Can Vmg exceed the V of a jet exhaust?
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Re:58 Harrier residual thrust?
Assume the air flowing into the (big) intake is all redirected down. There is a net forward net change of momentum to the air, just as slowing in a supersonic intake or a jetpipe, and thus there's a rearward reaction force on the aircraft. To hover a harrier without going backwards then the nozzles should have to point slightly to the rear.
I looked for images of hovering harriers after composing the paragraph, but when you get good side-on pictures, the big pylon hides the nozzles. There's also the issue of how much fine control is carried out by the little attitude control bleed jets.
I looked for images of hovering harriers after composing the paragraph, but when you get good side-on pictures, the big pylon hides the nozzles. There's also the issue of how much fine control is carried out by the little attitude control bleed jets.
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Sorry guys...
This thread has now sunk to the level of urban legends and vague rehashes of memories of badly understood physics lessons or popular science TV programmes....
Maths? Bah! So boring.
I think I'll unsubscribe...
CJ
This thread has now sunk to the level of urban legends and vague rehashes of memories of badly understood physics lessons or popular science TV programmes....
Maths? Bah! So boring.
I think I'll unsubscribe...
CJ
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Urban myths
Apologies to ChristiaanJ if I appear to be pedalling urban myths. Not intended. I'm just trying to get to the physics involved. Thanks for the discussion.
I appreciate that substantially increased pressures and temperatures exist in supersonic inlets, and that due to the receding slope of Concordes' deeper ramps (and the SR71's inlet structures downstream of the inlet cones) these surfaces are subject to very substantial forward pressure forces, and yes, these must be absorbed by the wing structure.
However, the air has still been slowed by the inlet, and none has spilled. Air enters the intake at M=2.0 (~570 m/s at 200K). To have the compressor ingest air at M<1, with no temperature change, would mean that the momentum of the airflow had more than halved. But the air has been shock heated, by a factor of up to ~3 (?). This would lessen the change in momentum, as the sound speed in m/s at the compressor face has increased by ~root(3), and so the condition that M<1 can be met at a higher flow speed. Nevertheless, there is still a slowdown, which requires a net forward force on the air (from the inlet shocks?) and thus a net rearward reaction force on the whole inlet.
Where is the discrepancy? Is it a key fact I'm missing or a wrong assumption I'm making, or is there just a bigger drag force on the front ramp and forward 1/3 of the intake (that supports the shocks) than the thrust force on the rear ramp.
If the section of wing incorporating the inlet was broken clear from a Concorde cruising at M2, all the way back to the compressor face, would the released inlet really accelerate forward due to this inlet thrust, no longer burdened by the airframe and engine behind? Think of the engine and bypass ducts still connected by extendable hoses if that removes some ambiguity.
If the inlet does head forwards, then where does it get the kinetic energy from, and is it not a perpetual motion machine?
I appreciate that substantially increased pressures and temperatures exist in supersonic inlets, and that due to the receding slope of Concordes' deeper ramps (and the SR71's inlet structures downstream of the inlet cones) these surfaces are subject to very substantial forward pressure forces, and yes, these must be absorbed by the wing structure.
However, the air has still been slowed by the inlet, and none has spilled. Air enters the intake at M=2.0 (~570 m/s at 200K). To have the compressor ingest air at M<1, with no temperature change, would mean that the momentum of the airflow had more than halved. But the air has been shock heated, by a factor of up to ~3 (?). This would lessen the change in momentum, as the sound speed in m/s at the compressor face has increased by ~root(3), and so the condition that M<1 can be met at a higher flow speed. Nevertheless, there is still a slowdown, which requires a net forward force on the air (from the inlet shocks?) and thus a net rearward reaction force on the whole inlet.
Where is the discrepancy? Is it a key fact I'm missing or a wrong assumption I'm making, or is there just a bigger drag force on the front ramp and forward 1/3 of the intake (that supports the shocks) than the thrust force on the rear ramp.
If the section of wing incorporating the inlet was broken clear from a Concorde cruising at M2, all the way back to the compressor face, would the released inlet really accelerate forward due to this inlet thrust, no longer burdened by the airframe and engine behind? Think of the engine and bypass ducts still connected by extendable hoses if that removes some ambiguity.
If the inlet does head forwards, then where does it get the kinetic energy from, and is it not a perpetual motion machine?
