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reversers and a/c speed

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reversers and a/c speed

Old 31st Dec 2010, 12:42
  #41 (permalink)  
 
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Having completed a spot of research into various websites I discovered the follwing statement on a NASA related subject; which was the Shuttle Training Vehicle.

Viewed from the engine's point of view... As the incoming air is compressed in the inlet (and compressor), it is also decelerated to quite low speed within the engine. The engine very nearly brings the air to a halt, creating a great deal of drag on the engine. After adding some heat, the engine then expands and accelerates the air through the exhaust nozzle (and turbine), creating thrust.

Using the terminology loosely, the net useful thrust of the engine is nozzle thrust minus inlet drag. Nozzle thrust and inlet drag are both typically several times the net thrust; an engine with 20klb of net thrust may well be generating 100klb of nozzle thrust and 80klb of inlet drag. (This is one reason why the net-thrust/weight ratios of jet engines are so puny compared to rocket engines, which are all nozzle and no inlet.)

So killing the nozzle thrust while retaining the inlet drag instantly gives you a "lot" of braking force, and it's not really necessary to divert the exhaust forward.

If the thrust reverse is from inlet drag, why does the pilot advance the thrusters after engaging the reversers? Is inlet drag related to throttle position? Correct -- it's more or less proportional to the amount of air the engines are swallowing, which is dictated by the throttle setting.
I'm somewhat surprised that none of my textbooks make any reference to "Inlet Drag" as defined by thrust reversal systems.

I might have to revise my thinking!

TCF
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Old 31st Dec 2010, 12:59
  #42 (permalink)  
 
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four engine jock

I've read that the engines on the B727 can be used in-flight - but not recommended!

The Trident (or more affectionally known as the "Ground-gripper" within British Airways) routinely used reverse thrust in the air and could achieve a rate of descent of around 10 to 15000 fpm. Reversers were routinely used during the landing flare; however, they weren't to be deployed without the implicit order from the handling pilot; otherwise the pitch-change came as a big surprise!

A captain (an ex Trident man) who I used to fly with whilst on the L1011 was recounting those days when he flew the Ground-gripper. One day when he was asked if he had enough height and speed to continue his approach he replied (in his Yorkshire accent) "Aye, ave got'n abundance of both"!

TCF
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Old 31st Dec 2010, 14:06
  #43 (permalink)  
 
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I have a tendancy to pick and choose my way through this discussion

My experience tells me that reverse thrust is difinitely a component of net thrust counter to forward thrust. Typically diverted forward at 45 degrees.

According to the explanation in the NASA discussion above (a good job of explaining) the inlet drag is significant to the total net thrust and always acts as a braking (against forward thrust) action in the typical subsonic jet.

I accept the explanation that a working engine does slow down the air and does increase the inlet drag contribution.

I submit (slightly off-topic) that a windmilling engine does not have the same significant internal air blocking (the burner is not working) and as such the drag component will be less. I also surmise that a seized engine (locked rotor) will not have the internal pressure blockage (from a still working compressor) and as such the inlet drag drops to mostly the efffect of the inlet minus an engine behind it (less yaw).

I expect that not all will agree with me
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Old 1st Jan 2011, 01:02
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B727 reverse thrust. Use airborne.

Four Engine Jock. In the B707 and B747 it is possible to introduce reverse thrust whether on the ground or airborne. The Thrust levers (Forward & Reverse) are protected by an inter-locking system which prevents simultaneous application of both forward and reverse thrust on any individual engine. To move the Reverse Thrust lever toward reverse the Forward Thrust lever must be at, or within a couple of degrees of Idle. When Reverse Idle is selected the reverser system moves to the full reverse position and allows further application of Reverse Thrust. As the Reverse Thrust lever is moved toward Maximum Reverse Thrust the Forward Thrust lever is mechanically locked against any forward movement toward Forward Thrust. There is no other "protection" of which I am aware to prevent selection of reverse thrust. I have never operated the B727, however given the system on both the B707 and B747 Classics, I would think that the B727 would be the same in respect to Engine control systems.

