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Thrust from Thrust Reversers on A380 & B747

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Thrust from Thrust Reversers on A380 & B747

Old 13th Sep 2010, 15:16
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Thrust from Thrust Reversers on A380 & B747

How much engine thrust is used when an A380 and B747 lands to slow down the plane? i am guessing that a lot of thrust from the engine is lost when the thrust is reversed. How much thrust is used during a typical landing?How much thrust can be used in an emergency (like if the plane touches down halfway down the length of the runway)? Can too much thrust through the thrust reversers damage the wings? one last question......what is the maximum speed that the braking systems on the wheels/landing gear can handle?

Thanks

(I teach physics to high school students and I am trying to incorporate some aviation dynamics into my classroom lessons)
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Old 13th Sep 2010, 17:55
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Cool

no thrust is lost as such it is just directed in a different direction.

It is done in such a way as not to damage the wings.

Most airlines will use idle thrust only and use the brakes to a greater extent.

Thrust may be limited using reverse thrust to say a setting of 60% rather than full thrust.
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Old 13th Sep 2010, 18:43
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In the 747-400 with GE engines the Electronic Engine Controls control the maximum thrust in normal or reverse. In these "high bypass" engines about 75% of the total thrust is from the large, low-pressure fan at the front of the engine. Because only the fan air is reversed, the high-pressure exhaust is always directed aft. Net result is about 40% of Takeoff thrust for full reverse.

Airline policies vary, but full reverse is available from touchdown to about 80 knots. Below that speed there is a significant increase in risk of engine damage due to re-ingestion of the exhaust gas and debris picked up by it. Below about 60 knots, idle reverse is allowed.

Brakes can also be used immediately on landing. Modern carbon brakes can handle full braking at max certified landing weight. Older steel brakes would likely overheat in that case if braking was not limited to a reasonable rate (normally regulated by the autobrake and anti-skid systems).

The brakes must be certified to stop the airplane at max takeoff weight from V1 (takeoff decision speed). However, brakes and tires will overheat in this case, and at least the safety fuse plugs in the wheels will melt, causing the tires to deflate instead of explode.
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Old 13th Sep 2010, 18:44
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Thrust, whether forward or reverse, is not only a function of RPM, air density and temperature, but also speed. Forward thrust decreases with speed and reverse thrust increases. When standing still the maximum reverse thrust can be as low as 5 to 10% of the maximum forward thrust. At 150 knots it can be 20 to 30%, but that number decreases rapidly as the aircraft slows down.

Most aircraft uses tires that have a speed limit of 195.5 knots, so that will be the maximum speed on the ground. This can be limited further by the energy the brakes are able to handle, but this is not a fixed speed as it is of course is depending on weight. It may be higher or lower that the tire limiting speed.
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Old 13th Sep 2010, 19:05
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Actually the thrust is lost. Reverse isn't so much a function of exhaust and fan gasses being directed forward (many reverse systems don't direct gasses forward at all), but rather a cessation or interruption of thrust. Gasses moving "aft," which provide for forward thrust of the aircraft, are interrupted or re-directed by vanes and doors, to preclude their pushing the airplane along.

At the same time, the engine is spooled to a higher value.

The net thrust produced by the engine is the exhaust value, minus any drag or friction from the engine. This applies to a turbine engine as much as any powerplant.

Drag produced at the engine inlet during the reverse process doesn't go away, simply because the engine is no longer producing thrust. Take away the rearward thrust, and all you have left is the drag.

Exhaust and bypass gasses being directed forward or sideward contributes a small measure to the slowing motion of the aircraft, but not a substantial amount. The retarding action of reverse thrust is a function of inlet or intake drag, rather than re-directed gasses.

Airplane thrust reversers (Henry Spencer; Mary Shafer)

I can't speak to the A380, but on the B747, we use an autobrake system to calculate landing distances, which has three settings. Each of these settings produces an acceleration (I'm not a physicist, so I still like to think in terms of deceleration) rate of a particular value. That is to say, on the "Minimum Autobrake" setting, the aircraft decelerates at 4 feet/second/second. At "Medium Autobrake," the airplane decelerates at 6 feet/second/second. This value is calculated by the antiskid system, regardless of aircraft weight. At the "Maximum Autobrake" setting, full braking pressure is applied, stopping just short of a wheel skid.

Brake energy is a big issue; stopping a big, heavy airplane results in hot brakes. Large, hot brakes take a long time to cool,which affects the turn-around times (the time between landing and departing again). Airplanes get paid to fly, not to sit and cool, so brake energy is a big issue. An airplane can't take off again until the brakes are cool enough, both for issues with hot brakes after takeoff, and for the possibility of a rejected takeoff.

