakes are not...unless it's a limited length field, I'll only get on the brakes below 100 kts, usually around 70 or so as I'm bringing the engines back out of reverse for the ground speed limitation.

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tdscanuck
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 5:53 am



Quoting RoseFlyer (Reply 6):
While reverse thrust is useful, it is not used in determining landing length.

It's not use for determining *dry* FAR landing length. All other conditions are up for grabs.

Tom.
 
Viscount724
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 6:30 am

On a slightly related hypothetical question, approximately how much distance would be required for an airliner such as a 747 to coast to a complete stop from normal landing speed without the use of any brakes or reverse thrust?
 
lowrider
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 2:32 pm



Quoting Viscount724 (Reply 9):

On level ground with no wind, I am not sure it ever would. Idle thrust is usually sufficient to keep you moving during the taxi, unless you are close to max structural weight.
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BMI727
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 4:37 pm



Quoting Vikkyvik (Reply 3):
Far as I know, for uncontaminated runway conditions, aircraft landing distances required are calculated with thrust reverse neglected.

That's correct. Reverse thrust is not considered for certification purposes.

Quoting Pilotpip (Reply 4):
Consider that some jets like the 145 are certified without them and performance is taken without them.

I think that Trans States has a fair number of ex-Crossair birds that don't have them at all.

Quoting Pilotpip (Reply 4):
You can also dispatch with them inop.

To certain airports. Some routes can't be flown with them inop I believe.
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timz
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 6:44 pm

Assuming no wheel brakes at all, would a typical airliner be able to stop on a typical runway with reverse thrust only? Maybe they wouldn't want to, but they could?
 
CosmicCruiser
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 6:45 pm



Quoting BMI727 (Reply 11):
To certain airports. Some routes can't be flown with them inop I believe.

I've never heard that but on the MD-11 it's co. specs or Boeing/McDon I'm not sure which but we can dispatch with 1 inop but no more and it doesn't matter where you're going.
 
pilotpip
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RE: Reverse-Thrust Force Compared To Brakes

Thu Feb 19, 2009 8:57 pm



Quoting BMI727 (Reply 11):
I think that Trans States has a fair number of ex-Crossair birds that don't have them at all.

Those are the ones I'm referring to. TSA also had a couple EPs floating around without them in american colors. I believe a few of Chautauqua's do not have them as well.

Don't get me wrong, they do help. However on a high bypass turbofan the cascades don't direct all the air in the opposite direction like the old clamshells. However the brakes do much more for you than reverse ever will. On both jets I've flown if one reverser is deferred you only get idle from the other one.

Quoting BMI727 (Reply 11):
To certain airports. Some routes can't be flown with them inop I believe.

I've never seen that. I've flown aircraft with a pinned reverser into MDW, HPN and LGA with no problems. Then again the 170 has fantastic landing performance if you need it.
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jetmech
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RE: Reverse-Thrust Force Compared To Brakes

Fri Feb 20, 2009 7:49 am



Quoting Dakota123 (Thread starter):

According to this link,

http://www.pprune.org/archive/index.php/t-287687.html

MAX autobrakes produces a deceleration rate of around 11 ft / s^2, or 3.353 m/ s^2. Apparently, manual braking can produce a higher rate of deceleration, but I cannot find an actual number for it.

If a RR powered 744 lands at its highest MLW of 285,764kg,

http://www.boeing.com/commercial/airports/acaps/747_4.pdf

the brakes would need to provide a force of around 958,167 Newtons to achieve the MAX de-acceleration rate ( ignoring aerodynamic drag and TR ). I don't recall where I read it, but I seem to remember that a thrust reverser must be capable of developing a reverse thrust of around 50% of the forward thrust. Again for a RR powered B744, this comes out to around 517,020 Newtons of force.

As others have mentioned, the amount of reverse thrust achievable goes down with aircraft velocity - and the braking force under AUTO operation is adjusted for aerodynamic drag and T/R operation - but if we ignore this, at the point of touch down at-least, it appears that reverse thrust provides about 35% of the total retardation force.

Regards, JetMech
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zeke
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RE: Reverse-Thrust Force Compared To Brakes

Fri Feb 20, 2009 10:20 am



Quoting Dakota123 (Thread starter):
So, in terms of percentage of braking force available, how much retardation is available by reverse thrust? For example, if braking is 100%, what is reverse thrust? 25%? Is it even that much?

Figures for the A330/A340, the amount of landing distance reduction based upon using 2/4 reverses operative with different runway conditions. Generally the less friction available from the runway surface, the greater the contribution of the reverse.

A330

Dry 2%
Wet 5%
1/4" water 8%
1/2" water 7%
1/4" slush 8%
1/2" slush 7%
Compacted snow 7%
Ice 19%

A340

Dry 3%
Wet 8%
1/4" water 13%
1/2" water 11%
1/4" slush 13%
1/2" slush 11%
Compacted snow 10%
Ice 27%
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dakota123
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RE: Reverse-Thrust Force Compared To Brakes

Fri Feb 20, 2009 5:43 pm

Thanks, all--

that all corresponds to what seemd to me to be the case.

Dakota123
 
FredT
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RE: Reverse-Thrust Force Compared To Brakes

Fri Feb 20, 2009 9:11 pm

Wing,
now you've done it. Facts will ruin the best of debates. Big grin
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tdscanuck
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RE: Reverse-Thrust Force Compared To Brakes

Sat Feb 21, 2009 12:50 am



Quoting JetMech (Reply 15):

MAX autobrakes produces a deceleration rate of around 11 ft / s^2, or 3.353 m/ s^2. Apparently, manual braking can produce a higher rate of deceleration, but I cannot find an actual number for it.

That's because there isn't one...max manual braking is just the most pressure you can apply before the tire locks up (equivalent to RTO autobraking on most models, I think). As a result, it will depend on the tire condition, the pavement condition, and the aircraft weight.

All the other autobrake settings are particular deceleration rates and the brakes will only come on as hard as necessary to reach that rate.

Tom.
 
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jetmech
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RE: Reverse-Thrust Force Compared To Brakes

Sat Feb 21, 2009 2:28 am



Quoting Tdscanuck (Reply 19):

Fair enough. I suppose max manual braking is the full 3,000psi to the brakes with the resulting braking force the maximum that the ABS system lets you have for the prevailing conditions.

Regards, JetMech
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tdscanuck
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RE: Reverse-Thrust Force Compared To Brakes

Sat Feb 21, 2009 6:17 pm



Quoting JetMech (Reply 20):
I suppose max manual braking is the full 3,000psi to the brakes with the resulting braking force the maximum that the ABS system lets you have for the prevailing conditions.

Exactly. Max manual and RTO are both full 3000 psi (or whatever that aircraft's brake system uses), with the ABS modulating each wheel as necessary. All the other settings are a/c deceleration rates, which is the aggregate effect of brakes, T/R's, spoilers, etc.

The only exception off the top of my head would be Airbus's brake-to-vacate system, which is targeting a particular speed at a particular point on the runway...I'm not sure how much pressure it's willing to apply before it gives up and just accepts that you're not going to make the turn.

Tom.
 
wingscrubber
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RE: Reverse-Thrust Force Compared To Brakes

Sat Feb 21, 2009 11:26 pm

One crucial difference between wheel brakes and reverse thrust which is not universally applicable, is that reverse thrust can be used in flight on some aircraft such as Concorde and Tu-154(?), and many turboprops. Wheel brakes obviously only work on the ground, with the gear down, with weight on wheels! If you belly land with your gear up, or you're scooting along the runway so fast the airplane still wants to fly, brakes are much less helpful...
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