Jet engine stall

[doppeldecker]

Me´s just a humble little student pilot, single engine piston, proudly commanding a C152 (most of the time the plane commands me). But who knows...

So I have one of those dreaded newbie questions about a thing I couln´t yet get figured out:

What is an engine/compressor blade stall in a jet engine? (correct me if I do not use the right terms)

[dartagnan]

if you push hard the nose down of a jet, the air around the blades will stall resulting in a flame out!.
unlike a 152, in case of emergency, you bank, then you descend.

[Mad (Flt) Scientist]

Me´s just a humble little student pilot, single engine piston, proudly commanding a C152 (most of the time the plane commands me). But who knows...

So I have one of those dreaded newbie questions about a thing I couln´t yet get figured out:

What is an engine/compressor blade stall in a jet engine? (correct me if I do not use the right terms)

The wiki (http://en.wikipedia.org/wiki/Compressor_stall) explanation isn't bad.

Blade stall is exactly the same, in aerodynamic terms, as a wing stall on your 152; the blades are, after all, little aerofoils, and if pushed to an AoA above their limit, they stall.

Surge is the actual engine level phenomon, though, and occurs when the flow through the compressor is sufficiently disturbed to actually be noticeable.

In aircraft terms, blade stall is analagous to a local separation on your wing, whereas surge is analagous to the full wing stall that you recognise as a pilot.

[doppeldecker]

In reply to dartagnan:

Somewhere I read it happened during or just after takeoff. That´s confusing me a bit.

[Chesty Morgan]

dartagnan are you telling me that I've been practicing, and indeed on one occasion actually carrying out, emergency descents incorrectly for over a decade?! Care to elaborate?

[Piper19]

The blades act the same as your propeller. Each blade has 2 airspeed vectors: 1 resulting from the rotating blade itself (RPM on your prop), 1 from your aircraft moving in a forward direction. Combine these 2 vectors and you get the blade's resultant speed. If that resultant speed gets too low, your blade stalls just like any aerfoil. This is not dangerous in itself, it only causes performance degrade on your engine. A stall can begin to rotate to other blades, that is when a following blade of the same stage gets in the area of air where a previous blade stalled and thus also begins to stall. Thus your area of stall will move with the blades, but at a slightly lower speed of RPM, so each blade kinda moves through the area.

A surge is more dangerous. this happens when not only 1 blade stalls, but a whole set of blades in a disk (say e.g. a whole stage). If the next stage of compressor blades still works, the much higher pressure there will cause the air in the engine to be pushed back to the inlet, so you get a reversed flow within the engine itself. A surge is a cyclic movement of air going forward and aft within the engine. A surge is much likely caused by ingestion/damage and will cause more damage.
See the Thomson 757 birdstrike video YouTube - ThomsonFly 757 bird strike & flames captured on video (http://www.youtube.com/watch?v=9KhZwsYtNDE), the bird causes a whole stage of blades to go bananas and next you'll see a pulsating flame out of the exhaust, which is that cyclic movement of air of a surge.

During normal operation, jet engines use surge valves to bleed air and variabel guide vanes to guide the air and prevent surge.

hope this helps, a picture could be much more explaining

[Mad (Flt) Scientist]

In reply to dartagnan:

Somewhere I read it happened during or just after takeoff. That´s confusing me a bit.

Engines are more likely to surge at high power settings because the engine is working closer to the "surge line". TO thrust is obviously one such case.

Any significant disruption to the flow into the engine in such cases increases the risk of surge, as does any kind of sudden change in the engine control. (Though modern FADECs provide a deal of protection these days)

[lomapaseo]

Aircraft stalls and/or upsets often lead to engine stalls and surges in the same event. Thus the proverbial eye witness reports of explosion and fire from the engines before the plane crashed.

Stall/surrge is typically only indicative of a short time recoverable power loss (1-2 sec) and usually not significant to the aircrafts performance and typically confirmed by eye witness accounts of a brief flash of flame out both the tailpipe and the inlet often associated with a loud noise if at high power.

However non-recoverable stall/surging equates to significant and/or permanent power loss for the engines involved. Eye witness accounts of such are continuous flame visible in the tailpipe.

Flameout is a loss of flame in the engine and does not correlate with witness accounts of visible flame associated with the engine.

the above is a primer for interpreting eye witness reports

[muduckace]

Engine stalls are not viewed in the tail pipe. I had a JT-9 stall on me on the fence once about dusk, the precussion knocked the wind out of me and a fireball was visible accompanied by some quite warm air.

Old low bypass engines could and puke and choke regularly, hang around any airport with old dc-8/9's 707/27's and you are sure to hear a compressor stall, usually a couple at a time before recovery..

