Mr Optimistic

I agree that most (but not all) of the rearward force on the nozzle is produced by the pressure in the jet pipe. But that does not mean that we cannot call it drag. Isn't Form drag caused by pressure acting on the forward facing surfaces of an aircraft?

I used the term DRAG because the force on the nozzle acts rearwards. If you prefer to use another word then you are free to specify one.

But the important point as far as this discussion goes is the fact that the force on the nozzle acts reawwards.

The pressure is actually acting on the forward facing surfaces of the nozzle. That's why it tends to push it rearwards.

Agrred, but does not answer the question.

Agreed, but still does not answer the question. IE. On what part of the engine does the thrust force that has been produced by accelearting gas through the nozzle act.

The nozzle would probably fly rearwards. The exception would be the a condi nozzle in which case the direction would depend on the relative magnitudes of the forward forces acting on that part of the nozzle downstream of the throat, and rearward acting forces on the part upstream of the throat. My vote would go for it flying reaward in most cases.

I do not agree.


I said that we don't neeed to consider a condi nozzle. You are correct that the pressure downstraem of the throat is greater than ambient. That is why it creates thrust on that part of the nozzle that is downstream of the throat.

Agreed. That "engineering factor" would include ensuring that the attachment of the nozzle was strong enough to stop it being torn off and thrown rearward.

But none of your post addresses the question at hand. On waht part of teh engien does the thrust caused by acceleration of air thropugh the nozzle actually act?

pressure
 

For the balloon, if the thing is tied then the internal pressure acts equally in all directions on the inner surface. Open the back end and there is an unequal distribution. Same with rocket engine ie its the net force obtained by the pressure in the chamber. Only force causes acceleration but conservation of momentum or energy often gets the result easier, however it is always a net force.

In a turbojet, not sure but there is the bypass flow generating a differential pressure and the reaction force on the fan blades (as someone else said).

Had to do the pressure integration myself a generation ago, for a rocket.

That was at Caltech, btw. Applied Physics. Doesn't make me infallible however.

Amusing how big fans, subsonic jets, supersonic jets, ramjets and rockets are all being dumped in the same basket when it comes to the 'internal dynamics'.

CJ

You are certainly right there ChristiaanJ.

There is no need to complicate the question with such things. In fact nothing useful will be achieved by doing so.

To simplfy the question as much as possible let's start by considering a metal cylinder which is closed at the front end and has a convergent propelling nozzle at the back end.

If the pressure in the cylinder is greater than that outside, then the air in the cylinder will be accelerated rearwards through the nozzle. This will produce a forward acting force which we conventionally call thrust.

But the force acting on the nozzle will be acting rearwards, so let's call it drag. Or if you prefer let's call it the Rearward Acting Force or RAF (a pretty appropriate abreviation really... the RAF always was a rearward acting force).

Now the thrust isn't pushing forward on the nozzle, so where exactly is it pushing forward?

If we can solve that one, then we can look at the slightly more complicated case of a zero by-pass jet engine where the front end of the jet pipe houses the exhaust unit and the turbines.

No maths required here, just a little bit of imagination and no emotion whatsoever (GRRRRRRR).

sorry
 

...get a bit stressed when at work. Wish I could draw a picture but skills not good enough. Reason for adding in the rocket etc was that the same principles apply and thought it easier to picture. The pressure on the inside of the nozzle will be backwards (normal to wall ignoring friction), so if the nozzle broke it would fly backwards. However there is a hole in the nozzle. View the whole thing as a lump of dumb metal: it doesn't 'know' about the conservation of momentum, the only thing which affects it are the forces acting on its physical surfaces and all forces count.

Now release your grip on the cone a little bit and blow really hard until you no longer hold the nozzle.

If the nozzle is producing thrust it will force its way into your mouth.

It is producing thrust and pushing it and you backwards slightly.

But it will actually fly out and away from you.

Well yes, because of the pressure on the inside acting backwards. Because there is no front surface (where your mouth just was) there is no front surface on which the internal pressure can act so all forces are rearward. Same is true in a rocket nozzle except that there is a front surface of the whole chamber opposite the nozzle on which the pressure can act, so the force on the nozzle (and any back face) is indeed rearwards but then there is this hole (no metal so no force), imbalance = thrust.

This is because the aerodynamic force on the nozzle is drag acting downstream.

Conventionally drag infers force due to motion, motion gives shear, shear gives lateral force on surface. Its pressure which gives the overwhelming force.

So if the force on the nozzle is drag, how does it increase the thrust?

Need a way of having hole at the back while letting gas out while keeping internal pressure high at that all important front internal surface (where the pressure pushing forward on the wall acts).

Nozzle is a means to an end, net thrust is generated elsewhere.

Always pressure, - don't shout at me for introducing kitchenware as well as bypass engines- but if you have a cupboard with two doors, slamming one shut may pop the other open but its not the 'rush of air' which pushes, its the rush of air which decelerates on getting to the door which then increases in pressure until the pressure pushes the door.