As for the discussion of what relative effect on speed retardation is provided by either "Inlet Drag" or "Thrust Reversal" I wonder why it is that aircraft/engine manufacturers go to the trouble and cost to design a "Thrust Reverser" system which enables acceleration of the engine to relatively high gas/fan discharge settings if it need not be done, i.e. simply removing thrust and allowing "Inlet Drag" to provide the retardation force. The adjustments to Landing Distance required with Reverse Thrust inoperative leads me to believe that "Thrust Reversal" is a significant contributor to the overall speed retardation achieved. HAPPY NEW YEAR TO ALL.
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Old 1st Jan 2011, 04:39
  #45 (permalink)  
 
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As for the discussion of what relative effect on speed retardation is provided by either "Inlet Drag" or "Thrust Reversal" I wonder why it is that aircraft/engine manufacturers go to the trouble and cost to design a "Thrust Reverser" system which enables acceleration of the engine to relatively high gas/fan discharge settings if it need not be done, i.e. simply removing thrust and allowing "Inlet Drag" to provide the retardation force. The adjustments to Landing Distance required with Reverse Thrust inoperative leads me to believe that "Thrust Reversal" is a significant contributor to the overall speed retardation achieved. HAPPY NEW YEAR TO ALL.
It's not needed, but nice to have. Pilots miss it if it's taken away. It's like being on drugs, You don't need it, but once its taken away you miss it. I suspect it also gets you to the gate faster

It's also is a big moneymaker in sales and spare parts so of course it will be offered. However it costs more in operational expenses than changing out brakes more often.

Ask any pilot and they want it and aint nobody gonna take it away.

For me, and the safety side of it, I would rather use a safety net for the few times you really wish you had that extra margin after screwing up a landing. But, yes I too will go with the flow on this one and let the data roll in.
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Old 1st Jan 2011, 05:34
  #46 (permalink)  
 
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Thrust Reverse Inoperative

Lomapaseo. I too would rather have Thrust Reverse operational than not. I suspect Qantas also has re-thought the "Idle Reverse - Flap 25" preferred technique which was in vogue when they ran a B747-400 off the end of Bangkok. I simply have a difficulty in understanding why "Thrust Reversal" is considered to play only a minor role in reducing speed on landing as distinct from "Inlet Drag". When used, it certainly feels effective. My last employer, onn the B747 Classic rarely used Auto Brake, except on contaminated or limiting runways, and always used Flap 30, unless unavailable, and Max Reverse. Manual braking was not usually initiated until at or below 100 Kts.
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Old 1st Jan 2011, 11:30
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I too am having difficulty in getting my head around this whole "inlet drag" scenario; however, I "think" I'm getting closer to the crux of the matter.

As my previous NASA clip suggests, a jet engine requires to slow down the gasses as they pass through the compressor; add fuel to the compressed gasses, big bang, and then accelerate those gasses, via the turbine, out through the back end as thrust. In a high bypass engine the fan (just like a propellor) accelerates the air through the bypass stage... as thrust.

In order to produce, say, 20 Klb of Net thrust, the engine needs to generate, say, 100 Klb Gross thrust, because 80% of that work is absorbed by the compressor. Now, here's the crux of the matter; and also the area where I find it difficult to assimilate; if you remove the thrust, you're left with the compressor absorbing all of the energy as inlet drag. Obviously, you cannot simply remove the thrust; however, you can (by use of the thrust reverser) redirect the thrust-line to anywhere but useful (in producing rearwards thrust). When you now increase engine power (with reversers deployed) there is no useful rearwards thrust, but that the compressor is now absorbing more work and bringing the inlet towards a choked condition (but not fully; due to the operation of compressor bleed valves/offload valves); hence the increase in inlet drag.

I'm not totally convinced... but I'm open to further discussion.

