reversers and a/c speed
 

Very straightforward question: why jet engine reversers are more effective at high speeds?

I have a couple of reasons in mind, but I couldn't find an "official" explaination!

I think that is due to characteristics of jet engine.
At high speed we have large amount of air mass, so engine works better and it produces more thrust.
You should also remember that on contaminated rwy, efficiency of wheel brakes decreases and the only proper way to brake are reversers.

cheers !

In fact the Jet Engine thrust is a little bit less at high speed, so the reverse thrust as well.
We can assume the produced reverse thrust is more or less same througout the landing roll. However at high speed due to lift, the efficiency of brakes are less. For the desired fixed deceleration rate at landing roll (e.g MED Autobrake Selection on an Airbus FBW), most of the deceleretion job is done by T/R at high speed moments.

The following figure is not based on speed but based on stopping distance, it may give a good idea the role of deceleration methods.

http://i1222.photobucket.com/albums/...g?t=1293192484

Edit reason: editorial

JABBARA

Thanks for that post:ok:

I love data.

Now since that stopping distance question keeps coming up on the forum your curve will always stick in my mind if.......... I could understand all parts of it.

Questions

The left axis appears to be braking effect

What are the units?

Since speed is not in the curve, I assume that the bottom axis of distance is for a typical landing for the specified aircraft?

I don't understand the aero drag being greater at the 3000m mark (end of roll)? so I probably have missed the understanding of the curves

Help

Also see http://flightsafety.org/files/alar_bn8-4-braking.pdf
Fig 3 shows stopping force vs speed, i.e. the contribution of each retarding device to the decelerating force vs speed.
Fig 4 (similar to #3) shows the % of stopping energy vs distance.

What indeed is the 'official explanation'? It is an oft-quoted mantra that 'reversers are more effectiv/efficient at high speed' but I think perhaps we should insert than wheelbrakes in there? Jabbara's 'weight on wheels v braking force' has some merit.

I have to check my literature on this. However, I believe that re-ingestion is one of the things influencing this. As speed reduces the reverse thrust plume re-ingests into the inlet affecting the efficiency of the thrust reversers. Reverse thrust decreases with decreasing speed.
Will have to check this when I am back in the office.

I think you will find that the reingestion is at a cross-over point and not linear until then. Affected by engine power setting (air out the reversers) vs forward speed.

Just above no-effect,
below a chance for some ingestion mostly through the fan (FOD) and increasing chance for some in the core and a surge

I was going to write exactly this as a response, because this is exactly the reason I wrote the initial question!

Knowing that:
1) reversers are more effective than brakes in the inital part of the landing roll

2) reversers are more effective than brakes if landing on a slippery rwy

So "reversers are more effective/efficient at high speed" implies that they are MORE EFFECTIVE THAN BRAKES and NOT more effective themselves for some particular reason that I am missing??

so...still looking for a definite answer!

ps: BOAC, the quoting on your profile is my favorite one!!

One would think that it could vary by aircraft type. For instance, the
DC-9 and 737-100/200 with their big external buckets would seem to be likely to provide more drag at high speed.

Some thoughts; by no means exact science.
Assuming that the same mass of air can be thrown forward by the engine reverse during the landing (unlikely to be true but good enough for the range of speeds on ground), then the change in momentum of that airmass relative to the static air is greater at high speed than at lower speeds. There should be a difference between; ‘v’ and ‘v squared’ amongst all that. Also, any ‘bucket effect’ - ‘v squared’ (as # 10).
Must stop thinking – back to the Christmas pud (now adding the brandy).

thread from 2003 on these boards:

Why are thrust reversers ineffective at low speed? [Archive] - PPRuNe Forums


'Checkboard' seems to give a very plausible and detailed answer near the bottom of the thread.

Net thrust of the engine is always gross thrust minus ram drag. Take away the net thrust, and you're left with ram drag, also referenced as inlet drag or intake drag. Spool the engine to greater power settings, and the drag rise increases.

Whether a straight turbojet or a turbofan, the inlet drag is considerable; the engine produces much more thrust than what is usable to propel the aircraft, but a certain portion of that thrust is used to overcome the drag produced by the engine. Higher velocities and higher power settings produce more inlet drag.

When reverse thrust is used, the thrust produced by the engine (by fan or by jet exhuast, depending on the powerplant), is diverted. Generally not directly against the direction of travel. The contribution to slowing the aircraft during landing, by re-directed exhaust gasses or fan flow is relatively small. The redirected gas path is often only slightly forward or at right angles to the direction of travel, and doesn't produce significant retarding action or deceleration (acceleration, to be correct).

Some aircraft will produce enough reverse airflow to back the aircraft, some won't. In either case, the contribution of this airflow to stopping the aircraft during landing is relatively slight.

