Thrust Reversers
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Thrust Reversers
I heard from a pilot who used to fly the Airbus 300 that a typical Front Fan thrust reversal system is more effective when the aircraft is at a higher forward speed. I was also told it is documented in the A300 Manual. Can anyone offer an explanation for this.
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To my thinking, that's actually not quite true.
Thrust reversers are effective down past 75 some knots, but the risk of FOD increases once we are below that, depending on aircraft and engine design. There is truth in that we do know thrust reduction along with stalls or surges are less likely to occur with the same level of reverse on a high bypass engine (mostly fan air) as opposed to a lesser ratio engine.
Brake wear costs are minor compared to maintaining, carrying the weight, and the use of thrust reversers. Early in the landing when the kinetic energy of the aircraft is very high, reverse is most effective when also considering airport noise requirements, maintenance, and fuel costs.
Thrust reversers are effective down past 75 some knots, but the risk of FOD increases once we are below that, depending on aircraft and engine design. There is truth in that we do know thrust reduction along with stalls or surges are less likely to occur with the same level of reverse on a high bypass engine (mostly fan air) as opposed to a lesser ratio engine.
Brake wear costs are minor compared to maintaining, carrying the weight, and the use of thrust reversers. Early in the landing when the kinetic energy of the aircraft is very high, reverse is most effective when also considering airport noise requirements, maintenance, and fuel costs.
Brake wear and maintenance is far cheaper on a per flight basis than reverser use.
OTOH big iron pilots have come to depend on reversers in their decision making so they don't like them being deactivated as the data shows that landing accidents go way up when they are locked out.
Lastly, reversers are a big money maker for the manufacturer so expect them for a long time to come.
OTOH big iron pilots have come to depend on reversers in their decision making so they don't like them being deactivated as the data shows that landing accidents go way up when they are locked out.
Lastly, reversers are a big money maker for the manufacturer so expect them for a long time to come.
I think there are multiple pieces to this puzzle. Thrust reversers redirect thrust forward, but that is not their only effect.
On the old stovepipe turbojets, clam-shell thrust reversers closed off the entire back of the engine nozzle. They not only redirected the engine exhaust forward, but the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe). In addition, the entire normal thrust flow was blocked, not just the fan bypass thrust. Finally, the clamshells stuck out into the slipstream, acting to a small degree as speedbrakes, not unlike the tail speedbrakes of a Bae 146.
That last effect is very speed-dependent, since drag varies as the square of the velocity. Slow by 50% and that drag drops to 25%.
On high-bypass fan reversers:
1) the engine core is still producing forward thrust, even when the reversers are deployed. Some tail-engined planes have full buckets that block core thrust, but many have a slot in the center, and mostly reverse only the bypass air. So the engine is pushing in both directions at once.
2) Some fan reversers of the "slide-back" type (Boeing, mostly) do not stick out into the slipstream, but others still stick out to the side (Airbus, e.g.) and will produce "speedbrake" drag as well as redirecting the thrust. But exponentially less drag as speed drops.
3) Even on the "internal" Boeing-style reversers, doors drop into the bypass flow, and are being pushed backwards directly by, and blocking, the fan thrust.
Combine those with the supposition (in the thread linked by DaveReidUK) that the reduction of ram air pressure into the engine intake as the aircraft slows reduces the thrust available to "reverse", and you can see how they add up to reduce reverser effectiveness with decreasing speed.
Less door drag - less internal backwards pressure on the reversing plates/doors/buckets - less total airflow - increasing (percentage-wise) interference with slowing from the remaining forward core thrust (at high throttle).
An aircraft can be stopped with reversers alone - but below a certain speed, it takes longer than shutting them down and going to wheel brakes, and increases the risk of FOD damage.
On the old stovepipe turbojets, clam-shell thrust reversers closed off the entire back of the engine nozzle. They not only redirected the engine exhaust forward, but the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe). In addition, the entire normal thrust flow was blocked, not just the fan bypass thrust. Finally, the clamshells stuck out into the slipstream, acting to a small degree as speedbrakes, not unlike the tail speedbrakes of a Bae 146.
That last effect is very speed-dependent, since drag varies as the square of the velocity. Slow by 50% and that drag drops to 25%.
On high-bypass fan reversers:
1) the engine core is still producing forward thrust, even when the reversers are deployed. Some tail-engined planes have full buckets that block core thrust, but many have a slot in the center, and mostly reverse only the bypass air. So the engine is pushing in both directions at once.
2) Some fan reversers of the "slide-back" type (Boeing, mostly) do not stick out into the slipstream, but others still stick out to the side (Airbus, e.g.) and will produce "speedbrake" drag as well as redirecting the thrust. But exponentially less drag as speed drops.
