OK, you are asking about the bleed air, but it wasn't clear from the context of your text.

True of the compressor blades (on a turbojet) but not true of the turbine blades which do provide thrust (reaction to the acceleration of exhaust gases), just like a propeller produces a higher than freestream velocity behind it.

From what I know of the design, the "inlet air" is everything that went into the mouth of the intake. Some of that was bled off the roof of the intake (about where the flow went through Mach 1.0. That bleed air was, I believe, ducted to flow around the outside of the engine as cooling air. The top and bottom passages you refer to are two bits of the same passage - it was just split to get cooling air to the lower part of the engine.

This cooling air has to be dumped as efficiently as possible - on Concorde this was done by using it as 'secondary flow' in the final propelling nozzle. It therefore contributes to thrust eventually, but would not be considered to be part of intake thrust as being discussed here.

Not sure what you are getting at, but certainly none of the intake thrust was made by combustion - unless you count that bit of the combustion energy that was used to drive "the inlet pumps" as described earlier!

I beg to differ!. Sure the turbine removes energy to drive the compressor, but if that was all it did there wouldn't be any thrust. The whole essence of a jet engine, at least as I understand it, is that the machine accelerates the air going in at the front to give it increased momentum when it leaves the engine. From what part (of a pure turbojet) does it get that increased velocity if it isn't the turbine?

To quote from Rolls Royce "the jet engine"

Italics are my explanatory addition, bold is my emphasis

This link was taken from the Rolls Royce book "The Jet Engine". It shows where the thrust is produced in subsonic flight.

http://www.pprune.org/spectators-bal...ml#post7234049

I stand corrected!

I never thought so much of the thrust was developed in the combustion chambers. :O

.. and the thrust so described is due to the application of gas pressures to the abutting structure.

The graphic gives the story for one engine ie internal story.

Now one has to look at what is happening in the intake system forward of the engine inlet face and the exhaust ducting aft of the turbine.

With a supersonic aeroplane (particularly) there is a tremendous amount of energy in the incoming airflow. As Owain observed earlier, the engine can't accept this supersonic airflow. We have a quite critically designed and controlled set of gadgets to provide a tightly managed system of oblique (ie the shock wave is inclined rather than normal to the direction of flow - such shocks are better for energy losses) shocks (shockwaves) ending up with the flow going through a final relatively weak normal (perpendicular) shock wave before it is massaged onto the forward face of the engine compressor inlet.

The engine itself is a comparatively highly strung thoroughbred stallion and doesn't take kindly to operating conditions which are too far off the design operating conditions case .. hence the various means of dumping excess air to keep the horse happy. With a bit of careful design, the varying air pressure can be managed to create a nett forward thrust on the whole of the inlet and outlet system.

Hence the figures which have been quoted indicating that, at high supersonic speeds (especially), the engine contributes only a small proportion of the total thrust pushing the aeroplane along ...

Owain...

I agree with Easy Street. I misread and assumed you were talking about compressor blades. The turbines are extracting energy from the airflow - similar to the air rotating the propeller when you have an engine failure. It produces lots of drag.

Let's say the airplane is cruising at altitude at lower speeds and 100% of the thrust is from the turbojet. Nothing from the inlet or nozzle. The thrust is 100,000 lbs total and is consuming 100 gal/hr. Now, let's speed up to the normal cruise where the turbojet is producing 8% of the total thrust. Let's say the thrust is 200,000 lbs for normal cruise. 8% is 16,000 lbs and the corresponding fuel flow would be 16 gal/hr for that thrust. So, does this mean that only 16 gal/hr are leaving the fuel tanks when the airplane is in cruise?!

It seems ludicrous that you can get 92% thrust for free! I'm trying to understand what is going on here.

John...

I understand what you said about managing intake air so that the engine only receives subsonic air by controlling the shockwaves but why can't the engine accept supersonic air? Is it because of the shockwaves that would develop as it went through the compressor section which would severely disrupt the airflow on the compressor blades?

I understand how a divergent duct can take sonic air and increase it to supersonic but I'm still not quite clear on the intake part and how it contributes to thrust.

Just came across this. It's a bit elementary but quite good I think for getting an idea of what's going on.

Rolls-Royce: Journey Through A Jet Engine

Hi italia458,
Have a look at M2dude's post at
http://www.pprune.org/tech-log/42690...ke-thrust.html - the total "thrust" is not for free. The figures mentioned are the proportions of thrust transferred to the airframe by the individual components.

why can't the engine accept supersonic air?