Keith,

Your 'cylinder' simplification takes us right back to this ancient picture (sorry, only a scribble on-the-fly)..

http://img.photobucket.com/albums/v3...iaanJ/puff.gif

Where does the thrust come from? From the pressure/force on the left side, which is not compensated by an equivalent pressure/force on the right.

Now if this were a proper rocket, with a con-di nozzle, and enough chamber pressure for the throat to go Mach 1, you'd get some additional thrust from the expansion in the nozzle (why does the shuttle engine have those large bell nozzles? It does help ...).

CJ

CJ
 

nice picture, indeed so, but wanted to get to the main point of the pressure at the front acting on the wall so could then expand to pressure on the back of turbine blades pushing fwd, on back of arms when swimming, pushing fwd etc. Sorry I got a bit priggish, it isn't rare behaviour afraid.

Now if you ask a) what makes a 'good' nozzle and b) how do you know you have made a good one....don't know, tried looking for this myself some months ago co-incidentally (and failed).

Re: 47: Intake and turbine
 

I completely agree with the picture in 46. The uncompensated pressure on the left-hand wall of the balloon and the change in momentum of the escaping gas are equivalent ways to get the leftwards thrust force.

To try to draw a link to the ice-cream cone, if you were to snip off the balloon's blowing-end, it would be blown off to the right, because the air is trying to drag it to the right, and being slowed down in the process.

-

Now, the main point I wanted to make is about the general discussion about thrust from supersonic intakes and the thrust on turbine blades mentioned in 47.

The turbine extracts energy from the exhaust, slowing it down. This reduces the rearward momentum of the exhaust, requiring a forward force on the air, and Sir Isaac demands a rearward force on the turbine in reaction. The exhaust stream does not push the turbine into the engine, it tries to pull it off. You could also consider this a pressure drop from upstream of the turbine to downstream.

I think if you have `front of the combuster' instead of `the back of turbine blades' in the first paragraph of post 47 then the statement is true. When you're swimming you're accelerating water backwards with your arms: that's an analogy to your arm being a fan blade rather than a turbine blade.

-

For supersonic intakes, I don't think that the idea that they produce thrust is correct. In the intake the airflow is slowed to allow subsonic combustion. This is the same as the case for the turbine: the rearward momentum of the airflow is reduced, pushing the intake backwards. This typically happens, along with direction changes, at carefully-controlled shocks. Only by then burning fuel in the flow is it reaccelerated to ensure a net gain in rearward momentum through the whole engine, and thus a forward thrust. I can't see how any arrangement of components, ramps and shocks can extract thrust from slowing an airstream.

I suspect that if the J58's rotating core in an SR71 is producing drag at M3, then it must be providing mechanical power to a compressor to get enough airflow to the ramjet-like afterburner that actually provides the thrust.

Where is this darn force acting?

I agree with the nice balloon picture and made many an eggshell steam cart as a kid. I was told in my service days that the thrust was felt on all those parts that add energy to the gasses as they transit through the engine. I always have trouble with this when I see a Harrier in the hover as quite frankly the thrust is acting at 90 degrees to those previously mentioned parts. My mental picture now is of the thrust acting on the area of the Nozzle. not the surface but the hole as it were.

Harrier swivel nozzle?
 

In the four swivel nozzles the fan/turbine exhaust is turned ~90 degrees. Changing the momentum of the air from horizontal to vertical requires a force on the air pointing inboard at 45 degrees down from the horizontal. Sir Isaac arranges for an equal opposite force on the nozzle: 45 degrees up from the horizontal and outboard. Each nozzle supports about 1/4 of the aircraft weight in the hover, so the thrust acts up through the nozzle hinge with ~4000 lb of force, and also tries to rip each nozzle off the aircraft with ~4000 lb. I reckon you could probably get an insight from the force required to hold a sharply bent firehose steady.

OK,

I think that most of us have now come to the conclusion that

A. The thrust force acts on the front of the chamber (in a simple rocket).
In a jet pipe it acts on the exhaust unit (which is just behind the rear
turbine), and on the rear face of the rear turbine.

B. The force on the nozzle acts rearwards.


Logically we can say that the resultant forward thrust is A minus B


This suggests that having the nozzle actually reduces the resultant thrust.


Which brings us to the real question.

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.

Once again, no heavy maths required (anyone who integrates will be excluded), just imagination and logical thought processes.

no nozzle = explosion
 

must be some measure of nozzle efficiency/effectiveness but no clear idea of it.

not the surface but the hole as it were.

To accelerate the body a force must act on the body. Application of 'control surfaces' often simplifies the maths but can mislead. The body doesn't 'know' what some parcel of ejected air is doing but remote parts of the air affect the air next to it which can be tracked back to the air next to the body and then to the body itself. Continuity. So in terms of whats happening at the hole, specifically 'no' but everything is connected to something else !.

On reflection, can't see that helping !

When I say "Why don't we just remove the nozzle?" I don't mean "why not get rid of the exit hole". If we consider a typical jet pipe I mean "why not just leave it parallel"?

We have already established the fact that the thrust is caused by the increased pressure acting on the aft facing surfaces of the chamber.