The "Barn-Door" principle is much more appealing to me; however, that wall of "reverse thrust air" isn't fixed to the aeroplane, but it does (perhaps) go a long way in explaining why reverse thrust is much more effective at higher speed.

Whatever the answer might be; thrust reversers are here to stay and... they do work as published!

Happy New Year to you all.

TCF
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Old 1st Jan 2011, 11:42
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Off Loading Compressor

CTF, I know that driving the Compressor takes a lot of Energy which is extracted by the associated turbine, however I do not know of any engine which has other than normal bleed air extracted during reverse thrust operation. The compressor bleed valve remains closed as far as I am aware. Also, the RB211 is limited to 90% N1 RPM at Maximum Reverse Thrust, a high percentage of normal limiting N1 which is around 104-106%.
To me, the term "Reverse Thrust" says it all and I relate it to Vectored Thrust. The engine is fixed in place and the only variable is the direction in which the mass air flow is vectored. I am not convinced by the "Inlet Drag" theory although I accept and appreciate the high degree of engine "power" taken to drive the compressors and ancillary items driven by the accesory gearbox.
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Old 1st Jan 2011, 15:10
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As my previous NASA clip suggests, a jet engine requires to slow down the gasses as they pass through the compressor

The compressor is accelerating the air into the combustion chamber, not slowing it down when the aircraft is static, or at at slow speed. 'Inlet drag' becomes a factor at higher Mach numbers, I believe.
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Old 1st Jan 2011, 16:28
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Intuitively the mechanics of reverse thrust are simple.

If we are talking braking action, we are talking forces along (roughly) the direction of the length of the fuselage, the x-axis in airplane-based coordinates. Braking means generating some force in the negative-x direction.

Let's consider how this is done by a jet or turbofan engine in the "reverse thrust" regime. You are sucking a lot of air in through the front of the duct, in the negative-x direction, and you are reducing its velocity in the x direction effectively to zero (you are chucking it back out perpendicular) or slightly negative (you are chucking it back out a few degrees forward). That momentum-reduction produces a sizeable force in the negative-x direction, which is experienced as braking force.

If you suck air in faster, there is more momentum to reduce to zero, and you get correspondingly more force if you so reduce. There are two ways in which you can suck air in faster. One is by having a faster free airstream velocity and absorbing it; the other is by revving your engine faster. The first explains why TR is more effective at higher ground speed; the second why you increase power on the engine to achieve higher braking.

It seems to me to be mostly definitional what you call this momentum-stopping force. If Guppy wants to call it "inlet drag", then I guess I'll call it "Annelise Merriweather" and invite him to explain why his term is more apposite than mine.

This business of a "disc of air" generating some kind of resistance seems to me to be poppycock. As usual, being a bit familiar with basic physics helps enormously.

(Of course, pedants could calculate how big a flat circular plate would have to be to generate the equivalent force simply by pure drag as the engine in "reverse-thrust" regime generates, and some PPRuNe wit could say "see, *that's* the circular disc of air generated by reverse thrust!" But that's just the fun of PPRuNe.)

PBL
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Old 1st Jan 2011, 17:53
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The Annelise explanation seems fine to me. The 'flat plate' theory becomes 'ad absurdum' at zero forward speed (and when backing off a stand).
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Old 1st Jan 2011, 18:25
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The compressor is accelerating the air into the combustion chamber, not slowing it down when the aircraft is static, or at at slow speed. 'Inlet drag' becomes a factor at higher Mach numbers, I believe.
twochai
Err, the air is considerably slowed-down in the compressor i.e. it's a convergent duct. The air is then accellerated in the diffuser (a divergent duct) and directed into the combustion chamber.
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Old 1st Jan 2011, 18:35
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Thrust Reversal Principals & Differences

There are two types of thrust reversal systems utilized, depending on the engine design. For engines like the JT8-D where the fan by-pass air is channeled around the engine inside the nacelle and re-mixed with the hot gas flow while still inside the nacelle, a clamshell deflector system is generally employed. This system was used starting on 727s, 737s and DC9s. Upon landing and throttle retardation to idle, the thrust reversal is initiated by clamshell doors blocking the axial exhaust flow of the entire engine and two deflectors deploying that redirect the airflow in a forward angle straight upward and straight downward. Once deployed, the reverse thrust power is governed by the amount of thrust power selected. It is effective throughout the complete roll-out distance and in combination with the brakes, very effective regardless of speed, hence the power back capability departing a gate.

