Can Vmg exceed the V of a jet exhaust?

[RJM]

As a lowly PPL who occasionally sits behind a piston engine, a question about jet turbines has been bothering me: Is the velocity of the output ie exhaust of a jet engine equal to the aircraft's Vmg in still air once it is cruising? If it's not, then how does an aircraft achieve, say, Mach 2 with a jet exhaust slower than Mach 2? If it does, is it something to do with acceleration? Please excuse the ignorance!

[renard]

A little bit rusty, and hopefully there will be some better answers to come, but here goes.

The aircraft can't fly faster than the exhaust velocity of the jet engine.

The thrust of the jet engine is mass flowrate x (exhaust velocity - inlet velocity)

If the aircraft moves faster than the exhaust you would get negative thrust, and the aircraft would go into reverse.

[hawk37]

The exhaust gases must have some rearward component, relative to the ambient air the aircraft is flying through. In that case, an aircraft at mach 2, would indeed have a speed differential compared to it's exhaust gases, of greater than the aircraft's true airspeed. But be careful about speeds here. There's an aircraft at 600 ktas (mach 1?) at -57 deg C, and there's exhaust gases at 600 ktas (mach .8?) at +900 deg C.

Same as a piston/propellor combination. With respect to the air mass the aircraft is flying in, if some air isn't being sent backwards, there's no forward thrust.

[RJM]

So as well as exiting the nacelle at some speed, the exhaust air exits both compressed and hot, with those factors adding to thrust?

[ChristiaanJ]

RJM,

renard already gave the basic answer and the basic formula.

A given mass of air per second comes in through the intake (at what is basically the airspeed) and gets ejected through the exhaust. So to get thrust (forward force) you have to accelerate that air backwards, i.e., it the jet velocity has to be higher than the airspeed.

(Note that "mass flow in" = "mass flow out" ; the mass of the fuel added is negligeable. It's the energy of the burning fuel that accelerates the air, plus the shape of the exhaust nozzle.)

"...the exhaust air exits compressed..."
Ideally, no. You let it expand in the nozzle to the outside atmospheric pressure, thereby accelerating it.
If the pressure at the end of the nozzle is still higher than the atmospheric pressure, the jet will expand further behind the nozzle, without contributing more thrust, so some of the energy in the jet is wasted.
In practice, unless you have a variable nozzle, the nozzle diameter is optimised for cruise, and you accept there will be some loss in the off-design case.

CJ

[Old Smokey]

F=MA.......... Basic Physics.:ok:

Force (Thrust) = Mass (of the Air) X Acceleration (of the Air).

Thrust (Force) is what we need for sustained flight. As mentioned earlier, the Mass of air "in" is the same as the Mass of air "out", and is thus a constant (ignoring the negligible addition of mass due to fuel added). Thus, if we want Thrust, we must Accelerate the air to a higher speed than that when entering the engine.

Regards,

Old Smokey

[RJM]

Thanks all. You've answered all aspects of my question. :ok:

[johndoe42]

Theoretically, ignoring for example drag on the airframe and internal engine parts, you can fly a small bit faster than the exhaust velocity, due to the added mass of the burnt fuel. How much depends on the ratio of air and fuel masses. At some point the deceleration of the air mass and the acceleration of the fuel reach an equilibrium, and you don't accelerate any further.

At the extreme of this air/fuel ratio, when reducing the air intake down to zero, you get a rocket engine. Rocket engines can accelerate to virtually any speed (if the fuel wouldn't run out at some point), irrespective of the exhaust velocity.

The underlying physical principle is the law of conservation of impulse. Newton's second law, as outlined above, only follows from that.

In reality however, with all the various drag forces acting on the aircraft, and with the small amount of fuel compared with the air mass, you will always fly a fair bit slower than the exhaust velocity.

[ChristiaanJ]

...you can fly a small bit faster than the exhaust velocity, due to the added mass of the burnt fuel.Just as well I read your post to the end before commenting....
You're right, of course.

At the extreme of this air/fuel ratio, when reducing the air intake down to zero, you get a rocket engine.Easier to understand, almost... you can use the "balloon" analogy. Nothing in, only "mass out".

What's not always stressed enough in the tale, is that for a jet engine it's very nearly all "mass in = mass out" (the added fuel mass being pretty well negliable), so the energy added by the fuel HAS to be converted into velocity, so the nozzle is a vital part of the system.

CJ

[james ozzie]

I think many of the posts above are incorrect (now thats asking for it!)

Consider the basic physics for a rocket - not quite the same but still a reaction motor. It is well understood that a rocket can only attain its own exhaust velocity if its full-to-empty mass ratio is equal to or better than "e" (2.718...), efficiencies aside. But if better than "e", it can and will exceed its own exhaust velocity and in fact needs to, to escape earth. This basic relationship does not depend on whether there is air present or not, other than the fact that rocket motor efficiency is degraded by the presence of external air pressure acting against the nozzle - the basic physics still applies.

Perhaps people are forgetting that what happens to the exhaust gases after leaving the nozzle is irrelevant.

