Peter...
 

The flow going into the compressor or fan is going to be at its design speed as long as the engine is turning at its design rpm...even when the airplane is standing still on the ground the engine will be sucking mass flow into the engine at about its design speed...


We can always do the math if we know the mass flow and the compressor (or fan) face area...and the design flow speed at the compressor face...which is almost always going to be ~ M0.4 to M0.5...


If we have a compressor inlet area of 1 m^2 and the flow speed is M0.5...which is ~ 170 m/s at sea level temperature...and knowing that SL air density is 1.23 kg/m^3...we know that mass flow is going to = 170 m/s * 1.23 kg/m^3 * 1 m^2 = 209 kg/s...


If we fly faster or slower that mass flow does not change...the engine is going to take in what the compressor can swallow...nothing more...of course as we go higher the air gets thinner and the density at FL350 is only ~1/4 of what it is at SL...


So if we are flying at M0.5 at ~SL the air flow in front of the engine inlet is going to be moving faster...due to the fact that the same 209 kg of mass flow moving at 170 m/s at the compressor face of 1 m^2 has to go through a smaller hole at the front...that is maybe 0.75 m^2...


Again we calculate that and get mass flow of 209 kg/s divided by area of 0.75...divided by air density of 1.225 = ~228 m/s...so even though our airspeed is M0.5 (170 m/s) and the speed of the flow at the compressor face is (M0.5) 170 m/s...the engine is sucking in the air in front of the engine due to the small “straw” opening...


So inside the duct we still see some static pressure rise as the air slows down from 228 m/s to 170 m/s...although this is not that much...we can calculate the static pressure rise as the kinetic energy from the moving flow that we have converted into pressure energy...kinetic energy being velocity squared divided by two...


The difference in speed is 58 m/s so that is a kinetic energy of ~1,700 m^2/s^2...the temperature of the flow would increase by the energy divided by the specific heat of air...which is ~1,000 J/kg per degree so we would get a temp rise of ~1.7 degrees...if the air was at 250 K to start with it means the temperature ratio ends up as ~252 / 250 = ~1.01...Pressure Ratio = Temp Ratio to the 3.5 power at these moderate temperatures...so the PR increases by 1.01^3.5 = 1.03...


Which means we get ~3 percent increase in pressure going through that diffusing duct...we can see from this that the more we slow the flow down the greater our temp rise...and the lower our starting temp to begin with...the higher our temp ratio...and therefore the pressure ratio as well...


So with Concorde where we slow the flow down from M2 to M0.5 we get a big temp rise...from minus 51 C to plus 127 C...a swing of 178 deg...we use absolute temp scale to get our temp ratio so temp compressor face of 415 K divided y freestream temp of 237 K gives TR of 1.75...


The pressure increase is the temp ratio to the 3.5 power so that is 1.75^3.5 = 7.1-fold pressure increase...that is the ideal isentropic (lossless) pressure increase...the actual static pressure increase in the airplane ends up as ~6.4 so the duct efficiency at recovering pressure is 6.4 / 7.1 = 0.9...90 percent...which is good...


However we see that this pressure increase results in the intake making thrust because it causes a large swing in momentum in the forward thrust direction when we take all forces in the intake into account...not because the increased pressure makes the jetpipe speed faster...although this occurs also...but now we are talking about a different way of accounting for the thrust...


So to wrap up a few loose ends...we see that it is useful to go through each component of the engine and figure out if its contribution is a net drag or thrust...but that does not change things...the whole thing needs to work together...without the fuel being burned the whole process comes to a stop...


So when we stand back and say “the intake produces x amount of thrust...”...well yes..but so does the compressor and the burner...and the turbine will produce a net drag...etc...it is just one way of looking at what is going on in the physical sense...


It was mentioned that the mass flow coming in to the engine is a net drag...and yes that is called ram drag and is the mass flow of the engine times the freestream velocity (aircraft speed)...but this is another method of accounting where then subtract that ram drag from the total thrust produced by the mass flow times its jet velocity out the tailpipe...


