Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Concorde engine intake "Thrust"

Tech Log The very best in practical technical discussion on the web

Concorde engine intake "Thrust"

Old 3rd Nov 2010, 22:37
  #101 (permalink)  
 
Join Date: Jan 2005
Location: France
Posts: 2,319
M2dude,

Thanks for a very interesting story, that puts it all into context !

You may want to glance at this old PPRuNe thread re the Tornado :
Is the Tornado GR4 still supersonic? [Archive] - PPRuNe Forums

It would seem your guess about the F3 was right.

CJ
ChristiaanJ is offline  
Old 5th Nov 2010, 11:03
  #102 (permalink)  
 
Join Date: Jan 2008
Location: FL 600. West of Mongolia
Posts: 462
Thanks ChristiaanJ, that Tornado thread was absolutely fascinating to read. The poor old Tornado was so down on thrust; a little more thought and effort in the powerplant design would have resulted in an otherwise good aeroplane becoming a GREAT aeroplane. (I stll remember the case of an F3 being left standing by Concorde during transonic acceleration).

Dude
M2dude is offline  
Old 26th Nov 2010, 06:17
  #103 (permalink)  
 
Join Date: Dec 2003
Location: On a different planet, so it appears...
Posts: 43
As Feathers said...

"In all the 40 years I've been reading about Concorde, I've never found any of this sort of information before.

Seriously this all needs to be copied over to one of the specialist Concorde sites and tidied up for permanent display. It's a complete treasure trove and it needs preserving."

Totally agree. If only we could get Dude, Christiaan, Exwok, Bellapheron and all the Concordians to somehow have their stories documented and thus preserved. So many stories and names I have never heard of - could keep on reading till the cows come home. But seriously....as Feathers said....its a complete treasure trove and it needs preserving......
speedbirdconcorde is offline  
Old 26th Nov 2010, 08:14
  #104 (permalink)  
 
Join Date: Jan 2008
Location: FL 600. West of Mongolia
Posts: 462
Thanks speedbirconcorde, possibly food for thought indeed. This particular thread I think deals with one of the most mystical and definately most 'clever' of all of the various 'Concorde Magic' aspects; that is how a 14' long box can end up providing 63% of the 'welly' pushing you through the air at Mach 2. After nearly 37 years of my own personal involvement in the project, it still fascinates the hell out of me.

Best Regards
Dude
M2dude is offline  
Old 26th Nov 2010, 23:09
  #105 (permalink)  
 
Join Date: Feb 2009
Location: Jungles of SW London
Age: 72
Posts: 354
Another great 'dollop' of my favourite 'indulgence'. Reading these two Concorde threads, especially this one - even though it makes my brain ache and my eyes wobble.

Dude wrote;
To find the root cause of a surge required a fair bit of forensic analysis; unlike AF, BA had a superb Plessey PVs1580 digital flight data acquisition and recorder (AIDS) system that monitored all the intake parameters; this enabled us to find the cause most of the time, by reading data from the Quick Access Recorder after the flight. (A fair bit of Midnight oil burned though; I still have bags under my eyes ). I remember that originally the singular most important parameter, inlet void pressure, was only sampled once every four seconds by the AIDS system, but once we showed that this was like an ETERNITY when you were trying to track down an engine 'hiccup', we got the sampling rate increased to once per second. Although even this was not ideal, it generally did prove to be sufficient for us.
This raises a couple of observations, one is really a question and is this; The Plessey Data Logger you coveted Dude, was surely only as good as the thermocouples, strain gauges and pressure transducers to which it was connected? Also the quality - or validity - of the data acquired from them was really only as good as the locations decided upon by the engine/intake group? Does it then follow that the production Concorde (BA and AF?) was routinely fitted with a whole outfit of sensors that were not normally monitored unless you and your mates needed to know something specific? (I've just realised there are about four questions there, but you probably know what I'm getting at?)

