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t1210s
Topic Author
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Engine Stall

Sat Aug 07, 2004 5:33 am

Can a jet engine stall on take-off. My mate said that when he flew out off luton UK the pilot said that because it was a short runway he would have to make a step ascent resulting in the engines stalling but he could restart them all right .Is this true or a load of b*****t. I'm going with the latter.
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KYIPpilot
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RE: Engine Stall

Sat Aug 07, 2004 5:42 am

Your friend is confusing what the word stall is relating to. Engines don't stall, the aircraft itself stalls, as a result of insufficient lift being produced by the wings. It is true that a steep ascent, if left unchanged, can bleed of airspeed and can stall the aircraft, but not the engines. Although engines can flame-out due to losts of water, debris, or some sort of mechanical problem, but it is usually rare.
"It starts when you're always afraid; You step out of line, the man come and take you away" -Buffalo Springfield
 
Klaus
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RE: Engine Stall

Sat Aug 07, 2004 6:11 am

Jet engines can stall indeed!

It´s even a related effect: Instead of the wing losing lift, stalling compressor blades will lose the ability to keep up the airflow; As a result, the already compressed air flows "upstream" against the normal direction instead of "downstream" into the combustion chamber.

The result will be a sudden loss of thrust and possibly damage to the engine´s compressor.

Modern engine designs have all kinds of automatic control mechanisms in order to prevent stalls, but it still happens occasionally (usually at high thrust settings, so yes, takeoff could be such a situation).

I can´t say much about the story described above... I´ve heard that some older turbojet engines were relatively robust against stalls, but it still sounds strange that a pilot would voluntarily risk stalling his engines. I´d be surprised if he was serious about that.


This 777 got stranded after suffering an engine compressor stall in flight:


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MD11Engineer
Posts: 13899
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RE: Engine Stall

Sat Aug 07, 2004 6:19 am

Turbine engines can certainly stall. This happens when the gas flow in the compressor section reverses and is indicated by a loud bang (like a cannon going off) and can actually destroy the engine (I´ve boroscoped engine like this with compressor blades missing). There are two ways it can happen on a modern two spool engine.
1) on accelerating the engine, either that not enough air can be sucked into the engine, causing the inlet pressure to drop to far (happened to us in a run up box on a CF-6-80), or if you accelerate the engine with a tail wind. It can also happen if the inlet guide vane of the high pressure compressor don´t open, causing a pressure build up at the 2.5 station, between the low pressure compressor and the high pressure compressor. Eventually this pressure will overcome the low pressure compressor and escape through the inlet.

2) On retarding power, if the VBV´s (variable bleed valves) don´t open. The high pressure compressor has less inertia than the LPC and will spool down faster. Therefore it can´t digest all the air forced into it. Result: a reverse flow. Not healthy for the engine. It is as if the blades run into a wall of solid air.

Jan
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JetMechMD80
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RE: Engine Stall

Sat Aug 07, 2004 8:47 am

A "stall" is not really correct, allthough it is the popular term. That would imply the the compressor stops turning. The proper term is a "suge".

"I get along great with nobody"~ Billy Idol
 
greasespot
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RE: Engine Stall

Sat Aug 07, 2004 11:16 am

You can have compressor stalls on the 72 engine number 3 when you climb out to steeply or in a cross wind. In fact we have some engines that are prone to stalling and therefore are not suitable for centerline engine install but are fine for pod install only. They actually come back from test cell runs with this stated in the documentation. But a JT8 can stall with out any damage most times.

Stall is the correct term and is actually used in the JT8 overhaul manual and was used in the 4 week course I was on with Pratt and Whitney. Klaus is correct in the definition.

Greasespot
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
 
JetMechMD80
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RE: Engine Stall

Sat Aug 07, 2004 1:16 pm

Still waiting for that ref, greasespot.
"I get along great with nobody"~ Billy Idol
 
MD11Engineer
Posts: 13899
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RE: Engine Stall

Sat Aug 07, 2004 3:42 pm

Acc. to the definitions I know, a stall is a breakdown of airflow over the whole area of a compressor stage, while a surge is a partial breakdown of flow. You can recognise a surge by a strange noise (hard to describe), high EGT and vibrations. It may turn into a full stall (of course the compressor keeps on turning, but the forces are big enough to damage compressor blades).
During a high pitch angle at high power, airflow into the intakes can be disrupted, causing a significant drop of total pressure in the intake and can cause a stall.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
exPratt
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RE: Engine Stall

Sun Aug 08, 2004 3:43 am

Theoretically, a compressor surge, more commonly referred to as a stall, can occur any time the engine is running. Some engines are more stall prone that others. The GE CFM56s and CF6s have been known to stall, but it is pretty rare. In comparison, the JT8Ds on the 727s, and especially the No. 2, center, engine or engine on the downwind side when there is a crosswind are known to stall.

The cause of a surge or stall is a disruption of the airflow. The disruption can be caused by crosswinds, injestion of a foreign object, airfoil damage, throttle movements (toc or tod), engine deterioration, system malfunction, aircraft movements. Stalls are usually typified by a loud bang (the bigger the engine, the bigger the bang) and flames from the inlet and tailpipe. Stalls can result in internal damage, usually overtemped turbine blades, or as with the JT8s, no damage usually. It is more likely for a stall to occur at the higher power settings as that is where there is the least surge margin although stalls can occur on off idle accels.
 
320tech
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RE: Engine Stall

Sun Aug 08, 2004 4:04 am

CFM56's can stall during ground runs, if you go to a high enough power setting and have a strong crosswind.

