Re: EPR v. N1 for turbofan power setting

From:         Crone <vcrone@u.washington.edu>
Organization: University of Washington
Date:         03 Mar 95 02:27:50 
References:   1 2 3
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> I remember that a design engineer on a military engine project said that 
> F/A-18 pilots used Exhaust Gas Temperature (EGT) to determine take-off power 
> setting. The pilot would throttle the engine to reach a specific EGT, once 
> reached the plane would then be catapulted off the deck.  It's easy to see 
> that you can get the same EGT with different EPR and N1's depending on the 
> condition of the engine.  So, when the engineer said this other designers (I 
> assume on other engine programs) went nuts.  They said in a severly degraded 
> engine N1 could be so low that the plane could be catapulted into the sea! I 
> cannot comment on any of this because I'm just not knowledgeable in this area; 
> but using EGT would seem to be very iffy proposition.
> 
> I know of no commercial engines that use EGT as a measure of thrust. Perhaps 
> somebody with more experience in this area could shed some light on this topic.

EGT is particularly useful in fighter aircraft because of (among other 
things) formation flight.  Generally two aircraft using the same EGT have 
nearly identical power output which is great for station keeping.  
Fighter engines tend to be low-bypass turbofans, or turbojets (which have 
only one RPM guage).  With these engines EGT is a pretty good indicator 
of power output although in the end I think the choice is simply a matter 
of how the pilot was trained to fly.  Having flown the Northrop T-38 I 
can tell you that it varied from pilot to pilot as to whether he 
preferred EGT or RPM for gauging his power.