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Not just a mere envelope for the engine: nacelles, like the CF6’s here seen, significantly reduce noise and fuel consumption.
In the development effort, cooperation between
engine maker and nacelle manufacturer
sets in early. “The nacelle people are in
on the game from the outset, to define interfaces
and mountings,” explains Wolfgang
Gärtner, GP7000 program director at MTU
Aero Engines. Many of the auxiliary systems
like fire extinguisher, pumps and electrical
adapters need accommodating in a maximally
space- and weight-saving manner. The many
interfaces make the nacelle a rather complex
piece of equipment. “The tricky thing is to
define the interfaces and neatly separate
roles and responsibilities.” Depending on the
program, either the airframer or the engine
builder is responsible for doing the integration
work and compiling the specifications to
be met. “An engine fairing must be a perfect
fit for engine and airframe, and that makes
communication with the airframe manufacturer
mandatory for us,” confirms Peter
Inman, manager business development at
Goodrich Aerostructures, which provides the
nacelles for the IAE V2500 and other
engines.
For the GP7200 engine fairings, the engineers
have gone the extra mile. For a first
time in their practice, they have made thrust
reverser actuation electrical, as they have
the opening mechanism of the fan fairing.
The heavy fairing opens electrically at the
push of a button, no longer hydraulically as in
earlier programs. When you are facing the
finished product, a weight around two tons
seems little considering the tremendous
size, what with an overall length of 8.50
meters. You could comfortably put an Airbus
A320 fuselage into the four-meter diameter
nacelle.
Apart from the enormous size difference, the
engine fairing is configured much like those
for the smaller PW6000 or other engines. For
turboprop engines, however, there are some
significant dissimilarities: unlike a turbofan,
a turboprop has no thrust reverser mechanism.
“Also, with propeller engines, the air
issuing from the intake below the propeller
hub—that’s what we call the chin intake—first
needs deflecting into the annular engine
The PW6000’s swing-up fairing provides optimum
access for maintenance.
inlet,” explains Dr. Wolfgang Gärtner, who
supervises TP400-D6 development at MTU.
“Apart from that, there’re hardly any differences
between fairings for commercial and
comparable military applications.”
When the design engineers worked on the
envelope for the GP7200, they pursued
some new avenues. In the lip of the air
intake, for instance, hot air from the engine
is swirled in cyclone fashion. The air intake
itself, much as on other nacelles, is lined
with composite elements to dampen the
noise. Visible joints have here been eliminated,
however. The upper layer has ports routing
the noise into the honeycomb core and
so damping rather than reflecting it. A further
perforated layer in the interior still
enhances that action. The thrust reverser
system, too, is noise-optimized through the
incorporation of 75,000 holes. Fitting the
engine with this envelope, also called podding,
takes about twelve days. First, in the
engine build-up (EBU) phase, specialists
install some 30 subcomponents, such as piping
and flanges. On the GP7200, the work is
performed by Goodrich. That done, the unit
goes to Aircelle, where it is fitted with the
intake lining, the mounting bracket and the
exhaust nozzle. On a special frame, the workers
then install the composite fan cowl and
the thrust reverser, checking them for perfect
fit.
While the novel nacelles are still in their
infancy, design engineers on either side of
the Atlantic are already working on future
technologies, mostly for noise damping.
Sawtooth-shaped rear nacelle edges might
help create further savings. These chevrons,
as they are called, optimize the mixing of the
fast air stream issuing from the engine with
the ambient air to reduce the noise heard on
the ground and by passengers in the aircraft.
That type of air swirling, however, at times
boosts fuel consumption, because engine
efficiency drops a little. To minimize that
phenomenon, chevrons made from a shapememory
alloy may be used. As a result of the
high temperatures, they will at takeoff bend
downward into the exhaust gas stream to
lower the noise. During cruise, they will cool
and retract into their original position to
again reduce fuel consumption. It’s still too
early in the game to use these chevrons in
actual applications. But chevrons or no,
today’s engine nacelles incorporate more
high-tech content than first meets the eye.
For additional information, contact
Wolfgang Gärtner
+49 89 1489-2803
For interesting multimedia services
associated with this article, go to:
http://www.mtu.de/107Nacelles
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