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Concorde engine intake "Thrust"

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Concorde engine intake "Thrust"

Old 13th Sep 2010, 14:22
  #41 (permalink)  
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Going full circle to the start of thread where I remarked that Concorde 'sucked its way through the air' it obviosuly wasn't too far from fact.

Still when one thinks about thrust one imagines what happens in subsonic flight where the propelling force comes from prop-wash,
jet-wash or down-wash in the case of a wing. All these involve momentum exchange iaw Newton to produce thrust.

I suppose part of the cognitive problem wjhen talking about 'intake' produced thrust is that it sounds a bit like 'one hand clapping' as one looses clarity of the momentum exchange process which is a prerequisit for thrust generation.

I suppose that the satisfactory explanation one was hoping to get was one that related the deceleration/compression of the incoming flow in the intake to a momentum exchange process that ledi to thrust.

There are of course other propelling forces that do not rely on momentum exchange such as tyre traction which requires friction and the dreaded drag forces produced by viscous friction.

Last edited by b377; 13th Sep 2010 at 14:37.
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Old 13th Sep 2010, 18:47
  #42 (permalink)  
 
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Mr Optimistic, Kelly Johnson of SR-71 fame described the the power plant as a pump to keep the intake alive (producing the thrust). Take it as a given that 2dude and christian know what they are talking about.
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Old 13th Sep 2010, 19:38
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Indeed I do.
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Old 13th Sep 2010, 19:42
  #44 (permalink)  
 
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M2Dude

Roger I'm so glad that my blurbish explanations are making a little sense, the subject drives me nuts too, and I started doing it thirty six years ago.

I quite like ChristiaanJ's analogy on explaing how an inlet can provide thrust, but the precise shockwave geometry that the Concorde intake required in order to do what it did best, was little more than mind numbing in terms of complexity and control; It is really difficult to imagine if it could ever be bettered aerodynamically, even now.
It is getting clearer all the time, many thanks to you both, but I have to take the math as read. I have my grandfather's grasp of the mechanical (electronics was before his time, but those too ),but not my late father's self taught grasp of mathematical concepts. They just give me a sensation very closely allied to vertigo.

Your latest diagram at #39 has, I think, finally wrapped it up about as far as I am going to be able to cope with. In return I could try and explain how MRI scanners work , but it would be a long way off piste and is just as much brain damage.

What I find so neat - I'm sure it is beautiful if the maths are included - is the way the design team 'conjured' the shock waves. The edges, dump gates and ramps are sort of obvious, almost simplistic in a sense, but the secret is all in the way these engineering 'magic wands' conjure a series of invisible, yet powerful 'force fields'. Force fields not directly connected to the doors and ramps necessarily, but the whole witches kitchen interacting to produce .... the thrust rabbit out of the intake hat.

I know engineers are regarded as soul less nerds, but the things they create are truly beautiful. Very few in Britain would disagree that Concorde is a beautiful thing to look at, like the Spitfire and the fan blades on a Trent 900, but how many could understand how beautiful she is on the inside?

There I go, waxing lyrical on a technical forum. I'll get my hat.

ROger.

PS: Thanks again guys, I just can't stop reading these threads.
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Old 13th Sep 2010, 19:52
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I suspect nothing as elegant as Concorde will ever fly again

...well, perhaps one of these super light things to some extent but not with the combination of elegance and power.
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Old 13th Sep 2010, 19:54
  #46 (permalink)  
 
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Originally Posted by b377
....the cognitive problem when talking about 'intake' produced thrust....
You're hitting the nail on the head. It's a cognitive problem.

Nearly all that is taught about jet propulsion (or aircraft propulsion in general) treats the engine, or rather the propulsive system, as a "black box", with mass going in, energy being added by the fuel, and basically the same mass coming out with added momentum.
Yes, in that case Mr Newton has the correct description of what's going on.
F= m x a !
The simplistic thrust formula T = m' x (Vo - Vi) is just another way of stating the same thing.

It's when you start looking in detail of what happens inside that "black box" that the "cognitive problems" start.

You're now suddenly dealing with a far more complex description of how and where the momentum exchange happens.
The basic F= m x a, while globally still perfectly valid, is no longer much use when you try to "get your head around" what exactly happens inside that "black box".

Inside the "black box", thinking in terms of pressure distribution, and in particular the forward components of those pressures, is an easier way of understanding what is going on.

