Re: Engine questions.....

Date:         28 Sep 97 00:53:43 
From:         b17864@vaxc.phx1.aro.allied.com
Organization: AlliedSignal Engines, Phoenix, Az
References:   1
Followups:    1
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In article <airliners.1997.2289@ohare.Chicago.COM>, "David A. Stuart, Sr." <dstuart@mail.bcpl.lib.md.us> writes:
> Two engine questions for the group:
>
> 1-
> [snip]
> The entire time the aircraft was on the ground the right engine was
> slowly (60-100 rpm) turning. I assume it was bleed air from the gpu. The
> questions is.....why?  Is it for lubrication purposes ? Was a bleed air
> valve left open or was it a faulty valve allowing blow-by?

What you probably saw was the wind blowing through the engine and
windmilling the fan.

> 2- (a little more involved) Could someone explain a 'compressor stall' ?
> [snip]

This is a little oversimplified, but should give you the basic idea.
A compressor stall is very similar to the stalling of an airplane wing.
The angle of attack of the air to the compressor blade is a function of
the speed of the air (which is proportional to the airflow rate) and the
rotational speed of the compressor.  The relative air velocity is the
vector sum of these two speed.  I hope the following ASCII art is clear
enough to help explain.  For a constant compressor speed, if the inlet air
flow is reduced, the relative angle of attack to the blade increases.  As
with an airplane wing, there is a critical angle of attack at which the
airflow separated and stalls.

                    /:
                  /  :                                :
    Relative    /    : Air flow       Relative      / :  Air flow
    Velocity  /      :                Velocity   /    :
            /        V                        /       V
            --------->                       --------->
          (   Compressor                    (    Compressor
         (      speed                      (       speed
        (                                 (
       (                                 (
  Compressor                         Compressor
    Blade                               Blade

As the airflow breaks down, the downstream higher pressure air wants to flow
forward to the lower pressure upstream.  With a very hard stall (or surge),
the hot gasses from the combustor can flow back out the inlet.  The result
can be a very spectacular fire ball shooting out the front of the engine.
If the engine is not very robust, severe damage can result from a heavy surge.

Normally, these events are not so dramatic.  A light surge may not even be
noticed except for a small pop or rumble.

Any way, this is probably more info than you wanted, but I hope this
explaination helps.

Mark Johnston
Sr Development Specialist
AlliedSignal Engines
Phx A