BMI727 From United States of America, joined Feb 2009, 15923 posts, RR: 27
Reply 1, posted (5 years 11 months 2 weeks 4 days 12 hours ago) and read 10993 times:
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.
Why do Aerospace Engineering students have to turn things in on time?
Lowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 2, posted (5 years 11 months 2 weeks 4 days 11 hours ago) and read 10970 times:
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.
Vikkyvik From United States of America, joined Jul 2003, 10474 posts, RR: 26
Reply 3, posted (5 years 11 months 2 weeks 4 days 10 hours ago) and read 10922 times:
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.
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.
RoseFlyer From United States of America, joined Feb 2004, 9926 posts, RR: 52
Reply 6, posted (5 years 11 months 2 weeks 4 days 7 hours ago) and read 10855 times:
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.
If you have never designed an airplane part before, let the real designers do the work!
DeltaGuy From , joined Dec 1969, posts, RR:
Reply 7, posted (5 years 11 months 2 weeks 4 days 7 hours ago) and read 10851 times:
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.
Viscount724 From Switzerland, joined Oct 2006, 26240 posts, RR: 22
Reply 9, posted (5 years 11 months 2 weeks 4 days 2 hours ago) and read 10770 times:
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?
Pilotpip From United States of America, joined Sep 2003, 3152 posts, RR: 10
Reply 14, posted (5 years 11 months 2 weeks 3 days 12 hours ago) and read 10565 times:
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.
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.
JetMech split the back of his pants. He can feel the wind in his hair.
Zeke From Hong Kong, joined Dec 2006, 9304 posts, RR: 76
Reply 16, posted (5 years 11 months 2 weeks 2 days 22 hours ago) and read 10388 times:
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.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 19, posted (5 years 11 months 2 weeks 2 days 8 hours ago) and read 10188 times:
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.
Tdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 21, posted (5 years 11 months 2 weeks 1 day 14 hours ago) and read 10062 times:
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.
Wingscrubber From UK - England, joined Sep 2001, 854 posts, RR: 0
Reply 22, posted (5 years 11 months 2 weeks 1 day 9 hours ago) and read 9987 times:
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...