When this system was applied to the underwing engine mounted 737 directly from the 727 tail mounted engines, it didn't work. The bottom deflector caused lift on the wheels resulting in ineffective braking. The system was replaced with a new cylindrical "target" deflector system skewed 35 from vertical, but the principle remained the same.

In the development and utilization of high by-pass engines, the thrust reverser system changed to fan reversal only. After touchdown, thrust reversal is initiated, blocker doors close shutting off axial fan by-pass flow while a cowing slides back exposing a series of cast aluminum cascade panels. Each panel is custom designed for a particular circumferential location (there are some common ones), for each aircraft/engine combination, usually by the engine designer/manufacturer. The turning vanes in these panels redirect the air radially forward, 40 - 50 from axial. There are left hand and right hand panels depending on which wing the engine is to be mounted on. During engine removal, the panels stay with the aircraft. Reversal of the fan stream affects the core nozzle in that the fan stream influence is removed. The core engine thrust is more than overcome by the fan and core flow ram drag. Ram drag is the largest of the forces in the pure engine. This becomes more complicated on wing of the aircraft as the reverse flow shrouds a part of the aircraft and changes aircraft drag. The reversers are used down to 60 knots at a minimum to prevent air re-ingestion into the fan, e,g, FOD potential. On Boeing aircraft, during a few seconds after thrust reversal, the hot core exhaust pattern is disturbed heating the pylon fairing under the wing (1000℉ for a couple of seconds). Over time, aluminum is not suitable and fairings are protected by shaped castings welded together, originally 17-4PH SS, but now Ti6-4 to save hundreds of pound of weight per aircraft.

Net reverse thrust is defined as fan reverser air, minus forward thrust from the the engine core, plus form drag.

Hope this is informative.

Turbine D
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Old 1st Jan 2011, 18:35
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I've read that the engines on the B727 can be used in-flight - but not recommended!
One hundred percent correct. However, in the 7,000 plus I flew the 727, I never had cause to deploy the reversers in flight. One more fact about the 727-100, on a maximum performance landing, at normal landing weight, the reversers were not effective, because by the time the engines spooled back up in reverse, you were almost stopped. The brakes are that good on the 727, assuming it was well maintained.
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Old 1st Jan 2011, 20:43
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twochai
Err, the air is considerably slowed-down in the compressor i.e. it's a convergent duct. The air is then accellerated in the diffuser (a divergent duct) and directed into the combustion chamber.
TCF: if the aircraft/engine is static, the outside air must be accelerated as it is being sucked into the intake by the fan and the compressor, after which the convergent duct does its thing to slow it down.

Last edited by twochai; 1st Jan 2011 at 21:24. Reason: clarity
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Old 1st Jan 2011, 20:49
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I'm somewhat surprised that none of my textbooks make any reference to "Inlet Drag" as defined by thrust reversal systems.

I might have to revise my thinking!
This is the typical reaction of most pilots when discussing the effects of ram drag or inlet drag with respect to reverse thrust. Most haven't been taught this, and react with distain; if it's not in my textbook, then it can't be true.

Then again, ask how many have been taught the basic thrust formula; gross thrust minus ram drag equals net thrust. Nearly none. The basic formula is all one needs to knkow to understand the effects of taking away the thrust; one is left with drag.

The common argument is that airplanes back up on reverse, therefore reverse airflow must be the primary mechanism of the retarding force during landing. This is a nonsensical argument, but it's commonly used. Thus, the "I've never heard of it" camp backs themselves up with "I can back up."