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awblain, you might work your way through the attached to gain some understanding. As for inlets producing thrust, aerodynamasists resort to all sorts of tricks to optimise performance, and even the inlets on your bog standard Airbus and Boeing produces thrust of its own accord, by careful design of the airflow path around the inlet lip.
Short Index of Propulsion Slides
During the supersonic cruise Concorde derived its thrust - 8% engine, 29% Nozzles, 63% intakes.
Short Index of Propulsion Slides
During the supersonic cruise Concorde derived its thrust - 8% engine, 29% Nozzles, 63% intakes.
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awblain
Back to the original question that started the thread: the engine's thrust is produced by accelerating the mass of air arriving at the intake and ejected from the exhaust.
When other replies say 'xxx generates yyy% of the thrust', they're not saying it generates thrust independently, out of nowhere; they're saying that of the thrust created by the engine, yyy% appears as an unbalanced force at component xxx. If you assume a pylon mounted engine, you still end up with a couple of bearings whose job is to convey that total unbalanced force (the engine thrust) to the airframe.
How does an inlet generate force? (ie how does an unbalanced force appear at or about the inlet components?). The purpose of the inlet is to slow down the air to a velocity that the compressor/fan can accept. In slowing down the air a (more or less) adiabatic compression takes place (ie a compression without any energy being added to the airflow). If the duct is just a cylinder, no unbalanced force will result; but if the duct has a profile that is narrower at the front (inlet) than the rear, a net forwards force will be generated. (The only way I can think of to explain this is if you simplify the inlet profile so that there's a step reduction in diameter, it's clear then that the higher static pressure of the air inside the inlet is pressing on the face of that step with a greater pressure than the air outside, resulting in a forwards force. That's still not a very good description... I think I've reached the limits of what I can easily explain without taking more time, text and mathematics. I'm pretty sure no-one would read that, and I can't be bothered writing it.)
The faster you go, the more the speed of the incoming air has to be reduced, and the higher the relative proportion of thrust will appear at the inlet. But this only happens because of the engine behind the inlet.
No perpetual motion required.
And all of this is only really relevant to the airframe designer, so that he knows why size bolts to use to hold the thing together. The engine is still the thing that produces thrust (I suppose you could say 'the engine installation' to make the point that the successful use of an engine is highly dependent on the inlet and exhaust arrangements).
If you're really interested, there are books (I mentioned a couple earlier in the thread, before getting thoroughly cheesed off with pointless, wrongheaded Gedanken experiments). Read those rather than trusting the highly variable quality of explanations available in internet forums (my explanations included!). These are matters of plain engineering fact, not opinion.
When other replies say 'xxx generates yyy% of the thrust', they're not saying it generates thrust independently, out of nowhere; they're saying that of the thrust created by the engine, yyy% appears as an unbalanced force at component xxx. If you assume a pylon mounted engine, you still end up with a couple of bearings whose job is to convey that total unbalanced force (the engine thrust) to the airframe.
How does an inlet generate force? (ie how does an unbalanced force appear at or about the inlet components?). The purpose of the inlet is to slow down the air to a velocity that the compressor/fan can accept. In slowing down the air a (more or less) adiabatic compression takes place (ie a compression without any energy being added to the airflow). If the duct is just a cylinder, no unbalanced force will result; but if the duct has a profile that is narrower at the front (inlet) than the rear, a net forwards force will be generated. (The only way I can think of to explain this is if you simplify the inlet profile so that there's a step reduction in diameter, it's clear then that the higher static pressure of the air inside the inlet is pressing on the face of that step with a greater pressure than the air outside, resulting in a forwards force. That's still not a very good description... I think I've reached the limits of what I can easily explain without taking more time, text and mathematics. I'm pretty sure no-one would read that, and I can't be bothered writing it.)
The faster you go, the more the speed of the incoming air has to be reduced, and the higher the relative proportion of thrust will appear at the inlet. But this only happens because of the engine behind the inlet.
No perpetual motion required.
And all of this is only really relevant to the airframe designer, so that he knows why size bolts to use to hold the thing together. The engine is still the thing that produces thrust (I suppose you could say 'the engine installation' to make the point that the successful use of an engine is highly dependent on the inlet and exhaust arrangements).
If you're really interested, there are books (I mentioned a couple earlier in the thread, before getting thoroughly cheesed off with pointless, wrongheaded Gedanken experiments). Read those rather than trusting the highly variable quality of explanations available in internet forums (my explanations included!). These are matters of plain engineering fact, not opinion.
