Compressor Stall?

[GravityHater]

Mini, I always understood it to be not enough airflow for the speed the compressor blades are already spinning at. In this situation, I can conceptualize a stalled blade.
I too, seek more detailed answers, good question flybzz.

 

[aucfi]

So..do the compressor blades (angle) actually change? Are they fixed? How do they "stall"?

The AOA the compressor blades have is a result of the inlet air velocity and compressor RPM. Think of how your fixed pitch propellers are efficient at only one airspeed. When the airflow is disrupted into the compressor, this airspeed changes and for that given RPM the rotors are at, the blades become inefficient and can "stall".


Compressor can stall for various other reasons. Some are excessive fuel flow caused by abrupt engine acceleration, excessive lean mixture caused by abrupt engine deceleration, contaminated or damaged compressors, damaged turbine components, and/or plain old exceeding the "red line" RPM.



A compressor stall can be best described as an imbalance between inlet velocity and compressor RPM.

 

[mar]

Inlets, diffusers and Bernoulli

I guess what you really need to understand is the importance of the inlet and exactly what's happening in terms of pressure, temperature and speed.

It's true the compressor blades are little airfoils that have an angle of attack, so to speak.

But even before the air reaches the blades it's modified by the inlet.

The engine inlet and the spinner both serve to diffuse the airflow (slow it down, increase the pressure) before it ever reaches the blades. The inlet and spinner also present the airflow to the compressor face for smooth operation...but it has limits.

Like others have mentioned, if the airplane has a high pitch attitude, low airspeed and high power setting, the airflow to the compressor face may actually be turbulent enough to induce a compressor stall.

In other words, the compressor blades are just not getting enough "clean" air to compress and send back into the engine for combustion.

The result is the banging (or backfiring, as some say) of the compressor stall. Actually it's the higher pressure air inside the engine trying to escape (because nature abhors a vacuum) through the FRONT of the engine.

Bad news. Best solution: reduce power, lower pitch and increase airspeed in order to regain a nice smooth airflow back into the engine.

Basically you want smooth, high pressure air shoved up against the compressor face or you're gonna ruin the engine with high EGT.

 

[minitour]

Cool. Thanks for the good info all.

-mini

 

[avbug]

Plenty of people care. But thanks for your opinion. Tell you what. If you're such a tough guy, why don't you email me your name. Just your name, nothing else. But you have to be honest about it. That way, when your airplane gets sold out from underneath you because your boss decided to goto to NetJets or dies, or goes out of business.... I'll remember your name when you apply and come in for an interview. You see, this is a small industry. We all win run into one another someday. If you're man enough to give me your name, I feel sorry for you if you ever think of applying to us or say... Southwest. You see, Southwest is full of ex-Netjets pilots and once a name is put on that blacklist, you're done.

You sound like a SCAB or a potential SCAB to me by making comments about a brother picketing for what he believes in.

So how about it? Got balls?

Now that sounds like the disgruntled netjets pilots we all know and love at Signature's everywhere. Keep up the consistancy!

 

[avbug]

Back to the topic at hand...a compressor stall occurs when the airflow through the compressor doesn't occur as it should. It can take several forms, and may have several different causes.

Ariflow through a turbine engine starts with pressure elevated slightly above ambient ("ram air rise") in the engine inlet, and then continues to rise under successive stages of compresssion...as the air is both accelerated and compressed. From there, the air is dumped into a "diffuser" that serves to further increase the airpressure before it's admitted to the burner section of the engine.

Air pressure in the compressor should be higher than that in the burner section. If airflow in the engine slows and pressure in the burner can increases above that of the compressor section that's feeding it, the airflow "backs up." A change in this flow, and a reduction in speed, changes the angle at which the airflow meets the various vanes and blades in the compressor section. These blades and vanes function very much like any airfoil to which you may be accustomed. They stall, too.

During engine start when airflow is low and burner can pressure is rising, "acceleration bleeds" are used to control the air pressure being developed in the compressor section. Too little airflow isn't good as the engine can overtemp, and a hot start will develop. Too much airflow isn't good, as the flame may be blown too far aft in the burner can and can go out...a flame-out.

Accleration bleeds automatically regulate the air pressure and flow during lower speed engine operation. Traditionally, compressor stalls can occur when some type of blockage or restriction occurs to the airflow into the engine, during operation of the engine at high angles of attack with low speed and high power settings, and during rapid power transients when temperature and burner pressure may increase faster than the compressor can keep up.

Compressor stalls vary from a low grade hooting hoise to shotgun like bangs and barks with flame coming out the front of the engine. The soloution, or the start of the soloution, is the same; reduce power to the point where compressor stalling ceases, then attempt to correct the condition. If the condition was low speed with high angle of attack and a high power setting, you've just corrected it by reducing the power. If the problem was too rapid an advance on power, then retarding power has corrected the situation.