So if the airplane is flying at 250 m/s and the mass flow is 100 kg/s...then the ram drag is 25,000 N...and if the speed of the jet blast is 750 m/s then the gross thrust is 75,000 N from which we subtract the ram drag and get net thrust of 50,000 N...which is why thrust = mass flow * jetpipe speed minus freestream speed...



It is another way of accounting for the forces going on...and the most simple way...the important thing is you can't mix and match different ways of doing the bookkeeping...if you start with one method then stick with it and don't mix concepts...


Regards,


Gordon.

Gordon,

Not wanting to go over old ground I wasn't going to do any more on this, but w.t.h. the golf course is unplayable, the river unfishable and its raining again ....


I make the compressor inlet area just about the same, and checking the mass flow from intake supply conditions a value of 189 lb/sec at FL580 looks about right also. The intake efficiency derived from the figures quoted would be around 94% which is also correct. That makes the compressor inlet Mach Number about 0.45.


You need to be careful with that 8050 lb figure - it is the thrust produced by an Ol 593 operating behind an intake that supplies 189 lb/sec mass flow at a total pressure of 8.5 psi. It does NOT include the thrust generated through the expansion of the flow in the secondary nozzle. Add on the 25~29% often quoted for this contribution and you are up to the 10,000 lb mark.

The actual cruise L/D was 7.5. Any Concorde pilot reading this could confirm, but I would expect that by the time the aircraft cruise climbed up to FL580 it would be at around 300,000 lb, which would fit well with the 10,000 lb per powerplant.

PM me if you need more detail

Going back to that 8050 lb thrust it is the thrust you would get from an engine and intake working together. The engine won't work without the intake and the intake won't work without the engine, so it is a bit pointless to consider them as separate units. As you suggest in a later posting, there are thrust and drag variations on the various components inside the engine proper (thrust on compressors and combustion chambers, drag on turbines etc) and nobody apart from the engine stressmen cares a stuff about those distinctions.

Better, I suggest, to consider the intake and engine together as a single powerplant and think of the intake as a zero stage compressor, or more accurately as a pre-diffuser set ahead of the first stage of the compressor.

That way you get to see the intake "thrust" as just another split in the breakdown of powerplant force distribution.

Again as you say. its all in the book keeping, and as an accountant friend of mine is fond of saying "What answer do you want?"

Thanks...Clive...
 

Okay...the numbers are starting to come together like they should...

I made a math boo-boo in calculating cruise mass flow...but now I get ~177 lb...oh well...that's in the ballpark at least...


It doesn't change the fact that thermal efficiency is well over 50 percent on this engine...something the high bypass fans are only now reaching...although with the smaller mass flow propulsive efficiency would end up at ~75 percent...still pretty good...I put overall efficiency right at 40 percent...again where the top airliners are at today...



What do you think about a bigger mass flow and lower jet velocity to increase propulsive efficiency...?...I will try sending you a PM...any data you can provide would be most appreciated...


Regards,


Gordon.


PS: I had to smile at your comment about all this drag and thrust stuff being important only to the structure guys...I'm sure you meant no slight...and it is a fact of course that their role is very much secondary...but it is a bit funny nonetheless...I'm sure they would point out very earnestly that if they did not do their job...that a lot of those carefully designed engine bits would come flying off the airplane long before it ever reached Mach 2...

Gornaut, I agree that the maths are straightforward . But I also agree that it can be somewhat unhelpful without some intuitive understanding of what is being described. I did find your diagram very helpful as to what the point of the calculations were.

I went back and reread post #5 which does indeed explain it very well. In light of that explanation (and CliveL's remark "nobody apart from the engine stressmen cares a stuff about those distinctions"), isn't the phrase "produced by" a little misleading. I can understand that 63% of the thrust is transferred to the aircraft through the inlet duct. But, couldn't I just as easily calculate and say that (obviously I'm making this up): 23% of the thrust is transferred through attachment bolt A, 15% through attachment bolt B, and 25% through attachment bolt C. I don't see where I would ever say that 23% of the thrust was produced by attachment bolt A.