In subsonic flow, the pressure waves moving upstream from an approaching body have time to cause the air to get out of the road, as it were, without too much disruption ..

However, in supersonic flow, the approaching body comes on the scene without an introduction and the flow, in deflecting abruptly, generates shocks. These are undesirable due to energy losses but can't be avoided. Hence the effort which goes into designing intakes to function as efficiently as practicable.

Now, if the incoming flow to the compressor were to be supersonic, the interactions would be complex to the point of chaotic.

The Concorde threads have plenty of commentary on intake design and might be worth a read.

As JT has explained, supersonic intakes employ a series of shock waves to decelerate the incoming air to subsonic speed. This converts most of the dynamic pressure into static pressure. The final part of the intake is a divergent duct, so as the (now subsonic) air flows through it, the deceleration process continues, causing a further increase in static pressure.

The overall result is that the static pressure inside the intake is much greater than that outside the intake. This high static pressure inside the intake pushes forward against the divergent sides, thereby exerting a forward force on the intake.

Provided the engine keeps running at a sufficiently high RPM it will draw the air out of the rear of the intake. This prevents the high static pressure from exerting a rearward force on the aircraft and also enables the air to continue to flow into the intake.

Shutting down the engine would remove the pumping effect. This would cause the airflow through the intake to break down, producing a very large pressure rise in the intake. The intake would then stop producing thrust and start producing a great deal of drag.

Italian

Hi. I consider both compressor and turbine to be propellers. One drives, the other is driven. The turbine can be considered to be windmilling.

I have a simple brain.

My stupid spell check prevents me from spelling your name correctly.

Italia

Hi Owain

It is true that Newton does not really explain Lift, but it is a much less generic way of looking at it than Bernouilli, same as Newton laws give much more detail about a body going downslope than energy conservation law does.

Darrol Stinton has a very nice explanation wich put the focus on the air particles rather than the wing. That is the best I have ever seen because the order in which events takes place is the correct one, in my opinion. That is: the wing pushes the air, the air is not easy to be compressed as a result, but pressure changes will create pressure gradients all around the wing, and the air will flow from higher to lower pressure, and the resulting motion, if we look at Newton third law, reveals the existence of Lift and Drag. According to this, the pressure changes come first and the speed changes are a result, and not the other way round, as bernouilli's explanation suggests.

Well, this one I must confess I am making an assumption. That assumption is that for a given set of conditions, the speed imparted to the air is a "given percentage". So the faster the air stream is, for that same "percentage" means that the air is imparted a higher speed. Something like: if airspeed is 100 kt, the air will be accelerated 10 kt (a 10%). If the airspeed is 200 kt, then the air will be accelerated 20 kt. Double acceleration, double force.

If I am not right, then there must be another reason why increasing speed increases lift, aside from more air being "processed" in a given instant. Otherwise speed wouldn't go squared.

I also hate the "theory" of the faster molecule above and the slower one below to reach the trailing edge at the same time, and I hate Koanda even more.

Coanda I like. But I like the best your thought that Bernoulli has it effect/cause, rather than cause, effect....

Air is 'transitional' for purposes of lift discussions, IMO. It can be thin, and weak, Stall, or solid, and abrasive, COLUMBIA. That makes it susceptible to those whose brain is well ordered, and given to Maths, and explaining things to simple people.

I am a Newton guy. Bernoulli was explained to me first by Hank, PA Captain. I revered this guy, being eleven, and worshipping all yhings flying.

I kept it to myself, but thought, Hank must be drinking again.

Ah, I see what you are getting at. It was the use of " more accelerated" that fooled me - not thinking straight ;) As you say, if you double the airspeed then the mass flow rate over the wing doubles and the downwash velocity doubles (at fixed AoA). The wing lift then increases by mass flow rate* downwash velocity i.e. by a factor of 4 (speed squared).

We might as well get the numbers right. In one of the postings in the Concorde thread M2Dude gave some figures from RR. The cruise fuel flow was 9700 lb/hour/engine and the engine thrust was quoted as 8050 lb, but this may have included the secondary nozzle contribution as it was shown on the accompanying diagram and RR might well claim that thrust as part of the engine firms' contribution to powerplant thrust. At mid-cruise weight the required thrust per powerplant (intake+engine+nozzle) would have been about 10000 lb.