The forward thrust is the Newton3 reaction to the rearward force that we have applied to the air in the jet pipe. It is the increased pressure in the jet pipe that is producing both the rearward acceleration of the air and the forward thrust. The increased pressure is tending to accelerate the air rearwards and accelerate the engine forwards.

The fact that the air gets accelerated rearward is (in a sense) just a by-product of the process.

The fact that we can use the eqaution T = mA to calculate the magnitude of the thrust does not mean that the acceleration actually causes the thrust. We could just as easily use the same equation to calculate the acceleration, but that does not mean that the thrust caused the acceleration, nor that the thrust and acceleration caused the mass.

I am not suggesting that getting rid of the nozzle would increase the thrust. It would not.

What I am doing is inviting readers to think about why we need the nozzle.


If

A. The thrust force acts on the front of the chamber (in a simple rocket).
In a jet pipe it acts on the exhaust unit (which is just behind the rear
turbine), and on the rear face of the rear turbine.

B. The force on the nozzle acts rearwards.

Logically we can say that the resultant forward thrust is A minus B

This suggests that having the nozzle actually reduces the resultant thrust.


Which brings us to the real question.

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.

Never a truer word spoken. It's amazing how often people think that F=MA implies the F is caused by A, rather than the other way round.

The fact that this misconception is so common is even more remarkable when we consider how easy it is to disprove it.

A runner on firm ground is clearly exerting a rearward force on the ground and the Newton3 reaction to that force is pushing him/her forward. But there is no detectable rearward acceleration of the ground.

The same runner on loose gravel is clearly accelerating some of the gravel rearwards, but he/she finds the going much more difficult. It requires much more effort to achieve the same speed.

This suggests that the rearward acceleration is actually an indication of the inefficiency of the propulsion process. It is.

<nod> Big M, small change in V, for efficiency.

Tie it into energy cost, 1/2mv^2 and pretty much everything about propulsive efficiency drops into place. Not to mention induced drag versus airspeed.

Trouble is, to really 'get' it, the student needs to be fluent with the conceptual differences between momentum and kinetic energy. Unfortunately many think that because they both have an M and a V in them they are basically interchangeable :(

pb

gravel
 

..because the loose surface can't sustain as high a load so get skidding/slipping. Running on water is more difficult, only a couple of us have managed that.

harrier
 

...as for the harrier the upward force in hover must be on the nozzle surfaces. Do the nozzles have to rotate slightly past the vertical for a stationary hover ? (as I would expect a residual fwd thrust).

Re:51 why a nozzle?
 

Nozzles are necessary evils?

Rocket expansion nozzles lose a lot of thrust, but they are necessary to be able to gimbal the exhaust accurately and ensure control of the whole vehicle. Also, as stated by Mr Optimistic, straight release of the 1000 atmosphere through a 2-foot hole would be an explosion. Stable combustion requires a steady flow. Also, they reduce and control instabilities in the shear layer between the exhaust and air and prevent acoustic damage.

The likely 2:1 thrust loss I quoted previously for the shuttle main engine is much much more than in a jet engine. It might be 800,000lb, but acting over the 2.4-m wide, 3-m long nozzle, it's only a viscous force of ~ 6 lb / square in. The exhaust speeds and pressures in even a military engine are much less, and so is the jetpipe surface area, leading to a much smaller fractional loss.

Note to ChristiaanJ: I accept this is ignoring changes in internal energy in the gas from temperature-pressure-density, the careful manipulation of which can give an efficiency gain.

I'm afraid that what you 'think', or 'can't see', doesn't change the facts.
I would suggest you dive a bit more into the relevant litterature....

On Concorde at Mach2, roughly a quarter of the thrust came from the intakes.

And this is no theoretical speculation..... to the structural engineers it was all hard fact, because those thrust forces had to be "led" from the intake structure into the wing structure and the rest of the airframe.....

CJ

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.

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. :ugh:

I think I'll unsubscribe...

CJ

now we can all play
 

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?

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.

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.

The NACA cowl used on radial engines is a good illustration of this. I once had occasion to ferry a vintage SE ship with the cowl missing. It cruised 15 kt slower than usual; the cowl contributed substantial thrust, as well as reducing external drag.

A more complete history of the NACA cowl see: N.A.C.A. Cowling

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.

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.

for rockets, this is about as clear as it gets
 

Mr Optimistic,

As I said ... PUFF

Maths or no maths....

CJ

CJ, yeah but I couldn't let go
 

I still think this is a wind-up

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.

Some sources say, without references, that "It is commonly cited that a large amount of the thrust at higher mach numbers comes from the inlet. However, this is not entirely accurate. Air that is compressed by the inlet/shockwave interaction is diverted around the turbo machinery of the engine and directly into the afterburner where it is mixed and burned. This configuration is essentially a ramjet and provides up to 70% of the aircraft's thrust at higher mach numbers."

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

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

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).....

You're right, the difficulty is that some concepts are counter-intuitive.

"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

these guys didn't have a diagram either
 

No, but with John Farley and Bellerophon taking part in the discussion, there were two people who know what they are talking about.

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

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.