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How does an inlet generate force? (ie how does an unbalanced force appear at or about the inlet components?). The purpose of the inlet is to slow down the air to a velocity that the compressor/fan can accept. In slowing down the air a (more or less) adiabatic compression takes place (ie a compression without any energy being added to the airflow). If the duct is just a cylinder, no unbalanced force will result; but if the duct has a profile that is narrower at the front (inlet) than the rear, a net forwards force will be generated.
A more complete history of the NACA cowl see: N.A.C.A. Cowling
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BryceM
Your xxx makes yyy% of zzz explanation, as a mounting-point breakdown of the total forces between engine and airframe makes absolute sense.
I suspect I was overinterpreting the earlier statements about inlet-engine-nozzle thrust ratios in the thread.
Your note on forward force on the increasing diameter inlet is the exact point I was trying to make described a `receding ramp' in the second paragraph of my post headed `urban myths' on 3 July.
I suspect most of any perceived disagreement is simply a scientist-vs-engineer culture clash, where my natural approach is to break down the system into simplified model bits, while an engineer probably wants to begin an explanation with that includes some carefully-wrought details of the finely-honed complete system.
I suspect I was overinterpreting the earlier statements about inlet-engine-nozzle thrust ratios in the thread.
Your note on forward force on the increasing diameter inlet is the exact point I was trying to make described a `receding ramp' in the second paragraph of my post headed `urban myths' on 3 July.
I suspect most of any perceived disagreement is simply a scientist-vs-engineer culture clash, where my natural approach is to break down the system into simplified model bits, while an engineer probably wants to begin an explanation with that includes some carefully-wrought details of the finely-honed complete system.
Last edited by awblain; 6th Jul 2009 at 18:30.
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Quoting Keith.Williams
Simply because the nozzle gives you much more thrust. The purpose of the nozzle is to accelerate the exhaust air and ideally expand the exhaust pressure to ambient. Higher jet velocity means more thrust. Nozzles become much more important with supersonic jet engines and reheated engines. Hence the use of variable geometry convergent-divergent nozzles.
The same goes for the nozzle on a rocket motor. It's not just there to steer the gas stream and vector thrust.
Why don't we just remove the nozzle and get rid of the rearward acting force?
The above results suggest that this will give us more resultant thrust.
The above results suggest that this will give us more resultant thrust.
The same goes for the nozzle on a rocket motor. It's not just there to steer the gas stream and vector thrust.
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My purpose in contributing questions to this thread was to encourage readers to think about the subject matter, rather than simply reciting things that they had previously read or been told.
As an ATPL Theoretical Knowledge Instructor I have found that far too many students devote very little effort to actually trying to understand anything remotely complicated. This approach may be OK if we accept the idea that modern pilots are intended to simply be button pushers, but I am not at all comfortable with this idea.
Some of the responses to my questions show quite clearly that readers are perfectly capable of giving references to textbooks, or mathematical explanations, but have not previously given any thought to fundamentals such as the direction of the force acting on the propelling nozzle.
Some of those who have contributed and the much greater number who have viewed this thread since my first post, have probably now realised that their initial “conventional wisdom” was wrong. Hopefully this will encourage them to give a little bit more thought to other subjects.
In that sense Mr Optimistic is correct in suggesting that “this is a wind up”.
My suggestion that contributors should avoid the use of “heavy maths” was not driven by any personal dislike for the subject. It has always been one of my favourite subjects, and I recognise the fact that the use of complex maths is absolutely essential in gaining a detailed understanding of the world in which we live. But the sad fact is that less than 1 in 20 of current JAR ATPL students in the UK are able to use maths in any meaningful way. A depressingly large number would have great difficulty in rearranging something like, Lift = CL 1/2Rho Vsquared S, to get the value of CL. These students represent the next generation of airline pilots.
Because of this lack of mathematical skill, the vast majority of students will instinctively turn away from any explanation that involves mathematics. The bottom line here is that if you want the majority of PPRUNE readers to understand your posts, then you must avoid any use of mathematics. If you are a scientist or a design engineer and you wish to be understood only by your peers, then by all means use as much maths as you wish.
BryceM has advised readers that
The simple fact is that most modern ATPL students will never consult such books. If a subject cannot be explained in simple terms then they will simply turn away from it.