A thread on the theory of "lift" in which the past 27 posts are about engines and not wings...

Unless we're talking about Harriers, isn't that a bit much?

Engines have wings too......

If Bernoulli is correct, why do they put wing designs in a wave tank to test them!?!?! :}
Actually, using a wave tank is wrong anyways, because while you can compress air, but you cannot compress water.

Just think if the surface vessel designers, designing boat hulls and trim tabs, would meet an aircraft wing designer, I bet the bottom of the wings would look much different (and work much better)

Lets face it, Newton is correct, the aircraft planes through the air, just like a boat on the surface of the water, and the waveform off the bottom of each is very much the same.

While the wave from the surface vessel tends to roll outward, the aircraft wing wake turbulence would as well, if the component from the top of the wing did not cause a rollup at the intersection of the influences.

Boeing is testing the 777-B on Puget Sound right now..

Hi Owain

something like that, but as I say it is kind of an intuition. We can investigate this further.

The first part, the increased mass flow, I think it is clear: the faster the airspeed, the more mass of air is pushed, the more lift is generated. The other part, the increased acceleration due to increased airspeed, that is another matter, but seems logical: the stronger the push by the wing, the steeper the pressure gradients around the wing, the faster the air will flow as a result and the stronger the upwash and downwash effect.

that would explain speed squared

The truth is that the empirical approach is clearly the best.

Hi Microburst,

As you say, it is almost intuition! I think that in reality everyone will go for an explanation that suits their mind set. For me, a one-time aerodynamicist, I tend to think non-dimensionally, so I seek an explanation in terms of flow angles - AoA and downwash angle off the trailing edge.

Classical (inviscid) theoretical aerodynamics couldn't explain either lift or drag until a chap called Kutta introduced a hypothesis that the flow should leave the airfoil at the trailing edge. To make that happen he had to introduce a new variable that he called circulation, so that the flow over the airfoil was a combination of freestream plus a "circular" flow which added velocity on the upper surface and reduced it on the lower. Trouble was, to get such circulation in real life you have to accept a 'starting vortex' when the air first starts to flow over the airfoil. But vortices are viscous phenomena so to make the (inviscid) theory work you have to rely on the existence of viscosity :D

Anyway, in my non-dimensional world the angle of the flow coming off the TE is only a function of AoA and wing camber. If you grant that, then the downwash velocity is proportional to freestream velocity as is the mass flow.
From that lift proportional to velocity squared follows directly.

yes

Darroll Stinton was also based on the circulation, I think. I wish I had that book here...

sometimes I wonder If I would be a happier person just using the theory of bernouilli as given in the books...

I think people should give some thought as to what use the various theories should be put to.

At the "why do wings fly" level, Newton clearly rules,to use the terminology of this forum. A wing is a machine for producing downwash, period. The fact that an unstalled wing deflects more of the air passing over its upper surface than that over its lower is only a detail, however important.

Unfortunately to get numerically correct results Newton's Laws should be applied to every air molecule passing over the wing, which is impractical even with today's computers. So there are the macroscopic theories, concepts of pressure and energy, Navier-Stokes equations, circulation, etc. But these theories are intended for calculation, not for explaining why a wing flies.

Finally, Bernoulli (and Bernoulli's equations are ultimately derived from Newton's Laws) is probably the best way of explaining the vital need of keeping the airflow attached to the upper surface, without going into the detail of either the atomic or the calculation-oriented approaches. For those whose day-to-day business it is to ensure that the airflow does stay attached, it's an approach that has worked for over a century - although how well it actually works may be questionable.

Just integrate the pressure over a control surface and have done with it

;). Potential flow plus circulation and diverting gaze from the odd singularity does it for me.

fizz57, I agree with you

and I would add, for the indoctrination of those whose job is to keep the wing unstalled, what is the real problem of stall.

For most the stall is a problem of lift, when it is clearly not. Lift of a stalled wing is much greater that lift for level flight. the real problem is the greatly increased drag and the control problems. If an airplane had powerful enough engines and could control attitude even at very high angles of attack, flying while the wings were stalled would just be a problem of fuel. But airliners can't do that, and that is why they will sink if they stay stalled.

So yes, what all pilot should know, and it was supposed to know if from the first lesson, is that you have to unstall whe wing to eliminate the problem. Apparently many pilots did not have that very clear, as the whole industry has had to meet and embarrass the pilot community. Because it was embarrasing...