The FAA doesn't permit the use of reverse thrust in calculating landing performance. Therefore, when we perform TOLD (takeoff and landing data) calculations, reverse thrust isn't considered. We use it, however.

The value of reverse thrust will vary with temperature, engine condition, and the velocity of the airplane. Reverse thrust is more effective during the landing at higher speeds, than slower speeds. The velocity of air entering the engine is higher at higher landing speeds than slower speeds, and thus the inlet drag or overall drag effect is higher at higher speeds.

With the autobrake system, acceleration remains constant, but the amount of work done by the brakes is reduced as reverse thrust is used. In other words, at "Minimum Autobrakes," we're still slowing at the rate of 4 feet per second per second, but the amount of brake energy absorbed, and the temperature increase in the brakes, is substantially less with reverse thrust in use. The autobrake system doesn't care what's slowing the airplane, just that the airplane is slowing at the prescribed rate.

Consequently, we use reverse thrust as much as possible.

We're limited to 70% fan speed on the particular engines I operate for normal reversing operations. In an emergency, we'll use all the reverse we can, of course. We also reduce reverse thrust as the airplane slows down, in part because reverse isn't as effective, and in part because reverse can cause the engine to begin ingesting it's own exhaust gasses at slow speeds, or cause debris to be blown up from the runway, where it could be ingested into the engine.

Landing with 70% fan speed for reverse is roughly equivalent to minimum autobrakes, requiring very little braking pressure by the autobrake system. The value it contributes is somewhat subjective, and I can't give you a hard number; it varies with field conditions. The part it actually plays in the landing roll also varies with aircraft weight; a heavy airplane may require more braking action than a light one, where reverse thrust stays relatively constant.

Sorry, I can't give you any specific values.
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Old 13th Sep 2010, 20:00
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So according to that theory there is no need to actually increase engine speed as it doesn't do anything?

Everytime I've landed (as a pax) in a 737 or 320 and the pilots give it max reverse they are infact wasting their time and fuel! Sorry don't buy that theory at all.
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Old 13th Sep 2010, 20:16
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Hmm, tell me how you similate it with FS? Do you bang your face on your monitor?
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Old 14th Sep 2010, 11:03
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Just a minor point, if you are doing sums on these things - the 747 has reversers on all four engines, the A380 just reverses the inboard pair - no 1 and no 4 have no reversers fitted.

TA
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Old 15th Sep 2010, 01:03
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So according to that theory there is no need to actually increase engine speed as it doesn't do anything?
It's not a theory, it's a fact. Net thrust produced by a turbojet engine is what's left over when the drag produced by that engine is deducted. Take away the thrust, and all you're left with is drag. Reverse is the act of taking away some or most of that thrust.

Spooling the engine increases drag, and thus the retarding force to which we refer as "reverse thrust."

While some retarding force in reverse is actually owing to reverse gas flow, in some aircraft, very little of that gas flow accounts for the acceleration (or deceleration, if you will) of the airplane. What's slowing the airplane is drag; this drag is highest at high engine speeds and high airspeeds.

Increasing the engine speed does everything. It increases induction drag, also referred-to as inlet or intake drag.
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Old 15th Sep 2010, 01:30
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I don't subscribe to SNS3Guppy's theory. However, at higher forward speed the net reverse airflow speed is higher. That may, in part, account for the greater effectiveness.
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Old 15th Sep 2010, 05:54
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It's not my "theory." It's a fact, and represents the function of reverse thrust.

The link cited above explains it better than me.
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Old 15th Sep 2010, 10:16
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Can I add a dumb but not-unrelated question here then. IIRC a Lauda Air aircraft crashed because a thrust reverser deployed in flight. Like most people I thought that "reverse thrust" meant just that. If I read the above correctly, then what brought this flight down was the extra drag?
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Old 15th Sep 2010, 10:33
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No it's not fact it's somebodies theory and I think it's rubbish, fact!
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Old 15th Sep 2010, 13:09
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It is correct that intake momentum drag makes up a part of 'reverse thrust'.

How much is difficult to say, but it is there and whether it's called drag or reverse trust doesn't matter. The effect is the same.
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Old 16th Sep 2010, 05:25
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The technical definition for net thrust is gross thrust minus ram drag/inlet drag/intake drag.

Take away the thrust by blocking or redirecting it, and what you have left is drag.
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Old 16th Sep 2010, 06:09
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Spooling the engine increases drag, and thus the retarding force to which we refer as "reverse thrust."
In that case, should I try to take off with idle (or 0) N1, so drag is reduced?

