Article 11491 of rec.aviation.military:
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From: jfb@usgp1.ih.att.com (Joe Baugher)
Subject: New Fighter Series--McDonnell Douglas F-15 Eagle (3 of 26)
Message-ID: <D2zKGB.r4@ssbunews.ih.att.com>
Summary: F-15A powerplant, F100.
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Date: Thu, 26 Jan 1995 00:09:47 GMT
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The F-15A is powered by two Pratt & Whitney F100-PW-100 afterburning 
turbofans.  The powerplant is designated F100-PW-100 by the company 
and JTF22A-25A by the USAF.  It is an axial-flow turbofan with a 
bypass ratio of 0.7:1.  There are two shafts, one carrying a 
three-stage fan driven by a two-stage turbine, the other carrying the 
10-stage main compressor and its two-stage turbine.  Normal dry thrust 
is 12,420 pounds, rising to a maximum thrust of 14,670 pounds at full 
military power.  Maximum afterburning thrust is 23,830 pounds.  Left 
and right engines are interchangeable with each other.  Unlike in the 
F-14 Tomcat, the engines in the F-15A are mounted close together in 
order to minimize asymmetric handling problems when one of them is 
out.  

The F100 dates back to August 1968 when the USAF awarded development 
contracts to Pratt & Whitney and General Electric for a next 
generation fighter engine, with the Pratt and Whitney engine being 
selected in 1970 by the USAF for further development.  A parallel 
version, the F401, had been proposed for the later models of the 
Navy's F-14 Tomcat.  However, the F401 was cancelled when the size of 
the Tomcat fleet was cut back in an economy move.  

The engines are fed by a pair of laterally-mounted straight, two 
dimensional external compression air intakes.  The intakes are swept 
forward from bottom to top, in order to ensure that an adequate amount 
of air is admitted to the engines at high angles of attack.  The 
intakes are pivoted at their lower edges and can be adjusted to angles 
of as much as 4 degrees above or 11 degrees below the horizontal.  The 
air intakes "nod" up or down under the control of an air data computer 
to keep the aperture facing directly into the airstream in order to 
maintain a smooth flow of air into the engines.  The angle of the 
intakes can also be adjusted to prevent more air than necessary from 
being taken in to the engines.  The intake surfaces have an additional 
function in providing extra maneuvering control, in a manner similar 
to the function of the canard foreplanes fitted to aircraft such as 
the SAAB JAS-39 Gripen.  At supersonic speeds, the effectiveness of 
the "nodding" intakes is almost a third of that of the horizontal 
stabilators.  The intakes stand away from the fuselage sides to 
prevent boundary layer air from entering the engines, making complex 
diffuser plates unnecessary.  Downstream of the intake are moveable 
ramps which control the amount of air admitted to the engines.  The 
exhausts of the F100 engine have fully-dilating nozzles to control the 
mass flow of air from the exhaust.  The dihlating nozzles were 
initially fitted with "turkey-feather" vanes, but these were later 
removed on most aircraft.  

The F100 engine had numerous teething troubles, which might have been 
expected for such a new and advanced aircraft engine.  Initially, the 
Air Force had grossly underestimated the number of engine powercycles 
per sortie, since they had not realized how much the Eagle's 
maneuvering capabilities would result in abrupt changes in throttle 
setting.  This caused unexpectedly high wear and tear on key engine 
components, resulting in frequent failures of key engine components 
such as first-stage turbine blades.  Most of these problems could be 
corrected by more careful maintenance and closer attention to quality 
control during manufacturing of engine components.  However, the most 
serious problem was with stagnation stalling.  

Since the compressor blades of a jet engine are airfoil sections, they 
can stall if the angle at which the airflow strikes them exceeds a 
critical value, cutting off airflow into the combustion chamber.  
Stagnation stalls most often occurred during high angle-of-attack 
maneuvers, and they usually resulted in abrupt interruptions of the 
flow of air through the compressor.  This caused the engine core to 
lose speed, and the turbine to overheat.  If this condition was not 
quickly corrected, damage to the turbine could take place or a fire 
could occur.  This was especially dangerous in a twin-engined aircraft 
like the F-15, since the pilot might not notice right away that one of 
his engines had failed.  To correct for this, an audible warning 
system was attached to the turbine temperature reading.  

Some stagnation stalls were caused by a "hard" afterburner start, 
which was a sort of mini-explosion that took place inside the 
afterburner when it was lit up.  "Hard" afterburner starts could be 
caused either by the afterburner failing to light when commanded to do 
so by the pilot or by the afterburner actually going out.  In either 
case, large amounts of unburnt fuel got sprayed into the aft end of 
the jetpipe, which were explosively ignited by the hot gases coming 
from the engine core.  The pressure wave from the explosion then 
propagated forward through the duct to the fan, causing the fan to 
stall and sometimes even causing the forward compressor stage to stall 
as well.  These types of stagnation stalls usually occurred at high 
altitudes and at high Mach numbers.  

Normal recovery technique from stagnation stalls was for the pilot to 
shut the engine down and allow it to spool down.  A restart attempt 
could be made as soon as the turbine temperature dropped to an 
acceptable level.  Of course, if this happened during the stress of 
combat, the pilot would be dead meat.  

There were frequent groundings and delays in engine deliveries while 
an attempt was made to fix these problem.  Strikes at two major 
subcontractors delayed the delivery of engines.  By the end of 1979, 
the USAF was forced to accept engineless F-15 airframes and place them 
in storage until sufficient numbers of engines could be delivered.  A 
massive effort by Pratt & Whitney helped to alleviate this problem, 
but the F-15 suffered from an engine shortage for a long time.  

The installation of a quartz window in the side of the afterburner 
assembly to enable a flame sensor to monitor the pilot flame of the 
augmentor helped to cure the problem with "hard" afterburner starts.  
Modifications to the fuel control system also helped to lower the 
frequency of stagnation stalls.  In 1976 the F-15 fleet had suffered 
11-12 stagnation stalls per 1000 flying hours.  By the end of 1981, 
this rate was down to 1.5.  However, the F100 even today still has a 
reputation of being a temperamental engine under certain conditions.  

Sources:

   McDonnell Douglas Aircraft Since 1920, Volume II, Rene J.
   Francillon, Naval Institute Press, 1990.

   Observers Aircraft, William Green and Gordon Swanborough, 
   Frederick Warne, 1992.

   Combat Aircraft F-15, Michael J. Gething and Paul Crickmore,
   Crescent Books, 1992.
   
   The American Fighter, Enzo Angelucci and Peter Bowers, 
   Orion, 1987.

   The World's Great Interceptor Aircraft, Gallery Books, 1989. 

   F-15 Eagle, Robert F. Dorr, World Airpower Journal, Volume 9,
   Summer 1992.


Joe Baugher			  **************************************
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