Usually? Maybe. "Period"? No. A wing in ground effect is not creating enough downwash to account for the lift.

I think you have to separate two questions.

1) How does one "explain" the empirical flow field that is observed around an aerofoil of a particular shape and at a particular AoA? (i.e. why does the air flow like that?)

2) How does one predict the lift produced by the aerofoil in that flow field?

"Newton" and "Bernoulli" as described here both help answer the second question. They don't help with the first. For that, one has to be satisfied with some waving of hands, or be comfortable with solving the equations associated with flow. I've never found much in between.

Yes, indicative of the attention deficit that seems to apply once the general principles are believed understood and the crowd rush to other more sexy problems. Field problems are generally not that easy to interpret by cause and effect thinking. Easier to figure out steady state solution and not worry too much about the initial transient which was how the system got there . Think the unsatisfactory discussion on starting vortices begins here!
However for a structure in airflow midchord flow is forced down so momentum theory is happy and the interrupted flow causes a reduced pressure behind the wing which accelerates local air. So reduced pressure and higher speed above the wing and everybody is happy :O

"Usually? Maybe. "Period"? No. A wing in ground effect is not creating enough downwash to account for the lift."

My thought on ground effect is that the wing functions in what is functionally denser air (ambient) due the piston effect of the wings. For a height of roughly one half the wing span, the air cannot escape the squeeze of the wing's downwash as quickly as in a fully 'open' region, the wing and ground proximity reduce the escape "area" between them; the local airmass is functionally, denser....

For the really masochistic or obsessed Abbott and Van Doenhoff's 'Theory of Wing Sections' answers everything...it is probably my favorite book in aerodynamics though...but it's a very rough read and you really have to KNOW math and physics {engineering level}otherwise it makes absolutely no sense, it's mainly a reference book:)

J_T great explanation...:ok:

I use this website, it has excellent notes not only on aerodynamics but also on stability and control.

http://www.flightlab.net/Flightlab.n...rse_Notes.html

I struggled with the theory of lift for 3 years at uni.
Couple of things I have in my head off hand which help me to understand:

Inviscid fluids cannot produce lift. This has been experimentally proven. There has been experiments with superfluid helium which prove this is true. Viscosity is essential for the production of circulation, which is a function of the Starting vortex (which you can see in the bath, push your flat hand through water at an angle of attack and you can see it). There is a theory (von Karmen I think) which states there has to be an equal and opposite vortex attached at the wing to maintain balance of forces.

2D wing sections do not produce lift. only 3D wing sections produce lift. 3 Dimensional effects are very important and form the heart of the "Bound Vortex" Theory of lift. This also explains downwash experienced by the fuselage aft of the wing and at the tailplane, changing pitching moment across the wing section and changing angle of attack experinced across the wing.

Without drag, there can be no lift. This comes back to both viscosity and also induced drag as a result of angle of attack of the wing to the freestream flow.

"You will never understand lift. Forget it. You haven’t got a chance."

And that is what makes me smile.... Whether from perversity or some other antisocial bent, I enjoy that man can not prove mastery over Nature. Some things are enjoyed in their mystery; it levels the field.

My motto regarding lift and it's tantalyzing resistance to explanation is,

Math hates RATE...... It's why pilots use their hands to explain things, and mathematicians use a pencil.

Abraham

Thnk you very much, reading your post has been delightful.

I always like to compare lift equation with friction force equation, when talking about lift theory.

Cambridge, Gottingen or MIT scientists could pursue and find an equation for friction which could be pages long, using all kinds of variables such as molecular mass and reach a point where physics would meet chemistry and even quantum theory, why not...

But then it is so much easier to make some experiments, tabulate, find relations of proportionality and find the constants for each material...

The equation obtained is so simple and beautiful, and so innocent because it does not challenge the mistery of creation, it only stares one of its laws. We could have an endless debate about "why is ther a friction force?" similar to "why airfoils lift".

That is why I love the lift Equation. It does not explain why air behaves the way it does when going past a wing. it accept it behaves that way. Then it describes how it affects the wing an what to expect if a few variables are known.

Yes The quantum. The transition. I really do not hate Math...much. I don't know if there will be a meeting of the maths with Mother Nature. But I say, soldier on!

