Re: Engine questions.....

Date:         01 Oct 97 19:57:50 
From:         mdubin@msn.com (M. L. Dubin)
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David A. Stuart, Sr. <dstuart@mail.bcpl.lib.md.us> wrote in article
<airliners.1997.2289@ohare.Chicago.COM>...

> Could someone explain a 'compressor stall' ?

Probably not exactly what you're looking for ( some aero guy can provide
the technical details) but perhaps the following will help.
In simple terms:
Stall occurs basically when there is a stoppage of air through the
compressor. The big bang one hears when an engine stalls is the air flowing
forward from the back (high pressure) stages of the compressor right out
the inlet. Many things can cause a stall: The Variable Inlet geometry not
per schedule,
hot gas ingestion, inlet air distortion, FOD, a dirty compressor which
erodes stall margin, Over-fueling during a transient accel, even during a
decel especially if the engine has a variable nozzle.
I've seen the result of a massive stall  on an Navy F/A 18A due to fuel
ingestion. This was an extremely interesting case,, fuel vapors from the
center line fuel tank via a loose gas cap was sucked into the engine
inlets. Both engines, at Max afterburner at the time, stalled within 0.5
sec of each other.  When torn down during the crash investigation the front
fans were OK but both compressors were corn-cobbed. The conclusion about
the fuel was made from data analysis of the "black box" . Then about 2
weeks after the accident board was closed an 8x10 picture was received,
anomonously, showing the aircraft (it was taking off and only about 20 ft
off the ground) with the fuel vapor trails clearly visible going into the
engine inlets and flowing over the wing feeding a fireball at the exhaust.
I"ve got a copy of that  picture.
Then there is the more subtle stagnation stall (AKA, hung stall, rotating
stall) The engine keeps running with very little air flow, speed rollback
occurs usually all the way to sub-Idle,
Turbine temp gets hot (900-1000C). A very bad scene, usually unrecoverable
and unless the engine is shutdown  in a hurry there can be a lot of hot
section damage.
I didn't answer the technical part of your ques but hope that this helps
MLD