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
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.
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
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
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