He has also made reference to “
The vast majority or PPRUNE members will never have access to facilities to carry out any practical experiments. This means that thought experiments are the only type that are available to them.
Having been away from internet access for several days I have just read through the posts that have been made since 3rd July. It is good to see that some contributors have been asking themselves some probing questions.
An example of this is the discussion of the divergent section of a supersonic air inlet. Here we appear to have two conflicting effects.
A. The increasing static pressure caused by the deceleration of the air produces a net forward force on the intake.
B. But the incoming air is being decelerated, so the intake must be applying a forward acting force upon it. The Newton3 reaction to this should be a rearward acting force on the intake.
So
Does the intake produce thrust or drag?
And
If it is producing thrust, then how does it overcome the rearward acting Newton3 force in item B?
I personally do not know the answer to these questions.
As an ATPL Theoretical Knowledge Instructor I have found that far too many students devote very little effort to actually trying to understand anything remotely complicated. This approach may be OK if we accept the idea that modern pilots are intended to simply be button pushers, but I am not at all comfortable with this idea.
Some of the responses to my questions show quite clearly that readers are perfectly capable of giving references to textbooks, or mathematical explanations, but have not previously given any thought to fundamentals such as the direction of the force acting on the propelling nozzle.
Some of those who have contributed and the much greater number who have viewed this thread since my first post, have probably now realised that their initial “conventional wisdom” was wrong. Hopefully this will encourage them to give a little bit more thought to other subjects.
In that sense Mr Optimistic is correct in suggesting that “this is a wind up”.
My suggestion that contributors should avoid the use of “heavy maths” was not driven by any personal dislike for the subject. It has always been one of my favourite subjects, and I recognise the fact that the use of complex maths is absolutely essential in gaining a detailed understanding of the world in which we live. But the sad fact is that less than 1 in 20 of current JAR ATPL students in the UK are able to use maths in any meaningful way. A depressingly large number would have great difficulty in rearranging something like, Lift = CL 1/2Rho Vsquared S, to get the value of CL. These students represent the next generation of airline pilots.
Because of this lack of mathematical skill, the vast majority of students will instinctively turn away from any explanation that involves mathematics. The bottom line here is that if you want the majority of PPRUNE readers to understand your posts, then you must avoid any use of mathematics. If you are a scientist or a design engineer and you wish to be understood only by your peers, then by all means use as much maths as you wish.
BryceM has advised readers that
“there are books (I mentioned a couple earlier in the thread….”.
He has also made reference to “
pointless, wrongheaded Gedanken experiments”
Having been away from internet access for several days I have just read through the posts that have been made since 3rd July. It is good to see that some contributors have been asking themselves some probing questions.
An example of this is the discussion of the divergent section of a supersonic air inlet. Here we appear to have two conflicting effects.
A. The increasing static pressure caused by the deceleration of the air produces a net forward force on the intake.
B. But the incoming air is being decelerated, so the intake must be applying a forward acting force upon it. The Newton3 reaction to this should be a rearward acting force on the intake.
So
Does the intake produce thrust or drag?
And
If it is producing thrust, then how does it overcome the rearward acting Newton3 force in item B?
I personally do not know the answer to these questions.
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Does the intake produce thrust or drag?
Ben Rich, the chief thermodynamicist who designed the SR-71 inlets, says the aircraft gets 65% of its propulsive thrust from the inlet, 25% from the engine and 15% from the ejector nozzle. He refers to the compressor as "a pump to keep the inlets alive". It is said the thrust produced by the inlet comes from the high pressure in the inlet acting on the spike.
SR-71 Retirement Ceremony speech by Ben Rich, Lockheed Skunk Works
So the answer would seem to be, THRUST. Thats if you take the words of the designer at their face value. NASA also says the inlets produce thrust in this particular application (SR-71) and I would tend to think they know of what they speak.
http://www.nasa.gov/centers/dryden/p...ain_H-2179.pdf
The inlet spike (fig. 4) translates longitudinally, depending on Mach number, and controls the throat area. The spike provides efficient and stable inlet shock structure throughout the Mach range.
At the design cruise speed, most of the net propulsive force derives from flow compression pressure on the forward facing surfaces of the spike.
At the design cruise speed, most of the net propulsive force derives from flow compression pressure on the forward facing surfaces of the spike.
Last edited by Brian Abraham; 11th Jul 2009 at 05:14. Reason: Add NASA bit
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Keith,
You have a fex excellent points.
