reversers and a/c speed
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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.
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
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)
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.
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
You really can't get much more "publicly-available" than wikipedia, however unscientific it may be.
Turbojet - Wikipedia, the free encyclopedia
Or, to further dumb it down to a "publicly-available" refernece:
Answers.com - What is gross thrust in a turbine engine verses net thrust
or
gross thrust: Definition from Answers.com
Or, to really, really dumb it down...
What is net thrust? and how can we increase it? - Yahoo! Answers
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/ca...1995114289.pdf
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.
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
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)
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.
Amazon.com: Aircraft Engines and Gas Turbines, Second Edition (9780262111621): Jack L. Kerrebrock: Books
Definitions would be helpful. Or references to them in publicly-available sources.
Turbojet - Wikipedia, the free encyclopedia
Or, to further dumb it down to a "publicly-available" refernece:
Answers.com - What is gross thrust in a turbine engine verses net thrust
or
gross thrust: Definition from Answers.com
Or, to really, really dumb it down...
What is net thrust? and how can we increase it? - Yahoo! Answers
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/ca...1995114289.pdf
this study shows that the economic cost of reverser use exceeds the comparative cost of brake replacement if reverse isn't used:
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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.
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.
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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/43758...ml#post6141941 and 31 http://www.pprune.org/tech-log/43758...ml#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:
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":
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/43758...ml#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
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/43758...ml#post6141941 and 31 http://www.pprune.org/tech-log/43758...ml#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:
Originally Posted by mustafagander
Thank you Guppy. Please explain.
Originally Posted by Guppy
I already did. Read.
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":
Originally Posted by Guppy
Mass airflow times TAS, subtracted from mass airflow times exhaust velocity, and there's your ram 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/43758...ml#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
Last edited by PBL; 3rd Jan 2011 at 08:33.
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Dimensions
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
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
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Originally Posted by HN39
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
Dimension of mass flow = m*t^-1
Dimension of TAS = l*t^-1
Dimension of mass flow times TAS = m*l*t^-2
PBL
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Now the only missing part is: flow of what mass of air?
You introduced the term "inlet drag" and said it was the phenomenon responsible for most of the braking effect of reverse thrust.
And you still haven't said!
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.
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!]
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.
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Another source. No reverse thrust modeled though.
EngineSim 1.7a beta
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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,
Another discussion game. Pass.
Could we maybe get back to discussing the mechanics of reverse thrust and the pertinent concepts?
PBL
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?
What do you think?
Could we maybe get back to discussing the mechanics of reverse thrust and the pertinent concepts?
PBL
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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
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
Last edited by HazelNuts39; 3rd Jan 2011 at 11:49. Reason: rephrased
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Originally Posted by PBL
Another discussion game. Pass.
EDIT::
Could we maybe get back to discussing the mechanics of reverse thrust and the pertinent concepts?
regards,
HN39
Last edited by HazelNuts39; 3rd Jan 2011 at 11:07.
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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
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
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Originally Posted by HN39
If you are serious, the concept is quite simple.
Originally Posted by HN39
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.
Originally Posted by PBL
...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
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
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Semantics and other things
Originally Posted by PBL
ChitterneFlyer, this isn't about semantics.
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
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Originally Posted by HN39
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.
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.
Originally Posted by HN39
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.
Originally Posted by HN39
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.
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
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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 ..
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 ..
Moderator
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.
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.
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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
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