I'm not a "propulsion" engineer, and I'm the first to admit when I first saw "the intake produces 75% of the thrust", that my first reaction was "huh"??? too.

And yes, it did take me more than a moment to work out what I now tried to say just above.

Luckily, the structural design engineers that designed the engine nacelle, and more specifically the intake, knew about this.
Because indeed, at Mach 2, those 75% of the actual propulsive force of the "engine system" was transferred to the wing structure and from there to the rest of the aircraft... by the intakes.
Just as well they were bolted on properly....

CJ
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Old 13th Sep 2010, 20:50
  #47 (permalink)  
 
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Originally Posted by Landroger View Post
It is getting clearer all the time, many thanks to you both, but I have to take the math as read. I have my grandfather's grasp of the mechanical (electronics was before his time, but those too ),but not my late father's self taught grasp of mathematical concepts. They just give me a sensation very closely allied to vertigo.
Roger,
A lot of it is not maths as such, but "getting your head around a concept".
Like supersonic flow, shock waves and all that.
I was lucky, I suppose... I learned all that when I was still a teenager, from somebody doing a few very simple demonstrations in a flow of water over a slightly inclined glass plate lit from below.
The same with electronics... my grandfather was a radio amateur (1920's !!), my father was an electrical engineer. It rubbed off very early on.

....I could try and explain how MRI scanners work...
Again, it's not just being an engineer, but being able to explain a concept in clear terms.
BTW, I know how MRI scanners work... after my early retirement I went into writing documentation for medical imaging software....

What I find so neat - I'm sure it is beautiful if the maths are included - is the way the design team 'conjured' the shock waves. The edges, dump gates and ramps are sort of obvious, almost simplistic in a sense, but the secret is all in the way these engineering 'magic wands' conjure a series of invisible, yet powerful 'force fields'. Force fields not directly connected to the doors and ramps necessarily, but the whole witches kitchen interacting to produce .... the thrust rabbit out of the intake hat.
Thanks, Roger.
It's usually only engineers that will recognise a particularly neat engineering solution as "beautiful".....

I know engineers are regarded as soul less nerds, but the things they create are truly beautiful. Very few in Britain would disagree that Concorde is a beautiful thing to look at, like the Spitfire and the fan blades on a Trent 900, but how many could understand how beautiful she is on the inside?
So true... sad, really that so few people can see the beauty in a truly well-done design, apart from rare exceptions such as Concorde or the Spitfire, where beauty of form join beauty of design.

There I go, waxing lyrical on a technical forum. I'll get my hat.
Please put your hat back on the hat-rack.
Because whether technical or not.... we've all been waxing lyrical here, one way or the other.
With Concorde, we did something special.
Apollo took men to the moon.
Concorde took us to the other side of the Atlantic in three-and-a-half hours, and in the end did so for twenty-seven years.
No, it didn't all work out, sure.
Apollo was abandoned, Concorde saw only sixteen aircraft built.
But I think we're all proud of what we DID achieve.

And as to Concorde.... she was beautiful in every way.

CJ
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Old 13th Sep 2010, 21:26
  #48 (permalink)  
 
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The concept (on a very, very basic level) of the Concorde powerplant / intake nozzle combination I keep having is basically squeezing the end of a hosepipe to 'focus' the forces already present.

Would that be close to the truth?
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Old 14th Sep 2010, 11:21
  #49 (permalink)  
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I think we can summarise all these muzings by stating that regardless of what component ( nozzle, intake, engine support) communicates the thrust ( force) to the airframe, the bottom line is that the POWER required for flight = drag X air speed comes from INSIDE the engine-core burning fuel.

The "intakes" may be doing the pulling but the energy: every Joule or Watt-sec or Newton-meter for the intake to do that work has to come from the engine fuel burn and there can be no other way unless this thread has successfully falsified Newtonian mechanics.

To say that the intake produces the thrust is not the same as saying that it produces the power, the engine-core does that. But if the engine (balck box) is taken to comprise of all its subcomponents: nozzle, intakes, compressor, turbine, burners etc then the thrust is generated by the 'engine'.

I think the cognitive problem lies here. Thrust on its own means very little if one talks about the power required for flight there can be no doubt that it comes from inside the engine and no where else.

