Re: Engine questions.....

Date:         28 Sep 97 00:53:44 
From:         Matthew Kranz <kranz@sprintmail.com>
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David A. Stuart, Sr. wrote:

> Two engine questions for the group:
>
> <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?

The most likely reason for the rotating engine is simple: wind. If there was
any breeze, it would have started the engine windmilling like you saw.
Actually, far from aiding the lubrication, it can be detrimental to the
engine and accessory section if allowed to continue for extended periods
and/or at high speeds. The slow rotation does not drive the oil pump quickly
enough to lubricate the bearings properly. Often, when airplanes are parked
for longer than overnight, inlet covers are installed to prevent windmilling
(as well as keeping birds from building nests and other FOD.)

> 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?

In short, yes. Each compressor blade is an airfoil which has been designed
to operate over a range of inlet velocities and rotational velocities. A
combination of relatively high rotational velocity and low inlet velocity
can create a condition where the blade's critical angle of attack is
exceeded, and -- voila -- compressor stall. Another way to create a
compressor stall is to disrupt the smooth airflow into the engine inlet,
usually by way of excessive _airplane_ angles of attack, which causes
airflow to separate around the lip of the nacelle and burble as it reaches
the fan.

A good example of both these conditions occurring in on old 727s. When they
first came out in 1962-3, the center engine was prone to compressor stall as
it was accelerated for takeoff. Think about it: high compressor speed and
low inlet velocity (that is, high thrust and almost zero airspeed), and air
having to snake it's way around that s-duct. These are the ingredients for a
compressor stall. (Boeing improved the situation by modifying the inlet and
adding vortex generators on the upper surface of the duct, but that's
another story.)

Hope that helps.

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    Matthew K. Kranz       COMM/Multi/Inst.
       Greensboro, NC         Flight Engineer
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