This misunderstanding is what caused eminent scientists (including the Royal Observatory Astronomer Royal, no less) in the 1930s/40s to declare "Space travel is impossible". So you are in good company.

[ChristiaanJ]

I think many of the posts above are incorrect (now thats asking for it!)James,
I would say we simply shouldn't have dragged rocket engines into the discussion....
Rocket engines carry their own 'reaction mass' with them, which is then accelerated rearwards "from standstill" (relative to the vehicle).
Jet engines differ in that they get their 'reaction mass' from the outside (neglecting the tiny mass contribution of the fuel), coming in a the speed of the vehicle. So it has to be accelerated to a greater speed at the exhaust to provide a reaction (thrust). The thrust formula is fundamentally different.

CJ

[awblain]

Force (thrust) is rate of change of momentum. That's the time derivative of the product of mass and velocity coming out versus going in.

Thrust = [d(mv)/dt at exhaust] - [d(mv)/dt at intake]

In the case of constant exhaust and intake velocity, this is renard's formula from post 2.

Thrust = (v_exhaust - v_intake) dm/dt

Fuel burnt adds a little to mass, bleed air takes a bit away.

-

For a rocket, the product of m and v at the intake is zero, and the change of momentum comes entirely from the exhaust speed and the burn rate of the fuel and oxidiser.

Thrust = (v_exhaust m/s) x (burn rate - Kg/s)

-

How fast you end up going (horizontally) is controlled just by drag.

If drag is very small, then a little bit of thrust is enough to reach a speed that can be much higher than v_exhaust: in fact the exhaust velocity hasn't got much to do with it, other than for efficiency.

In the limit that drag is very high you get a test rig, where there's a substantial static force on the pylon, but no motion (OK - with the exception of a tiny change in earth rotation).

[james ozzie]

CJ - I think most of us are well aware of the differences between rocket motors and jet turbines; specifcally in the fact that the rocket carries its own reaction mass with it while the jet turbine gathers it up as it goes. No problem.

But the point of referring to the rocket motor is that it is a reaction motor, like a jet. A jet creates thrust from the reaction of expelled mass, as does a rocket. The exhaust nozzle does not know if the reaction mass has come from a tank or from the air in front of it. The point of referring to rockets was to point out that there is no fundamental reason why a reaction motor cannot travel faster than its own exhaust velocity, which is what the original Poster asked.

If I recall correctly, even the the most powerful chemical rocket fuel pairs (hyrdrogen/flourine) cannot attain an exhaust velocity as high as that required for orbit (sorry, do not have the figures) and yet a single stage to orbit rocket is feasible and was being developed. That project was dropped not because the basic physics was wrong.

Furthermore, the notion that the reaction force is related to the airspeed implies that engine thrust diminishes as air speed increases - even during take off roll. This is clearly not the case.

Thank you to the original Poster - a good discussion!

[ChristiaanJ]

...there is no fundamental reason why a reaction motor cannot travel faster than its own exhaust velocity, which is what the original poster asked.For a jet engine, there is a fundamental reason, and that is that the reaction mass flow enters at the same speed as that of the 'reaction motor' (the aircraft) and has to be expelled at a higher velocity to create thrust.

Furthermore, the notion that the reaction force is related to the airspeed implies that engine thrust diminishes as air speed increases - even during take off roll. This is clearly not the case.I think what you've missed is that the mass flow into the engine rapidly increases with the airspeed at takeoff. More mass to accelerate, more thrust.

CJ

[Mr Optimistic]

if you regard the a/c at rest and the air moving past it (at the air speed of the a/c natch), to gain thrust (ie accelerate or just overcome drag to maintain same speed), each 'parcel' of air ingested must be accelerated, so it leaves the exhaust at a speed higher than the airspeed. So in the moving a/c reference, the exhaust has a rearwards velocity relative to the undisturbed air.

[james ozzie]

Sorry CJ but I don't buy your maths. Methinks thou does't confuse a moving and static frame of reference (or something).

I remain convinced that a turbine aircraft can fly faster than its exhaust gas velocity and can see no reason why not, once you abandon the intuitive feeling that the exhaust gas has some effect after it has left the engine. I am happy to be proved wrong and I was rather hoping a mechanical engineer or physicist might come to my rescue (where are they when you need one...??) But the arguments presented on this thread so far do not convince me I am wrong. As Poirot would say: "Ze truth will reveal itself"

As a matter of interest, I had a quick squiz at the jet turbine data sheets and amongst the heaps of technical data, I could find no mention of exhaust gas velocity. Surely if it were a limiting factor in the engines use, it would be specifed?

The other point to note is that high bypass turbofans have low speed (subsonic) gas flows (albeit at a high mass transfer) and yet a high bypass turbofan can fly at M0.85 (easily) and perhaps more - as far as I can see, the only limit to higher speeds is if there is enough thrust to overcome the steeply rising drag.

I am away from the net for a day or two, so I will not be participating in this interesting discussion for a while.