I 'll be patient. In no other pursuit I know, is Faith a component of Science. It is why the old ones were called back to fly the 747. Gray hair and four gold, something to believe in. If he thinks it will fly, maybe it will.....

soldier on
 

@extricate:
if you prepare exam, flight licences, Atpl instructor qualification, etc. or if you want a well paid carrier in aerospace, stay on 1.
If you want to understand how birds fly, how unloading the wing works, how spacecrafts come back on earth, how butterfly flies, ...how nature allows lift goto 2!

1. If you want to get your exams, please, just write as it is written in YOUR books. Your teachers teach the lift as these crazy theories give them their salaries, to get famous, for some of them to get Nobel prices. And forget... (maybe you will finish like AF447 in the ocean, take a good insurance to help your family : all that is only statistics! "So many millions NM anf flights have not accident..." STOP.:uhoh: GOTO END

2. You want really fly like birds! Just look seriously the sky : can you really imagine the air is continuous, homogen, isotrope? NO NO NO. So forget ALL the continuous equations: Bernouilli, speed model, pressures equations like navier-stokes or Kutta,Lanchester :First learn Fractals,(it is difficult),dynamic systems (sampled fractals...), and plastic rupture! It is a nice story. Good kuck. And never forget. STOP.:ok: GOTOEND.

END.

I do fully agree with the first sentence, but at the same time do fully disagree with the second.
The problem of stall is not lift, it is the slope of the lift-over-AoA Curve! The Airplane can fly at any Cl, given the right speed is applied. So a drop in lift can be compensated by more speed, which can be easily demonstrated in Gliders which have two Cl max peaks in the lift-over-AoA Curve. In full stall some of them fly fully stable, but with a higher speed than the lowest possible one. (You can easily demostrate that when flying in formation)
The equilibrum of forces perpendicular to the flight path (or the airflow) is only stable, if there is a positive slope of the lift-over-AoA Curve. Lift automatically equals weight as long as the slope is positive, mother nature controls this for us. Any lack in lift results in increase of (downward) vertical speed, which results in AoA increase which restores lift. If however the slope of the lift-over-AoA Curve is negative, then lack in lift results in increase of (downward) vertical speed, which results in AoA increase which further reduces lift, hence the airplane switches from the modern rules of aerodynamics to the ancient law of gravity.

Nobody worries about the theory of gravity, neither should we worry about where lift comes from. As long as we know how to loose it, and avoid that situation, we are fine.

Volume

I don't agree with your disagreement.

Lift coefficient at very high angles of attack is very high, and in the stall, even well inside it, it is very high too, compared with typical cruise angle of attack.

Of course, if you keep pulling and pulling and your aoa increases a lot, then even CL will be low. We agree in that. But even in that case, if you have enough power and means to maintain a given attitude steadily, you can fly the airplane. However this is extremely difficult, specially the control part.

So if pilots were indoctrinated in this, maybe using modern fighter jets as an example, they could be convinced than pushing the stick is the right thing to do whenever they face a stall or impending stall situation.

In my flight school, long time ago, in the piper arrow we practiced a manoeuvre called something like "characteristic stall", which was a progressive stalling of the airplane in landing configuration. The exercise was successful if after the stall warning you recovered the stall... without losing more than 20 ft. Many times this exercise was carried out at merely 500 ft, over the runway!

I had lots of arguments for that with instructors, but I only had about 120 hours. Now I know I was right. The whole industry met only to clarify my point.

Negative training is intensive and extensive and it is everywhere. One day they should address it seriously, it deserves more attention.

Many times this exercise was carried out at merely 500 ft, over the runway!

How quaint. How does one spell "silly" again ?

While specific techniques may vary between Types, the general rule with stalling is that alpha has to be reduced somehow in order to escape ..

I'm really loving this discussion...I wish I had time to really join in the way I'd like to...:8

but I don't have the time really so if I make make two brief points
1...the best mind in aerodynamics who ever existed was SzQllQskizlaki Von Karman Todor...better known as Theodore Von Karman:ok::ok::ok:

2...a designer picks whatever equations s/he need to meet the design the theory of differential equations goes on seemingly forever...:)

One of the main reasons I don't have enough time to post what I'd really like to say sometimes is that my laptop (girlfriend) keeps hogging my desktop...confused?:confused:
well here's what I mean

http://press.princeton.edu/images/k7999.gif

:}:}:}:ouch:











ps I think she's doing a vg diagram;)

http://ts1.mm.bing.net/th?id=I492057...h=155&c=7&rs=1

what he has written on the board is what exactly i've been rambling about all along...:)