BTW, what is your reference? It's hard to fathom how drag is increased when flow through the fan is facilitated by power from the turbine...
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Old 16th Sep 2010, 06:36
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In that case, should I try to take off with idle (or 0) N1, so drag is reduced?
I have no idea what you're attempting to say here. Should you attempt to take off with no thrust or idle thrust?

Every engine has internal friction which reduces it's efficiency. In piston engines, the internal friction of the many moving parts is substantial. The actual output of the engine is substantially less than the power the engine is making; the largest share of power produced by the engine is taken overcoming it's own internal friction, drag, and inertia.

Likewise, a turbine engine overcomes internal friction from a number of sources, ranging from bearings to accessory drives, but the net thrust output is defined, technically, as gross thrust output, minus ram drag. Ram drag increases as a function of airspeed, and inlet drag is a function of mass airflow. Increase the N1, one increases thrust from the fan, one also increases drag. One gets more thrust out of the engine than drag, hence one has something that's measurable (and usable) as net thrust.

The engine produces a lot more power than what you're able to use, because a lot of it is nullified by the drag increase.

This really isn't that hard to understand. The faster one goes, the higher form drag, parasite drag, etc, increases. Transonic issues also come to bear on nacelle inlets just as they do on flying surfaces and every other part of the aircraft. Just as certain parts of the aircraft react differently and are affected differently by these effects, the engines likewise have unique changes in airflow, drag rise, etc.

Pick up a textbook. Read the definition of net thrust. This is hardly something I've invented. It's unfortunate that you find it confusing or disturbing. Perhaps it's new to you. It's none the less a basic definition and element of powerplant design, operation, and measurement. Net thrust is gross thrust minus ram drag. Ram drag is related to forward speed, as well as the engine operational condition. A windmilling engine doesn't experience the same ram drag (also often referred to as inlet drag or intake drag, which are related and part of the same value, but not the same thing) as an engine operating at high power settings.

This falls into somewhat the same type of category of misunderstood concepts as ground effect. Even today ground effect is being taught as a cushion of air beneath the wing, when it's nothing of the kind. Add to the same mythology the popular notion of reverse thrust in turbojet/turbofan installations, and mixture and carburetor usage in light piston airplanes...all commonly misunderstood topics.
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Old 16th Sep 2010, 15:47
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As previousy stated, large fan jet engines, approx 70% of the thrust is from the 'cold stream fan air'. During T/rev this cold stream air is directed via some form of blocker door/ cascade vain to an angle of about 110 to 120 degress from the normal 'aft' path, i.e. slightly in a foward direction. This can clearly be seen when T/rev used on a wet runway, (seeing the spray 'blown forward'). Engine power is increase up to about 70% of the full power. (normaly limited by the engine control). Many airlines now only use reverse at idle power, and rely on the brakes to slow the aircraft down. This is because the modern carbon fibre brakes are a lot more efficient, and the wear/costs associated with using them is less than the fuel and wear on the engine if full power reverse used all the time. Airports with a perhaps a shorter runway (Newark is one I believe), the reverse would be used a lot more than say Heathrow. Noise is another consideration nowdays as well. When landing the engines are put into reerse more as a safety issue, i.e. just incase they need them incase of brake failure etc, then the engines are all ready in reverse and time is not wasted while they are deployed.
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Old 16th Sep 2010, 15:58
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We always make use of reverse thrust. We're often landing close to our maximum landing weight of 630,000 lbs, which means hot brakes if we don't use reverse thrust.

Our landing distance doesn't change with or without reverse, but the brake energy does. In autobrake mode, the system only cares about how fast the airplane is slowing, not what's slowing it down, so the more reverse that's used, the less the brake is applied, and the cooler the brakes are when we park. This is especially important in hot climates, where the brakes tend to cool very slowly, and large fans must sometimes be used to cool the brakes after landing.

When light, such as an empty repositioning leg, we hardly use braking at all, relying largely on reverse thrust.
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Old 18th Sep 2010, 16:39
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Well thanks SNS3guppy, I've learned something new, although when reading your 'drag' theory, my first reaction was 'complete BS'. But I've been reading on the subject for a couple hours and now I'm a firm believer.

However...
Reverse isn't so much a function of exhaust and fan gasses being directed forward
...should be seen in the context of large, high bypass fan's (like the original poster specified A380/B747). As I've witnessed low bypass fans with cam-shell type reversers in action (redirecting flow 20+ forward), making the aircraft taxi backwards.
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