The drag rise in the engine, specifically at the inlet, accounts for most of the retarding action used during landing. The faster the airplane, and the higher the power setting (the faster the engine is spooled, the more thrust produced, but take away that thrust, and the more drag remains. It's this drag that accounts for the effectivity if reverse thrust during landing.

Re-ingestion affects engine longevity, but doesn't account for a change in effective reverse thrust during landing.

Lomapaseo

I guess the answer to your question about the unit of vertical axis is in the Figure 4 of the link given by Safetypee.

About the second part of your question, I guess there is a bit misunderstanding. The following examples may explain better:

At given airplane and given spesific conditions,

1. if pilot wants to stop airplane at 2000m with manual brake and Max Reverse, the Kinetic Energy which airplane has at touchdown is killed by
10% Tire Brake and Rolling drag combination
40% Max Reverse
50% Aerodynamic Drag (= parasit Drag)
Total 100%

2. If the pilot uses only Max Reverse but never wheel brake, the airplane stops near 3000m, and this time the Kinetic Energy which airplane has at touchdown is killed by
3% Only by Rolling drag
42% Max Reverse
55% Aerodynamic Drag (= parasit Drag)
Total 100%


A few word about dragging force of the reverse thrust vs. speed:

I believe Figure 3 at the link is not precisely drawn for engineering purposes but only to give an idea to pilots about the effect of tools they are using for deceleration. But still, the curve for Max Reverse (darker dashed line) shows practically the the dragging force produced by Max Reverse changes only a little bit vs. speed till the moment it is deliberately closed to idle below 80Kts:ok:

I've always understood that the airflow from the reverser buckets can be likened to an aerodynamic "barn door" shoved out into the airflow. By that I mean that the plume of air from the reversers is "seen" by the ambient airflow as a (semi) solid obstruction and hence creates drag which varies directly with airspeed.

The catastrophic destruction of the circulation around the wing of the Lauda B767 which had the uncommanded reverse at high altitude all those years ago would seem to support this simplistic theory.

OTOH, I may well be completely on the wrong tack - please add learned comments as necessary.

This is not the case, at all.

No, it doesn't.

Disruption of lift doesn't imply that a wall of air creates drag. Multiple factors contribute to the hazard of a deployed reverser in flight, among them lift disruption.

I may be picking at too fine a detail here but;

I had thought that the loss of lift was over a very small portion of wing and that the loss of thrust itself resulted in the aircraft rolling into that engine.

This roll could not be easily corrected by instinct since the available inboard airelons at that flight regime behind the engine were disrupted by the reverser.

happy to be corrected if I'm confused :)

I don't think it is any different than the effectiveness of a reversing propellor, which is greater at high speeds than at low speeds.

For instance, on a turboprop aircraft, the flight idle blade angle of the controllable pitch propellor normally produces zero thrust at the aircraft's stall speed. That same idle blade angle produces positive thrust when the aircraft is static, but a negative thrust component at speeds above stall.

Immediate selection of reverse at high speed (on a flapless touchdown, say) is very effective, but as the forward speed decays the effectiveness of reverse also decays.

Thank you Guppy. Please explain.

I already did. Read.

Low speed ingestion.
Had to use emergency reverse at low speed at the old Gotenborg on a DC 9 -51 to stop us sliding off the cliff at the end.
Engines surged repeatedly - characterised by a very loud bang as the airflow re established itself and ignited the fuel in the back end.
Reduced the thrust until the banging stopped (emergency reverse - white knuckle reverse).
Fortunately we hit some ice with some braking action otherwise I wouldn't be writing this.
Borescope check reviewed no damage.

Reverse is much more effective at high speed and ineffective at low speed - so if in doubt I always pulled a little too much when initially setting it.

Once watched a skipper reverse off a stand in lhr - not a good idea because of FOD and the risk of standing the aircraft on it's tail.

Couldn't be done on a VC 10 because of the castoring nosewheel.

I agree with Mustafagander

Put your hand out of the window of a high speed car and slow down.:cool:

Using Reverse Thrust.
 

Whether it is the "hot stream" or the "cold stream" mass air flow being diverted to provide reverse thrust the effective speed retardation is achieved by redirecting the gas or mass air flow foward, ideally fully opposite to the forward flow direction, but generally at around 45 degrees forward. It has been mentioned that it is inlet drag which is the primary contributor to speed reduction. This is not supported by either Rolls Royce (The Jet Engine) or General Electric (The Aircraft Gas Turbine Engine and its operation), each of whom stress the reversal of gas or cold stream flow as being the means by which speed is reduced with no mention of the effects of inlet drag. Reverse thrust is very effective in "washing off" speed to a value where less brake energy is required and is usually fully cancelled by 60 Kts to preclude re-ingestion of gas flow or ingestion of FOD.

I'm not sure that either RR or GE dispute the effects of inlet drag. After all the design, and operating characteristics of the reverser falls under the domain of the aircraft designer.