[ChristiaanJ]

Sorry CJ but I don't buy your maths. Methinks thou does't confuse a moving and static frame of reference (or something).Sorry James, it's not my maths. The basic formula is already in post #2 (by renard). Otherwise Google is your friend, there's quite a lot about the subject on the the web, and clearer than I could write it.

The trouble starts when you begin describing it in words... everybody has his own way for "getting one's mind around it". After that, some find a moving reference frame easier, others a static one. The end result is the same.

Big fans (much like props) accelerate a lot of air a little, but they still accelerte it.

CJ

[james ozzie]

CJ - it sems to be just you and I left in this thread but I would really like the original poster to get the right information, since he/she posed the original question.

I have no problem with the maths in post #2 regarding mass flow and velocities. If you care to, just add a value of Mach 1 to all velocity variables on both sides of the equation (or any other velocity). Obviously, the equation still holds and Sir Isaac is happy. You may well end up with a negative exhaust velocity but all that means is that the exhaust gases are momentarily "chasing" the aircraft as they slow down - remember, they cannot affect the thrust measured at the nozzle after leaving the nozzle. By adding a velocity to the equation, all you are doing is changing from a static reference frame to a moving reference frame.

The key word is velocity, a vector quantity. There is nothing wrong with a negative velocity - it just signifies movement in the opposite direction, whereas a negative speed is undefined.

Consider for a moment the practical implications of your position: As soon as airspeed exceeds exhaust velocity, thrust goes negative (according to post #2) So in a steep dive, all our turbine driver needs to do is pull off the power to get thrust reversal! But his panel would have placards warning him of the extreme danger of throttling back in level flight, in case the dreaded negative thrust kicks in.. etc.etc.

I think awblains post says it all - it warrants a careful re-read. He/she clearly has a better grasp of the theoretical mechanics than I.

"If drag is very small, then a little bit of thrust is enough to reach a speed that can be much higher than v_exhaust: in fact the exhaust velocity hasn't got much to do with it, other than for efficiency."

To really prove this thing once and for all, does anyone out there know what the maximum exhaust velocity is of the exhaust gases in the engine powering the SR71 at M3? I would venture to say below M3 but of course we need the figures in m/sec, not Mach numbers.

[yaarpanjabi]

http://www.me.com/ro/yaarpanjabi/Galleries/100154/Slide1_1/web.png?ver=12461771980001

See if that helps.

Cheers

[james ozzie]

ypj, thanks but I could not open the link but am curious..

CJ - As you suggested, I looked a bit further on the net. Lets look at some approximate numbers to illustrate the point.

Take a JT9D - I found some Static sea level figures (not verified but believable):

Fan airflow: 1248 lb/sec @ 885 ft/sec (=270m/s)
Turbine exhaust: 247lb/sec@1,190 ft/sec (=363m/s)
Thrust 43,500 lbs.

Fan contribution to thrust: approx 3.5 x turbine exhaust contribution (by applying mass flows & velocities) i.e fan thrust is dominant.

Lets bolt this JT9D onto a heavy airliner and take off. We will apply the incorrect notion that forward airspeed must be subtracted from the engine exhaust velocity.

The captain figures out that he needs to rotate the plane at 130kt (67m/s). He also knows that his airspeed in the controlled airspace is 250kt (129m/s). He also has a barber pole on his ASI at 350kt (180m/s).

Does our captain know that when he reaches his rotate speed his trusty JT9s have already lost almost a quarter of the thrust they had when he first applied power at the start of his take off run (270m/s-67m/s)? And he has not even left the ground!

And does he know if he now loads his plane up so that Vr is increased to 175kt (90m/s) he will have lost a full third of his static thrust at a time when he really needs that thrust? Aha, easy answer - as your take off weight increases, REDUCE your rotate speed to make more thrust available!!

But wait, it gets worse. He accelerates to 250kts, maintaining (near) sea-level due to traffic congestion. Shucks, now the JT9D is only producing one half of its static thrust (270m/s-129m/s). And he has not even begun his climb yet. He asks, "Gee, with the air-resistance at 250kt, is there ANY surplus thrust left to climb with?

And could he get to barber-pole speed (180m/s), where his thrust will have now diminished to only one third of the static thrust?

The more I research this thing, the more I see it has been discussed - I think it is an old chestnut. The great Wiki has an entry which supports your view CJ but it is debated at length on the discussion tab. I think the vast majority are comfortable with the incorrect but intuitive view that forward airspeed directly reduces thrust. And, yes, I am ignoring the host of highly complex issues of air behaviour in the inlet ducts and other jet engine performance aspects about which I know nothing.

[awblain]

I think the discussion so far is getting close. Any incorrectness in the discussion so far appears to be based on terminology and modulated to account for intuition.

The gas entering the engine must leave the engine faster: unless there is acceleration there isn't any forward thrust. The exhaust jet must move backwards compared with the swallowed air. All viewers agree: people on the ground, any people floating in the air, people on the plane.

