Thrust Reversers

[Boyington]

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.

[DaveReidUK]

Discussed (somewhat inconclusively) here:

http://www.pprune.org/tech-log/88502-why-thrust-reversers-ineffective-low-speed.html

[vapilot2004]

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.

[lomapaseo]

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.

[pattern_is_full]

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.

[Intruder]

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.

[DaveReidUK]

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. :O

[Chu Chu]

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.

[Smilin_Ed]

Quote:
Originally Posted by pattern_is_full http://www.pprune.org/images/buttons/viewpost.gif (http://www.pprune.org/tech-log/567347-thrust-reversers.html#post9109943)
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. :O
Re directing the exhaust forward is accelerating it in the opposite direction. That acceleration (A) is the same as in F=MA which produces a force (F). That force slows the aircraft.

[Centaurus]

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.

[pattern_is_full]

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.

Probably because the steerable pods are far less streamlined when turned sideways, and add a huge amount of drag in the water.

http://www.boatnerd.com/pictures/special/macklaunch/macklau2005_0402AO.jpg

[DaveReidUK]

Re directing the exhaust forward is accelerating it in the opposite direction. That acceleration (A) is the same as in F=MA which produces a force (F). That force slows the aircraft.

No argument there. But the OP appears to be suggesting that there are two separate effects that contribute to the net reverse thrust:

They not only redirected the engine exhaust forward, but the exhaust was also pushing backwards on the clamshell as it was redirected

There aren't two lots of F=MA going on here. :O

[pattern_is_full]

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?

[DaveReidUK]

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?

Yes, of course. But the force on me is precisely because I'm accelerating (redirecting) the airflow, as Smilin_Ed explained in his post.

That's still only one lot of F=MA.

Where do you think the additional contribution to the reverse thrust comes from?

[KenV]

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?

The force on your body is a measurement taken outside the frame of reference of the aircraft. That will result in erroneous results when viewed from the frame of reference of the aircraft.

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.

[KenV]

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.

[lomapaseo]

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.

Tailwind is your friend when backing using reversers. The idea is to keep the eflux from crawling far enough forward that it get's over-run by the inlet moving forward or caught in the suction of the inlet.

Lots of ways to manage this effect (forward speed vs eflux size, tailwind or aft speed to create your own tailwind)

[mustangsally]

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.

[Amadis of Gaul]

Just use 100-jigawatt efflux capacitors rather than 50-jigawatt ones, and you'll be good to go.

[lomapaseo]

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.

The reverser is part of the aircraft powerplant pod and not the engine. As such it is designed and certified under the aircraft regulations, not the engine regs. It can be manufacturerd by any supplier under the oversight of the aircraft certifier as part of the installation.

Tis true that engine makers are happy to manufacture and sell them as middlemen themselves.

[Machinbird]

Just an aerodynamic thought from a guy who has never had reversers on his jet:
Doesn't the efflux from the deployed reversers act as a dam to block a lot of the airflow around the the engine pods and create a major change in the flow field around the aircraft? In essence, not only do you have some reverse thrust, you also have much more drag from an effectively larger, less aerodynamic aircraft. For that reason the reversers will be more effective at higher speeds. The greater the efflux from the engine, the larger the aerodynamic dam being formed around the engine and the greater the aerodynamic drag.

[Plumb Bob]

Exactly my thoughts! Well written!
But what made me think like that?

The DC-10 has a more or less sideways-exhausting APU.
The opening is partly covered in-flight by a small door, which is some kind of a shield when the APU is running in-flight. It could be, for electric purposes but not for pneumatics (or the other way around, forgive me that I’ve forgotten that)!

Now running the APU brings a considerable penalty for take-off performance. If my memory serves me right the runway and climb limited weights are to be reduced by 4.7 “something”. Or even 6 ‘something’.
It may be 4.7 k pounds, but, as we were a metric operator, with metric numbers all over the MDC-J1030 FAA Approved Airplane Flight Manual (AFM), it could wel be 4.7 tonnes. On the other hand, I doubt every little AFM appendix was issued in a metric version as well.

Anyway, this detail many years ago prompted me to consider the powerful sideways exiting air as a sort of invisible speedbrake. Indeed an invisible ‘dam’ of air.