This discussion (which is fascinating!), sort of reminds me of the story of the man who upgraded his car with: a carburetor that claimed fuel savings of 40%, spark plugs which saved 30%, tires that saved 30%, and an exhaust which saved 20%. He had to stop every few miles and drain his fuel tank.

Goarnaut
My remark wasn't meant to denigrate anybody's work, and with hindsight I can see that the same criticism of my words could be applied to all those people who struggle to improve the efficiency of all the component parts of an engine. They all care very much.
What I really meant to say was that people OUTSIDE the engine companies don't really care about the fine distinctions.

Crabman
That was exactly the message I was trying to convey

Brilliant conversation!

I've got to say that the numbers and physics can be brain-numbing. And this just emphasises the achievement of all those brown-overalled slide-rule operators who didn't let common sense get in the way, and made an airliner do Mach 2, and go from London to New York in under three and a half hours whilst burning about the same amount of fuel as a 747-2.

The Blackbird was a phenomenal achievement, a magnificent aircraft, but to achieve supersonic flight with an airliner, using more restrictive legislation (TSS) than existed for blunties at the time was nothing short of miraculous. Our equivalent of the Apollo programme.

But as a simple pilot who lived under those brown overall coat-tails (and was bloody luck to do so), <<without the use of reheat and with staggeringly low fuel flow values>> brought back to mind one of my most treasured memories of the experience (apart from those nights in pubs around Filton):

You plugged in reheat (the proper name for afterburners) at 0.95. She climbed at Vmo. M 1.0 went past without a gin & tonic spilt down the back. The ramps dropped at (from memory - Dude?) M1.3. But the real cool bit was at M1.7. You had a burn of about 12000 KGS/HR per engine. You cut the reheat. The fuel flow dropped to about half that - 6 tonnes per hour / engine. She didn't drop out the sky. She just carried on reaching - at FL500 hitting M2.0 - and just did what the designers intended - cover distance at about 20 nms/min. No fuss. No reheat. Day in - day out.

I suppose that's the practical experience of this thread's title.

Brilliance. Pure, original physics and engineering brilliance.

Hey ho.

goanaut, any chance you could resize that image at post 119 please. It's doing my head in just trying to follow the maths never mind scrolling back and forth at the same time.

Thanks again to all the contributors. :ok:

Great to Hear Pilot Perspective
 

NW1...it's good to hear from you...your numbers fill in some blanks...How long did it take to reach cruise altitude...?...


Crabman...the inlet is not transferring the thrust force from the engine to the airframe...that's the job of the engine pylon...the inlet is actually making thrust...think it through logically...the engine adds momentum to the mass flow and hurls it out the back with greater force than it came in...the engine is hurled forward as a reaction...momentum is conserved...


Now why don't we put that same kind of nozzle on the front of the engine instead of a diffuser...?...the flow would likewise speed up increasing the momentum going INTO the engine...but what would happen...?...how would the engine react...?...if the flow going OUT is speeded up and the engine moves forward...then logically if the flow coming in is speeded up then the engine will move backwards...we get drag...not thrust...


That is why you don't see a nozzle on the front of the engine...so continuing in this logic...if the flow COMING IN is now slowed down...what happens...?...how does it all balance out...?...remember momentum must be conserved...where does it go...it can only be forward or back...there are no other choices...


Turin...you can decrease your browser resolution by simultaneously pressing ctrl and minus keys...

goarnaut:

That much, at least, my simple mind understands. That is how I always viewed the situation. That is the way I'll need to continue viewing the situation.

This is the wording that I find confusing. I can understand that some (most) of the thrust is being "actualized", "transferred", or [... fill in another word..] due to momentum changes in the airflow which are occurring in the inlet duct which are transferred to the walls of the inlet duct (and/or passed along with the airflow to the next stage), which are transferred to the engine structure which are transferred to the craft through the engine pylon. I also can understand that this (plus and minus changes to the momentum of the airflow) is occurring all through the engine.