However, my personal knowledge of internal aerodynamics is limited, so all I can do is quote existing litterature, which all agrees on supersonic intakes producing a significant part of the thrust, be it SR-71, Concorde, or military jets.
Brian,
You found a rather interesting quote:
"Ben Rich.... refers to the compressor as "a pump to keep the inlets alive"
It should answer awblain's query:
"If the section of wing incorporating the inlet was broken clear from a Concorde cruising at M2, all the way back to the compressor face, would the released inlet really accelerate forward due to this inlet thrust...."
The answer is 'yes' , on condition that you have some way to continue "sucking" out the same amount of air from the back (at the compressor face location) to maintain the flow conditions inside the inlet itself.
Brian,
Your initial quote in italics does really only apply to the SR-71.
On Concorde virtually all the air from the intakes goes through the engine, and only a small amount is bled around the engine to cool the engine accessories.
Also, Concorde does not use reheat (afterburners) in cruise at Mach 2, only during supersonic acceleration to "get over the hump".
CJ
You have a fex excellent points.
However, my personal knowledge of internal aerodynamics is limited, so all I can do is quote existing litterature, which all agrees on supersonic intakes producing a significant part of the thrust, be it SR-71, Concorde, or military jets.
Brian,
You found a rather interesting quote:
"Ben Rich.... refers to the compressor as "a pump to keep the inlets alive"
It should answer awblain's query:
"If the section of wing incorporating the inlet was broken clear from a Concorde cruising at M2, all the way back to the compressor face, would the released inlet really accelerate forward due to this inlet thrust...."
The answer is 'yes' , on condition that you have some way to continue "sucking" out the same amount of air from the back (at the compressor face location) to maintain the flow conditions inside the inlet itself.
Brian,
Your initial quote in italics does really only apply to the SR-71.
On Concorde virtually all the air from the intakes goes through the engine, and only a small amount is bled around the engine to cool the engine accessories.
Also, Concorde does not use reheat (afterburners) in cruise at Mach 2, only during supersonic acceleration to "get over the hump".
CJ
What's wrong with maths ?
Without a heart I couldn't think, but does my heart think ?
Thrust = force.
Take any geometry inlet you like (without the engine behind it), drop it into an airstream and what will happen ? Will it zoom off on its own ?
Take an undriven compressor stage of an engine and put it in the airstream. The airstream will drive it so it windmills. You could get electricity from that with a generator: it's taking energy from the air, slowing it down, and at the same time feeling the drag/push back.
In compressing a gas work is done on the gas be it by a fwd moving nozzle or driven turbine blades. In doing that work you expend energy. If all you have is kinetic energy you will slow down. Drag if you like, but not thrust.
The issue isn't 'maths' its 'concepts'; that's the source of the difficulty. Took the human race ~7000+ years to get to Newton's simple equations (which are actually definitions of the constructs of force and inertial mass), but once they were accepted look what happened in the next 300 years. Those concepts are deceptively simple looking: it's not human nature to think that way.
All these devices heat gas and then extract useful work from it. How that produces 'thrust' (ie a force) is easiest to see for a rocket motor with the distribution of pressure. However, perhaps imagine that the 'inside surface' - engine side- of the compressor blades and the 'inside' surface of the inlet are the surfaces upon which the pressure acts: the increased pressure produced by burning fuel in the gas and heating it, and the pressure produced by driving the compressor stage.
To say that x% of thrust comes from this and y % comes from that only makes sense if the thing is a whole: this and that wouldn't produce the thrust without everything else maintaining the pressure differences.
Forget jet engines and rockets. Get in your car and drive. Put your foot down and accelerate. How ? Must be a fwd acting force on the car. Assume there's no atmosphere (eg near Swindon). where did this force come from ? Must be tyres on road as there is nothing else, agreed ? Take your foot off and you decelerate. How ? Must be tyres on the road - agreed ? One scenario the tyres are a source of thrust, other they are a source of drag.
Hmmm. Given that to avoid slipping the rubber in contact with the road is moving at the same speed as the road, what mechanisms are these that produce thrust and drag ?
Think compression and tension in the tyre at the zone of contact (producing pressure at the interface).....
Thrust = force.
Take any geometry inlet you like (without the engine behind it), drop it into an airstream and what will happen ? Will it zoom off on its own ?