That does means that the exhaust jet has to travel in the opposite direction to the aircraft as far as the undisturbed local air is concerned, although not necessarily at any particular speed. (Thanks to the following post for prompting the edit). Viewed from the undisturbed air, the exhaust gas must move backwards to produce a forward thrust. That's not so for a rocket.

-

The extreme case of exhaust speed (efficiency) is probably the space shuttle main engine: mass flow rate at liftoff is 1000 lb/s (from a ~500s burn and ~500,000lb of fuel/oxidiser per engine); quoted thrust is 480,000 lbs/2.1 Meganewtons. 500 Kg/s mass ejected and 2.1 Meganewtons thrust implies an exhaust speed of 2,100,000/500 = 4200 m/s, which isn't going to change much with height. However, orbital speed is about 7500 m/s. So, above some point in the climb, the exhaust still moves in the same direction as the shuttle compared with the earth, by about 3 km/s when the fuel is cutoff.

-

For low-speed thought experiments, you need to be careful when quoting numbers, because the speed that air hits the fan is faster than the pitot airspeed from the nose: the airflow has already been sucked faster when it enters the nacelle.

[BryceM]

To keep the jet aircraft moving at a constant velocity, you need a force which balances the drag force.

That force is obtained from the engine exhaust.

F = Ma

That is, the force (thrust) developed by the engine depends on the mass of air it accelerates, and the size of that acceleration.

The force generated by the engine is in the forwards direction, so the acceleration of the air must be in the reverse direction.

So the air coming out of the back of the engine is moving faster than the air coming in the front.

Relative to the engine, the speed of the air coming in the front is the aircraft's airspeed; so the air going out the back has been accelerated relative to that velocity; so the air coming out of the engine is moving faster than the aircraft's forward speed.

For a rocket, the situation is completely different - you're just throwing mass out of the back to generate the force, and you don't care about its velocity (because it's starting velocity in the rocket's frame was zero, so any non-zero exit velocity means that the gases have been accelerated, meaning a forward thrust has been generated).

Not hard to understand, surely?

[ChristiaanJ]

Out of the mouth of babes....
In this case I'm strictly referring to the post title... "high school physics".
Well done, Bryce!

Go and stand on a skateboard with a bag of sand.
Throw it backwards, and you'll move forwards. Rocket engine.

Now stand on your skateboard and have somebody throw a bag of sand towards you. Catch it and you'll move backwards. Put your own energy into it and throw it backwards faster than it arrived. You'll move forwards. Jet engine.

CJ

[Brian Abraham]

Out of interest I ran the figures for the CF6 through NASAs educational software and it came up with the following
ALTITUDE SPEED CORE FUEL FLOW GROSS RAM NET
(MPH) FLOW (LB/S) (PPH) THRUST DRAG THRUST

0 0 254 13820 45,171 0 45,171
0 170 262 14062 47,415 8,716 38,699
3,000 282 251 13511 47,256 14,146 33,109
35,000 .85M 103 6501 23,183 11,382 11,801

It should be noted that the air meeting the compressor face is not at free stream velocity (speed of the aircraft). The inlet is designed to slow the airflow so that it is presented to the compressor at a speed of .5M or less, to avoid choking the compressor. The ram drag above, is the drag associated with slowing down the free stream air as the air is brought inside the inlet. Even the SR-71 at 3.2M has the air delivered to the compressor at approx .5M.

Of interest also is the J-58 installed in the SR-71. At 3.2M, 54% of the thrust is provided by the differential pressure between the internal and external surfaces of the inlet spike. Of the remainder, 17% is provided by the engine and 29% by the ejector (the afterburner, which is in ram jet mode at this speed - air being tapped off the 4th stage compressor, diverted around the turbine, and injected into the afterburner)

I've tried reformatting with little luck - hope you can make sense of it.

[james ozzie]

OK, I am convinced now! The post from Brian Abraham above shows quite clearly how a forward speed increase at sea level directly and signifcantly reduces net thrust, which concurs with the principle that exhaust speed cannot be less than forward speed. So I am happy to admit I have learned something, which in retrospect I should have been able to figure out.

Thank you to those posters who so patiently took the trouble to explain it and to those who were able to resist the temptation to be condescending.

Most importantly, I thank the original poster for initiating the discussion and I hope he/she enjoyed the postings.

[Keith.Williams.]

Ok, so now that we have got that sorted out, let’s look a bit more closely at the propelling nozzle.

Most texts tell us that the nozzle produces forward thrust by accelerating the air rearwards.

But if we examine it closely we will find that the aerodynamic force on the nozzle is actually trying to tear it off the jet pipe and push it rearwards.

Anyone who does not believe this can try the following simple experiment.
Buy an ice cream cone and throw away the ice cream (OK you can eat it if you really must)

Now bite off the pointed end to leave a convergent nozzle.

Put the narrow end between your lips and blow through it.

You will find that a slow warm airstream comes out of it. (because it is a divergent duct)

Now put the wide end between your lips and blow through it.