Newer engines, high By-Pass with variable stator blades to maximize the aoa of the compressor stators as the engine speed increases do not stall as often but are much more susceptable to damage as a result as like anything with more moving parts.

Most of the above posts did not mention the relation between the compressor blade and stator. It is not merely an airfoil it is one that compresses air trapped between the blade and the stator continously achieving a higher level of compression. If for some reason (advancment of thrust in most cases) the pressure of a fwd stage is unable to be guided to further compress we get a violent backflow of air out the inlet. That is the simpilest explanation as I understand and have experienced.

[SNS3Guppy]

Doppeldeker,

Without spending a lot of time on turbine theory, you can think of a turbojet engine as a device that takes a whole lot of air and moves it into a small space. A turbine engine uses four main parts. One is a compressor to draw the air in and mash a lot of it into a little space. Another is a diffuser which takes the compressed air and causes a pressure rise before dumping it into a burner can. The next is the combustion chamber, which is just what it sounds like...it serves a function somewhat like the fuel injected combustion chamber in a piston engine where fuel and air is mixed and burned...and finally comes the turbine section. The turbine section is where the exhaust gasses go. The burning or burned gasses blow across the turbine blades, which turn a shaft...which moves the compressor. So much for turbine theory...mashing air into a small space.

Ever go through a revolving door? Ever try to go through a revolving door as part of a crowd of people all trying to go through the door? The door mashes up a lot of people into a little space, and the ability of the door to admit a lot of people depends on the folks who pass through doing so in an orderly fashion.

Imagine someone trips, or slips,or bumps into someone else. There's a chain reaction, people slow down, people bunch up at the front of the door. People maybe even get stuck in the door. The chain reaction ripples back and people get shoved and pushed, and find that they can't get through the door. If people find they get trapped in the door they may even react violently, pushing and shoving not inward, but back outside into the cold street. Such is a revolving door, and such is a compressor stall.

To help many turbine engines function properly, special "bleed valves" or "acceleration bleeds" are installed to help prevent air from bunching up or backing up...to help air flow smoothly through the engine. These valves are like little side doors that might open up in the revolving door to allow extra people through...except it's in the engine. These valves are doors or relief ports which will allow some of the air passing through the engine to be vented off until a good, steady flow takes place. They mostly open and close automatically to help "unload" the engine or relieve it of some of the air flowing through, to prevent it from backing up or...you guessed it...stalling.

As others have indicated, a stall is nothing more than airflow exceeding a critical angle of attack of an airfoil, and it can happen as easily in a turbine engine full of little airfoils as it can to a wing or rotor or propeller...a stall is a stall. A compressor stall is more than just an issue with angle-of attack of individual blades, however, and is a problem with the overall airflow through the engine (particularly the compressor)...and may stem from multiple causes ranging from a sticky acceleration bleed to a dirty compressor to too high an engine demand with too low airflow, too high an angle of inlet airflow, a birdstrike, or anything else that adversely alters airflow through the engine.

That's what causes it. What it is from the cockpit is something else. It may be bouncing needles on a cockpit indication. It may be a low, subtle hooting noise like an asthmatic beagle begging to be let in, or it may bellow out there like an upset walrus. It can bang away like nobody's business and sound like shotguns going off, or simply chug and vibrate and shake. It may be subtle, or may be something that can't be ignored.

Something like this, though for very different causes, can take place in the engine of your Cessna 152. This doesn't happen because of the propeller, but for other reasons that can range from a slipping magneto to a sudden change in engine operation. You can get backfiring, through the induction, or afterfiring, through the exhuast. The closest you might get in the piston engine to a compressor stall is a backfire...which can bark out through your engine intake and can damage your carburetor, carb air box, air filter, or induction.

[muduckace]

To simplify things further the simple suck-bang-blow phenom. are the fundimentals just as in piston engines. A compressor stall is simply a suck and bang with the the blow going in the opposite of the intended direction.

This was the 101 explanation I recieved in A&P school.

[BelArgUSA]

Jet engine compressor stalls...
Axial flow compressors are small airfoil blades. Any airfoils can be stalled.
Angle of attack of the air circulating on that airfoil.
xxx
Compressor stall control - read about compressor variable inlet guide vanes.
See also compressor bleed valves... or variable stator blades (J-79/CJ-805).
All above are controlled by the fuel control unit.
xxx
Also to mention is the location of engines.
Tail mounted engines are often subject to stall because of inlet upsets (from fuselage/wing).
To avoid "compressor stall" (when reducing power to idle for descent) -
My trick with JT3D, JT8D and JT9D, was to operate wing thermal anti-ice.
This bleeds the compressor and seems to help avoid compressor stalls.
This also warmed-up the leading edge flaps reducing their failures... (727-747).
xxx
:}
Happy contrails

[Piper19]

As a side note, all this made me thinking about those russian planes doing cobras and backflips, without damaging their jet engines...