My problem is this. From the point of view of the aircraft, all the thrust is coming from the pylon. I still think it would be an odd usage to say that the engine pylon is "actually making thrust".

I'll bow out now and continue reading this thread. I don't want to appear obstinate or argumentative. Many thanks to you and CliveL (and also to M2Dude who earlier attempted to explain this).

Crabman

I see where you are coming from, but Concorde didn't have a pylon. On subsonic aircraft all the powerplant loads are transmitted to the airframe via the pylon as you say. But the Concorde powerplant had three distinct components, the intakes, the engines and the secondary nozzles, and all three were attached to the airframe separately.
I tried, and failed, to find a suitable illustration.
Since the components are individually attached, it seems not extraordinary to think about how the powerplant forces are shared between them. This is why people talk about the thrust transmitted through the intake attachments. That, for me, is not saying that the intake makes thrust - it just carries its share of the powerplant forces.

Clive,
Belated thanks for the shockwave explanation. Inevitably I must spend time with the basics.

Gordon,
Thanks for your explanation.

I always thought the intake momentum drag was one of the costs of bringing the air aboard. Hence the appearance of flt velocity in the expression.
However, since the air never slows down beyond about M0.5 at the engine face the air is never brought to rest relative to the aircraft. Does this mean that the momentum drag is really a function of axial vel at compressor entry also?
Have I gone off the rails here or is it something to do with airframe, ie inlet, and engine accounting?

Crabman...maybe the word thrust is making it hard for you...
 

If you prefer... think of the inlet making a forward force...that's all it is...and that force is very real..the mechanism of it is that the diverging duct shape decreases flow speed and increases pressure...that increased pressure yields a forward force...after all force is pressure time area...


Since the outlet of a diverging duct has a bigger area than the inlet...and since the pressure is also increased...the result has to be a force pointing forward...there can be no other outcome in the physical world in which we live...


A converging duct...nozzle...is the opposite...I think you have stumbled on that “transferring” thrust part...that wording is unhelpful and does not actually have any place in this explanation...there is no transferring going on...the forward force is made right there in the duct...


Now of course it is true that the duct by itself cannot move the airplane forward...we need the entire engine and all of its bits...but the same is true for any other component...the gas generator without the nozzle will make no thrust...


So the bottom line is that if we look at the engine component by component...each of them makes either thrust or drag...doesn't change the fact that we still need each and every one of them...


I just pulled a good engine book off my shelf and looked this up...Aircraft Propulsion by Farokhi...Fluid Impulse is covered in chapter 2 and there is a whole section on it...with worked examples for each engine component...the inlet...compressor...burner...turbine and nozzle...there is a drawing that shows which components are making thrust and which ones drag...the inlet...compressor and burner all make thrust...while the turbine and nozzle make drag...


Now I agree with Clive that this is not hugely important to break things down in this way...but it is the physical reality and we must appreciate that...


As far as the Concorde inlet is concerned the goal was to have good aerodynamic performance...a diffuser is a difficult item...the flow is moving from low pressure to high pressure...an adverse pressure gradient...going uphill if you like...which can cause flow separation and the resulting turbulence eats up energy that we wish to recover as pressure...


So those were the design goals...I'm sure Clive and the team spent zero time thinking about how much “thrust” was going to be made in the inlet...that part we have no control over...it is just the way it works out if we do a proper job in recovering as much pressure as possible...


My aim here was just to shed a little light on this because it is fascinating...like many physical phenomena...it is another example that there is nothing for free in nature...when the air flow going at M2 gives up its momentum...it is the engine that gains...similarly if we inject cold fuel into a hot combustion chamber the fuel heats up and vaporizes...but the surrounding air loses the exact same amount of heat that the fuel gained...and the air temperature goes down...it is always a 2-way street...the action reaction thing...