The observation is; a picture is beginning to emerge of one airline who cherished and took care of an extraordinary asset, while another, broadly speaking, did not. Yet another remarkable aspect of Concorde, that the same aeroplane should have been held in such diametrically opposed regard.

Dude also wrote;
the final Tornado intake was a very poor design, with excessive levels of fuselage boundary layer ingestion and shockwave control. The RB199 engine (a totally 'political' design, with a never to be repeated, for a supersonic engine that is, 3 shaft layout) was already down on thrust, but now had even worse Mach 2 performance due to a wholly inadequate intake design. (MBB had the design principles thanks to BAC, but were totally out of their depth when it came to applying these principles into a practical intake).
This highlights yet another example of how British Industry/ government have habitually managed to snatch defeat from the very jaws of victory, especially in aviation. I am currently re-reading Genesis of the Jet - the story of Sir Frank Whittle and the struggle he had to bring his revolutionary creation to life - and although one is very proud that Sir Frank was one of the good guys ( a Brit ) the story is ultimately quite depressing. The attitude of the government of the day, British Thompson - Houston's reluctance and Rover's deviousness all conspiring to delay what should have been a war winner.

One thinks of the TSR2 which, on the limited amount of flight data acquired by the single flying airframe, appears to have been an aeroplane in the same astonishing mould as Concorde. With later digital electronics the TSR2 would have made an unbeatable bomber, allowing the money and effort 'squandered' (?) on MRCA to be spent on an agile interceptor replacement for the 'Electrifying' Lightning.

One also thinks of the P1154 and later, the offer of joint development on a supersonic AV8B succesor. All of these and many more until even reading about them becomes too depressing and frustrating.

Finally, Dude wrote;
This particular thread I think deals with one of the most mystical and definately most 'clever' of all of the various 'Concorde Magic' aspects; that is how a 14' long box can end up providing 63% of the 'welly' pushing you through the air at Mach 2. After nearly 37 years of my own personal involvement in the project, it still fascinates the hell out of me.
I completely agree and what is more, your fascination mirrors my own with my day job. Although MRI scanners are my day job, I still - after more than twenty years - look at some of the images it can produce and think; " That is bl00dy clever!" I still haven't got over the 'novelty'.

Thanks once more to all the Concorde respondents - amazing stuff.

Roger.
Landroger is offline  
Old 27th Nov 2010, 05:39
  #106 (permalink)  
 