Now back to fan reversers.
Once this ‘deflected’ way of looking at so-called ‘reversed’ airflow is accepted, one can think of the plume of ‘reversed’, or, rather, deflected fan air as a drag device by itself. As if a giant drag chute is deployed around the engine. Enormous drag (and lift loss) at speed (remember the Lauda Air 767 that was not recovered above Thailand when one reverser deployed in-flight at FL 280 at M 0.76).
Much less dramatic drag at landing speeds, but quite useful during the speedy part of the landing roll, and very little remaining at taxying speed.
Note that the working area for the dragchute-like disturbed airflow is less when the airplane is rolling on the runway, since the runway surface is normally not ripped up in the process, and the wing provides a shield as well.

This also helps understanding the strange fact that the tail engine of a DC-10 yields much more reverse effect than one wing engine. There are very interesting graphs in either the Airplane Flight Manual OR the Flight Crew Operating Manual – Performance that show the amount of reverse thrust versus airspeed for various N1 settings. At landing speeds, the reverse effect of the No. 2 (tail) engine by itself amounts to 80% of the combined effect of both wing engines together.
The tail engine is high in the air, and its ‘virtual dragchute’ affects more air than the reverser of a wing engine that is partially shielded by the ground and the wing.

Also, the reverser air of the tail engine (composed of blocking doors in the normal fan exit and many individual cascade panels in the ring of louvres that is exposed by the fan reverser cowl transiting aft) deflect the fan air mainly upwards (but at an angle away from the rudder panels) and sideways (but at an angle aiming above the nearby elevators). Here the air is not really reversed forward!
The panels in the lower sectors of the reverser louvres direct the deflected air more or less straight down and sideways, but again not forward, in order to create “dragchute drag” without really reversing the flow direction since that would rip off the elevators).

This remarkable phenomenon came up one day when a DC-10 tried more reverse at low speed on a very slippery surface and the crew felt the airplane almost being thrust forward instead of really braked by the supposedly reversed air.

Indeed the performance graphs showed that going below some 30 knots airspeed the net reverse thrust of the No.2 engine changes into a moderate amount of forward thrust, because the hot air is still directed aft, while all the cold air is deflected more sideways and upwards/downwards than really ‘reversed’ to any great extent. This is interesting, since the very long intake could otherwise nicely enable reversing on the tail engine down to zero speed (in an emergency), which the wing engines may not do without protest because of re-ingestion of disturbed air at speeds well below 60 knots.

Note this is a phenomenon of the later operational variant with the technically awkward turbine reversers deactivated or removed: just fan air reversers operative. Also, at lower N1 speed the effect is much reduced.

[Amadis of Gaul]

Hey Bob, any chance of a Cliff's Notes version of that? That's way too many letters for my feeble mind, but it looks interesting.

[Turbine D]

Machinbird & Plumb Bob,
Good posts!

To add some information, when the outer shroud translates aft, blocker doors close off downstream flow path and cascade boxes (we called them egg crates) are exposed through which the fan flow exits in a forward direction. The smooth annular surface over which the flow turns outward into the cascades is called the Dagmar. Therefore, the fan flow first turned radially outward where it approaches the cascades. The cascade boxes are comprised of turbine-stator turning vanes which accelerate the flow to the minimum section at the exit. Most of the cascade boxes have radial outward exhaust in a direction 40 to 50 degrees axial. Some boxes provide other than radial outflow by skewing of the turning vanes or by virtue of axial vanes in an arrangement that resembles an egg crate.
Each cascade is special as to location, whether it is to be used on a lefthand or righthand engine, or a center engine as on a DC-10 or MD-11. It requires close teamwork with the aircraft manufacturer to establish the proper parameters of the cascade design in relation to the surrounding features.
Reverse thrust of the engine system alone results from three sources. The unaffected core engine thrust is more than overcome by the fan flow exhausting forward and by the fan and core flow ram drag. The ram drag is the largest of of the forces.
Reversers deployed after touchdown are generally used to speeds just above the speed at which reverse flow is re-ingested or cross ingested so as to affect engine operation. The range of speed is generally 120 knots down to 60 knots. In this speed regime, the aircraft drag is a large retarding force. However the reverse flow shrouds a part of the aircraft and changes the aircraft drag. This is why close coupling between the aircraft manufacturer and the thrust reversal system manufacturer is very important to gain the most efficient overall reversal system.