The story that started the thread sounds like baloney, or else is not well-described. As if a pilot on a commercial jet would (a) deliberately stall the aircraft (nearly impossible to stall the engines intentionally, especially in flight), and (b) tell the passengers he would do so.
The primary function of the design engineer is to make things difficult for the manufacturer and impossible for the AME.
 
greasespot
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RE: Engine Stall

Sun Aug 08, 2004 9:45 am

You will have to wait until I get back to work on Monday for the ref's. I guess I should be more like you and keep this stuff at home.

Greasespot
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
 
lowrider
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RE: Engine Stall

Sun Aug 08, 2004 11:31 am

I have always used the compressor stall label, since the individual blades actually do stall. The first time I saw it in writing was in reference to the PT6 engine and the possible symptoms that the P2.5 valves had stuck in the closed position. That particular book is now buried in the basement, but you might want to check out Aerodynamics for Naval Aviators, Chap 2. The terms stall and surge are used interchangably. So the whole stall/surge arguement basically boils down to arguing if the earth is round or spherical.
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MD11Engineer
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RE: Engine Stall

Sun Aug 08, 2004 11:40 am

Since the compressor blades move at the same angular velocity, but each section, starting from the hub outwards has a different track speed, just like a prop, the inner section can go into an aerodynamic stall (being slower), while the outer parts are still ok (ok, the blades are slightly twisted, just like a prop, but there are limits). Therefore you can have a partial breakdown of the airflow through the compressor. This is what I know as being defined as a surge. A full stall would mean a total breakdown over the whole diasmter of the compressor blade.

A crosswind or a tailwind will disrupt the airflow into the inlet, the same can happen at high angle of attacks, esp. on planes with an inlet on top of the fuselage.

Older engines are usualy more robust and can take punishment modern engines won´t accept.


Jan
Je Suis Charlie et je suis Ahmet aussi
 
kalakaua
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RE: Engine Stall

Sun Aug 08, 2004 4:27 pm

If the operating conditions imposed upon the compressor balde departs too far from the design intention, breakdown of airflow and/or aerodynamically induced vibration will occur. The blades may stall because the angle of incidence of the air relative to the blade is too high (positive incidence stall) or too low (negative incidence stall). The former is a front stage problem at low speeds and the latter usually affects the rear stages at high speed, either can lead to blade vibration which can induce rapid destruction. If the engine demands a pressure rise from the compressor which is higher than the blading can sustain, a "surge" occurs. In this case there is an instantaneous breakdown of flow through the machine and the high pressure air in the combustion system is expelled forward through the compressor with a loud bang and a resultant loss of engine thrust.
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LineMechQX
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RE: Engine Stall

Mon Aug 09, 2004 4:04 pm

JetMechMD80-
Do airplane wings quit moving when they stall?

Late
PC
 
JetMechMD80
Posts: 370
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RE: Engine Stall

Thu Aug 12, 2004 9:14 am

**Do airplane wings quit moving when they stall?**

I didn't know that airplane wings moved to begin with. Rotors on hellos move, but aiplanes? I always thought thats why they were called "fixed" wings.  Smile
"I get along great with nobody"~ Billy Idol
 
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Starlionblue
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RE: Engine Stall

Thu Aug 12, 2004 9:32 am

LinMechQX, they keep vibrating if that's what you mean. My guess is that since wings don't normally stall all at once (some parts stall first) they vibrate more during a stall.
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MITaero
Posts: 485
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RE: Engine Stall

Thu Aug 12, 2004 9:51 am

LineMechQX was referring to an above reply by JetMechMD80.
 
LineMechQX
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RE: Engine Stall

Thu Aug 12, 2004 10:17 am

My point is airflow continues over a wing even during a stall. Yes the airplane is moving, yes the wings must be as well. Just pointing out that a stall is more of a disruption of air flow over an airfoil then it is loss of speed or loss of flow. It all depends on the airfoil when and how it stalls. True "stall" may not be the best terminology for this phenomenon, but its not completely inaccurate either. Not to mention its accepted through out aviation along with "surge". Which even that brings its own questions, wouldn't "back surge" be more accurate. To me surge sounds more like a sudden increase in thrust. Not trying to be nickpicky here, but yes a helicopter rotor can stall, and still be moving. So a compressor wouldn't have to stop, to "stall". How this all clears up what I was saying.

Late
PC
 
XXXX10
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RE: Engine Stall

Thu Aug 12, 2004 7:17 pm

I was on a BY 757 which suffered a surge just after take off, it sounded like a series of loud pops, the engine was shaking and there was some smoke in the cabin.

Some of the pax said that they had seen flames coming out of the back of the engine.

It was frightening for the 10 seconds that it took , after that everything was normal.

The captain said that he thought it was due to high crosswinds and that the engine was ok now.

We went back to LGW where the emergency services were waiting for us.
 
greasespot
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RE: Engine Stall

Sat Aug 14, 2004 2:12 am

Ok now that I am back from vacation I can provide a reference If you go to the JT8D-1/17AR Engine manual (PW P/N 481672) Revision Oct 15/03
Section 72-00-00 testing 07 talks about Compressor stalls.

"Following procedure may be used after engine repair to determine
if an engine has sufficient margin to operate without
encountering off idle, high compressor stalls during normal engine
operation after sufficient warmup."

I hope that is sufficient for you that P & W does use the term compressor stalls.

Greasespot
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"

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