Join Date: Jan 2008
Location: FL 600. West of Mongolia
Posts: 462
Landroger
Does it then follow that the production Concorde (BA and AF?) was routinely fitted with a whole outfit of sensors that were not normally monitored unless you and your mates needed to know something specific? (I've just realised there are about four questions there, but you probably know what I'm getting at?)
Actually Roger the Plessey PVS1580 system generally used 'spare' outputs from existing system sensors as far as the powerplant went, the one exception to this being T3 (HP compressor delivery temperature) which had it's own dedicated thermocouple. An engine surge would be 'indicated' on the recorder playback as a spike in EGT (Exhaust Gas Temperature), Pv (Inlet Void Pressure - a static absolute pressure measurement above the fwd intake ramp) as well as a collapse in P7 (Exhaust gas total pressure). The surge itself might only be evident for one to three recorded frames, before the engine recovered. The E/O would usually give you a head start by pressing an 'event' button on the system control panel, which put a marker on the recording. You then needed to decide (by looking at the frames immediately BEFORE the surge) whether the engine or the intake caused the surge in the first place. (Not always easy and straightforward). The recorded frames were generally one second samples, so there could often be quite a bit of 'searching' involved.
The observation is; a picture is beginning to emerge of one airline who cherished and took care of an extraordinary asset, while another, broadly speaking, did not. Yet another remarkable aspect of Concorde, that the same aeroplane should have been held in such diametrically opposed regard
I see your point Roger, but personally I found that it was more a case of one airline not taking matters (and the aeroplane) for granted and approaching everyday Concorde 'life' in a disciplined and professional manner and the other, very clearly, not. I remember one visit to CDG where I was horrified by the state of the aeroplanes there: The airframes were generally filthy and festooned with repair 'patches' (one large one over the main passenger entry door looked absolutely awful) and the flight deck panels had at least one broken switch and a couple of cracked instrument glasses. (I thought 'surely they don't actually FLY the aircraft like this?' .... they did!!). When I witnessed with my own eyes how aircraft defects were being investigated I could only wonder if 'they' had any clue how the aeroplane really worked. It was blessed relief when I returned to LHR that evening, and could see two gleaming Concorde aircraft in the hangar.
One thinks of the TSR2 which, on the limited amount of flight data acquired by the single flying airframe, appears to have been an aeroplane in the same astonishing mould as Concorde. With later digital electronics the TSR2 would have made an unbeatable bomber, allowing the money and effort 'squandered' (?) on MRCA to be spent on an agile interceptor replacement for the 'Electrifying' Lightning.
I can only agree with you here Roger, the story is one of complete political and industrial madness; the designer and manufacturer of the finest quality and most capable aircraft in all of Europe being relegated to a producer of mere sub-assemblies and spare parts for certain European 'partners'
(Who must now be rubbing their hands together in glee at this short sighted British stupidity). As has been said here before; 'WHAT A WASTE'
I completely agree and what is more, your fascination mirrors my own with my day job. Although MRI scanners are my day job, I still - after more than twenty years - look at some of the images it can produce and think; " That is bl00dy clever!" I still haven't got over the 'novelty'.
Oh I can echo that 1000%. After having an MRI done last year (a very sick Dude indeed) I was absolutely AMAZED at the quality and detail of the resulting images. (Being a true engineer of course I drove everybody at the hospital nuts, wanting to know all the ins and outs of this incredible (but rather LOUD) machine). Truly amazing stuff Roger, I can see your attraction here.

Regards
Dude

Last edited by M2dude; 27th Nov 2010 at 10:05. Reason: spelling as always
M2dude is offline  
Old 21st Nov 2012, 11:27
  #107 (permalink)  
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Intake Thrust is Due to Impulse

I realize this thread is two years old...but I just came across it and would like to clear up some of the misconceptions here about what causes the high amount of thrust produced by the Concorde inlet duct...

The reason is very simple and it has to do with the fluid impulse principle...the air entering the inlet duct has a very high momentum (mass flow times velocity)...but it slows down inside the duct from about M2 to ~M0.5...as a result there is a large decrease in fluid momentum...

Since momentum must be conserved...the aircraft gains that much forward momentum...which is thrust...

This momentum change happens in every stage of the engine...the inlet...the compressor...the combustor...and the nozzle...the total thrust of the engine can be computed by computing the impulse (the change in momentum) at each station in the engine...

Even in subsonic flight a diffusing inlet that slows down the flow will produce a thrust impulse...the compressor also...and the burner...the turbine produces a net drag force...and so will the nozzle...

The basic math is that the net Force (in the longitudinal x axis) produced on the engine inside wall is equal to the (mass flow times velocity at the inlet...plus the pressure times area at the inlet)...MINUS... (the mass flow times velocity at the outlet...plus pressure times area of the outlet)...

Fx = (mdot * V1 + P1 * A1) - (mdot * V2 + P2 * A2)

Looking at the right hand side of that equation we have the intake duct opening as the first term..and the fan face as the second term...in order to make thrust in the forward direction we need Fx to end up with a negative value...which means the second term must be bigger...

The mass flow-velocity product decreases as the flow slows down in the duct...but the pressure-area product ends up much bigger so we end up with the second term being bigger than the first term...and we have a change of momentum to the left (ie negative impulse) and we get thrust...

So while it is true that flying at M2 we can greatly increase the static pressure going into our engine by converting dynamic pressure to static pressure...that does not tell us why this causes thrust...the reason is the change of momentum...or impulse...