This time you will get a faster colder airstream. (because it is now a convergent duct)

The above results are exactly what Bernouli would lead us to expect.


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.

But it will actually fly out and away from you.

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

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

This question is not unrelated to the subject of this thread.

[Mr Optimistic]

Pressure forces air out of the cone. It is that pressure acting on the cone which propels it away. In a jet, what ever force that accelerates the air must have a reaction, ie thrust. For a rocket you integrate the pressure over the internal space, same would work for anything else, as it must unless you want an argument with Mr Newton.

[BryceM]

Ummm... Keith - without trying to be offensive: your question, though stated reasonably enough, is not actually very reasonable.

You say 'most texts state ...' then go on to discount the information in those texts.

Maybe you meant to say 'I don't really understand this, please explain', but it reads as 'the texts are wrong - explain that!'.

You're not (I expect) denying that there is thrust (I mean, we've all seen that jet aeroplanes fly, right?) - just questioning where in the engine it appears. Which surfaces, that is, are acted upon by the exhaust to propel the aircraft?

I don't know. I would send you back to the texts you mention - if they're decent books, they'll tell you, or tell you which other book to read. From my reading, Cumpsty and Kerrbrock's books are decent starting points.

If you want a guess, based on having read a bit about how gas turbines work (but by no means being an expert of any kind) - any compressor fan (large effect in high bypass engines), plus the nozzle (small effect in high bypass engines).

[Keith.Williams.]

Mr Optimistic

I have no argument with Mr Bernouli.

And as far as I can see I have no argumnet with anything that you have said.

Pressure forces air out of the cone.

Agreed

It is that pressure acting on the cone which propels it away.

Agreed.
But it is an aerodyanmic force acting rearwards so we (usually) call it drag.


In a jet, what ever force that accelerates the air must have a reaction, ie thrust.

Agreed, provided the acceleration is rearward.

For a rocket you integrate the pressure over the internal space, same would work for anything else,

Agreed.

Now let's get back to what I actually said.

Many people believe that the force acting on the nozzle is thrust. They are wrong it is actually drag.

It is made up of friction forces, pressure forces and inertial forces acting on the forward facing (internal) surfaces of the nozzle. But the overall force acting on the nozzle is in a rearward direction.

You are correct in saying that the thrust is the Newton3 reaction to the pressure that is accelerating the air through the nozzle. But this thrust force is not acting on the nozzle.

Now the question becomes, "So if the thrust isn't acting on the nozzle, what is it acting on"?

BRYCEM
I'm not saying that the texts are wrong. Some cover this aspect of the subject and some do not.

I'm saying that many people draw the wrong conclusions from the texts. The majority of people walking away after completing a JAR ATPL Theory course (and a good many other courses) probably think that the force on the propelling nozzle is thrust.

I could of course go and read more books, but my purpose in asking the question is not get an answer. It is to get the readers to think a bit more about the subject.

[BryceM]

I could of course go and read more books, but my purpose in asking the question is not get an answer. It is to get the readers to think a bit more about the subject. Today 13:12
I could of course go and read more books, but my purpose in asking the question is not get an answer. It is to get the readers to think a bit more about the subject. Today 13:12
I could of course go and read more books, but my purpose in asking the question is not get an answer. It is to get the readers to think a bit more about the subject.

Keith - some people (not me, obviously) might construe that as you 'wasting their time', and general clever-dickness.

If you already know the answer to the question, please supply it; otherwise, what exactly are you trying to achieve, except education by irritation?

On modern (high bypass) engines, the bulk of the thrust is obtained at the fan.

I'm done with this, BTW.

[ChristiaanJ]

Many people believe that the force acting on the nozzle is thrust. They are wrong it is actually drag.Some people first define what they're talking about. A con-di nozzle above Mach 1 usually produces at least some of the thrust.

On Concorde (like the SR-71 quoted earlier) most of he thrust came from... yes, the intakes.

On the SR-71, above Mach 3 under certain circumstances, the engine itself actually produced drag, evidenced by it moving backwards on the suspension points.

CJ

[FE Hoppy]

Now the question becomes, "So if the thrust isn't acting on the nozzle, what is it acting on"?

It's a long time ago but if my memory hasn't faded too much are we thinking about a choked nozzle and an increase of pressure behind the nozzle and therefore the "thrust" acting on the surface area of the nozzle itself?

It's past midnight here so I'll sleep on it and get back to this in the morning.

[ChristiaanJ]

FE Hoppy,

Most of the maths just deals with a jet engine as a black pipe (rather than a black box :) ) with a hole at the front and a hole at the back. The slightly more subtle maths adds front and back surface and local pressure at the "interface" into the equation.

We now are getting into the internal dynamics.... I'll leave that to the experts. About the only thing I'm familiar with in that field is the classic rocket engine with a con-di nozzle with Mach 1 in the throat. Took me enough time to get my mind round it at the time, when actually "playing" with real ones.

CJ

[awblain]

Origin of most of the thrust in subsonic engines: the fan bearings.
The fan blades accelerate the air backwards, and Sir Isaac pulls the fan blades forward, dragging the aircraft along with them.