Take an undriven compressor stage of an engine and put it in the airstream. The airstream will drive it so it windmills. You could get electricity from that with a generator: it's taking energy from the air, slowing it down, and at the same time feeling the drag/push back.
In compressing a gas work is done on the gas be it by a fwd moving nozzle or driven turbine blades. In doing that work you expend energy. If all you have is kinetic energy you will slow down. Drag if you like, but not thrust.
The issue isn't 'maths' its 'concepts'; that's the source of the difficulty. Took the human race ~7000+ years to get to Newton's simple equations (which are actually definitions of the constructs of force and inertial mass), but once they were accepted look what happened in the next 300 years. Those concepts are deceptively simple looking: it's not human nature to think that way.
All these devices heat gas and then extract useful work from it. How that produces 'thrust' (ie a force) is easiest to see for a rocket motor with the distribution of pressure. However, perhaps imagine that the 'inside surface' - engine side- of the compressor blades and the 'inside' surface of the inlet are the surfaces upon which the pressure acts: the increased pressure produced by burning fuel in the gas and heating it, and the pressure produced by driving the compressor stage.
To say that x% of thrust comes from this and y % comes from that only makes sense if the thing is a whole: this and that wouldn't produce the thrust without everything else maintaining the pressure differences.
Forget jet engines and rockets. Get in your car and drive. Put your foot down and accelerate. How ? Must be a fwd acting force on the car. Assume there's no atmosphere (eg near Swindon). where did this force come from ? Must be tyres on road as there is nothing else, agreed ? Take your foot off and you decelerate. How ? Must be tyres on the road - agreed ? One scenario the tyres are a source of thrust, other they are a source of drag.
Hmmm. Given that to avoid slipping the rubber in contact with the road is moving at the same speed as the road, what mechanisms are these that produce thrust and drag ?
Think compression and tension in the tyre at the zone of contact (producing pressure at the interface).....
Last edited by Mr Optimistic; 12th Jul 2009 at 14:40.
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Originally Posted by Mr Optimistic
The issue isn't 'maths' its 'concepts'; that's the source of the difficulty. [....] Those concepts are deceptively simple looking: it's not human nature to think that way.
"What falls faster in a vacuum, a feather or a stone?" The 'intuitive' answer is 'a stone' because it's heavier. It takes a moment's thought (or a demo) to arrive at the right answer.
Think of that other ancient trick often used to demonstrate how a wing works.
Take two sheets of stiff paper.
Fold over one edge of each so you can hang them over two pencils.
. ^ ^
. | |
Now curve the two sheets.
. ô ô
. ) (
(I suppose you get the idea, I'm too lazy to do a drawing and post it...)
Now blow down between the two sheets.
The 'intuitive' answer is that blowing between them will push them apart.
Again, it takes thought, and knowing the airflow in the resulting 'venturi' causes a pressure drop, that pushes the two sheets together, to arrive at the right answer.
Or take a wing... again the 'intuitive' notion is that it's the 'wind' pushing on the underside of the wing that produces the lift.
It's not until you actually see a picture of the pressure distribution over a typical lift-producing wing, that you realise it's the lower pressure on the upper surface which produces nearly all the lift, rather than the higher pressure on the lower surface.
With intakes (supersonic intakes in particular) it's much the same story.
The fact that "sucking" air through a con-di intake (whether with ramps or a spike doesn't matter) will result in a forward force on the intake structure (hence thrust) is at first counter-intuitive, yet it's a proven fact.
I was rather hoping somebody here (this is Tech Log, after all!) would come up with a drawing of the pressure distribution in a supersonic intake, much like the wing pressure distribution I mentioned above.
After all, a drawing says more than a thousand words.
CJ
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Originally Posted by Mr Optimistic
these guys didn't have a diagram either
The long and short is about the same... put a donk behind a supersonic intake to "suck" the air through, and the intake will produce thrust.
Some of us will accept that the pressure distribution in the intake will be such that it'll produce thrust, others would like to see a diagram to better understand.
Personally I have not the slightest difficulty in accepting the facts, but I would love to see some diagrams to better get my mind round it
CJ
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Could it be that the supersonic intake produces thrust in an indirect way, rather like the convergent propelling nozzle?
The force on the propelling nozzle acts rearwards, but the overall effect of the nozzle is to increase the total thrust. But the extra thrust does not act directly on the propelling nozzle.
The Intake Momentum Drag thread cited by Mr Optimistic above includes the following text.