By itself an increase in pressure ratio will result in somewhat more thrust...and a six-fold increase in static pressure would result in about a 40 percent increase in thrust...compared to an engine that is at atmospheric pressure...but that does not tell us which components of the engine are actually making thrust and which ones are making drag...only a component-by-component breakdown of the impulse will give us the answer...
goarnaut is offline  
Old 22nd Nov 2012, 11:45
  #108 (permalink)  
 
Join Date: Dec 2010
Location: Europe
Age: 83
Posts: 286
I realize this thread is two years old...but I just came across it and would like to clear up some of the misconceptions here about what causes the high amount of thrust produced by the Concorde inlet duct...
I’ve also just come across this thread after two years, and although I for sure don’t want to repeat any of the millions of electron-volts expended in the excellent discussions I thought it might be of interest to have some additional comments ‘from the inside’.
The first thing I would like to say is that we always thought of the total system as “powerplant”. As in any aircraft powerplant (engine) the forces on the various components vary – some produce drag, others thrust. Part of the intake had a thrust component, part had drag. Similarly with the engine itself and the propelling nozzle. It is really a bit misleading to say that “the intake produced thrust”. For us it was just one component of the powerplant that gave a thrust .- and we weren't particularly bothered whether that came from pressures on the diffuser walls or momentum changes, What mattered for us was that the intake efficiency (pressure recovery and intake external drag) was as high as possible. (not intake drag as high as possible of course). I can't stress too highly that the powerplant had to be considered as a whole!



Dude’s remark:


The important concept to grasp is that you have to consider the powerplant as the 'engine' if you like. It's the intake, engine and nozzle assembly that were able to work together in such perfect harmony, but each component was totally codependant on the others.
was absolutely on the mark. Other things I found interesting:


Post #15 B377
Wonder how much of this was learned from the use of the 593s on the 1950s Victor bombers albeit these were not supersonic?
Not a lot!
Post #31 Landroger
The whole concept of 'Supercruise' is quite stunning and the thought occurs; was it played for, or a happy outcome of the way the design froze?
Very definitely played for !
You are saying though, that it could not maintain Mach 2 without reheat
Yup!
Post # 33 M2Dude
The problem was that having decided that an analog intake system would never be able to provide the level of control and stability required for certification, the technology almost had to be invented. In 1970, when relatively late on, in project definition terms, it was decided to use 'throw 'away' the analog system and replace it with a completely new digital one, there came a problem; there was no such thing as an airborne digital control system, and so one had to be 'invented' in Bristol.
The analogue system wasn’t thrown away exactly – we retained the inner ‘analogue’ system that did the closed loop dynamic control of ramps etc. and wrapped a digital system around it to define the control ‘laws’ to which the intake operated.

Post #39 Mr Optimistic

Of course you would need 10^-9 now, don't suppose safety was quite the game it was now.
Actually Concorde was the first civil aircraft to be certificated using this 10^-9 approach, although the rules were of course contained in TSS Standards not enshrined in FARs. Also we adopted a far more pragmatic ‘engineering’ way of working than the almost blind faith in numbers that seems to be the case today!
Post #56 M”Dude
For a supersonic aircraft, how good your whole 'engine' in this context is relies on solely how well designed the 'front bit', the 'rotating middle bit' and the 'back bit' are, and how they work together. Weakness in any one of these three is gonna cost you performance and/or fuel (and trans-Atlantic range is just not possible; ask Tupolev).
Post #66 Pugilistic Animus
and it illustrates that the Great Art of Aerodynmics is Experimental - a little twist here a little waxing and polishing,..., a little area cut away from there...and it---- Still Flies
Ain’t that the truth!
Post #56 again
And all this done with slide rules, protractors and the backs of hundreds of cigarette packets, without any mathematical modelling in sight.
Also true, but only two of the team smoked (mind you they made up for the others)
Post #67 M2 Dude
If I may, I would now like to mention the 'some oil lamps and diesel oil' story.
True, but we also did the ‘Hi-tech’ bit. I thought people might be interested in the actual flow visualisation results we got from the lamp black and diesel fuel trick:




Post #100 M2Dude
The next part is pure comic opera; MBB then approach BAC for financial compensation with regard to this copyright infringement, (yep!! BAC design and develop the Concorde intake, give all the necessary knowledge and 'know how' to MBB for the Tornado and then are required to PAY THEM for the pleasure??).
but there is a bit of poetic justice; the final Tornado intake was a very poor design, with excessive levels of fuselage boundary layer ingestion and shockwave control.
In fairness to MBB, one should say that the Tornado had conflicting design requirements in that there was a need for efficient subsonic performance to give loiter (CAP) endurance, so they were forced to compromise a bit on the purely supersonic efficiency.


Hope I haven’t hogged too much space!

Last edited by CliveL; 22nd Nov 2012 at 12:12.
CliveL is offline  
Old 24th Nov 2012, 15:32
  #109 (permalink)  
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Interesting Stuff

Clive...thanks for chipping in with some very interesting info...


Always good to hear from one of the design team members...just curious about a couple of things...what was the mass flow per engine at M2 cruise...?...and what about the jet velocity out the tailpipe...?...my rough estimate puts it at ~300 lb/s and ~ 1,100 ft/s over and above the freestream velocity...


That would put propulsive efficiency at ~80 percent...which means engine thermal efficiency would be just over 50 percent...


Some other interesting figures...total flight power at M2 would be over 140,000 thrust hp...nearly three times the flight power of a triple seven...fuel burn would have been most impressive at over 13 lb per second...compared to 5 for the triple seven...


The triple seven's GE90 engine is good for a TE of ~50 percent...and propulsive efficiency of ~0.8...really quite amazing that a Mach 2 transport could still be the high water mark after all these decades...


Regards,


Gordon.

Last edited by goarnaut; 25th Nov 2012 at 03:45.
goarnaut is offline  
Old 24th Nov 2012, 20:27
  #110 (permalink)  
Second Law
 
Join Date: Jan 2008
Location: Wirral
Age: 72
Posts: 114
Bloodhound SSC

I'm fascinated by this thread too chaps, addicted to is a better way of putting it.

I know it's not Concorde but how will Bloodhound SSC be addressing these problems do we think?

1,000 mph at 0 agl must be some challenge too and not exactly "off the shelf"....

CW
chris weston is offline  
Old 25th Nov 2012, 04:53
  #111 (permalink)  
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Tupolev Tu144

It was mentioned earlier that the Tu144 did not supercruise...and that Soviet designers never did solve the technical challenges in creating an efficient engine inlet...

In the spirit of fairness and accuracy...this is not correct...it is true that the initial version of the airplane was not ale to supercruise...due to using inefficient Kuznetsov turbofan engines... instead of more efficient turbojets which are better suited to supersonic cruise...the Russian designers were aware of this but there was not a suitable turbojet engine available at the time...


The D model of the airplane switched to Kolesov RD36 turbojets when those became available...and was able to cruise at M2 without reheat...SFC of the Kolsesov engine was comparable to the Olympus at ~1.2...although the Kolesov was a bigger more powerful engine with `12,000 pounds of cruise thrust compared to the Olympus's 10,000...


The more efficient engines greatly increased the range of the airplane to 3,500 nm...largely closing the gap with Concorde's 3,900 nm range...an extended range model was in the works that would have exceeded Concorde...when the program was cancelled in 1983...


The original intake ducts were not as efficient as Concorde...but this was rectified with subsequent models...starting with the S model that preceded the D...


In the 1990s Nasa obtained a Tu144LL which was a D model retrofitted with Kuznetsov fan enngines due to the fact that the more efficient Kolesov jets were no longer in production...a comprehensive flight test report is available from Nasa...


http://dcb.larc.nasa.gov/DCBStaff/ebj/Papers/TM-2000-209850.pdf


The report notes that "the engine inlets appeared insensitive to sideslips (up to 4.5 degrees) or to pitching moments..."...this points to good inlet design...something that is not always present in supersonic aircraft...for instance the F14 which was known to have compressor stalls at high yaw angle...