In a rocket/turbojet/ramjet, it's the combustion chamber pressure, pushing on the closed front end of the chamber, reduced by viscous forces as the exhaust streams along the nozzle, and in some cases a turbine.

Thrust from shuttle engine again: about 400,000lb. The chamber pressure quoted is about 2800 psi, the throat of the exit of the chamber is roughly 24 inches across (about 430 square inches), which would give 1,200,000lb: so the price of stabilizing (by slowing and expanding) the flow out of the nozzle seems to be 800,000lb of thrust. The nozzle is thus pulled off the engine with 800,000lb of force, while the combustion chamber is pushed up into the engine by 1,200,000lb, giving a net 400,000lb. This suggests that the link between the chamber and the nozzle need to be tougher than that between the engine and its mount. I think that makes sense. Rocket scientists?

[GarageYears]

Taking the simple approach - the expanding burning gases will take the path of least resistance, which in this case is out the rear of the engine, since the front is 'blocked' by the highly compressed air entering the engine. The difference in pressure at the nozzle exit (and fan-bypass exhaust) is the thrust produced.

The amount of fuel burnt is proportional to the additional energy imparted to the airflow (less internal friction loss, etc).

-GY

[john_tullamarine]

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

I think where Keith is going is to look at

you integrate the pressure over the internal space

A common presentation is to look at the pressure forces throughout the engine. Especially for the FJ folk, a lot of the net thrust comes from careful and clever intake design (the airflow has to be slowed down to subsonic flow at the engine intake face) and then there are bits and pieces throughout the engine where local net forces are forward or aft.

The bit coming out the back is only part of the equation ...

[Keith.Williams.]

BRYCEM
If you manage to control your emotions for a moment you will find that over-dosing on grump is clouding your vision.

Your statement that

On modern (high bypass) engines, the bulk of the thrust is obtained at the fan.

is ture but not relevant to the question

Now the question becomes, "So if the thrust isn't acting on the nozzle, what is it acting on"?

We are talking about the nozzle at the back end of the jet pipe.

By the time the air reaches this nozzle it has already passed through the low pressure turbine so its effect on the fan is almost zero (back pressure effects only).


Cristiaan J
We are not talking about a condi nozzle, just a plain old convergent one.

FE Hoppy
If we were taliking about a condi-nozzle then some of the thrust would be acting on that part of the nozzle that is downstream of the throat.

Awblain.
Your statement that

The nozzle is thus pulled off the engine with 800,000lb of force, while the combustion chamber is pushed up into the engine by 1,200,00lb, giving a net 400,000lb.

is along the right lines.

JT
You are of course correct (as usual).

But I think that your statement about

integrating the pressures......

probably leaves many readers none the wiser.



The best answers (those from which we learn the most) are often not the ones that we get from books, but the ones that we get from within our own heads. Spending a little bit of time pondering this type of question can often be very useful (even if the questions are sometimes rather irritating).

[john_tullamarine]

But I think that your statement about


Quote:
integrating the pressures......
probably leaves many readers none the wiser.


Keith's wisdom supersedes my writing with the brain only partly engaged ..

Folk should recall that fluid flows are about pressures. Pressure gradients and deltas generate fluid flow. Static and flowing fluids generate forces on things like engine bits (and wings, tails, fuselages, etc.) by exerting pressure on surfaces.

Pressure acting over a surface (area) results in a force. Depending on the net orientation of the bit of surface in question (in, say, an engine) the force associated with the fluid pressure will have a net forward or aft direction (and, mostly, a lot laterally .. with which we are not terribly interested).

When one adds up all the bits of forces so calculated, one gets a net thrust (forward or aft).

"Integration" is a mathematics buzzword which really just means "adding up all the little bits and pieces" in the calculation. It comes from the integral calculus which is a really neat way of doing a lot of this stuff but it's not necessary for the pilot folk to have any competence in the mathematics per se.

[ChristiaanJ]

Thanks, John.... you've now probably confused even more of the readers even more....

Integrating from intake to exhaust is a nice idea but, more often than not, you don't have enough pressure data to do more than an approximation.

To me, T = m* x deltaV does it every time, with the small refinements where needed.

Otherwise, I just like looking at shock diamonds, or at the plume during a shuttle launch, when the air pressure becomes less than the pressure at the nozzle end.

CJ

[Mr Optimistic]

The high pressure in an engine or jet would tend to push the back end off: nothing to do with drag force on the nozzle surface, just pressure acting on the rear surface. Nozzle turns pressure into flow speed. Blow up a balloon, turn it around so it faces away from you and let go of the untied end. Would it fly away from you owing to drag over the aperture or into your face because of the thrust ? Put a weak joint between the nozzle of a rocket motor and the case and see what happens (owing to the pressure inside the case) - from half a mile away that is. And the adiabatic expansion or compression on flow through a cone would cause no discernable heating or cooling (convergent at subsonic speeds would in any case heat). Don't need de laval nozzle theory here. NB look at most rocket plumes, they bulge out ie exit pressure is still above ambient. Small drag losses in a nozzle I suspect are just accounted for by an 'engineering factor' in practice.