I hope I have been able to help a little...if you still want to keep working through it I am certainly willing to help...


Regards,


Gordon.




.

Yes, I appreciate that but then all the text gets reduced and my eyes are not what they used to be. :{

And just one more thought...
 

Clive mentioned that the engine structural designers were probably the only ones calculating the component thrust and drag...you can be sure that is the very first thing they did...


In order to keep the engine together you have to know how much force is pulling in what direction...think about the burner and turbine interface...the burner is attached to the compressor...which is attached to the inlet duct...all of which are pulling forward with great force...and then you have the turbine making a drag force and pulling in the opposite direction...how strong do you make those bolts that hold things together...?...


So this stuff is more than just a curiosity...it is very real and that engine would have flown apart on the test stand if someone did not calculate all this in advance...


Regards,


Gordon.

Peter...now you are mixing and matching accounting methods...
 

Yes...ram drag is what we subtract from the gross thrust...ram drag is freestream speed (aircraft speed) times mass flow...so that is why thrust = mass flow * (jetpipe exit speed – freestream speed)...


However...if we want to do a control volume analysis of the engine...then we do it as mentioned...btw we can also do the whole engine as a single control volume and we will get the thrust the exact same way as I did for the inlet duct...using the same math...


And again it helps to make a sketch...we see from such a sketch that we can again use the impulse method to calculate thrust...


Let's say again we have a mass flow of 100 kg/s...and an inlet area of 1 m^2...at the nozzle the area is 0.5 m^2 and the pressure is let's say 2 times the atmospheric pressure...the speed of the jepipe exit is 500 m/s...the aircraft is flying at 250 m/s...~M8 at FL 350...


The simple formula for thrust is mass flow times jetpipe velocity...minus ram drag which is mass flow times freestream V...so our ram drag is 25,000 N...and our “gross” thrust is 50,000 N...from which we subtract ram drag of 25,000 N and we have net thrust of 25 kN...


We can get the same result if we now do a control volume analysis and take into account the pressure times area at the front inlet and aft nozzle respectively...the atmospheric pressure is 22.5 kN/m^2 so the front pressure pushing the engine 22.5 kN/m^2 * 1 m^2 inlet area = 22.5 kN...


The nozzle pressure is double...but the area is smaller by half...so it is a wash...we then take our inlet momentum (ram drag) and our exit momentum and we find that we have 25 kN thrust...


We could do this control volume thing for each component and still will get the same net thrust at the end...


The important thing is not to mix and match ideas...pick one bookkeeping method and stick with it...




Regards,


Gordon.



PS: being an Ontarian myself...I'm curious if you are related to the TV newsman of the same name...?


http://i49.tinypic.com/1t0cra.jpg

Turin, Here is a smaller version
 

Peter,

It's just the accounting. Normally to calculate thrust one takes the momentum change from freestream conditions (ahead of the intake) to the exit conditions just behind the jet pipe nozzle or fan. Between those two 'stations' the momentum is fluctuating as the flow goes through intake diffuser/compressor stages/ combustion chambers etc. Momentum drag is calculated from that initial state ahead of the powerplant.

Hello, Sir.

How far in front of the intake is freestream?

No relation Gordan. I've disappointed a few people tho'(dentist,etc)

goarnaut and CliveL, you two gentlemen seem to have the background to be able to answer this question. Many assert that airflow through a turbine engine may reach and/or exceed sonic conditions with respect to the rotating components. I would have thought that unlikely, what with shock waves bouncing around inside the rotating bits. Would it be safe to assume that the temperature rise as the air passes through keeps the airflow subsonic?

Many Thanks.

Supersonic Speeds Inside Engine...
 

Brian...yes the tip speed of the compressor and fan will almost always be supersonic in a modern turbine engine...typically from M1.2 to M1.7...even a turbocharger compressor will go supersonic at its higher pressure ratios...