[Rainboe]

doppeldecker, you are being led astray by frankly some of the craziest answers I´ve ever seen! It´s an unfortunate fact that the people who don´t know tend to answer first in Pprune.

A jet engine surge or stall is caused by the pressure build up through the engine reaching excessive levels. There are bleed and pressure relief valves in the engine to bleed-off excessive pressure. If, for some reason, the pressure behind a stage of compressor blades versus the pressure in front, reaches an excessive level, the pressure will surge forwards like a 'cough'. It can sound like a canon shot, and at night it looks spectacular with sparks and smoke 'coughing´out of the front of the engine. It lasts a fraction of a second and then often recovers. But sometimes the gas temperature and burning of fuel gets so disrupted that the engine will start overtemperaturing. The blades are not 'stalling'! When that happens, you are really into a problem with the engine.

Some of those early answers are really off the wall! Post 2 belongs in a lunatic asylum!

[smudgethecat]

well worth buying poster number 2
The Jet engine - Rolls-Royce (http://www.rolls-royce.com/about/publications/jet_engine_book/)

[Michael Birbeck]

Came across this video of two Aussies who built there own home made gas turbine in the 80's.

A couple of the early runs on this crude engine clearly demonstrate compressor surging.

YouTube - Jet Engine Backyard - Episode 1 (http://www.youtube.com/watch?v=4pTUeuEv8Uc)


YouTube - Jet Engine Backyard - Episode 2 (http://www.youtube.com/watch?v=moX6Lps9w70&feature=related)


For a more cogent analysis of compressor stall and the dynamics of this complex phenomenon then Cranfield engineering school is a good starting point. This thesis provides a good intro.

https://dspace.lib.cranfield.ac.uk/bitstream/1826/2027/1/Compressor%20Rotating%20Stall.pdf

If you are really into the physics then try some of the introductory papers provided by JPL, MIT or NASA (some of their stuff is available free on the internet). :ok:

Part 2 of the Turbofan Jet Engine failure recognition series covers surges at a basic level.

YouTube - pt. 2 of 3 Turbofan Jet Engine failure recognition (http://www.youtube.com/watch?v=osAT6mwkr94)

[lomapaseo]

Very interesting post above from Micahael Birbeck with all the great links:ok:

A couple of comments.

The engine surging shown in the model testing ain't the same as what was first discussed in this thread, but it's entertaining just the same. The model testing shown in the first links is with a centrifugal impeller and not an axial flow compressor with blades and vanes. hence the aerodynamic effects of aifoil stall leading to surging are unlikely. The gross surge event seen in the early video might be only a burner stability issue

The dependence on rotating stall (in the document link) of the axial compressor to limit turbine overspeed in a shaft separation event does have merit for a multi-spool/shaft machine protecting the aft low speed fan-drive turbine should a turbine drive shaft fail. However for a multi spool machine the separation of the high turbine drive shaft will still have enough air left in the burner behind the compressor to drive that turbine way above red-line operating speeds.

In the later sequence videos of fan blade failure testing RR-GE & P&W engines are shown. In some of these sequences you can see behind the fan the generation of an engine surge as a ball of incandescence flame spiraling forward through the fan blades.

And last but not least, all pilots should pay attention to the surge recognition video link posted above. That whole study and recognition program was the direct result of lots of data studies showing that most pilots did not understand the instruments, sounds or vibrations associated with a high power surge event in the current generation engines.

[muduckace]

The engine surging shown in the model testing ain't the same as what was first discussed in this thread, but it's entertaining just the same. The model testing shown in the first links is with a centrifugal impeller and not an axial flow compressor with blades and vanes. hence the aerodynamic effects of aifoil stall leading to surging are unlikely. The gross surge event seen in the early video might be only a burner stability issue

A prime example of this is the common APU (centrifugal impeller) that will stall often when the surge bleed valve fails closed. These APU's are designed to vent bleed pressure when not in use by the ACM's or during engine start. I have avoided this issue in a crunch befor by advising aircrew to keep the PACK's running until just prior to engine start. Oops chock it down to tribal knowledge.

[Michael Birbeck]

Lomapaseo/Muduckace

Thanks for the correction guys.

Coming onto this forum and reading some of the technical wisdom here is a license to listen and learn. :ok:

[doppeldecker]

Agreed. I´m still reading, will reply again. For now, THANKS!