[Keith.Williams.]

Mr Optimistic

The high pressure in an engine or jet would tend to push the back end off: nothing to do with drag force on the nozzle surface,

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.

just pressure acting on the rear surface.

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

Nozzle turns pressure into flow speed.

Agrred, but does not answer the question.

Blow up a balloon, turn it around so it faces away from you and let go of the untied end. Would it fly away from you owing to drag over the aperture or into your face because of the thrust ?

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.

Put a weak joint between the nozzle of a rocket motor and the case and see what happens (owing to the pressure inside the case) - from half a mile away that is.

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.

And the adiabatic expansion or compression on flow through a cone would cause no discernable heating or cooling (convergent at subsonic speeds would in any case heat).

I do not agree.


Don't need de laval nozzle theory here. NB look at most rocket plumes, they bulge out ie exit pressure is still above ambient. Small drag losses in a nozzle

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.

I suspect are just accounted for by an 'engineering factor' in practice.

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?

[Mr Optimistic]

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.

[ChristiaanJ]

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

[Keith.Williams.]

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).

[Mr Optimistic]

...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.

[ChristiaanJ]

Keith,

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

http://img.photobucket.com/albums/v324/ChristiaanJ/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

[Mr Optimistic]

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).

[awblain]

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.

[FE Hoppy]

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.

[awblain]

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.

[Keith.Williams.]

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.

[Mr Optimistic]

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 !

[Keith.Williams.]

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.

[Capt Pit Bull]

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.

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.

[Keith.Williams.]

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.

[Capt Pit Bull]

<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

[Mr Optimistic]

..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.

[Mr Optimistic]

...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).

[awblain]

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.

[ChristiaanJ]

For supersonic intakes, I don't think that the idea that they produce thrust is correct...... I can't see how any arrangement of components, ramps and shocks can extract thrust from slowing an airstream. 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

[awblain]

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.

[ChristiaanJ]

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

[Mr Optimistic]

Interactive Nozzle Simulator (http://www.grc.nasa.gov/WWW/K-12/airplane/ienzl.html)

[awblain]

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?

[Brian Abraham]

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 (http://www.grc.nasa.gov/WWW/K-12/airplane/shortp.html)
During the supersonic cruise Concorde derived its thrust - 8% engine, 29% Nozzles, 63% intakes.

[BryceM]

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.

[barit1]

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 NACA cowl (http://en.wikipedia.org/wiki/NACA_cowling) 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 (http://www.oldbeacon.com/beacon/naca_cowling.htm)

[awblain]

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.

[lefthanddownabit]

Quoting Keith.Williams

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.

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.

[Mr Optimistic]

http://media.wiley.com/product_data/excerpt/59/35274068/3527406859.pdf

[ChristiaanJ]

Mr Optimistic,

As I said ... PUFF

Maths or no maths....

CJ

[Mr Optimistic]

I still think this is a wind-up

[Keith.Williams.]

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 “there are books (I mentioned a couple earlier in the thread….”. 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 “pointless, wrongheaded Gedanken experiments” 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.

[Brian Abraham]

Does the intake produce thrust or drag?
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 (http://www.wvi.com/~sr71webmaster/benrich_main.htm)

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/pdf/88507main_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.

[ChristiaanJ]

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

[Mr Optimistic]

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).....

[ChristiaanJ]

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.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

[Mr Optimistic]

http://www.pprune.org/tech-log/9366-intake-momentum-drag.html

[ChristiaanJ]

these guys didn't have a diagram eitherNo, 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

[Keith.Williams.]

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.


“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 .”

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.

“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.”

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.

[ChristiaanJ]

Keith,

In VERY bief, I think you're still confusing a supersonic intake being 'dragged through the air' as such and the same intake with an engine behind it.

CJ

[ChristiaanJ]

Could it be that the supersonic intake produces thrust in an indirect way...As said before, there's nothing "indirect" about it.
The forces acting on the complete intake structure, which finally are part of the forces that propel the aircraft, are acting forward.

... 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.You're slightly confusing the issue, because we're now talking about supersonic con-di nozzles, where again the resulting forces on the nozzle structure act forward.

CJ

[Keith.Williams.]

As said before, there's nothing "indirect" about it.
The forces acting on the complete intake structure, which finally are part of the forces that propel the aircraft, are acting forward.

I am quite prepared to accept the possibility that the supersonic con-di intake produce sthrust directly, but I would like to hear an explanation of how this is achieved. If it simply a matter of the balance of static pressure ahead and behind the intake, then an explanation would help.


You're slightly confusing the issue, because we're now talking about supersonic con-di nozzles, where again the resulting forces on the nozzle structure act forward.

My only reason for including a reference to the convergent propelling nozzle is because it produces additional thrust indirectly. As we have already obeserved the force acting on the convergent nozzle is rearward acting.