3) Even on the "internal" Boeing-style reversers, doors drop into the bypass flow, and are being pushed backwards directly by, and blocking, the fan thrust.
Combine those with the supposition (in the thread linked by DaveReidUK) that the reduction of ram air pressure into the engine intake as the aircraft slows reduces the thrust available to "reverse", and you can see how they add up to reduce reverser effectiveness with decreasing speed.
Less door drag - less internal backwards pressure on the reversing plates/doors/buckets - less total airflow - increasing (percentage-wise) interference with slowing from the remaining forward core thrust (at high throttle).
An aircraft can be stopped with reversers alone - but below a certain speed, it takes longer than shutting them down and going to wheel brakes, and increases the risk of FOD damage.
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There is no doubt that reverser effectiveness IS higher at high speed, but it is difficult to find a thorough explanation in quickly-found documents. One relatively simple graph is in Figure 3 of http://flightsafety.org/files/alar_bn8-4-braking.pdf. A simplistic answer may be that at higher forward speed, the relative [negative] velocity of the reversed airflow is also higher, imparting more [negative] force.
I can't find a good reference to link to, but I've read in several reports about ships with propellers in steerable pods that were designed to "crash stop" by turning the pods 90 degrees inward or outward. I don't understand why that works. But presumably its more effective at higher speed (a stationary ship that set its pods that way would not accelerate at all).
Whatever the phenomenon, it could be related to why thrust reversers are more effective at high speed -- at least they also create a lot of sideways flow.
Whatever the phenomenon, it could be related to why thrust reversers are more effective at high speed -- at least they also create a lot of sideways flow.
Quote:
Originally Posted by pattern_is_full
the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe).
That part sounds a bit like the aeronautical equivalent of pulling yourself up by your own bootstraps.
Originally Posted by pattern_is_full
the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe).
That part sounds a bit like the aeronautical equivalent of pulling yourself up by your own bootstraps.
We did some unofficial, unmeasured trials, in a 737-300 simulator using a dry runway and then a slippery (ice patches) runway high speed rejected take off. In the case of the dry runway, spoilers and brakes only were used and as expected stopping distance on the dry runway was excellent and there was plenty of runway remaining. The brakes were very hot afterwards. Repeated this time with slippery runway and we left the runway at 40 knots (10,000 ft runway sea level 15C)
Then with slippery runway we did full reverse, max manual brakes and spoilers, and pulled up with 200 metres to spare. It was obvious the full reverse with its significant deceleration effect effort at high speed was the key to stopping short on a slippery runway-ice patches, wet, contaminated) etc especially as the braking efficiency of the brakes themselves was a lot less on the slippery runway. From my experience I would much rather be flying a aircraft with thrust reversers than a brakes only aircraft.
Then with slippery runway we did full reverse, max manual brakes and spoilers, and pulled up with 200 metres to spare. It was obvious the full reverse with its significant deceleration effect effort at high speed was the key to stopping short on a slippery runway-ice patches, wet, contaminated) etc especially as the braking efficiency of the brakes themselves was a lot less on the slippery runway. From my experience I would much rather be flying a aircraft with thrust reversers than a brakes only aircraft.
I've read in several reports about ships with propellers in steerable pods that were designed to "crash stop" by turning the pods 90 degrees inward or outward. I don't understand why that works.
http://www.boatnerd.com/pictures/spe...005_0402AO.jpg
A little thought experiment.
Remove the reverser buckets, attached chains to the rear engine nacelle, and stand in the exhaust flow holding the chains. You are now the reverser plate/bucket.
(yeah, yeah, assume a heat-proof suit!)
Run the engine up - will there be a substantial force pushing you backwards, and straining both your arms and the linkage chains? And by way of the chains, pulling backwards on the nacelle?
Y or N?
Remove the reverser buckets, attached chains to the rear engine nacelle, and stand in the exhaust flow holding the chains. You are now the reverser plate/bucket.
(yeah, yeah, assume a heat-proof suit!)
Run the engine up - will there be a substantial force pushing you backwards, and straining both your arms and the linkage chains? And by way of the chains, pulling backwards on the nacelle?
Y or N?
That's still only one lot of F=MA.
Where do you think the additional contribution to the reverse thrust comes from?
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A little thought experiment.
1. Remove the reverser buckets, attached chains to the rear engine nacelle, and stand in the exhaust flow holding the chains. You are now the reverser plate/bucket.
(yeah, yeah, assume a heat-proof suit!)
2. Run the engine up - will there be a substantial force pushing you backwards, and straining both your arms and the linkage chains? And by way of the chains, pulling backwards on the nacelle?
Y or N?