Overall the Tu144 is a bigger and heavier aircraft than Concorde and was able to reach a higher speed...M2.4...although cruise was at M2...the prototype used a lot of titanium although this was cut down on production models...unfortunately...


Regards,


Gordon.
goarnaut is offline  
Old 25th Nov 2012, 06:50
  #112 (permalink)  
 
Join Date: Dec 2010
Location: Europe
Age: 83
Posts: 286
Interesting stuff

Gordon,

Without doing a lot of sums (which I have probably forgotten how to do), the best I can offer is:

58000 ft, M 2.0, Ambient temperature - 52 deg C, Ambient pressure 1.2 lb/sq.in
Engine intake conditions
Total pressure 8.5 lb/sq.in, Total Temperature 127 deg C, Mass flow 189 lb/sec.
Fuel burn 9700 lb/hr
Jet pipe exit (not final nozzle) Total pressure 16 lb/sq.in, Total Temperature 687 deg C
Engine thrust 8050 lb.

The jet pipe nozzle was choked of course and the petal angle was 10 deg, but I don't have the value of the actual area handy.

Clive

Last edited by CliveL; 25th Nov 2012 at 06:52.
CliveL is offline  
Old 25th Nov 2012, 10:22
  #113 (permalink)  
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Thanks for those numbers, Clive...

I had estimated the compressor inlet area at ~1 m^2...based on the engine static mass flow of ~450 lb/s...and an assumed inlet Mach of 0.5...


Seeing as cruise mass flow is a lot less...perhaps some of my assumptions were off the mark...


Also surprised to hear a cruise thrust figure of just 8,000 lb...a Concorde website with lots of info gives 10,000 lb per engine...Wikipedia gives a Lift to Drag ratio of ~7 at M2 cruise...which would imply that ~50,000 lb of total thrust would be needed...since maximum weight is ~400,000 lb...fuel weight is ~200,000 lb...and fuel flow with max power and reheat is ~50,000 lb/hr...


This means even if it takes a full hour at the maximum fuel flow the airplane would still weigh at least 350,000 lbby the time it reached cruise speed and altitude...since thrust must equal drag in straight and level unaccelerated flight...it means 50,000 lb of thrust would be required...


Looks to me like there is a fair bit of confusing information floating around...hope you can clear some of that up...would like to do a full cycle analysis on the engine at cruise power...that SFC figure of 1.2 is very impressive...


Regards,


Gordon.
goarnaut is offline  
Old 25th Nov 2012, 12:54
  #114 (permalink)  
 
Join Date: Jan 2008
Location: Boston
Posts: 31
First, thanks for resurrecting this most interesting thread. At one point I thought I was getting this, but clearly I wasn't.

Gornaut states that momentum is being transferred from the supersonic incoming air flow to the engine (and hence to the aircraft). I can't visualize it. Isn't the momentum already in the aircraft?? (It is the aircraft that is moving through the air, not the other way round.)

As I read the discussion, it seems we are getting the ultimate free lunch (which I know isn't right). I'm sure that if we turn off the fuel flow, the net thrust must drop to zero. So how can we say that 63% (or whatever) of the thrust is coming from the inlet duct.

I'm really trying to understand this.
Crabman is offline  
Old 25th Nov 2012, 14:14
  #115 (permalink)  
 
Join Date: Jul 2009
Location: France - mostly
Age: 80
Posts: 1,689
Crabman,

Perhaps post #5 explains:
in the final analysis all powerplant thrust of course is really generated by the engine, what we have been looking at how this thrust is transmitted to the airframe.
HazelNuts39 is offline  
Old 25th Nov 2012, 16:08
  #116 (permalink)  
 
Join Date: Jun 2009
Location: Bedford, UK
Age: 65
Posts: 1,193
Gornaut states that momentum is being transferred from the supersonic incoming air flow to the engine