I am aware of the fact that the divergent part of a con-di propelling nozzle produces thrust directly. In effect it is recovering some of the energy that would otherwise be lost when the high pressure exhaust gas expands in all directions behind the aircraft.

But even in a con-di propelling nozzle the force on the front (convergent) section is drag and only that on the rear (divergent) section is thrust. The overall resultant force is the sum of these two forces.

Similarly I accept that the force on the rear (divergent) section of a supersonic intake is thrust. But the force on the front (convergent) section is drag. Once again the resultant is the sum of the two.

What I have a problem with, is the idea that the static pressure inside the intake is somehow greater than the total pressure ahead of it.

[Mr Optimistic]

Jet engines: fundamentals of theory ... - Google Books (http://books.google.co.uk/books?id=VpJEm7cFVE4C&pg=PA64&lpg=PA64&dq=supersonic+intake+shock&source=bl&ots=z2aXC6vwEG&sig=cmcsMn0MKU8XzA9PDixHni9QdcI&hl=en&ei=F3dcSu-oEZCc8wSfsL3lDQ&sa=X&oi=book_result&ct=result&resnum=1)

[barit1]

Keith.Williams.
What I have a problem with, is the idea that the static pressure inside the intake is somehow greater than the total pressure ahead of it.

I suspect you also have a problem with the curious fact that a fore-aft-sliding bubble canopy, left in an unlatched state, is likely to be forced forward by the difference in static pressures.

[Mr Optimistic]

http://web.mit.edu/ga3/www/qual_exams/Exam-2002.pdf

see page 12 (MIT asking similar question)

Not sure I believe intakes ever produce thrust of themselves, however always ready to learn !

[Keith.Williams.]

Thanks for the link Mr Optimistic.

I was already familiar with most of it, and I am quite familiar with how a supesonic intake compresses the air.

The link confirms my earlier comment that the TOTAL PRESSURE reduces as air flows through a shockwave.

But nothing in the link explains how the thrust exerted by the STATIC PRESSURE acting on the rear surfaces of the intake, can be greater than the drag exerted by the TOTAL PRESSURE acting on the front face.

I don't think that anyone can sensibly deny that the increased pressure and density caused by the intake will increase the thrust produced by the engine.

[ChristiaanJ]

Not sure I believe ... It's a fact, and belief doesn't enter into the equation.
That's why I insisted earlier some of it is totally counter-intuitive.

...the thrust produced by the engine.Not sure this was already mentioned, but in the SR-71, in a slightly off-design state (not standard Mach 3 cruise), ALL the thrust comes from intake and exhaust and NONE from the engine. The engine actually causes a small amount of drag, and moves backwards on the engine mountings.

CJ

[Mr Optimistic]

The engine increases the enthalpy of the flow: whilst the engine may take momentum out of the gas flow the subsequent expansion in the nozzle gives the thrust. The intake prepares the flow but unless the engine increases the pressure against a surface it can't produce thrust. Otherwise I look forward to a diagram showing how this miracle can happen (hence the ram jet example which makes the principles clear and also begs for a nozzle).

[Keith.Williams.]

barit1

I suspect you also have a problem with the curious fact that a fore-aft-sliding bubble canopy, left in an unlatched state, is likely to be forced forward by the difference in static pressures.

Well, in this situation the front facing (inside surface) of the canopy is shielded by the windscreen so it is not being subjected to total pressure pushing it aft.

And the slipstream will probably be sucking air out of the cockpit. So the pressure in front of the canopy will be less that that behind it.

If you think about it for a moment you will see that this siutation is totally different to that of an intake.

Now imagine that the windscreen blows off and the canopy becomes unlocked. Which way will it now slide?

You will of course need to switch off your sarcasm mode for a moment to see what I mean.


CJ

Not sure this was already mentioned, but in the SR-71, in a slightly off-design state (not standard Mach 3 cruise), ALL the thrust comes from intake and exhaust and NONE from the engine. The engine actually causes a small amount of drag, and moves backwards on the engine mountings

I believe that at that point it is using reaheat so it is effectively using a ramjet. The ramjet then becomes the engine.

Now if the pilot were to shut down the fuel flow to the reheat would it still produce thrust?

[RJM]

I'm the OP, also responsible for post #5:

Thanks all. You've answered all aspects of my question.

In the light of recent posts, I'll retract that... :bored:

[Mr Optimistic]

The trouble is that in trying to focus on essentials the arguments become diffuse. Hence the supersonic intake producing thrust argument. If the engine can hold the shock at an advantageous position in the inlet then yes the net force obtained by integrating the pressures over the inlet can be a positive thrust but not without the engine ! The thrust is not an intrinsic characteristic of the inlet nor of the nozzle and I think that's the source of confusion.

[Brian Abraham]

We might dwell for a moment on the designers (Bob Rich) statement that the compressor is a pump to keep the inlets alive. It goes without saying that thrust produced by the inlet requires the turbo machinery in order to function, and by extension, fuel.