A pressure of 18 psi inside the intake sounds impressive, particularly when the ambient pressure is only 0.4 psi. But is it really as good as its sounds?
Let’s start by assuming that the external (front facing) area of the intake is equal to the internal (rear facing) area. Any pressure on the external surface will produce a rearward acting force and any pressure on the internal area will produce a forward acting thrust force. The relative magnitudes of these forces will be determined by the relative magnitudes of the pressures.
The pressure pushing aft on the external surface is static pressure plus dynamic pressure.
But dynamic pressure acts only in the downstream direction, so the pressure pushing forward on the internal surface will only be the (considerably increased) static pressure.
If the total pressure has remained constant as the air flowed into the intake, then the total pressure inside will be equal to the total pressure outside. This means that the static pressure pushing forward on the inside surface can be equal to the static plus dynamic pushing aft on the outside, only if all of the dynamic pressure has been converted into static pressure. This would require the air to be brought to a full stop inside the intake. This clearly does not occur. So the rearward force exerted on the external surfaces must be greater than the thrust force exerted on the internal surfaces. This does not explain where the extra thrust comes form.
In reality the situation will be somewhat worse because some of the total pressure is lost when air flows through shockwaves.
So by what other means might the supersonic intake be producing extra thrust?
Let’s look again at that quote.
That represents a pressure ratio of 45 to 1, which is greater than that achieved by many jet engine compressors. What the intake has done is to recover kinetic energy from the incoming airflow and convert it into static pressure energy.
This will have (at least) the following effects.
A. Increase the air density and mass flow rate of air passing through the engine.
B. Increase the pressure of the gas at all of the subsequent stages of the engine. If the compressor pressure ratio is 20 to 1 for example, then the pressure at the compressor outlet would be 360 psi with the intake but only 8 psi without it.
Both of these factors will increase the thrust produced by the engine. But the extra thrust will not actually act upon the intake.
This explanation might also explain why the extra thrust is lost if the engine is shut down. Without the benefit of the secondary compression process carried out by the engine, and the additional energy provided by the combustion of fuel, the kinetic energy that has been recovered from the incoming air is simply wasted windmilling the engine.
The force on the propelling nozzle acts rearwards, but the overall effect of the nozzle is to increase the total thrust. But the extra thrust does not act directly on the propelling nozzle.
The Intake Momentum Drag thread cited by Mr Optimistic above includes the following text.
“The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi. This extremely large pressure differential led to a forward thrust vector .”
Let’s start by assuming that the external (front facing) area of the intake is equal to the internal (rear facing) area. Any pressure on the external surface will produce a rearward acting force and any pressure on the internal area will produce a forward acting thrust force. The relative magnitudes of these forces will be determined by the relative magnitudes of the pressures.
The pressure pushing aft on the external surface is static pressure plus dynamic pressure.
But dynamic pressure acts only in the downstream direction, so the pressure pushing forward on the internal surface will only be the (considerably increased) static pressure.
If the total pressure has remained constant as the air flowed into the intake, then the total pressure inside will be equal to the total pressure outside. This means that the static pressure pushing forward on the inside surface can be equal to the static plus dynamic pushing aft on the outside, only if all of the dynamic pressure has been converted into static pressure. This would require the air to be brought to a full stop inside the intake. This clearly does not occur. So the rearward force exerted on the external surfaces must be greater than the thrust force exerted on the internal surfaces. This does not explain where the extra thrust comes form.
In reality the situation will be somewhat worse because some of the total pressure is lost when air flows through shockwaves.
So by what other means might the supersonic intake be producing extra thrust?
Let’s look again at that quote.
“The inlet system created internal pressures which reached 18 psi when operating at M 3.2 and 80,000ft, where the ambient pressure is only 0.4psi.”
This will have (at least) the following effects.
A. Increase the air density and mass flow rate of air passing through the engine.
B. Increase the pressure of the gas at all of the subsequent stages of the engine. If the compressor pressure ratio is 20 to 1 for example, then the pressure at the compressor outlet would be 360 psi with the intake but only 8 psi without it.
Both of these factors will increase the thrust produced by the engine. But the extra thrust will not actually act upon the intake.
This explanation might also explain why the extra thrust is lost if the engine is shut down. Without the benefit of the secondary compression process carried out by the engine, and the additional energy provided by the combustion of fuel, the kinetic energy that has been recovered from the incoming air is simply wasted windmilling the engine.