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Theory on lift

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Theory on lift

Old 26th Jul 2012, 08:30
  #41 (permalink)  
 
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I wasn't talking about a turbojet, I was asking about the intake system that creates 63% of the thrust at cruise speed.
OK, you are asking about the bleed air, but it wasn't clear from the context of your text.

Regarding the multi-stage propeller bit: I understand that the blades act like a multi-stage propeller but I was saying their purpose is to compress the air and not to directly provide thrust like a propeller.
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.

What is the "inlet air"? Where does that "inlet air" go? In the picture it appears that some air goes along the top, following the first arrow and bypassing the engine intake, through a tiny passage and then gets dumped at the rear of the engine right before the divergent exit. There is also another passage on the bottom of the engine where air bypasses the engine intake and meets at the rear of the engine right before the divergent exit. Does the air that exits out of those two passages contribute to the "inlet thrust"?
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.

Edit: 8% of the thrust is from the engine in cruise - does that mean that the fuel burned is only related to that 8% that is from the engine? NONE of the thrust from the inlet is made by combustion?
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!
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Old 26th Jul 2012, 08:35
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The turbine removes energy from the exhaust gases in order to power the compressor. In no way does the turbine provide thrust, so the propeller analogy is wrong!
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"

From C to D (expansion through turbine and nozzle on a working cycle chart) the gases resulting from combustion expand through the turbine and jet pipe back to atmosphere. During this part of the cycle some of the energy in the expanding gases is turned into mechanical power by the turbine and the remainder, on its discharge to atmosphere, provides the propulsive jet.
Italics are my explanatory addition, bold is my emphasis

Last edited by Owain Glyndwr; 26th Jul 2012 at 08:47.
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Old 26th Jul 2012, 08:59
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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
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Old 26th Jul 2012, 09:07
  #44 (permalink)  
 
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I stand corrected!

I never thought so much of the thrust was developed in the combustion chambers.
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Old 26th Jul 2012, 09:24
  #45 (permalink)  
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.. 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 ...
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Old 26th Jul 2012, 09:29
  #46 (permalink)  
 
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Owain...

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.
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.

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!
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.
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Old 26th Jul 2012, 09:39
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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.
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Old 26th Jul 2012, 09:51
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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
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Old 26th Jul 2012, 10:37
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Hi italia458,
It seems ludicrous that you can get 92% thrust for free! I'm trying to understand what is going on here.
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.
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Old 26th Jul 2012, 11:05
  #50 (permalink)  
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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.
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Old 26th Jul 2012, 11:52
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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.
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.
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Old 26th Jul 2012, 13:43
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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.
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Old 26th Jul 2012, 13:46
  #53 (permalink)  
 
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My stupid spell check prevents me from spelling your name correctly.

Italia
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Old 26th Jul 2012, 14:36
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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.

the faster the air is, the more accelerated it is
Why would that be?
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.
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Old 26th Jul 2012, 15:04
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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.
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Old 26th Jul 2012, 15:11
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the faster the air is, the more accelerated it is
Why would that be?
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.
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).
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Old 26th Jul 2012, 15:18
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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?!
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.

Last edited by Owain Glyndwr; 27th Jul 2012 at 04:17. Reason: thrust corrected
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Old 26th Jul 2012, 19:14
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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?
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Old 26th Jul 2012, 19:38
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Engines have wings too......
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Old 26th Jul 2012, 20:28
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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..
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