Re: Engine questions

Date:         28 Sep 97 00:53:43 
From:         GWilson404@aol.com
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In article <airliners.1997.2289@ohare.Chicago.COM>, "David A. Stuart, Sr."
<dstuart@mail.bcpl.lib.md.us> writes:

>2- (a little more involved) Could someone explain a 'compressor stall' ?
>I understand the concept of an airfoil stalling when the angle of attack
>is excessive to the point that the laminar flow is disrupted and
>separates from the airfoil surface. In a compressor however, I assume
>(always dangerous !!) that the airflow across the compressor blades is
>constant relative to the angle of attack of the blades. Does the term
>'stall' have the same meaning in both contexts?

In essence its the same for compressor blades as for wings. At a given
rotational speed the compressor can deliver a range of flow and pressure
ratio : more flow at lower PR to less flow at higher PR. The actual level,
known as the "operating point" or "running line", is determined by the
"matching" of the engine. The fan running line is primarily determined by the
size of the nozzle. As the running line is "raised" higher PRs are generated
at lower flow and the angle of attack of the air relative to the blades
increases. Go too far and the blade stalls : this point is known as "the
surge line". When this happens at high power the effect rapidly propogates to
the whole engine and the whole airflow of the engine goes through a rapid and
violent pulsation. At low power it is possible for a "stable" mode of
operation to exist known as "rotating stall", where the stall cells fail to
propogate.

Hope this answers the question.

Gerald Wilson