That would be drag as something has to slow the air down. This thread caused endless discussions a while back. The way it was originally phrased you would think you had the basis for a perpetual motion machine, ie just bend metal into a sophisticated shape and off it would fly ! Need to take a holistic view of the whole power plant to really get the picture, hence the quotes above. My view was that it was really about the efficient recovery of pressure for the intake but what do I know: certainly the intake on its own isn't a prime mover or the active source of any thrust.
Mr Optimistic is offline  
Old 25th Nov 2012, 17:42
  #117 (permalink)  
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Trying to Post A Sketch ut It Won't Let Me...

The math is quite straightforward and can be found in any good engine text or fluid mechanics text...but that does not help us understand the physical principles behind it...


I have already given the momentum equation involved which is a statement of Newton's Second Law of Motion...which we mostly know as F = mass times acceleration...


In fact the Second Law tells us that a force results from a change of momentum...In differential form F = m * a becomes...F = m * dV / dt...which means a mass that experiences a velocity change (differential of V with respect to time) is experiencing a momentum change...


Of course momentum must be conserved...just as mass or energy must be conserved...if a car collides with another car it loses its momentum as it stops...but the other car gains momentum as it is shoved forward...


Same thing with a flowing fluid... if it goes into a control volume such as an engine intake with a certain amount of momentum...and then loses a good part of that momentum...what happens to it...?...it is transferred to the control volume...ie the engine...which is attached to the airplane...


An impulse is simply a change of momentum...when we accelerate a hot gas stream out the tailpipe we impart a change of momentum to that mass flow...the result is reacted by the engine which is attached to either an airplane or a test stand...in either case a change of momentum in the forward direction means thrust...

If we were to take an engine inlet duct (a diffusing duct that has a smaller hole at the front than at the back) and bolt it to the floor inside a wind tunnel... and then blow air at 500 mph through that duct...what do you think would happen...?


If we allowed the mount to slide back and forth...which way would it slide...forward or backward...?


What if we turned the duct around and now had the big hole at the front and the little hole at the back...?...which way would it move now...?...isn't this how a wind sock works...?...which way does it want to go...?...back of course...


And have you ever seen a wind sock where the wind is blowing into the little hole...?...and coming out the big hole in the back...?...of course not...


So why would that intake duct that is shaped just like a wind sock that is turned around front to rear want to move back...?...of course it is going to move forward...


The reason is that the impulse (change of momentum) gets bigger as we go from front to back...

Now we can work out an example like the illustration...we have an airplane flying at 250 m/s...and the inlet opening is 1 m^2...the mass flow is 100 kg/s...let's say the opening at the aft end of the duct is 1.25 m^2...which has the effect of slowing down the flow and increasing static pressure...at the same time the flow loses momentum as it slows down...


We recall that Force = mass flow * velocity...so the force at the front of the duct is 250 m/s * 100 kg/s = 25,000 newtons...a little over 5,000 lb of force...


If the flow slows down to 125 m/s at the aft end...its total force will now be 125 m/s * 100 kg/s = 12,500 N...(mass flow does not change of course...owing to conservation of mass)...


So the fluid flow has lost 12,500 lb of force...but we need to account for all the forces acting on that control volume...we see that there is pressure from the wall pushing on the fluid that is equal and opposite to the pressure of the fluid that is pressing on the wall...since pressure acts exactly perpendicular to any wall...we see that a lot of the arrows are canted in the forward (thrust) direction...this gives us a clue that the sum total of all those little arrows is going to end up as a net forward force...


We can integrate all the pressure forces across that surface and expend a lot of math...or we can just do a simple multiplication of the pressure times the area of the inlet and outlet respectively...that will give us the answer...


So we see that our static pressure at FL350 is 22,500 N/m^2 (22.5 kPa)...and our intake area is 1 m^2...so the total pressure force pushing back is 22,500 N...