While a compressor stall may include actual aerodynamic stalling of compressor blades, generally it's not really a compressor stall so much as it is an engine stall...the engine is "stalling" by slowing or reversing the airflow momentarily. Surging in a compressor stall occurs as the situation rights itself and then creates itself again, and again. During these surges, the engine may flame itself out by moving the flame in the burner cans toward the rear of the can and ultimately eliminating the ability for it to propogate itself. It also creates a situation in which not enough cooling airflow through the burners is available, and the burner cans can be torched and crack.

Approximately 75% of the airflow through the comprressor gets used to cool the burner cans and keep the flame in the proper spot inside of them...to protect them from being torched. Reducing this airflow, such as occurs during a compressor stall or surge, can result in the flame touching the think burner can walls and damaging, eroding, or cracking them. Again, an important reason why a reduction in power is necessary during a compressor stall.

 

[Workin'Stiff]

I've always thought of it like a boat... Have you ever heard an outboard engine cavitate when you're sitting still and you firewall it??? You throw it to the boards, tons a fuel is sent to the engine, in turn spinning the prop extremely fast. However, there's in sufficient "grip", so to speak, to produce any thrust. The prop just spins and make a helluva sound...

 

[mayday1]

Bad news. Best solution: reduce power, lower pitch and increase airspeed in order to regain a nice smooth airflow back into the engine.

thanks, good info... is recovery that easy? can you develop unrecoverable compressor stalls? for instance, that RJ that crashed earlier this year..?

 

[avbug]

A mechanical compressor stall, wrought on by damaged compressor blades or stator vanes, inlet blockage, excessive compressor contamination or FODding,or other mechanical interruption, may be unrecoverable, or the engine may be operated in some cases at a reduced power setting.

If the engine flames out due to the airflow interruption or surging, then it may or may not be restartable, depending on a number of variables.

 

[mar]

Just a little more

Ariflow through a turbine engine starts with pressure elevated slightly above ambient ("ram air rise") in the engine inlet, and then continues to rise under successive stages of compresssion...as the air is both accelerated and compressed. From there, the air is dumped into a "diffuser" that serves to further increase the airpressure before it's admitted to the burner section of the engine.

These topics are always difficult to generalize because we operate these machines over such a vast range of settings and conditions.

But I wanna add just a little more.

I'd say the first part of Avbug's statement is true during "cruise" conditions (let's generalize and say more than 250 kts). At speeds greater than 250 kts (generally) you have to factor in "compressibility" in everything aerodynamic.

Therefore, yeah, the air pressure in the inlet is "above ambient." But that's not the only reason.

At speeds less than 250 kts (when you can consider the inlet air being "sucked" in rather than "crammed" in) the inlet will *still* induce a pressure rise by the act of diffusion (which is the exact opposite of Bernoulli's constricted flow idea).

I'm not arguing with Avbug's point that a "diffuser" exist behind the compressor section but I didn't want to gloss over the importance of the design of the inlet and spinner that the engineers use to mitigate compressor stalls.

Not only that but I'd also like to expound just a little more on Avbug's use of the word "accelerate" in the context of what's occuring in the compressor section.

Is the air "accelerated"? In the sense that it's direction has changed (centrigually), Yes! A physicist would consider that an acceleration.

But would a pilot consider the air flow (axially) to be accelerated? I'm not so sure. I would certainly admit that the compressor blades are small airfoils and require smooth airflow to operate efficiently and, thus, conform to Bernoulli's theorum. But I'd be careful to consider the airmass (in general) as "accelerating" through the compressor section.

In fact, quite the opposite is occuring. The airflow, in general, from the lip of the inlet until combustion in the hot section, is slowed and the pressure rises along with temperature.

One final point: In the original question it was asked if the blades in the compressor can change their "angle of attack" or are they fixed.

As far as I know, compressor blades are fixed (in the angle they're mounted) but they actually "float" in fittings. Sort of like loose teeth that wiggle and can be removed if you're clever about it, but the centrigual force from the rotation of the compressor rim actually holds them in place during flight.

What does change is the angle of the stator vanes (on some engines). The purpose of the stator is help direct the air flow from a previous compessor stage on to the next compressor stage. Stators do not rotate and do not exactly compress the air but they do slow it down and present it for further compression.

Like I said, on some engines, the angle is changed on the stator vanes pneumatically by engine bleed air. For the most part they change angles based on power setting.

To answer mayday's question about the last RJ crash and "unrecoverable compressor stalls"...that wasn't just a compressor stall. That was a seized engine where all rotation stopped.

In a plain vanilla compressor stall you won't find a seized "spool" but as Avbug says, there may be more damage than you counted on: http://www.tsb.gc.ca/en/reports/air/2002/A02P0021/A02P0021.asp