The reason is that high wheel speeds are necessary to achieve high rates of work...remember kinetic energy is a function of speed squared...and compressor and turbine work are a function of rotational speed...so the higher our wheel speed the greater the work output per given mass flow...


The sonic shock waves do contribute to losses of course...although the shock energy actually helps to compress the air...the axial gas speeds through the compressor and fan will generally be ~M 0.4 to M 0.5...


The axial gas speed through the first turbine nozzle guide vane ahead of the turbine wheel will usually be choked...ie Mach 1...although this will typically increase to just slightly over sonic speed just aft of the NGV...about M1.1 or 1.2 at most...as the annulus increases in size...giving a bit of converging-diverging nozzle effect...


The turbine wheel tip speeds are generally subsonic...about M 0.8...or M 0.9...however since the speed of sound increases with temperature...and at hot section temps it will be more than double that of freestream speed of sound...the turbine actual speed in m/s (or ft/s) will be similar to the cold section...


It is really quite something to stop and consider the amazing power that modern gas turbines produce...a single turbine wheel of ~0.75 m diameter will make close to 40,000 hp...with a mass flow of about 100 kg/s through the engine core...about the size of a 50,000 lb thrust fan engine you would see on a widebody airliner...


Regards,


Gordon.


PS:One more thought about the intake thrust...the very first post said it best...with the intake making over 60 percent of thrust...the airplane is basically “sucking” its way through the air at M2...that's it in a nutshell...

Not closer than the point at which the streamtube that contains the engine flow is first affected by the forward influence of the intake. Otherwise as far ahead of the intake as you wish to define it.
For subsonic intakes the closest point will vary with the ratio of engine mass flow to intake capture mass flow for that flight Mach number and intake area.If in doubt go further forward.
For supersonic intakes it will be the point of the cone for axisymmetric intakes or the leading edge of the ramps if 2D

"Inlet Thrust"
 

Hello All,
I have been reading and studying for many, many years the topic of "Supersonic Inlet Thrust. I do not hold a degree in Aeronautical Engineering or physics, but feel that I have my head around the physics of propulsion, at least to a degree.

I am not posting to irritate others or create a flame war, I only seek the truth, and there is MUCH confusion regarding the issue of how supersonic inlets work, even among the engineering staff at NASA. I know, because I have spoken with them and exchanged much mail and documents regarding this interesting issue.

First of all, in trying to get of grasp of this, one must have an understanding of supersonic and subsonic fluid flow. Supersonic fluid flow is not magic, but it behaves very differently than fluids at subsonic flow, and the transition from one to the other must be kept in mind too. To state that "Most of the thrust of the SR-71 or Concord" comes from the intake is a misunderstanding of jet propulsion. Yes, there is positive pressure recovery in the inlet, and yes it is greater than free stream static pressure. Jet engine propulsion creates thrust thru momentum change, accelerating gas out of the exhaust. The "positive pressure" in the inlet system or that acting on the rear faces of compressor blades is what is know as "PRESSURE THRUST". It is used to explain rocket and jet propulsion to high school students. It is a VERY INEFFICIENT way of producing thrust, and is avoided by engineeers.

If you look at rocket nozzle designs, you will see a different flow regions the relationship of "pressure thrust" to change in momentum. Engineers would like to convert all pressure thrust to a change in momentum, however some pressure thrust will always be created. It is NOT the lions share of thrust, even in the case of the SR-71 or the Concord. The supersonic inlet on the SR-71 is very efficient and it is thru that (at high mach speeds) it performs the lions share of compression, freeing up compressor stages of the J58 engine. At the 4th or 5th stage (can't remember which one), 6 bypass ducts pass the compressed air directly into the afterburner inlet, so a large portion of the airflow is acting as a ramjet engine. One could state that (at high mach) the inlet is responsible for the compression and ramjet action, BUT it is NOT producing the greatest portion of total thrust anymore than the Pistons in you car create the greatest portion of the horsepower generated.