It's a TAS issue.

Here is a good question.
Can you use Reverse Thrust in flight on a B727?

Blind Pew
 

Slight thread drift !

There was a page in the VC10 Flying Manual detailing all the reasons why it "should not" be done. I think the risk of using brakes while reversing was the major concern (Tail on ground, nose in air...means no tea, unless the cabin crew have climbing gear.)

I have been present when a 3 or 4 point turn was required near the end of the runway before departure, (many many moons ago), it worked. :eek:

Inlet drag
 

Lomapaseo. I did not say that either RR or GE dispute the effects of inlet drag. What I did say was that neither mention it when explaining how thrust reversal assists in slowing the aircraft. I mention inlet drag only because it has been cited by another contributor to the thread as being the primary contributor to slowing the aircraft and that the contribution of reversal of the gas flow, or fan discharge, is relatively small. This I find difficult to accept.

Regards

Old Fella

Whilst that Inlet Drag does contribute to overall decelleration it's only a small effect. Old Fella is right to find it difficult to accept that Inlet Drag is the prime contributor... because it isn't!

Mustafagander and Old Fella are both on the right track and were right to challenge SNS3Guppy.

TCF

four engine jock:

Question: 'Can you use Reverse Thrust in flight on a B727?'
Answer: No.

Though you can reverse the aircraft on the ground, blow back from the gate, do three point turns, etc. Have done all these, though one must be careful if using the #2 engine, as it is very likely to compressor stall. Three point turning can place a lot of strain on the nose gear.

Lomapaseo didn't dispute the effects of inlet drag.

The formula for net thrust is gross thrust minus inlet drag. Pure and simple; take away the thrust, and you're left with drag.

While reversing the airflow accounts for some retarding force, it's not much at all.

Many reversers don't divert the airflow forward at all, and it's not the diverted airflow that's accounting for the retarding force; it's the inlet drag.

Take away the thrust, all you have left is the drag. Very simple.

You mean if it is physically possible to get them into reverse?? :E :E

Don’t know if its true or not.
I heard a few guys used the thrust Reverses in flight to slow them down on the B727.

Are you talking about that 727-100 into Brussels?

SNS3Guppy; I stand corrected and have edited my previous post.

However, on the continuing saga of what provides the decelleration force from a thrust reverser...

Please explain how you have managed to remove "thrust" from your formula. Indeed, if you were to turn-off the fuel supply, you'd produce drag from a windmilling engine; however, this is not the case. If you were to shut-down an engine in-flight there isn't a huge ammount of drag to overcome. Now do the same with a reverser deployed; yes, there would be an increase in drag, but not as much as there would be with the engine running.

If, during flight, you experience an inadvertant thrust reverser deployment (accomanied by loss of airspeed, buffeting/vibration), most (if not all) Emergency Checklists instruct you to shut-down the engine. Why do you think this is? In this example we've removed the "thrust" and regained control of the aeroplane. Your theory implies that the removal of the thrust would further increase the drag!

With regard to thrust reverser design... indeed, not all thrust reverser systems divert the thrust in a forward direction (usually perpendicular to the free stream air); however, the net effect is the same. As Mustafagander has said, this produces a "barn door" type of effect to the free-stream airflow and, as trimotor has correctly stated, is proportional to TAS.

Happy New Year

TCF

Dont Know but it could be the one in EBBR.

Thrust reversal
 

SNS3Guppy. If you believe that the contribution made by reversing the gas flow and/or fan discharge is such a minor one in the reduction of forward speed, why is it that so much work goes into the design of the reversal system, whether it be clam shell, bucket or blocker doors? Further, if simply removing the thrust would lead to the desired speed reduction, why are the engines accelerated to the relatively high thrust setting attained at Maximum Reverse? Also, it is common for the thrust to be redirected forward by as much as 45 degrees. Just look at the cascade vanes on a RB211 or JT3D, or the buckets on a aircraft so equipped.

Surely this is simple? To change the direction of motion of a mass requires a force. Turn a jet's airflow back on itself and you require a rearward (decelerating) force. QED? Exactly the same as 'adverse intake momentum drag' for the now deceased Harrier and other S/VTOLS

How the reverser force changes with a/c forward speed someone else will have to explain:)

Totally incorrect, I'm afraid. The jet engine is a reaction engine. The blades in the turbines force the molecules of air in a particular direction (or the reverser cascade, in this case), and as every force has an equal and opposite force the engine (and anything attached to it) is forced in the opposite direction.

What happens to those molecules after they have left the engine is irrelevant, as those molecules (being in a free gas stream) have no way of transmitting any force back to the aircraft. Your "barn door" is built of gas - not wood!

My question for the B727 thrust Reverses is still not answered. CAN YOU USE THEM IN FLIGHT????