Engine questions.....

Date:         25 Sep 97 01:39:42 
From:         "David A. Stuart, Sr." <dstuart@mail.bcpl.lib.md.us>
Organization: ABSnet Internet Services, Inc. - (410)-685-2000 - info@abs.net
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Two engine questions for the group:

1- Today, I took a relative to to National (DC). While waiting for her
plane to arrive a 757 was waiting at the adjacent gate to off load / on
load pax. The 757 had its on board ground power unit operating (evident
from the hot exhaust gases) and was possibly connected to electrical
power from the gate.

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?

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 accross 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?

Thank you in advance,