1. Remove the reverser buckets, attached chains to the rear engine nacelle, and stand in the exhaust flow holding the chains. You are now the reverser plate/bucket.
(yeah, yeah, assume a heat-proof suit!)
2. Run the engine up - will there be a substantial force pushing you backwards, and straining both your arms and the linkage chains? And by way of the chains, pulling backwards on the nacelle?
Y or N?
In experiment #2 while your body is experiencing much more "negative" thrust you are neglecting to account for the higher "positive" thrust produced by the engine.
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A lot of the effect of thrust reverser performance is related to inlet performance. At slow air speeds, the air entering the inlet must accelerate. This acceleration inside the inlet results in the inlet being pulled forward. Another of way of explaining it is that at slow airspeeds, the inlet produces a significant amount of thrust. As airspeed increases, the air entering the inlet no longer accelerates and in fact begins to decelerate and create negative thrust (which is accounted for as "ram drag"). So when the aircraft is at flying speed, the inlet is producing no forward thrust and, with thrust reversers deployed, the fan thrust is negative (the core thrust remains positive). As the aircraft slows down and the engine RPM remain high, the air in the inlet must accelerate more and more, producing more and more forward thrust in the inlet, effectively reducing the net reverse thrust.
There are two other reasons airliners don't use reverse thrust at low speed.
1. FOD. The thrust reversers not only direct airflow forward, but also radially outward. Thus the bottom section of the engine nacelle directs airflow downward toward the runway/taxiway, potentially throwing up debris into the engine.
2. Re-ingestion. As the airspeed goes down the reverser output (efflux) angle moves forward. Eventually, the reverser efflux will re-enter the engine inlet. This causes all sorts of problems for the compressor and will often result in serious compressor stalls which can seriously damage an engine.
BTW, the C-17 is unique in that it has both fan and core reversers. Further, the lower section of the reversers are blocked off, resulting in all the reverser efflux moving forward and upward. Very little moves downward. Further, the flow is carefully tailored so that the reverser efflux does not enter the engine even with negative airspeed (i.e. a tailwind), meaning that the reversers can be used to back the aircraft using engine power alone.
There are two other reasons airliners don't use reverse thrust at low speed.
1. FOD. The thrust reversers not only direct airflow forward, but also radially outward. Thus the bottom section of the engine nacelle directs airflow downward toward the runway/taxiway, potentially throwing up debris into the engine.
2. Re-ingestion. As the airspeed goes down the reverser output (efflux) angle moves forward. Eventually, the reverser efflux will re-enter the engine inlet. This causes all sorts of problems for the compressor and will often result in serious compressor stalls which can seriously damage an engine.
BTW, the C-17 is unique in that it has both fan and core reversers. Further, the lower section of the reversers are blocked off, resulting in all the reverser efflux moving forward and upward. Very little moves downward. Further, the flow is carefully tailored so that the reverser efflux does not enter the engine even with negative airspeed (i.e. a tailwind), meaning that the reversers can be used to back the aircraft using engine power alone.
reverser efflux does not enter the engine even with negative airspeed (i.e. a tailwind), meaning that the reversers can be used to back the aircraft using engine power alone.
Lots of ways to manage this effect (forward speed vs eflux size, tailwind or aft speed to create your own tailwind)
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Boeing?
Interesting contribution. But, I really think Boeing has very little to do with the design of the reverser system. Has much more to do with the designer, manufacture of the engine. The aircraft designer may make some preliminary input, but it is the GE, Pratt etc.. that put it all together.
New braking systems have greatly improved. The 747 classic series had a hot day short runway over heating brake problem. Reverse would be applied at first touch down and when the airspeed slowed to something below 100 knots the brakes could be applied. We would normally try to wait until something below 80 knots and then one firm solid application of the brakes. This would keep the brakes cooler. I can remember engineers slowly pouring bottled water on top of the tires scary thought.
Thank you Bendix and the others for new carbon braking systems. Little to no heating problems.
New braking systems have greatly improved. The 747 classic series had a hot day short runway over heating brake problem. Reverse would be applied at first touch down and when the airspeed slowed to something below 100 knots the brakes could be applied. We would normally try to wait until something below 80 knots and then one firm solid application of the brakes. This would keep the brakes cooler. I can remember engineers slowly pouring bottled water on top of the tires scary thought.
Thank you Bendix and the others for new carbon braking systems. Little to no heating problems.
Last edited by mustangsally; 9th Sep 2015 at 21:53. Reason: adding comment
Interesting contribution. But, I really think Boeing has very little to do with the design of the reverser system. Has much more to do with the designer, manufacture of the engine. The aircraft designer may make some preliminary input, but it is the GE, Pratt etc.. that put it all together.
Tis true that engine makers are happy to manufacture and sell them as middlemen themselves.