TD

[Centaurus]

An aircraft can be stopped with reversers alone

Just a point of interest. If you have no brakes for what ever reason, and have only reverse thrust available then if you just manage to stop right at the end of the runway with full reverse still applied, consideration should be given to cutting the engines while in full stopped reverse.

The reason being if you cancel the reverse quickly when stopped, forward thrust caused by the engines winding down (no brakes, remember) can cause you to start moving again.

[vilas]

Yes, if engines haven't stalled already.

[lomapaseo]

The reason being if you cancel the reverse quickly when stopped, forward thrust caused by the engines winding down (no brakes, remember) can cause you to start moving again.

I would like to see the internal thrust balance proof of that or even some sort of real life test in an out-of-the-way run-up stand.

[Tourist]

Plumbob Machinbird

I'm having a problem understanding the mechanism which would make your air-dam theory slow the aircraft.

Any force acting on the air blown forward would blow the air backward. How would this be transmitted to the airframe?


pattern-is-full

Newton is turning in his grave.....

If you could increase the thrust of an engine by fitting it backwards and then bouncing it off something then fighters would have done it long ago.

[Journey Man]

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).

This is akin to attaching a large fan to a sailboat and blowing air at the sail expecting it to move...

[Judd]

I would like to see the internal thrust balance proof of that or even some sort of real life test in an out-of-the-way run-up stand. I have personally seen that demonstrated in more than one Level 5 B737 FFS.

[Chu Chu]

Actually, a fan blowing air at a sailboat sail would cause the boat to move. They say a sailboat on a close reach "makes its own wind" -- with a fan toward the bow, it would literally be true.

And if you put the fan at the stern blowing forward, the system would act like a jet engine with a thrust reverser (only facing the other way, of course). At least I think the curvature of the sail would deflect enough flow aft to generate net forward thrust.

[Journey Man]

The boats don't 'make their own wind', so much as the relative wind vector increases as the boat speed increases allowing the boat to accelerate until drag stabilises the speed.

And as you've no doubt spotted, in your case the 'wind' is generated by accelerating a mass of air by a fan mounted on the boat so you wouldn't have an increase in the relative wind. Nice try though.

[DaveReidUK]

This is akin to attaching a large fan to a sailboat and blowing air at the sail expecting it to move...

Yes, though I prefer the "pulling yourself up by your own bootstraps" analogy in my earler post. :O

[Chu Chu]

Journey Man --

I was being a little cute -- by "make its own wind" in this context, I just meant that the fan would in fact fill the sail and cause the boat to move. But once the boat started moving, the fan would be accelerating air that was already moving aft relative to the sail. So I think the apparent wind velocity at the sail would increase as the boat started moving faster.

As you point out, of course, the additional thrust as boat velocity increases would be offset by the increased drag through the water, as well as by the fact that the apparent wind direction would move further toward the bow of the boat.

Needless to say, all this applies only if the fan is facing somewhat aft to begin with -- and it might well be more efficient in the end to lower the sail and just have a fan boat.

[Journey Man]

I was being a little cute -- by "make its own wind" in this context, I just meant that the fan would in fact fill the sail and cause the boat to move. But once the boat started moving, the fan would be accelerating air that was already moving aft relative to the sail. So I think the apparent wind velocity at the sail would increase as the boat started moving faster.


Both premises are wrong.

[Tourist]

This is all scaring me. Please tell me none of you guys are engineers?

[plhought]

Pfft. Stuff the reversers. Just fly a 146.

😆

[lomapaseo]

This is all scaring me. Please tell me none of you guys are engineers?

Some of the engineers on this board are very good at working out conveyor belt takeoffs as well, especially after a bottle of the red stuff. :)

[Chu Chu]

What about the effect of reversers on braking performance?

Presumably any air dam effect would disrupt flow over the wings, tending to destroy residual lift and putting more weight on the wheels. And to the extent the reversers deflect engine efflux upward, this would create downward thrust, also increasing the effective weight on the wheels.

[Tourist]

It gets scarier.......:eek:

[lomapaseo]

Tourist

It gets scarier.......

:D

Actually some of the comments sound way out of the box but do stimulate the mind which is encouraging as long as the mind remains open.

When I hear these from wanabee engineers I sit them down (and sometimes myself) with a few equations and a calculator and tell them to have at it.

[Tourist]

a few equations and a calculator and tell them to have at it.

Don't even need that, just a diagram and pair up the forces....:ok: