reversers and a/c speed [Archive]

SNS3Guppy

24th Dec 2010, 19:18

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.

JABBARA

24th Dec 2010, 22:44

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:

mustafagander

25th Dec 2010, 09:03

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.

SNS3Guppy

25th Dec 2010, 09:15

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.

This is not the case, at all.

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.

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.

lomapaseo

25th Dec 2010, 12:48

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 :)

twochai

25th Dec 2010, 13:15

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.

mustafagander

26th Dec 2010, 07:40

Thank you Guppy. Please explain.

SNS3Guppy

26th Dec 2010, 13:58

I already did. Read.

blind pew

29th Dec 2010, 07:21

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.

bugsquash1

29th Dec 2010, 09:45

I agree with Mustafagander

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

Old Fella

29th Dec 2010, 09:46

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.

lomapaseo

29th Dec 2010, 12:49

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.

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.

trimotor

30th Dec 2010, 05:13

It's a TAS issue.

four engine jock

30th Dec 2010, 06:50

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

Sir Richard

30th Dec 2010, 07:34

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

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:

Old Fella

30th Dec 2010, 09:24

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

TheChitterneFlyer

30th Dec 2010, 10:20

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

trimotor

31st Dec 2010, 03:14

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.

SNS3Guppy

31st Dec 2010, 07:32

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.

Maurice Chavez

31st Dec 2010, 07:59

Here is a good question.
Can you use Reverse Thrust in flight on a B727?You mean if it is physically possible to get them into reverse?? :E :E

four engine jock

31st Dec 2010, 09:17

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.

Maurice Chavez

31st Dec 2010, 09:21

Are you talking about that 727-100 into Brussels?

TheChitterneFlyer

31st Dec 2010, 09:34

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

four engine jock

31st Dec 2010, 09:42

Dont Know but it could be the one in EBBR.

Old Fella

31st Dec 2010, 09:51

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.

BOAC

31st Dec 2010, 10:55

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:)

Checkboard

31st Dec 2010, 11:06

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.

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!

four engine jock

31st Dec 2010, 11:16

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

TheChitterneFlyer

31st Dec 2010, 11:42

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

TheChitterneFlyer

31st Dec 2010, 11:59

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

lomapaseo

31st Dec 2010, 13:06

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 :E

Old Fella

1st Jan 2011, 00:02

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.

lomapaseo

1st Jan 2011, 03:39

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.

Old Fella

1st Jan 2011, 04:34

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.

TheChitterneFlyer

1st Jan 2011, 10:30

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

Old Fella

1st Jan 2011, 10:42

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.

twochai

1st Jan 2011, 14:10

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.

PBL

1st Jan 2011, 15:28

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

BOAC

1st Jan 2011, 16:53

The Annelise explanation seems fine to me. The 'flat plate' theory becomes 'ad absurdum' at zero forward speed (and when backing off a stand:)).

TheChitterneFlyer

1st Jan 2011, 17:25

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.

Turbine D

1st Jan 2011, 17:35

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

con-pilot

1st Jan 2011, 17:35

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.

twochai

1st Jan 2011, 19:43

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.

SNS3Guppy

1st Jan 2011, 19:49

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.

PBL

1st Jan 2011, 20:56

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

HazelNuts39

2nd Jan 2011, 14:31

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

PBL

2nd Jan 2011, 15:32

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)

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

And how is "inlet massflow" defined?

PBL

HazelNuts39

2nd Jan 2011, 19:00

PBL;

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

regards,
HN39

SNS3Guppy

3rd Jan 2011, 02:44

We have a number of eminently more qualified personnel who participate here, than I, to address the topic mathematically. Accordingly, I will say up front that errors here are my own failing at proper presentation. I'm neither a computer guru, nor a mathematician. I'll link some common public sites with multiple formulas spelled out, which I can neither type here, nor properly duplicate, and therefore won't try. This doesn't change the fact in discussion regarding ram drag, net thrust, and components of reverse applicable to slowing or stopping the airplane.

One can produce husk diagrams and introduce thermodynamic and specific power equations all the live-long day, and one can complain that this text or that text didn't break down the basics enough, but the fact remains that the elemental equation for the most important part of thrust that we get to use (net thrust) is very simple: gross thrust minus ram drag. Take away the net thrust, and we're left with ram drag. Load a whole lot of air into the front end of an engine, don't squirt anything useful out the back, and what you have is a lot of drag. That's reverse thrust.

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.

It's not a case of "his term" or "my term." You can call inlet drag (ram drag, etc) by whatever term you prefer, but I'll stick with the proper terminology appropriate to the thrust formula in which each is utilized.

You can break down the drag into numerous components, including inlet drag, ram drag, spillage drag, cooling drag, etc. In fact, you can break down the entire thrust equation to be as complicated as you want to get. You can break down for specific thrust, mass airflow, nozzle performance, inlet performance, engine core efficiency, etc. Yes, every aspect has it's definition, and yes, you're absolutely free to look it up, if that's the direction you wish to go.

Jet engine performance - Wikiversity (http://en.wikiversity.org/wiki/Jet_engine_performance)

Simply put, the basic thrust equation for net thrust is gross thrust minus ram drag, where ram drag serves as a composite for everything that takes away from the gross thrust produced by the engine. Consider your net thrust (the stuff you get to use), consider your gross thrust, and the difference between the two is collectively ram drag. Net thrust is the most basic representation of what we're getting out of the engine; gross thrust is X, but in order to know what our usable net thrust is, we need to subtract ram drag, Y. X minus Y equals our number, net thrust, what we get to use to do work, as Z. Specifically, Fn=Fg-Fr, where Fn is net thrust, Fg is gross thrust, and Fr is ram drag.

Airplane thrust reversers (Henry Spencer; Mary Shafer) (http://yarchive.net/space/thrust_reversers.html)

To expand slightly, net thrust is the sum of mass airflow and fuel flow, multiplied by exhuast stream velocity, minus intake ram drag, written as Fn=(Mair+Mf)Vj-MairV, where Fn is net thrust, Mair is the engine mass airflow rate, Mf is fuel flow rate, Vj is exhaust gas velocity, and MairV is intake ram drag. This may be further reduced to Fn=Mair(Vj-V), where Fn is net thrust, Mair is rate of engine mass airflow, Vj is exhaust gas velocity, and V is true airspeed, if fuel contribution is discounted. In a nutshell, this simplification accounts for ram drag without having to spell it out: considering the relationship between mass airflow, exhaust stream velocity (made under the assumption that mass airflow out equals mass airflow in, which is to say, disregarding bleed usage for the time being, and other intrinsic losses to operational use of the jet engine), and true airspeed, we see that a significant loss occurs between what the engine is doing, and what we get out of it. Take away the TAS as representative of inflow velocity, remembering that if we've got any forward thrust that the exhaust velocity must exceed TAS (free airstream) by some measure, then what we're left with is the loss. Mass airflow times TAS, subtracted from mass airflow times exhaust velocity, and there's your ram drag.

You also see a telling relationship as you throw in numbers: ram drag is low, nearly nothing, at a standstill. Net thrust is greatest at that time; net thrust increases as we slow during the landing, while the ram drag decreases. This harks back to the original question of why we go for the reverse at higher speeds than lower, and why reverse is more effective at higher speeds than lower. Assuming we're diverting all the net thrust during the rollout, we're getting less and less effective reverse thrust as we slow down; the component of reverse thrust contributed by redirected fan or exhaust gasses is greater as we slow, but the component of ram drag is less...and it's the ram drag that is slowing us down and providing the greatest retarding force during the rollout; not the redirected fan and exhaust airflow/gasses.

Ram drag/inlet drag is determined by mass airflow and the effect the engine has upon that mass airflow. Put a large mass of air into the engine and slow it down, the engine is experiencing a lot of drag. Take away the thrust at the other end by diverting it somewhere other than the traditional thrust axis, and now you've got a lot of drag and no useful thrust. You have a retarding force that is greatest at high power settings (high mass airflow) and higher airspeeds (higher inlet and ram drag).

Inlet drag is not equivalent to flat plate area, and hence a discussion of flat plate aerodynamics is nonsensical and irrelevant.

If they don't occur in textbooks, it is surely not surprising that many people don't know what they mean.

How about Aircraft and Gas Turbine Engines (MIT Press, Kerrebrock, 1992)?
Amazon.com: Aircraft Engines and Gas Turbines, Second Edition (9780262111621): Jack L. Kerrebrock: Books (http://www.amazon.com/exec/obidos/ISBN=0262111624)

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

You really can't get much more "publicly-available" than wikipedia, however unscientific it may be.

Turbojet - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Turbojet)

Or, to further dumb it down to a "publicly-available" refernece:

Answers.com - What is gross thrust in a turbine engine verses net thrust (http://wiki.answers.com/Q/What_is_gross_thrust_in_a_turbine_engine_verses_net_thrust)

or

gross thrust: Definition from Answers.com (http://www.answers.com/topic/gross-thrust)

Or, to really, really dumb it down...

What is net thrust? and how can we increase it? - Yahoo! Answers (http://answers.yahoo.com/question/index?qid=20061203111527AAWf8ES)

As an aside, an interesting study (albeit fifteen years old) regarding the reasons for thrust reverser use among airlines (included are a number of well-recognized and respectable operators); this study shows that the economic cost of reverser use exceeds the comparative cost of brake replacement if reverse isn't used:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950014289_1995114289.pdf

twochai

3rd Jan 2011, 03:32

this study shows that the economic cost of reverser use exceeds the comparative cost of brake replacement if reverse isn't used:

Probably true, except for the cost of airframe replacement when the use of reverse is neglected!

SNS3Guppy

3rd Jan 2011, 04:25

Perhaps. Given that landing performance is predicated on no reverse, the issue isn't really not getting stopped, but brake energy and wear.

The contributing airlines, mostly major world players with large fleets, noted that the cost of reverser and engine maintenance exceeded the cost of replacement brakes by a significant margin.

We use reverse largely because our operational policy is to use autobrakes, and given a constant rate of acceleration, any reverse use means cooler brakes for the same stopping distance. Given typical two or three hour turns and usual max weight landings, we're concerned with keeping the brakes cool to minimize turnaround delays. For us, it's not the cost of engines or brakes, but the turn time that interests us most.

For the respondents to the paper cited above, the cost of reversing was higher than that of brakes. As reverse use is often cited as a way of "saving the brakes," I find the observations in the report to be of interest.

PBL

3rd Jan 2011, 06:45

An awful lot of guff, Guppy, but I think you made my point for me quite well.

You introduced the term "inlet drag" and said it was the phenomenon responsible for most of the braking effect of reverse thrust.

So I wondered what you meant by the term "inlet drag". (You may presume I had tried to find out!) I still don't know! And you still haven't said!

The point is that you used a concept in notes 13, http://www.pprune.org/tech-log/437583-reversers-c-speed.html#post6141941 and 31 http://www.pprune.org/tech-log/437583-reversers-c-speed-2.html#post6151425 with (to me) some degree of confidence to "explain" a phenomenon (braking under reverse thrust), but apparently you cannot tell us what the concept means. That makes your "explanation" moot.

(I say "confident" because of the following exchange: Thank you Guppy. Please explain. I already did. Read. Which led me to wonder "he did?")

You hint you're not a mathematician, but the very first reference you cite is full of equations. Can we presume, then, that you are not actually familiar with the material in that reference? If so, I am at a loss to imagine why you cited it. (As I noted, it doesn't use the term "inlet drag").

AFAIK, the only place in internet-accessible documentation in which the term "inlet drag" is used is in the comments from 2001 on the sci.space.shuttle newsgroup by Henry (btw, both Henry and Mary are correspondents of mine from the '80's and 90's, although I haven't heard from either of them for a long time), and in a Master's thesis at the Naval Postgraduate School in which the term is also not defined. BTW, it's ironic that a term such as "inlet drag" should occur of all things on a shuttle newsgroup, because of course the shuttle, being driven by a rocket engine, has no inlet, and therefore no "inlet drag" (whatever that may mean)!

A term it seems you now want to use is "ram drag":
Mass airflow times TAS, subtracted from mass airflow times exhaust velocity, and there's your ram drag.
Is that the same as "inlet drag"?

I agree with what I take to be your point, that the braking force attibutable to use of reverse thrust is the force generated by taking all that momentum and stopping it in the x-direction, indeed I said so in my post 50 http://www.pprune.org/tech-log/437583-reversers-c-speed-3.html#post6153587. But my formulation is preferable to yours, because you mix up your units [Edit: too quickly said: maybe not. See HN39's post below!]

Just as a point on discussion etiquette, I don't think it's helpful to use lots of technical vocabulary, of which it seems you may not know the meaning, and link them with lots of qualitative math (such as "mass airflow times TAS") which it seems you also do not quite master. Wouldn't it have been easier to say you really just meant what I wrote in my note cited above? Then we'd all agree with you!

PBL

HazelNuts39

3rd Jan 2011, 07:42

PBL;

Basic dimensions: m=mass, l=length, t=time

Dimension of force = m*l*t^-2
Dimension of mass flow = m*t^-1
Dimension of TAS = l*t^-1
Dimension of mass flow times TAS = m*l*t^-2

regards,
HN39

PBL

3rd Jan 2011, 08:31

Dimension of force = m*l*t^-2
Dimension of mass flow = m*t^-1
Dimension of TAS = l*t^-1
Dimension of mass flow times TAS = m*l*t^-2

So it is! If your "mass flow" is what Guppy means by "mass airflow" then the units cohere. Now the only missing part is: flow of what mass of air?

PBL

SNS3Guppy

3rd Jan 2011, 08:46

Now the only missing part is: flow of what mass of air?

Inlet.

You introduced the term "inlet drag" and said it was the phenomenon responsible for most of the braking effect of reverse thrust.

Not my term. If you mean "introduced" it in this thread, then yes. If you mean I introduced the term, then of course not. It is responsible for most of the braking effect of reverse thrust, however.

And you still haven't said!

I have, repeatedly, and no, I don't presume.

You hint you're not a mathematician, but the very first reference you cite is full of equations. Can we presume, then, that you are not actually familiar with the material in that reference? If so, I am at a loss to imagine why you cited it.

I don't think I hinted, at all. I was quite direct. You should not presume.

The equations and the information are cited precisely because it was requested. It is unnecessary, however, as the point stands without the equation.

But my formulation is preferable to yours, because you mix up your units [Edit: too quickly said: maybe not. See HN39's post below!]

Too quickly said, correct. My rationale for inclusion of more than one term into a composite has already been given, for the sake of simplicity. While one can certainly divide ram drag into components, it is unnecessary and does not contribute further to the point that it is not reverse flow of fan airflow or exhaust gas stream that provides the braking effect in reverse thrust operations, but ram drag (inlet drag, if you will. Also, if you will not).

Just as a point on discussion etiquette, I don't think it's helpful to use lots of technical vocabulary, of which it seems you may not know the meaning, and link them with lots of qualitative math (such as "mass airflow times TAS") which it seems you also do not quite master.

Perhaps you'd prefer to put words in my mouth and speak for us both?

HazelNuts39

3rd Jan 2011, 08:49

Now the only missing part is: flow of what mass of air?
What do you think?
HN39

chksix

3rd Jan 2011, 09:08

Another source. No reverse thrust modeled though.
EngineSim 1.7a beta (http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html)

PBL

3rd Jan 2011, 09:13

Guppy,

this bores me. I am participating to discuss the mechanics of reverse thrust, not to belabor discussion styles. This happens so often on PPRuNe, which is why I don't participate that often.

So, one last time:

I have asked you what you meant (that is, to define the term "inlet drag", which you said is responsible for most of the braking effect of thrust reverse).

You still won't say.

I now know where you are coming from.

HN39,

[PBL: flow of what mass of air?]
What do you think?

Another discussion game. Pass.

Could we maybe get back to discussing the mechanics of reverse thrust and the pertinent concepts?

PBL

HazelNuts39

3rd Jan 2011, 09:42

chksix;

Thanks for that most instructive link. At the bottom of that page is a link to "Beginners Guide to Propulsion", which may be helpful.

regards,
HN39

HazelNuts39

3rd Jan 2011, 10:36

Another discussion game. Pass.
Not my game, yours!

EDIT::Could we maybe get back to discussing the mechanics of reverse thrust and the pertinent concepts? If you are serious, the concept is quite simple. In forward thrust mode, the net propulsive thrust of a jet engine is gross thrust (aka 'nozzle thrust') minus intake momentum drag (aka 'ram drag'). In reverse, for the same fuel flow, N1, N2, etc., the engine air flow and hence intake momentum drag is unchanged and part of the 'nozzle thrust' is deflected sideways and forward. On modern high bypass-ratio engines, only the bypass air is deflected, the gas generator exhaust is not. Hope this helps.

regards,
HN39

TheChitterneFlyer

3rd Jan 2011, 11:26

Guys, let's not get bogged-down with semantics. This whole discussion has been most interesting and has, certainly, provided me with a whole raft of information that I hadn't previously considdered.

There will allways be differences of oppinion; especially with the varying terminology that's used within International English. The problem with the use of forums such as PPrune is that not everyone is capable of presenting the precise nature of their discussion without making the odd (unintentional) error (of which I too am guilty of). If all of the contributors to this thread were to be within the same room and conducting the same conversation we'd, no doubt, come to an ammicable conclusion. Also, frustrating as it might be, there will always be the occasional heckler who will want to throw a spanner in the works (I'm not inferring that this is the case within this particular thread so, please, don't shoot me down for making this statement of fact)!

My thirty-five years spent within the aviation industry doesn't exonerate me from making mistakes; nor does it mean that I'm the font of all knowledge.

Where were we... Inlet Drag, Ram Drag... terminology; don't you just hate it?

TCF

PBL

3rd Jan 2011, 12:32

If you are serious, the concept is quite simple.

Well, I thought so, as I said earlier. But unfortunately my simple explanation doesn't appear to satisfy everyone (although I suspect you know it's right, even if Guppy can't tell).

In forward thrust mode, the net propulsive thrust of a jet engine is gross thrust (aka 'nozzle thrust') minus intake momentum drag (aka 'ram drag'). In reverse, for the same fuel flow, N1, N2, etc., the engine air flow and hence intake momentum drag is unchanged and part of the 'nozzle thrust' is deflected sideways and forward. On modern high bypass-ratio engines, only the bypass air is deflected, the gas generator exhaust is not.

And how does this square with
...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 ?

Answer: it doesn't. Some of these values are changed during use of reverse thrust. However, you and Guppy seem to suggest that some things are unchanged. So let me ask more precisely: which of these values do you think will remain unchanged, and which stay the same, if said engine is in "thrust reverse" configuration?

ChitterneFlyer,

this isn't about semantics.

PBL

HazelNuts39

3rd Jan 2011, 15:23

ChitterneFlyer, this isn't about semantics.
PBL,

Please forgive me if I start my reply with your last remark. I was about to reply exactly the same but, after carefully re-reading your post #50 of Januar 1st, I must agree with ChitterneFlyer that the discussion is to a large part about semantics. What you call "momentum reduction (...) experienced as a braking force" is what I would call "intake momentum drag", and others "ram drag". The term "inlet drag" is somewhat ambiguous, because it could refer to the skin friction or pressure forces acting on the intake duct.

That brings me to the forces 'produced' by engine internal components that you quote from J.D. Anderson's book. Without an explanation of how they are derived, these figures don't mean anything to me. I suspect they have to do with the structural design of the engine carcass, loads on bearings (see discussion on QF32 threads), etc., but they don't seem particularly relevant in the context of engine performance.

Now that we agree on the intake side, let's turn to the exhaust side of the engine, which to me seems to be slightly 'under-exposed' in your account. For a given engine and flight condition, the inlet mass flow (the mass of the air that enters the engine intake) is, as you say, a function of engine RPM. In principle, that relation is the same for forward and reverse thrust regimes, except that deployment of the thrust reversers may result in a change of the effective final nozzle area, and thereby affect the internal 'matching' of engine components.

In other words, for a given RPM, deployment of the reversers doesn't change very much on the intake side, but affects mostly the 'nozzle thrust' by varying degrees of effectiveness, depending on the design of the reversers. In addition, as pointed out by others, the 'reversed' exhaust plume can have adverse effects on the aerodynamics of wing and control surfaces, and can be re-ingested by the engine. Due to these adverse effects, reverse thrust usually needs to be cancelled or reduced at low speed, which adds to the perception of ineffectiveness at low speed.

regards,
HN39

PBL

3rd Jan 2011, 16:19

What you call "momentum reduction (...) experienced as a braking force" is what I would call "intake momentum drag", and others "ram drag". The term "inlet drag" is somewhat ambiguous, because it could refer to the skin friction or pressure forces acting on the intake duct.

Well, so there's yet another new term: "inlet momentum drag". It's a shame we can't find vocabulary on which we can agree, and to which we can stick.

Some of the air passing outside the duct will encounter reverser air, become quite slowed in the negative-x direction and thereby contribute to momentum reduction. I wouldn't want to call that "intake..." or "inlet..." anything.

forces 'produced' by engine internal components that you quote from J.D. Anderson's book. Without an explanation of how they are derived, these figures don't mean anything to me.

I think they should. My point in citing them is as follows. If you include all the forces from inlet up to turbine, there is still a net forward force (positive-x). So since thrust reverse generates a net backward force, there is a fair bet, to understate the case, that some of those forces will be significantly different in the reverse-thrust configuration. If they are significantly different, then saying "thrust is this-minus-that; take away this; you're left with minus-that" is a misleading way to try to explain the braking effect caused by reverse thrust.

Now that we agree on the intake side, let's turn to the exhaust side of the engine, which to me seems to be slightly 'under-exposed' in your account.

I wouldn't yet say that we agree on the intake side. But I am quite willing (obviously - one can check!) to agree that I haven't really addressed what happens with the exhaust.

There are various ways to slice the pie when talking about the contributions of various architectural components of an engine to the braking effect under reverse thrust. I think the contributions of the components to load mostly change. The difference between the normal-thrust contribution of a component at a given RPM and the reverse-thrust contribution of that component at the same RPM could be called the "drag due to <the component> when in reverse-thrust". I note that the total momentum-reduction will be larger than the sum of the drags due to the components when in reverse-thrust. That is why I resist giving the entire momentum-reduction phenomenon a name which suggests it has to do with a specific engine component such as the intake or the inlet.

PBL

SNS3Guppy

3rd Jan 2011, 16:55

I now know where you are coming from.

Clearly not.

john_tullamarine

4th Jan 2011, 01:25

If I may jump into the pool for a moment or two.

I have avoided comment in this thread as (having done a couple of engine design courses in years long departed) I am comfortable that the down and dirty details are dreadfully complex and my understanding is not sufficient to make much in the way of any useful contributions.

Similarly, my involvement with pilot theory training in a previous life (and long standing observations of typical pilot texts and the like) convinces me that a lot of theory "training" is responsible for creating a lot of the real world understanding problems within the flying community. With engines, in particular, the dreadful variety of "definitions" is a never ending problem ..

Engine detail workings and design, like many topics (atomic theory is a good one which comes to mind) can be looked at from a range of complexity viewpoints, according to the needs of the discussion and the folk involved. When, such as in this present discussion, we find folk with different preferences, backgrounds and needs, the discussion can become a tad confusing and, even, a little heated.

With my engineer's hat on, if I were an engine designer (which I'm not and don't want to be) then I would be interested in precision in definitions/jargon, details of what this and that do, details of how this and that interact, etc.

With my pilot's hat on, I view an engine as a tool to be used and am interested in the basic guts of what's going on - there are bits which we would rather not have but are stuck with (the drag things), bits which we admire and seek (the thrusty bits) and bits which we can put to some sort of good use (net thrust available to make the airframe do useful things). Other pilots (SNS3Guppy is, I think, a good example) seek as much knowledge as they can come upon and are interested in digging down a lot deeper although not always successfully when it comes to jargon precision - that is not a criticism of such folk at all - just a fact of what happens when any of us seek to get into the nitty gritty of complex matters without going the full journey to where we might acquire an expert level of expertise.

Folk such as PBL, essentially, are academic in their background and conditioned to seek precision in discussion - hence his thrust in this thread.

Presuming that we all can keep our emotions in reasonable check, the real value of this sort of thread is that a variety of knowledge and technical competence levels can toss ideas around with the useful outcome that we might be able to go away from the discussion with a bit more of an understanding of what is going on in a particular process - whether engine operation, flight standards considerations .. whatever.

One useful comment I noted earlier is .. If all of the contributors to this thread were to be within the same room and conducting the same conversation .. the limitation of a non-refereed on-the-fly print discussion model is never far from being obvious ..

john_tullamarine

4th Jan 2011, 01:46

Just keep the insulting attitude in check please.

I guess we all have our moments from time to time .. however, the need for decorum doesn't extend to the heights of political correctness .. only that we don't allow ourselves to cross the fence into the paddock of abuse.

Within the aviating community the necessary requirement for individual high ego levels dictates the occasional bit of to and fro ... comes with the territory, I'm afraid.

One of the considerations of life is that we represent a continuum of interpersonal styles and appearances. We have to be able to manage both our individual styles and modes of interaction to effect a useful management of group activities. I suggest that a cursory view of any successful large aircraft captain (or purser, for that matter) will, near invariably, reveal a person with such attributes ? .. doesn't mean that one has to be PC, nice all the time, or whatever ... but one needs to be able to interact effectively and in a goal oriented manner.

PBL

4th Jan 2011, 09:58

After reading JohnT's wise words I'd like to add a couple of my own.

I have done quite a bit of on-line searching for any information concerning the science of reverse thrust and haven't found anything. Guppy's on-line references don't discuss it in any depth. It may be that there is something in Kerrebrock's classic text, but I don't have a copy. Maybe someone who does can tell us what it says? I don't have the Rolls book either; something might be in there.

In my experience, persistent quibbles over terminology are symptoms of fundamental difficulties with concepts. Everyone says "oh, it's just words", but in fact it is much more than that. I am involved in international standards work, and a huge amount of drudge work is involved with terminology. It's like bookkeeping: everyone wants to do "real work", but any real work is impossible without it. To my mind, the reason is that terms are used because they denote concepts, and if your concepts aren't clear, then neither is your understanding.

Where we all agree is that the idea, found in many "explanations" of reverse thrust, that braking is mainly achieved primarily through the exhaust, is misleading. I think HN39, I, and others agree that the braking effect is achieved through momentum reduction of some air mass.

Now, here is where terminology becomes important. With which engine component or components is that momentum-reduction primarily associated? The answer so far seems to be: nobody here really knows.

Isn't it worth finding out? I think so.

Maybe DevX, who seems to work for or around RR, or Turbine D, could help?

PBL

HazelNuts39

4th Jan 2011, 10:48

With which engine component or components is that momentum-reduction primarily associated?Answer: The thrust reverser, and it's not a component of the engine but of the nacelle (or exhaust duct). I wonder if you have ever seen the demonstration of a Harrier at a Farnborough Air Show, and thought about how it "vectors" its exhaust thrust and controls the engine that produces the exhaust "momentum"?

regards,
HN39

PBL

4th Jan 2011, 11:01

With which engine component or components is that momentum-reduction primarily associated?

Answer: The thrust reverser, and it's not a component of the engine

Then it isn't an answer to the question, is it? :)

But interpreting you more widely, if the aircraft component primarily associated with the momentum reduction is the reverser itself, why would anyone want to call the resultant force "inlet ...", or or "intake...." anything? Why wouldn't they call it "reverser vane load"?

PBL

HazelNuts39

4th Jan 2011, 11:14

PBL;
In my vocabulary I wouldn't call it a momentum 'reduction', because the energy added by the fuel burned in the thermodynamic cycle increases the momentum of the air mass flow that passes through the engine. The reverser merely changes the direction of the exhaust momentum, and doesn't appreciably affect either the intake momentum or the internal cycle of the engine itself.

regards,
HN39

SNS3Guppy

4th Jan 2011, 13:20

But interpreting you more widely, if the aircraft component primarily associated with the momentum reduction is the reverser itself, why would anyone want to call the resultant force "inlet ...", or or "intake...." anything? Why wouldn't they call it "reverser vane load"?

Because that's not the case.

If blowing exhaust gasses on cascade vanes or reverser buckets were to produce a retarding force, we'd have a whole new law of physics at play (and we'd be revisiting Newton's third). We could transfer that new law to sailing ships, and let the ships use giant fans to blow on ship-mounted sails to propel the ship along.

Using a rearward engine-produced force to act on an engine-mounted or nacelle-mounted reverser mechanism to produce a greater retarding force is nonsensical.

You can certainly attempt to separate components of drag into lip drag, form drag, intake drag, or any number of other components of the ram drag equation (already provided, here); but it's largely irrelevant and meaningless. Particularly to one sitting in the cockpit.

Ram drag is a collective term which accounts for numerous components. One could blame the compressor, the diffuser, the intake, etc; "ram drag" is a useful collective term with a viable equation (again, already given here, with link provided) with accounts for the retarding value that is what slows us down during reverse action on the runway.

So far as exhaust mass gas flow being used to account for reverse thrust, it's the wrong flow. We're considering intake flow (hence, the acceptable use of "intake drag" or "ram drag," used here interchangably), not exhuast mass airflow. If we are to consider airflow through the exhuast on a high-bypass turbofan, we can only consider it as a portion of the mass airflow through the nacelle inlet; it's the mass airflow through the inlet that accounts for the retarding force in reverse operations. Considering the fuel flow in the equation, when it's only added to a small component of airflow through the engine assembly is important to the thermodynamic equation of thrust production, but not necessarily to reverse thrust. The chief value of burning fuel during reverse operations is to increase RPM and mass intake airflow, and thus intake or ram drag.

PBL

4th Jan 2011, 14:37

But interpreting you more widely, if the aircraft component primarily associated with the momentum reduction is the reverser itself, why would anyone want to call the resultant force "inlet ...", or or "intake...." anything? Why wouldn't they call it "reverser vane load"?
Because that's not the case.

I'll let you argue that with HN39.

If blowing exhaust gasses on cascade vanes or reverser buckets were to produce a retarding force, we'd have a whole new law of physics at play (and we'd be revisiting Newton's third). We could transfer that new law to sailing ships, and let the ships use giant fans to blow on ship-mounted sails to propel the ship along.

New laws of physics? I think we can do that with the ones we have. I can think of at least two designs where that would work. Not that it would necessarily be efficient, mind.

It would work for airplanes also. Mount the engines in the other direction, blowing onto the wing. Smoothly, of course. And rise into the air vertically. Better idea: move the wing in the air. You'd probably have to make it go round and round to do that though. Wait, hasn't somebody had that idea already?

Using a rearward engine-produced force to act on an engine-mounted or nacelle-mounted reverser mechanism to produce a greater retarding force is nonsensical.

Really?

You can certainly attempt to separate components of drag into lip drag, form drag, intake drag, or any number of other components of the ram drag equation (already provided, here)

So now you are saying that "intake drag" is "a component of the ram drag equation", whereas, before, you were saying that ram drag and intake drag were two names for the same thing:

you're left with ram drag, also referenced as inlet drag or intake drag

But wait a minute ...... you weren't only saying it before, you're saying it again now!

We're considering intake flow (hence, the acceptable use of "intake drag" or "ram drag," used here interchangably), not exhuast mass airflow.

So, if you would please, I'd like you to decide. Is "intake drag" a "component" of "ram drag"? Or is "intake drag" identical with "ram drag"? You can have one, but not both. And when you've chosen, I'd like you to stick with the choice.

If we are to consider airflow through the exhuast on a high-bypass turbofan, we can only consider it as a portion of the mass airflow through the nacelle inlet; it's the mass airflow through the inlet that accounts for the retarding force in reverse operations.

Well, no. There is also the mass of accelerated air which passes just outside the cowl, which will be arrested in the x-direction by the reverser exhaust. That loses momentum also, so that is going to enter the momentum-loss equation too.

PBL

SNS3Guppy

4th Jan 2011, 15:14

Well, no. There is also the mass of accelerated air which passes just outside the cowl, which will be arrested in the x-direction by the reverser exhaust. That loses momentum also, so that is going to enter the momentum-loss equation too.

No, it's not.

The perception that diverted fan or exhaust gasses cause a "wall of air" which somehow serves to slow the airplane down is largely a myth. Only where such airflow causes aerodynamic losses for the airplane can it possibly serve to slow the airplane, or where some forward component of that gas path exists. It's effect on the free airstream, or even the boundary layer adjacent to the cowl, is largely inconsequential to the process of slowing the airplane on landing.

While re-directed gas path may impinge on the free airstream and divert of slow it, this doesn't cause the airplane to slow. Only the forward component of that exhuast gas path has an effect. It's not a sail, and it's not a solid, attached component to the airplane. It's a fluid, and once vectored by the cascade vanes or buckets, it's done, insofar as any benefit to the airplane.

It would work for airplanes also. Mount the engines in the other direction, blowing onto the wing. Smoothly, of course. And rise into the air vertically. Better idea: move the wing in the air. You'd probably have to make it go round and round to do that though. Wait, hasn't somebody had that idea already?

No. If you're talking about helicopters, find a helicopter that works by mounting turbojet engines on the rotors and using the exhuast gas to flow over the rotor to produce lift. Then again, find an airplane where the same thing happens. Even then, you're talking about something entirely different than considering exhuast gas velocity with respect to reverse; entirely different mechanisms, absolutely apples and oranges.

A rotor passing through the air operates in a very similar manner to a wing passing through the air. This has nothing to do with reverser operation, however.

You seem to have run in circles and are entertaining yourself with nonsensical teasers that do nothing but cloud the issue.

Really?

Yes, really.

So now you are saying that "intake drag" is "a component of the ram drag equation", whereas, before, you were saying that ram drag and intake drag were two names for the same thing:

For the purposes of discussion, as explained here ad nauseum, yes.

When considering whether redirected exhaust or fan gasses serve as the chief retarding force, or whether the chief retarding force in reverse operation is airflow through the engine inlet, yes. We will refer to the collective inlet force interchangeably. You're welcome to break it down ad infinitum into numerous components, even to attempt to decide which compressor stage, diffuser, or even fan blade absorbs the most energy from the ram mass airflow influx, but I have no need to do so. Collectively, the drag produced by the various components that compose the total retarding force may be referred to as ram drag, and yes, I also interchangeably use intake drag, for reasons previously cited. Deal with it.

PBL

4th Jan 2011, 17:29

My, my, we do seem to be getting confused!

There is also the mass of accelerated air which passes just outside the cowl, which will be arrested in the x-direction by the reverser exhaust. That loses momentum also, so that is going to enter the momentum-loss equation too.

No, it's not.

Oh, yes, it is!

(And the response is.....?)

The perception that diverted fan or exhaust gasses cause a "wall of air" which somehow serves to slow the airplane down is largely a myth.

Yes, I seem to remember someone saying something like that earlier in the thread. Who was it now? Maybe...

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.

Your suggestion about ships was that you needed to change the laws of physics to get a self-blown sail to work. It took me a minute to think of two designs where that would work. If it takes you longer than a minute, we may conclude that I am better at aerodynamics than you are.

Concerning blown wings,
It would work for airplanes also......

No. .... find an airplane where [that] happens.

Does this count? Powered Lift: Novel GTRI Design (http://www.gtri.gatech.edu/casestudy/powered-lift)

If not, why not?

PBL

Pugilistic Animus

4th Jan 2011, 17:45

Does this count? Powered Lift: Novel GTRI Design (http://redirectingat.com/?id=42X487496&xs=1&url=http%3A%2F%2Fwww.gtri.gatech.edu%2Fcasestudy%2Fpowered-lift&sref=http%3A%2F%2Fwww.pprune.org%2Fnewreply.php%3Fdo%3Dnewre ply%26noquote%3D1%26p%3D6159135)

If not, why not?no...it does not:E

why? boundary later control is what those designers are discussing, blc adds weight and complexity that's why it never is really implemented [as of yet] in a successful commercial design...I could expand this...if you wish

any questions?:8:E
we may conclude that I am better at aerodynamics than you are.
But are you better than Lester:E ?

bearfoil

4th Jan 2011, 17:52

PA

I think someone should inject Coanda, No??

Pugilistic Animus

4th Jan 2011, 18:07

Bear,

Coanda effect is based upon the law of conservation of momentum 'p'

p=mass*velocity

if mass flow is held constant then the only effective change in momentum can come about through a change in the velocity so if the velocity vector is changes this is a momentum change

a momentum change through a certain time interval produces a force [written dmv/dt] the most common example is cambered surface.held to a moving jet of a fluid...such as a spoon being held so the it's curvature faces a running faucet...also practically speaking one is adding momentum to the boundary layer...similar to what the designers mentioned in PBL's post were aiming for...in essence Coanda effect is exploited to modify the boundary layer in such a manner as to produces an aerodynamic force..

a practical example of Coanda effect is Demonstrated in the Boeing/ McDonnell Douglass NoTaR system...I hope I'm being understandable if not let me knowin fluid mechanics the concept is derived from a concept known as 'continuity'http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wibble.gif... but I try to temper my posts...sometimes:)

PBL

4th Jan 2011, 18:12

So, what's the answer, PA?

Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?

Do notice I said "physically possible" and not "a practical proposition"!

bearfoil

4th Jan 2011, 18:14

If Boeing can do it.....I think the solid need not be cambered, though that is the default shape in a flexible mass due friction. A ceiling fan has greater efficiency and a broader disc when it is close to the ceiling. Sticky has its place. Actually it is all about placement. No mysteries then.......

lomapaseo

4th Jan 2011, 18:20

It gets murlier and murkier the further that we go in this discussion.:(

I thought I was satisfied several pages back and posted my undrestanding of the issue for any further enlightment of myself. Since then I have seen a lot of personal semantic arguments between a few individuals. So I would propose adding some new paricipants eager to learn communication skills which might satisfy most of us lurkers.

I'm really not sure that I yet have this correct but here's where I'm at right now.

There are two parts of the engine as a propulsion system assisting in the braking of the aircraft.

The first is the deflection of some of the engine's propulsive force (energy) forward against the motion of the aircraft. This is accomplished by turning the air entring the inlet by more than 90 degrees (reverser cascades, buckets, clamshell deflectors etc.) This is only practical at high aircraft speeds due to the inherent risk of reingesting the air and runway debris.

The second means of assisting the braking of the aircraft is to increase the engine nacelle drag (frontal area of the engine) by slowing the air down that passes through the engine without producing significant forward thrust or turning the reverser air forward enough to cause reingestion.

This is accomplished by an idle condition on a running engine where the compressor and fan work are just enough to slow the inlet air down and maintain the engine cycle with very little net thrust produced. Thus the effective drag on the engine nacelles assume a plate blockage (not just the lip) while at the same time without effective rearward thrust the net effect is almost entirely a perceptive increase in drag assisting the brakes.

If the reduction of the engine reverser efflux is done at low aircraft speed and low thrust settings, the pilot should perceive no changes in retarding force.

OK, I'm quite sure that I haven't got this all correct so anybody is welcome to tweak it or entirely discard my understanding. Either way I should learn something more from the discussion.

Pugilistic Animus

4th Jan 2011, 18:43

I think the solid need not be cambered

You are correct it can be a flat plate but C'mon it's Boeing:}

Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?

Do notice I said "physically possible" and not "a practical proposition"!

No it's not possible....a similar proposition to a conveyor belt'

That article discusses boundary layer control i.e an air jet blown though slots add energty to the boundary layer, delaying turbulent flow separation beacause of the increased kinetic energy...but that energy is very expensive and heavy...and exhaustive of fuel;)

It gets murlier and murkier the further that we go in this discussion.http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/sowee.gif

I think Guppy's explanations should suffice and perhaps a review of Davies

otherwise we are all just beating Tin Lizzie:}

mSqltiTvbTc&feature=fvst

PBL

4th Jan 2011, 18:52

Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?

Do notice I said "physically possible" and not "a practical proposition"!

No it's not possible

I see. How do you then explain the function of wind tunnels?

PBL

bearfoil

4th Jan 2011, 19:00

Wind Tunnels test aerodynamics by supplying free energy. Free in the sense that the test model isn't producing power, that's later, at Edwards.

Did you not say "Its own engines"?? Is this a trick question, Professor??

Pugilistic Animus

4th Jan 2011, 19:04

or,

you can not fly an aerodynamic aircraft, as opposed to an aerostatic aircraft...without a relative wind...where is the Wind PBL?:ugh:
:ooh:

PBL

4th Jan 2011, 19:24

No, it's not a trick question, bearfoil. It's a despairing question.

Let me explain. You can generate lift in an airfoil by blowing air across it.

I hope we can agree on this.

Suppose you attach an airfoil to a fuselage with pilot. Can you lift it vertically? Yes, by blowing air across the airfoil.

How much air? Enough.

Can you generate "enough" air from devices mounted in front of the wing? Theoretically, yes. Try the kinds of solid-rocket motors used to power the Space Shuttle.

Can these be attached to the aircraft, or must they be physically separated? Doesn't matter.

Let's go back to ships.

Mount a rotating cylinder vertically on a ship. Mount a keel under the cylinder to keep the ship from moving perpendicular to its bow. Mount a fan on the ship, let's say on the left side of the ship, which blows air across the cylinder perpendicular to the bow.

Will the ship move?

In which direction will the ship move?

My problem at the moment is that I seem to be discussing aerodynamics with people more interested in repartee than they are interested in physics or aerodynamics. Further, the repartee is not very entertaining. What a waste of time!

Except for lomapaseo's intervention, which I missed first time around (sorry!)

Loma, consider. A mass of air is accelerated towards the engine inlet duct (by sucking). Some of it goes in the duct, some of it goes around. But it all gets stopped (relative to the aircraft); the stuff inside by plates, the stuff outside by encountering reverse-velocity air (formerly the stuff inside).

You have to put energy in to stop all that air, but stopped it gets.

Now, that air had momenttum. All that momentum goes somewhere (conservation of, and so on). Where does it go? If you have designed things cleverly, it goes into force. In which direction? The same: negative-x (that is what conservation of momentum means). That negative-x direction force is experienced as braking by the airplane+occupants.

Now, exactly how which bit of air gets momentum-reduced, and by how much, is a question no one here has yet answered. Some people don't care; they can stick with the story above. I care, and I'd like to find an answer. I will find it, but I don't think any more through this discussion.

PBL

HazelNuts39

4th Jan 2011, 19:53

Do you think it is physically possible to lift an aircraft by blowing the wing from its own engines?The schemes you propose in your last post could work in principle but are blatantly inefficient.

I'm intrigued by another question of yours, but am not sure that I understand it:
How do you then explain the function of wind tunnels?As I see it, the function of wind tunnels is to provide wind (moving air) in a laboratory environment. You can place (models of) various objects (buildings, cars, bridges, cyclists, aeroplanes, pitot tubes, etc.) in the tunnel to study the flow around them by various visualization techniques, or to measure the forces acting on them, or the pressures on their surfaces. What more needs to be explained?

regards,
HN39

PBL

4th Jan 2011, 20:24

The schemes you propose in your last post could work in principle but are blatantly inefficient.

Yes and yes.

The point of the wind-tunnel comment is that you generate lift on an airfoil by blowing air across it. If you blow sufficient air across it sufficiently fast, the airfoil generates more lift than the weight attached to it, and we know from commercial aviation that this can be more than enough to lift numbers of hundreds of human beings. You just have to blow sufficient air, etc, etc.

Now, does it matter where this "relative wind" comes from? No. Just that it is present. Is there enough energy present in sources known to mankind to generate a relative wind of the required energy? Obviously, yes. Could we attach such an energy source structurally to the wing, in front of the wing? In theory, yes.

Suppose we do that, and we do it in such a way as to generate uniform airflow across the wing (by channelling it). And we start the whole contraption up. Does the aircraft rise vertically?

Some here want to say that can't possibly happen.

What do you think?

PBL

HazelNuts39

4th Jan 2011, 20:47

Does the aircraft rise vertically? (...) What do you think?
Yes, initially. In the longer term, you need to take care of drag and moments. It's called powered lift. The FAA has even proposed a set of regulations for large airplanes using that principle. The Breguet 49something (forgot the number) came in that category.

Any chance of returning to subject?

regards,
HN39

SNS3Guppy

4th Jan 2011, 21:14

Suppose we do that, and we do it in such a way as to generate uniform airflow across the wing (by channelling it). And we start the whole contraption up. Does the aircraft rise vertically?

No. The rearward vector of the thrust provided by the fan will provide forward propulsion. If you intend to provide enough thrust to cause adequate airflow over the entire airfoil to develop lift, you're going to be providing enough thrust to drive the entire vehicle forward, too; it's not a vertical operation, but a forward operation while developing lift.

Some here want to say that can't possibly happen.

What do you think?

I think you're still playing spin doctor with ridiculous theories. You might be alluding to coanda effect and attempting to reinvent the airplane and helicopter, but you're mixing your efforts and disregarding basic physics.

You're dangerously close to resurrecting the old treadmill hypothesis, which had no merit from the start. Your attempt at a parallel with a wind tunnel is without merit.

While certain aircraft have used bleed air injected along the top edge of the airfoil to enhance boundary layer aerodynamics, and various devices such as slats and slots are used to enhance boundary layer aerodynamics or delay separation, and some multi engine airplanes do benefit from propeller airflow over the wing to some degree, mounting engines or fans over the span of the wing to produce wing lift vertically isn't happening.

While I suspected early-on that you had some clue whence you speak on these matters, I'm more and more convinced that you're neither pilot nor engineer, and appear to be tossing smoke in the air for lack of something intelligent to offer. Your posts are tantamount to flame-bait.

Loma, consider. A mass of air is accelerated towards the engine inlet duct (by sucking). Some of it goes in the duct, some of it goes around. But it all gets stopped (relative to the aircraft); the stuff inside by plates, the stuff outside by encountering reverse-velocity air (formerly the stuff inside).

You have to put energy in to stop all that air, but stopped it gets.

Now, that air had momenttum. All that momentum goes somewhere (conservation of, and so on). Where does it go? If you have designed things cleverly, it goes into force. In which direction? The same: negative-x (that is what conservation of momentum means). That negative-x direction force is experienced as braking by the airplane+occupants.

The mass of air accelerated toward the engine duct occurs in part by "sucking," and in part by ram effect (hence the term, "ram drag."). The airflow outside the nacelle is irrelevant. Whether it it is accelerated or not by reverse ran gas flow is irrelevant.

The function of reverse gas flow (by blocker doors or cascade vanes) is that of Newton's third law. The opposite reaction to vectoring gas flow forward is a retarding force which contributes to what we experience as "reverse thrust." The reverse flow diverted fan and exhaust gasses do not push against the free airstream outside the nacelle, in order to slow down. While an interaction with the diverted airflow will certainly occur, that interaction doesn't contribute to slowing the airplane down during reverse thrust operations.

The second means of assisting the braking of the aircraft is to increase the engine nacelle drag (frontal area of the engine) by slowing the air down that passes through the engine without producing significant forward thrust or turning the reverser air forward enough to cause reingestion.

Frontal area isn't the issue, nor the equation: this isn't a flat-plate drag issue.

You're probably already familiar with the concept of a windmilling propeller causing substantially more drag in a light twin than a stopped prop: you're hopefully also familiar with the fact that the windmilling propeller produces more drag than a flat plywood disc of the same diameter as the propeller. The issue, then, isn't simply frontal area or flat plate aerodynamics.

While energy is extracted from the slipstream to rotate a propeller and engine, and all it's associated drag, the effects of the turbine engine are substantially more complex with respect to the energy extracted. This extraction occurs at multiple points during entry and processing of the mass airflow, from the inlet through the compressor and diffuser and turbine, to say nothing of the fan itself.

Disregarding airflow through the engine and it's various complexities, consider only the airflow through the fan, bypassing the core. The fan acts as a series of propeller blades or airfoils, each one producing "lift" in a rearward direction: thrust. A function of the production of lift in an airfoil is also induced drag. Airflow through the fan isn't simply imparted energy to thrust without any loss; that loss occurs as drag across the fan. This is but one of many areas in which energy is absorbed in the engine, or lost from the airstream, which account for the difference between gross thrust and net (usable) thrust. That difference is properly termed "ram drag," and is a collective figure comprising various components of drag.

If the reduction of the engine reverser efflux is done at low aircraft speed and low thrust settings, the pilot should perceive no changes in retarding force.

Very little benefit will be perceived (or found) at slower speeds. The disparity between gross and net thrust diminishes as ram drag diminishes, at lower power settings and lower speeds, just as previously shown.

When at a standstill, the only tangible retarding force, or reverse force, will be the redirected fan or gas flow.

It's called powered lift. The FAA has even proposed a set of regulations for large airplanes using that principle.

Powered lift is entirely different: that's the V-22 Opsrey, and other aircraft of the same pattern design.

TheChitterneFlyer

4th Jan 2011, 22:14

I'm now somewhat seriously confused by trying to follow all of the contributions to this thread.

I have now considdered those "hot and cold nozzles" of the Harrier and, how we have managed to balance several tons of airframe upon a vertical stream of thrust; all of that "inlet drag", or whatever each individual might wish to call it, has stopped the aeroplane from plummeting to earth by redirecting the thrust path into a "more useless path" other than a "forward thrust" direction!

I've no wish to become "Devil's advocate", but...

Do forgive my lack of scientific understanding of what has been previously debated, but, "How does that work"?

TCF

Pugilistic Animus

5th Jan 2011, 01:59

-----:eek:-----
PBL as Guppy asks where's the forward component of thrust?

:\:\:\:{

The Encyclopaedia Britannica: a ... - Google Books (http://books.google.com/books?id=GKMMAAAAYAAJ&pg=PA555&lpg=PA555&dq=bernoulli+perpetual+motion+second+kind&source=bl&ots=TuKb7JluTK&sig=-SrLWczDIN2amrrTOQagqbJyGgQ&hl=en&ei=3eQjTcXpOYOdlge7tOysDg&sa=X&oi=book_result&ct=result&resnum=4&ved=0CCcQ6AEwAzgK#v=onepage&q=bernoulli%20perpetual%20motion%20second%20kind&f=false)

a brilliant example of mathematical induction from Bernoulli senior--that's totally wrong

SNS3Guppy

5th Jan 2011, 03:00

PBL as Guppy asks where's the forward component of thrust?

Say again?

PBL

5th Jan 2011, 07:30

While I suspected early-on that you had some clue whence you speak on these matters, I'm more and more convinced that you're neither pilot nor engineer, and appear to be tossing smoke in the air for lack of something intelligent to offer. Your posts are tantamount to flame-bait.

Guppy, who I am is no secret, and neither is elementary physics, although both these things seem to be a mystery for some here.

I admit here I really don't know how to discuss with you. Since we're talking physics and aerodynamics, it would seem that a common understanding of the basics of these sciences is necessary to have a fruitful discussion. That isn't there, because one of us apparently doesn't have those basics.

You think it needs new laws of physics to get a ship to sail forward under own-generated wind (your example), whereas any aero student would fail a first course if heshe couldn't see how to do it. You think a wing can't fly under self-generated wind alone, without "forward thrust", whereas in the standard intro-aero text to which I often refer, thrust isn't even mentioned until half-way through, after four hundred plus pages of how airfoils work. We're worlds apart, and I am not about to alter mine, because my day job and those of my colleagues are way too bound up with it. And I'll suggest that yours is too!

When I find out the answers to my questions about the relative contributions of the various engine parts to the loads under reverse thrust, I'll share them here. Thank you, Guppy and HN and others, for a lively debate!

PBL

de facto

5th Jan 2011, 08:22

Im lost,jeez requesting radar vectors:E

SNS3Guppy

5th Jan 2011, 08:31

You think a wing can't fly under self-generated wind alone, without "forward thrust", whereas in the standard intro-aero text to which I often refer, thrust isn't even mentioned until half-way through, after four hundred plus pages of how airfoils work.

Thrust isn't necessary for an airfoil to develop lift, obviously.

That a text doesn't address thrust as one of the four forces of flight until lift and drag have been thoroughly treatised is no surprise. Again, however, you mix apples and oranges.

Your mythical airplane which develops lift by blowing air over it's airfoil, independent of any motion of the free airstream, is more akin to a perpetual motion machine than the result of anything more than junk science. These magic fans, spaced the full span of the wing, will produce thrust if attached to the airframe, if producing enough thrust to produce lift. That amount of thrust will produce forward motion, whether or not you desire it, whether or not the airfoil requires thrust to create lift.

Of which airplane in the world are you aware that develops lift and rises vertically by employing fans to blow over it's airfoil, without need nor means of forward propulsion? This miracle method of yours, of which every first-year student is intimately aware: why is it not standard in the industry (or more precisely, why is it not employed at all)?

When I find out the answers to my questions about the relative contributions of the various engine parts to the loads under reverse thrust, I'll share them here.

Do what will float your boat. As I stated at the outset, I'm not particularly interested in the specific values to which each component may contribute: it's enough to address the salient point that ram drag, and not reverse gas flow, accounts for the retarding force in reverse thrust landing operations. More appropriate to the point of the thread is the relationship of rollout speed to the effectiveness of reverse thrust; this is owing to the effects of ram drag (and such components as may interest you, of which the collective ram drag is composed).

HazelNuts39

5th Jan 2011, 10:33

These magic fans, spaced the full span of the wing, will produce thrust if attached to the airframe, if producing enough thrust to produce lift. Guppy, I agree with most of what you write, but here there might just be a slight misunderstanding. You are quite right if the "wind" over the airfoil is generated by propellers or fans mounted in front of the wing. But that's not how I interpreted PBL's hypothetical proposition:
Now, does it matter where this "relative wind" comes from? No. Just that it is present. What if the source of the "wind" is a large reservoir of compressed air?

it's enough to address the salient point that ram drag, and not reverse gas flow, accounts for the retarding force in reverse thrustWhy not both?

regards,
HN39

PBL

5th Jan 2011, 10:56

What if the source of the "wind" is a large reservoir of compressed air?

Or, as I mentioned above in Note 98 (http://www.pprune.org/tech-log/437583-reversers-c-speed-5.html#post6159357), ..... solid-rocket motors

And to the question "what airplane flying would do this"?, I gave the answer in Note 91 (http://www.pprune.org/tech-log/437583-reversers-c-speed-5.html#post6159213): Do notice I said "physically possible" and not "a practical proposition"!
In both these cases, it would obviously be more practical to turn the compressed air, respectively rocket motors, towards the ground, rather than towards a wing which was to lift one off the ground. This is a matter of understanding physical principle, not of practice.

As to the suggestion that Guppy, I agree with most of what you write I am having trouble finding any agreement between attributing most of the braking force to "intake drag"/"inlet drag"/"ram drag" (which we are told all mean the same thing) and attributing most of the momentum-reduction to [t]he thrust reverser, and it's not a component of the engine but of the nacelle (or exhaust duct)
And lomapaseo had this difficulty also, in Note 93 (http://www.pprune.org/tech-log/437583-reversers-c-speed-5.html#post6159229).

PBL

HazelNuts39

5th Jan 2011, 11:26

Why not both?
Let's consider a straight (non-bypass) jet engine with thrust reverser. Let's assume that the reverser buckets can be set in four positions and deflect the exhaust gas stream without loss of momentum (hypothetically). In the first position the buckets are stowed around the jet pipe: forward thrust mode (zero deflection). In the second position the buckets intercept the exhaust gas stream and divide it in two 'plumes', deflected 60 degrees from the x-direction, still producing forward thrust, but less. In the third position the buckets deflect the gas stream 90 degrees from the x-direction for zero forward thrust. Finally, 120 degrees deflection from the x-direction results in negative thrust, i.e. drag, or thrust for power-back from stand-still. Then we could have for example the following thrust or drag forces:

Deflection (degrees) ........... 0 .... 60 .... 90 ... 120
Gross thrust ................. 100 ... 100 ... 100 ... 100
X-component of gross thrust .. 100 .... 50 ..... 0 ... -50
Ram drag ...................... 40 .... 40 .... 40 .... 40
Net thrust .................... 60 .... 10 ... -40 ... -90

regards,
HN39

P.S. Just in case you don't have your slide rule handy: cos 60 = 0.5 and cos 120 = -0.5
P.S.2 As said earlier, the ratio between gross thrust and ram drag changes with airspeed and power setting.

SNS3Guppy

5th Jan 2011, 16:22

Why not both?

I did address both from the outset, specific to the point that redirected gas flow is the minor contributor, and that the effects of ram drag variable with airspeed (and RPM) are very germane to the question of why we apply reverse at higher speeds, earlier in the landing, and remove it later on.

In both these cases, it would obviously be more practical to turn the compressed air, respectively rocket motors, towards the ground, rather than towards a wing which was to lift one off the ground. This is a matter of understanding physical principle, not of practice.

Do you realize that you just responded to your own quote? You're quoting yourself.

You're doing so incorrectly, and answering yourself incorrectly, but you're actually talking to yourself. I find that rather telling.

You're now attempting to draw a parallel between a wing which produces it's own lift with no forward motion, to rocket motors, or airflow directed not at the wing but downward? You're now introducing thrust again (after going to the trouble of telling us that it's irrelevant and isn't found until the four hundredth page of your favorite text. You've gone from blowing airflow over a wing and making it rise vertically, to directing thrust downward. Why not get rid of the wing entirely, in that case?

All of which is entirely irrelevant to the matter of reverse thrust. While redirected gas flow, often at a shallow angle, does little to contribute to the slowing action of an airplane during reverser operation on landing, certainly ram drag and the inherent loss of efficiencies between the gross thrust and net thrust produced by the engine do account for the lions share of the change in velocity when in reverse range.

What if the source of the "wind" is a large reservoir of compressed air?

What if it is?