In this case intake_thrust x airspeed= engine_core fuel burn engery x efficiency factor. (ignoring thrust contributions from nozzle etc which also exist of course)
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Old 14th Sep 2010, 13:38
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b377,
Most of the problems lie in the fact that, when discussing a subject such as this, a lot of terms such as thrust, force, power, momentum, energy, etc. are used very loosely, and as a result the discussion can easily go off on a tangent, if the terms, and their context, are not defined very clearly beforehand.

You're right, the aircraft is finally propelled by the chemical energy in the fuel being released in the engine.

But you have to be careful with the term "power"... as discussed in an earlier thread, for instance: what is "power" for a Concorde?
When it is standing at the start of the runway, with all four engines at full dry thrust, you have about 120 000 lbf thrust, but the power is... zero, because that force isn't moving.

In our current context, nothing stops one from defining the "power" of the intake as the 75% (at Mach 2) of the forward-acting force on the propulsive assembly, multiplied by the speed.
With that definition, 75% of the propulsive power comes from the intake. Nothing wrong with that statement.

The mistake being made is considering the intake as a closed system, and then considering the thrust of the intake as "free power".
This is wrong, of course... nothing would work without lots of fuel being burned each second in that engine right behind the intake, to maintain the airflow, even if the engine itself produces little thrust (8%) in the process.

Maybe we could say, that the "power", in the sense of forward-acting force x speed, is "expressed" (or finally does its "work", if you like) for 75% in the intake.

Its really a matter of semantics, or terminology... saying the intake 'produces' the power is indeed misleading, as you say.

CJ
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Old 14th Sep 2010, 13:50
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A minor snippet about the SR-71 (also see page 1).

On Concorde at Mach 2, the engine itself provides only about 8% of the total thrust.

On the SR-71 at Mach 3, that percentage is even less.
And in certain flight conditions, the engine itself no longer produces any thrust at all, but some drag.
Result? Instead of the engine 'pulling' the aircraft along, it's now the aircraft that 'drags' the engine along, and yes.... the engine moves rearward in the engine mountings !!

CJ
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Old 14th Sep 2010, 15:30
  #52 (permalink)  
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ChristiaanJ

I fully agree... my simple expression for power says just that.

i.e. at constant forward speed the thrust just compensates for drag and the power required is thrust x air speed = total drag x air speed.
i.e. thrust= drag.

The power lost to the "induced" drag component of total drag goes into putting the air in motion as down-wash from the wing to keep the plane in the air. Parasitic drag heats the air and a/c skin. The latter losses represent all the power the engine has to provide to maintain constant forward speed. But of course the engines also have to provide the power to run electricity generators and supply bleed air etc...that is extra.

75% of the thrust ( not power) requirement to compensate for drag is provided from the intake assembly.
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Old 14th Sep 2010, 15:54
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Let's make this a tad more complicated still.

From "thrust (force) = drag (force)", does it follow that "power (developed by engine) = power (lost due to drag)"? Or, for that matter, is the opposite implication true?

Hint 1: If we stand right behind the aircraft with its engines on take off power with reheat, can we hear the engines? Is it perhaps also a little windy? If we step close enough, or drive a small car behind the aircraft, would anything noteworthy happen?

Hint 2: If we were able to position ourself right behind the engines with the aircraft in cruise (i.e. when thrust force does actually equal drag force), is it likely that the engines would be inaudible and the exhaust would not be felt?

What I am getting at is of course that the engines also do work on other things than the aircraft. Therefore the engine will develop more power than that being lost due to drag, whether induced drag or the other kind.

Edited to add: Great minds think alike... And some are 1 minute faster than others...
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Old 14th Sep 2010, 15:55
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what is "power" for a Concorde?
When it is standing at the start of the runway, with all four engines at full dry thrust, you have about 120 000 lbf thrust, but the power is... zero, because that force isn't moving.
absolutely true.

If you push against a wall you do no work on the wall as it does not move, but your muscles are still expending the same engergy - you get tired, muscles heat up, heart works harder to pump against contracted muscles etc.. basically effort wasted.

