flyingfalcon401

I need some help with a couple of questions:

1. When will a compressor stall most likely occur?
2. What determines the most desirable cruise altitude for jet aircraft?

FlyerJosh

It's been a while since I studied engine theory however, compressor stalls (or engine surging) is caused by flow reversal in the engine (and HP air from the combustion chamber escaping forward through the compression stages).

IIRC, the highest potential for engine surges is when power is added abruptly (rapidly increasing pressure in the combustion chamber without having adequate time to spool the LP compression section). That said, modern advances in engine technology such as blow off ports and bypass air flow have helped to alleviate much of the concern for engine surge.

As far as most desirable cruse altitude- do you mean relevant to each leg, or relevant to an engine from a design standpoint?

With respect to each leg, consideration for winds aloft, time enroute and fuel burn all come into play (as well as performance restrictions to a climb based on takeoff weight and the air temp/density which may limit ceiling). In general, when flight planning, I look for an enroute altitude that meets the following conditions in order:

1) Meets requirements for MEA and obstacle clearance and other governing regulations.
2) Provides reasonable balance of enroute speed vs enroute time (IE can we spend a few extra minutes climbing up higher to achive a better TAS and still arrive at the same time as if we stay lower?)
3) Provides good conditions for passengers.

All things being equal, if the enroute time at FL350 is the same as FL410, I'm going to fly at FL410 and save a few hundred pounds of fuel. Of course if you know that FL410 is unattainable due to ATC or route restrictions (IE the flight is only 150 NM), then you pick an appropriate altitude.

With respect to the engine/airframe, that is a factor considered by the necessary performance of the engine, the design parameters of the aircraft and required mission. Factors such as cost, range, efficiency and speed will all come into play.

FlyerJosh

Here's a good site to read through a bit of info that's posted by people that know a lot more than I do (or at least seem to...):

http://www.eng-tips.com/viewthread.c...=132538&page=1

andy171773

The Variable Geometry Vanes in most modern jet engines help prevent this "flow reversal"...they modulate their position based on engine demand to provide the smoothest front to back airflow..i guess you could kind of relate it to a sort of "check valve"....sorta

To put it simply..without going into the countless variables that could determine / change the most "desirable altitude" the main goals are to find the...

1.) most efficient fuel flow in respect to altitude..obv specific engine performance dependent
2.) highest true airspeed with respect (as said above) to MEAs and of course the hemispherical rules of what altitudes to fly with directions
3.) most advantageous tailwind

and then any other factors that make sense. You're not going to climb to FL350 from JFK to BOS to get an 80 knot tailwind..just not practical on such a short leg

__________________
1/20/2009