compressor stall, and bleed valve questions.

[The Sizzler]

I searched and found a similar thread to this back on 10-4-04 but I’ve still been looking for a good explanation or definition of a compressor stall, more specific to a P&W PT-6 Turboprop engine. Im studying a manual which mentions compressor stalls and I sort of have an idea of what it is but I need to be able to have a more clear understanding and clearly explain it to someone else.

Why does it happen? Is it the operators fault or the engine by itself? How much do the built in safety measures help such as the bleed vales at station 2.5? And why does my manual say the compressor stage bleed valves main function are to prevent a stall at start up and low power? Im unclear on what’s happening to possibly cause a compressor stall during that startup/low power phase. Thanks.

 

[Kingairrick]

The PT-6 has two different types of compressors. Axial flow and centrifugal flow. Air entering the engine is compressed by the axial flow compressors first and then the centrifugal compressor, sometimes called the "impellor." The axial flow compressors (2 or 3, depending on the engine series) are more efficient at low RPM and the centrifugal compressor is more efficient at high RPM. The compressor bleed valves open to relieve the pressure built up by the axial compressors that can't be swallowed by the centrifugal compresssor at low RPMs because of its (the centrifugal compressor) inefficiency.

In the PT-6, it is only needed during startup and under very hard acceleration as I understand it.

I hope that helps. I'll give it another try if not. Sorry if I only confused you more...

 

[CitationLover]

the easiest analogy i can think of for a compressor stall is a clogging toilet. you are trying to shove too much compressed air into a small area and it "backs" up.

the 2.5 valve is fully open at low N1, 72%, and slowly closes as it approaches 90%, where it is fully closed (PT6A-67D).

 

[nosehair]

It's like a wing stall...the compressor blades are like little wing airfoils. If you spool up the compressor too fast, it's like rotating an airplane wing at too high an angle of attack too soon on the runway - increase angle of attack too slow, and you stall the wing. Spool up the compressor blades too quick so the air can't flow over 'n under smoothly and you stall the blades.

This may not be perzactly what it is, but it is enough to understand the pilot's actions.

 

[The Sizzler]

Thanks a ton fellas, it makes total sense now! Not to mention re reading the diagrams and aircraft book for an hour. Appreciate the help.

 

[Kaman]

Hello,

The P&W PT-6 series of engines uses one of the most beautifully simple engineering solutions to solve the problem of flowback during the low-RPM regime. Specifically, through the use of P2.5 air that flows foward and discharges via the "piccolo holes" to create an intake swirl mode. If this simple device wasn't incorporated the PT-6 would be much bigger, more complex, more expensive and heavier. For instance the GE T-58 engine uses fixed inlet guide vanes, and variable stator vanes that are driven by an actuator that uses fuel pressure to regulate the amount the stators "schedule" during start.
The PT-6 is an outstanding engine in all respects, and the beauty is in it's simplicity, rugged durability, and versatility. The PT-6 not only powers turboprop fixed-wing, but also versions of the Bell UH-1 helicopter. I don't miss pulling phase inspections on the old T-58 and removing, cleaning and replacing the dynamic fuel filter!!! But that's another story in and of itself.

Regards,

ex-Navy Rotorhead

 

[Kingairrick]

Well then I guess it's...

Trivia Time!
Q: What does the "A" stand for in the phrase "PT6-60A" or "PT6-42A?"

Hint: Alaska based pilots are inelgible for prizes, so keep it quiet.

 

[Flyairjason]

I know, I know!!!

A is for AIRPLANE! You guys did a great job of explaining those 2.5 bleed valves!

 

[Kingairrick]

I guess that wasn't such a mystery. Bonus round:

Q: What's another series? As in: PT6-60"?"

 

[Ganja60Heavy]

Stalls of the compressor

Others have put down some great explanations and analogies here.



If I may add one, it may help with an understanding of compressor stalls and the accesories ("bleed valves") designed to prevent this.



Think of each compressor blade as a wing. At low airspeed (low N1), the wing is much more sensitive to stall from airflow disturbance (turbulence) than it is at a higher speed. The bleed valves are a way for the engine to reduce the compressor blade angle-of-attack (without actually moving the blades). The decrease in pressure downflow from the compressor has the same effect as "pushing the stick forward" in an impending stall.



All jet engines (this includes the PT-6) rely on very high pressures and tight mechanical tolerances to compress large amounts of air over very short distances. There is very little margin of error: the compressor blades are designed to operate at very high angles of attack (AOA). If they were designed to run at low AOA, the turbines would have to spin much faster to compress the same amount of air...very inefficient since the power turbine would need to supply more energy to run the compressor, thus leaving less available for thrust (either prop or fan). If their AOA were any higher, the engine would be very susceptible to compressor stall, much like a wing at very low speed in turbulence.



Engineers design the compressor blades to operate at an AOA roughly equivalent to Vx.....maximal air compression over a given "distance" (compressor rotations). Vx is close to the stall in a fixed wing, much like it is in a turbine blade. We know a climb in a C-152 at Vx in turbulence can be exciting, with an intermittent stall warning horn. It's the same excitement inside the engine! The bleed valves allow unloading of the airfoil to prevent the stall. Low engine speed allows small areas of turbulence within the engine to have much more of an effect on airflow, and so the bleed valves are active to decrease AOA during startup and acceleration.



Usually the engineers remove the pilot from the loop when it comes to compressor stalls. The pilot needs to know the idiosyncrasies of his particular engine, but that's about it. Most phases of engine operation are pilot-proof, especially in the most modern versions of any given engine.

 

[crash-proof]

What about the old JT8 on the 727, MD-80 and so on? Why does it seem to "backfire" (compressor stall) so much when in put in reverse, as opposed to other turbofans?

 

[Ganja60Heavy]

Only a guess

What about the old JT8 on the 727, MD-80 and so on? Why does it seem to "backfire" (compressor stall) so much when in put in reverse, as opposed to other turbofans?

Dear Crash:

I'm guessing here, using my general knowledge. Someone with more insight may pounce on me as a bull**CENSORED****CENSORED****CENSORED****CENSORED**ter, but here goes:

The operative word is OLD. The JT8 was designed in the 60's, and much of the functions of that engine is mechanical. The fuel control is mechanical (as opposed to electronic), and subject to limitations of space, weight, physical design (cams, cables, valves, etc.) and lack of real-time feedback/logic. Thus, a rapid advancement of thrust levers causes the same mechanical input to the engine as slow advancement, only it occurs faster (duh!). This is different on more modern designs.

More modern engines incorporate microprocessors into the system which provide real-time feedback and control of the fuel system. If you slam the thrust levers forward on a 777's GE90, the computer provides fuel to the engine according to a schedule which monitors parameters in real-time. The fuel computer is designed to allow maximal engine acceleration (because you slammed the thrust levers forward) while monitoring performance, preventing misadventures within the engine (stalls, "backfires").

The center engine on a 727 is notable for acceleration stalls, probably due to the S-duct's effect on airflow (turbulence) while the plane is standing still. Try sucking chocolate milk thru a Crazy-Straw!