Concorde with its brakes on and engines running at full tilt at the end of the runway does not gain kinetic energy so it takes no energy from the engines - but the engines still do substantial work if not the same, after all its burning the same amount of fuel, but it all goes into moving fast air as jet blast (jet-wash) noise, heat etc.
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Old 14th Sep 2010, 16:01
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[the engines also do work on other things than the aircraft
yes that is part of the efficiency figue - heat, noise, bearing friction, and turbulent motion in the jet-wash itself that just increases the entropy of the world.
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Old 14th Sep 2010, 17:19
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Talking Phew!!! This is becoming one HELL of a topic

Wow this subject has generated one hell of a lot of debate, but intake 'thrust' really became a truly fascinating subject once the SR71 became reality. (Keep the posts coming guys, this is great).
What I've tried to suggest before (in my own confusing way ) is that the nozzle and inlet components should maybe considered as part of the whole 'engine' if you like, and this rotating bit is where the chemical energy conversion ,lighting of the fires occurs and sucking and blowing occurs.
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).
The mistake being made is considering the intake as a closed system, and then considering the thrust of the intake as "free power".
This is wrong, of course... nothing would work without lots of fuel being burned each second in that engine right behind the intake, to maintain the airflow, even if the engine itself produces little thrust (8%) in the process.
Well said ChristiaanJ, I think this is the main 'thrust' of the argument (sorry 'bout the pun ).
75% of the thrust ( not power) requirement to compensate for drag is provided from the intake assembly.
Not forgetting of course the 12% negative component due to to front ramp loading, giving us 63% net thrust. (numbers are really mind blowing I know).
In all my ramblings I've not even mentioned the incredible complexities of arranging and generating the inlet shock system, and how controlling the intake was as complex as any single system that I personally seen on ANY aircraft, old or new. And all this done with slide rules, protractors and the backs of hundreds of cigarette packets, without any mathematical modelling in sight. (And also some oil lamps and diesel oil, but that's another story).

Dude
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Old 14th Sep 2010, 17:58
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Now it can be told. Imagine your hose has a fitting that varies the water spray. It is set on "fan", a wide chord of accelerated water. See It? Now imagine that instead of exiting the nozzle, it is reversed, and entering instead. Each drop makes its way to the inlet and barges in with all its mates. Consider that it (the hose) has a forward velocity, creating a dynamic system. The "Cone" of entering water (air) is larger consistent with the size and setting of the "system". It is not incorrect to say that the entering air creates (potentiates) a very low pressure (energetic) cone for the nozzle, hose and airframe to enter.

Nomenclature is all, unless you have envisioned this new system before, and can enfold it into prior bias of the pilot mind (guilty). For one, the top of the wing lifts the a/c, for another it is the bottom pushing. I hope ChristiaanJ appreciates my hose "chops", I mean it in good humour!

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Old 14th Sep 2010, 20:34
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bearfoil,
IF I follow your way of describing things... what you describe is what happens at low speed, when indeed a 'cone' of air is sucked into the inlet.
Once supersonic, to use the popular old-fashioned way of describing it: there no longer is any "warning" for the air ahead of the intake, and the intake neatly slices a squarish "pipe" from the arriving airflow and performs its magic.

bjornhall, b377, et al,
This thread really started off on the problem of getting ones mind around the question on how an intake could actually produce thrust.
We seem to have that sorted out to a large extent.

What happens to the energy actually being liberated by the burning fuel is a slightly different story, and there your reasoning is perfectly right... some ends up being used to power the engine accessories, some ends up as hot air, literally, some ends up as noise

At Mach 2, most of it is still used to move the aircraft.

CJ
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Old 14th Sep 2010, 20:47
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Originally Posted by M2dude View Post
In all my ramblings I've not even mentioned the incredible complexities of arranging and generating the inlet shock system...
I think you already did.
....and how controlling the intake was as complex as any single system that I personally seen on ANY aircraft, old or new. And all this done with slide rules, protractors and the backs of hundreds of cigarette packets, without any mathematical modelling in sight.
You may have mentioned this already in the other 'Concorde question' thread.
(And also some oil lamps and diesel oil, but that's another story).
I suppose you're talking about Casablanca?
Maybe we could add that story here, now that most of us agree an intake can produce thrust?

CJ
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Old 14th Sep 2010, 20:53
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The only part I left off (arguably) is the part about the size and shape of the cone being relative to setting and configuration of the "system". This means velocity, and at velocities in excess of a specified value, the shape is as you describe. "Down the Rabbit Hole, Alice." I leave this hole at the end of every essay. One must allow the student some room to extrapolate, reason, and experience the "Aha!", No?

Arrogant enough?

Your water sheet on glass, was that in re: Laminar flow?

bear
 

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