Certainly where redirection of exhuast or fan gasses can be used as a contributing factor without causing re-ingestion problems or aerodynamic problems, a manufacturer will do so. After all, there's no reason not to use every available element of force to retard the airplane during landing. That the re-directed airflow isn't the primary mechanism for slowing the airplane, however, is misunderstood; largely misunderstood because many seem to believe that if they don't know it or haven't heard it, then it can't be true.

Manufacturers generally don't go into an explanation of ram drag or inlet drag any more than they present the basic thrust formulas. Operationally, the pilot is more interested in when thrust must be applied and when it should be discontinued during the landing. Re-ingestion and FOD issues are much more compelling operationally than the base force which exists with or without pilot input. The drag is always there; removing the rear thrust component (or most of it, in the case of re-directing or blocking fan thrust, because jet thrust remains) does nothing more than allow the same force that was already present, to still be present.

Increasing the power setting increases the drag. It would increase thrust, but thrust has been blocked or divered; what we get is a drag rise.

By utilizing various cascade vanes, blocker doors, and other means, some component of the re-directed gas or airflow can also be used to slow the airplane on landing. The term "reverse thrust" misleads one to believe that this is the primary mechanism for slowing, when it is not. In fact, many bucket systems and reverser systems don't direct the fan or exhaust gas path forward at all, or very much forward at all. The retarding force we feel during reverse activation is much more simple than what we tend to think; we've done little more than remove the net thrust component of the equation, allowing what was already present, to remain. Our biggest concern as we slow is to protect the engine and to prevent FOD damage and watch for a compressor stall.
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Old 1st Jan 2011, 21:56
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Terms such as "ram drag" and "inlet drag" are by no means ubiquitous. This may account for why so many people do not know what they mean. And they sure don't occur in textbooks such as Anderson's. If they don't occur in textbooks, it is surely not surprising that many people don't know what they mean.

Definitions would be helpful. Or references to them in publicly-available sources.

In particular because, if one does look in textbooks, such as John D. Anderson's Introduction to Flight (which is ubiquitous), one will find (courtesy of Rolls-Royce PLC) in Figure 9.19, p720, a division of the x-axis components produced by a jet engine in normal thrust-producing mode: in sequence from front to back,

Compressor produces about 19,049 lb forward
Diffuser produces 2186 lb forward
Combustion chamber produces 34,182 lb forward
Turbine produces 41,091 lb rearward
Exhaust unit/nozzle produces 5587 lb rearward

It should be obvious with a little thought that this situation can only happen when gas is passing through, not when gas is stopped and diverted perpendicularly. So using the same terminology for what happens in the two cases is somewhat apples-and-oranges, I feel.

PBL
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Old 2nd Jan 2011, 15:31
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Originally Posted by PBL
Terms such as "ram drag" and "inlet drag" are by no means ubiquitous. (...) Definitions would be helpful.
Terms such as "ram drag" and "inlet drag" or "intake momentum drag" are commonly used in jet engine performance analysis.

Intake momentum drag is equal to the product of inlet massflow and free stream velocity.

regards,
HN39
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Old 2nd Jan 2011, 16:32
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Originally Posted by HN39
Terms such as "ram drag" and "inlet drag" or "intake momentum drag" are commonly used in jet engine performance analysis.
Then it really shouldn't be too much to ask for someone knowledgeable in jet engine performance analysis to define these terms for us, should it?

(Then, for example, we would be able to tell if a statement such as "braking effect in reverse thrust is due to inlet drag" is just a tautology, or if it has substantial content)

Originally Posted by HN39
Intake momentum drag is equal to the product of inlet massflow and free stream velocity.
And how is "inlet massflow" defined?

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Old 2nd Jan 2011, 20:00
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PBL;

Are you asking for a definition of "inlet" or of "massflow"?

regards,
HN39
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