This misconception has been perpetuated and is a misunderstanding of how the entire propulsion system works together. When some state that "The engine falls back in it's mounts while the intake system transmits forward thrust to the airframe", you should be willing and able to offer proof. DO the math, you will not find "positive pressure" in the inlet system "pushing" on the rear end of inlet components, or doors to amount to "75% or whatever number" of the total thrust. If you are careful to study ALL of the inlet system, you will find, again at high mach much of the "overpressure" air in the inlet is dumped overboard thru ducts and vents, as the inlet can take in more mass flow than the J58 can effectively deal with with some bypassed around the core and mixed in the complicated exhaust system again with it's blow-in doors and de-laval type noxzzle.

I am not posting to offend anyone, I like most everyone here am interested in the exact workings supersonic inlets, and propulsion. If, I have overlooked anything or failed to take into account forces not considered, please call me on it and educate me. The supersonic inlets and exhaust systems on the SR-71 are extremely interesting and work together, but it is not "Sucking itself thru the sky. I find it incredibly that all of this was conceived, worked out and tested in the 1950's with slipsticks.................
Tomtech

Thrust Distribution in a Jet Engine...
 

Tom...please refer to my previous posts in this thread...I have even drawn little sketches to illustrate the physics...I do have a degree in aerospace engineering and I work in the industry...

It is no secret how an intake produces thrust...this subject is covered in all textbooks...here is a book from Rolls Royce that has an entire chapter devoted to this...you can view the entire book online here...

Rolls Royce - The Jet Engine

Go to Chapter 20...Thrust Distribution...Figure 20-1 on page 208 shows the thrust distribution in the engine...the compressor...diffuser...and burner each make a forward thrust force...or gas distribution as the book calls it...

The turbine section and the jet nozzle make a rearward force or drag...when you subtract all the drag forces from all the thrust forces you end up with the engine's overall thrust...

This is the principle I have tried to explain in detail...it has to do with the fact that the sum of momentum and the pressure times area product is greater at the aft end of the compressor and diffuser and burner...than it is at the forward end...creating a force in the forward direction...

An engine inlet is a diffuser...that is why it makes a forward force...please read the chapter and try to do the math in there and you will understand how it works...

Regards,

Gordon.

Gordon,
Many thanks for replying to my post, I will, when I have time look at the materials you mention. No disagreement on the inlet being a diffuser. I think the mail area of confusion lies in the statements that point to the inlet static pressure being much above the outside static pressure, no argument there. The aircraft is not standing still, the inlet or the entire engine is not "thrusting against static pressuer outside the aircraaft". It is, rather thrusting against the "ram pressure", whic is MANY times geater than static pressure. It is the energy in ram pressure that the inlet can recover and convert to pressure rise, this, is one measure of inlet performance. As the (mach 3.1 or so) air is slowed thru a series of shocks, (in the case of the SR-71) it is terminated in the inlet section. The diffusion process builds pressure but at the expense of DRAG. Inlets produce drag, not thrust. You are not thrusting against outide pressure (which at 50,000 feet ain't much), but against the RAM pressure. If you want to say the the air compressed by the Sr-71 that is bypassed and fed to the afterburner inlet produces the majority of thrust, so be it, I am sure that is correct. I have "done the math", seems like everyone posting is not familiar with PRESSURE THRUST, which is covered in elementary text on any type of jet or rocket propulsion. To go even further and state that the engine is just processing airflow, while the inlet is doing the lion's share of the work is a false statement. It was stated by Kelly Johnson himslef, but later he was corrected by engineering and he, among friends admitted he was initailly wrong. Yes, I have done the math, and adding up and summing drag and positive pressures inside the engine is not necessary or even correct, unless you want ot calculate Pressure Thrust. In the end it is momentum exchange in the exhaust gases that creates thrust, and yeah, some state the "exhaust system " produes X amount of thrust. All propulsion components work together to provide thrust. I will study the book you mentioned, and try to see what you are saying. Again, thank you for the response, I know you for sure have done the math, and I will continue to educate myslef in this area. I will get back with you when I have done some more studying. I still find this an extremely intersting subject!

