Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!cs.utexas.edu!rutgers!tut.cis.ohio-state.edu!att!cbnews!military From: shafer@drynix (Mary Shafer) Newsgroups: sci.military Subject: Fighter engine smoke Message-ID: <7906@cbnews.ATT.COM> Date: 30 Jun 89 20:10:45 GMT Sender: military@cbnews.ATT.COM Lines: 68 Approved: military@att.att.com From: Mary Shafer David Gardiner writes: >A rule in the board game "Flight Leader" indicates that the engines >used in US F-15s are only smokeless if they are tuned appropriately. >Apparently, this limits engine life so that most or all of the engines >have been "detuned" to extend engine life. Does anyone know (a) if this >is true and (b) any more details? The term is "de-rated" and I'm probably going to tell you more about this than you really want to know. When the F-111 first came out, it had a _terrible_ problem with engine stalls. The engine would stall during ground engine runs, even. This was due to a bad inlet design. A lot of work went into fixing this and if you look down the inlet of an F-111 you'll see vortex generators all around the inlet. This is the most visible fix. Engine stalls aren't the same as aircraft stalls, but they are caused by airflow problems. When the engine stalls, the thrust drops off rapidly. Damage to the engine is also possible, mostly from overheating. The Air Force and the Navy were so alarmed by all this that they decided, when the F-14 and F-15 came along, that they weren't going to take the chance of having fighters that were prone to compressor stalls. The solution was to de-rate the engine to the point that it _couldn't_ stall. The way to do this is to adjust the stator vanes (the fixed vanes in the compressor stage) so that they won't stall. However, doing this means that the engine isn't going to operate to its full capacity. Also, since you're not using the full capacity, you'll have lower pressures and lower temperatures and, hence, less wear and longer engine life. But you will also have less thrust and/or more fuel burn, since efficiency also is reduced. The smoke would result from an out-of-spec engine, because you would be so far off the design point that you would get incomplete combustion. However, I've never seen a smoking F-15 or F-16, either here at Edwards or at Langley AFB. I assume that the tolerances are fairly broad and that you would see other engine problems like elevated temperatures, in addition to the smoke, that would make you down the a/c and fix it. The F-15 HIDEC (Highly-Integrated Digital Engine Control) project here at Dryden uses a digital engine control system to run the engine. By scheduling the engine parameters on aircraft parameters like angle of attack, a non-derated engine can be used, since the engine control system takes care of avoiding stalls. Thus the HIDEC gets over 10% more thrust because it can use the thrust available without engine stalls. Here's a quick explanation of the compressor. The compressor stage of a jet engine consists of several sets of moving and stationary vanes. The front vanes are relatively small and the space between the moving and stationary vanes relatively large. As you proceed through the stage, the vanes get larger and the space gets smaller, so the air is compressed. -- M F Shafer shafer@elxsi.dfrf.nasa.gov NASA Ames Research Center arpa!elxsi.dfrf.nasa.gov!shafer Dryden Flight Research Facility DON'T use the drynix address Of course I don't speak for NASA A MiG at your six is better than no MiG at all. --Unknown US fighter pilot.