At the aft end our pressure is 1.5 times higher...this is a subsonic plane and slowing down the flow from ~M0.8 to ~M0.4 will result in a static pressure increase of that amount...


Our duct exit is 1.25 m^2...so our total force at the back is 22,500 N * 1.5 * 1.25 m^2 = ~42,000 N...So we have gained nearly 20,000 N from the pressure on the engine duct inside surface...!...


We subtract the 12,500 we lost from the flow slowing down...and we are still ahead y more than 7,000 N...an engine of that mass flow would make perhaps 30,000 N total thrust so that would be a good part of it right there...nearly ¼... of total thrust...


So yes the engine inlet is making thrust...that is very real...it is just that we are used to thinking purely about thrust as mass flow times velocity coming out of the tailpipe...and that is the simplest way to account for the sum total of all the physical forces going on in the engine...


However...if you want to do a more thorough line-by-line accounting...then we must start at the tip of the engine and do this impulse analysis for each component...we would then go on and do this for the compressor...the burner...the turbine...and the jet nozzle...and we would find that the net thrust still remains the same...but it is the physical forces of pressure and momentum and impulse that do their work at each point along the way...


Regards,


Gordon.

Last edited by goarnaut; 26th Nov 2012 at 06:08.
goarnaut is offline  
Old 25th Nov 2012, 17:52
  #118 (permalink)  
 
Join Date: Aug 2011
Location: Grassy Valley
Posts: 2,123
Source/Site

Power/Work

I think unpacking any aeronautical question leaves room for almost absurd conjecture. Of course the nacelle does not provide power. It is the site, instead, of the Work. Or at least most of it, in modern Turbofan engines. Powered aircraft need, well, power. This happens well aft of the Work.

The Wrights used unducted fans. The power came from a combustion engine that provided mechanical energy to the fans.

Given extreme latitude in architecture, how is the Concorde different from the Wright Flyer? Distance from power to work?

I am still trying to suss the Wright's accomplishments. There were few shoulders to stand on, they made their own engine, for goodness sake.

The idea is to move mass, energetically enough to gain useful work, after all. It is an attempt at lazy magic that pulls people into heated discussion that requires a suspension of the basic Laws.

Mr. Optimistic has busted out the thread, imho.
Lyman is offline  
Old 25th Nov 2012, 17:56
  #119 (permalink)  
 
Join Date: Nov 2012
Location: Europe
Posts: 15
Here is the Sketch Hope It Works

goarnaut is offline  
Old 25th Nov 2012, 23:11
  #120 (permalink)  
 
Join Date: Jan 2012
Location: Ontario
Age: 69
Posts: 81
Perhaps it helps, just for a moment, to view the engine as a flow inducer. This is the term used by 'Kelly' Johnson to needle P&W J58 engineers. This merry banter should be seen for what I'm sure it was, just that.

However there is merit in viewing the engine as such since another angle on any issue can only help to get a better picture.

This particular view imo helps to come to terms with the ever-increasing relative contribution of the intake to the total thrust.

At the same time though the engine is producing ever more gross thrust from an ever-increasing release rate in its engine and jetpipe combustors. Everything can be traced back to the fuel. The burning fuel induced the flow which, by virtue of its finer details, must produce forces in some direction or other.

Imagining what's going on in the combustors should help prevent downplaying the loser in the relative stakes.

On a slightly different tack:

Would the inlet momentum drag be zero when the flight speed is the same as the engine entry speed, say M0.5, assuming, of course, that the intake design flow was matched to the engine flow at that flight speed?
After all, if I watch a plane go right to left at M0.5 the compressor face is going left at M0.5 and the air is going right at the same speed, ie is stationary like it was before the plane appeared.

Alternatively, if I'm sitting on the Rosemount probe and looking ahead I see air approaching at 0.5 and if I look behind I see it going away from me into the compressor at 0.5, again no speed change.

Last edited by peter kent; 25th Nov 2012 at 23:33. Reason: ification
peter kent is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.