Gordon,
I have taken time to look at the book you referenced. I must say, I hope, as a practicing engineer you are not using that material, it was written for the "Sunday Reader". In turbojet engineering school, you would have worked out velocity triangles for axial flow compressors? Well, the bulk of energy put into the air is swirl energy, that is slowed (diffused) by the stator blades, resulting in compression correct? The stator blades absorb forces in the rearward direction releiving what the compressor "rotor" blades put into the foward direction (Pressure thrust again). Yeah, there is an overall foward pull on the compressor shaft. If so much "foward thrust" was generated by the compressor blades, then the Harrier would be moving foward while it is in hover? The book even shows gas flow for the Harrier. I have seen thrust distributions for turbojet engines before in far greater detail than the Rolls Royce book, If I get a chance, I will dig thru some and foward them to all for study. I am not trying to poke fun, I am glad you took the time to respond, as I my learn something. I will be back, thank's for taking time to read and think.

Concorde Inlet Thrust
 

Tomtech,

As one of the original Concorde inlet design team and as someone who has spent a fair bit of time recently trying to unpick the intricacies of the SR-71 design, I have to disagree with you gently.

The 'split' of Concorde's cruise propulsive forces generally referred to first appeared in Ken Owen's "New Shape in the Sky" many years ago. The data was given to him by members of the design team, and I think it is stretching things a bit to suggest that they 'misunderstood' jet propulsion principles.

I won't try to answer for the SR-71, but on Concorde (this is after all a Concorde thread :)) the intake, engine and final nozzle all had their own attachments to the airframe so it is reasonable and possible to enumerate their relative contributions. They all produced thrust. How you divide the total up between them is a matter of book-keeping conventions and as any accountant will tell you those are convenience variables!

Specifically, the 'split' will depend on how one divides the momentum drag out between the components. I can't remember (if I ever knew) the book-keeping that went with the Ken Owen's picture, but any reasonable division results in a considerable part of the total thrust acting on the intake mounts and another sizeable chunk on the final nozzle mounting.

What I think you may be missing is that although, of course, the thrust is obtained by imparting momentum to the mass flow through the engine, and although the design objective is to make sure that the static pressure of the flow exiting the final nozzle is as near ambient static as possible, the forces which must be applied to that mass to accelerate it to its final velocity are reacted as pressures on solid surfaces on the aircraft (or engine of course). In fact all forces on the aircraft (other than gravitational) ultimately come from pressures acting on solid surfaces.

So "pressure thrust", far from being an undesirable, is in fact an essential.

I'm going to be away for a few days, but if you feel you want more detail feel free to PM me.

I think some ealier posts had the view that the engine could be deconstructed into individual parts which could be characterised separately with the total engine being the sum of the parts.

Gentlemen,
Well, I have gone back over all the fine posts made on this forum and re-introduced myself to aircraft propulsion and must offer my apology for my prior post (#143). I STAND CORRECTED and thank all of you for having patience with me. My misunderstanding was thinking of the intake as lone device "creating" thrust, not, rather as contributing to thrust by virtue of the great pressure recovery at high speeds. Also, I will take this time to correct myself, I completely miss applied the term "pressure thrust". As Gordon, and others have pointed out, ALL reaction forces in the engine and intake must be eventually felt as pressure forces distributed throughout the engine, intake and exhaust. I was using the "pressure thrust" term of positive pressure at the exhaust opening, which was not fully expanded to velocity, thereby contributing to thrust, but not as efficiently if it were converted to gas velocity. It was my miss application of the term that led me to a "serious miss understanding" of propulsion. I feel I have moved my understanding forward by quite a leap, and have you all to thank. I will read more in an effort to rid myself of ignorance and thank you all again for putting up with my lack of understanding and taking time to post and offer diagrams, it helped me much.
Tom