Infrared suppression exhaust duct system for a turboprop propulsion system for an aircraft

The invention is an exhaust duct system for an aircraft, the aircraft having at least one turboprop propulsion system mounted on the wing within a nacelle, the propulsion system having a circular shaped exhaust port generally aligned with the longitudinal axis of the aircraft. In detail, the invention includes a generally shallow S shaped duct having a longitudinal axis aligned with the longitudinal axis of the exhaust port. A duct includes a circular shaped inlet section, a rectangular exhaust port section and a transition section therebetween. The inlet has a larger diameter than the exhaust port of the propulsion system and is positioned there about. The inlet includes a plurality of flexible finger like members extending inward such that they are in slidable engagable with the external surface of the exhaust port. A first pair of struts are pivotally connected at the center of the rectangular exhaust port section by their first ends and extend upward and outward from the longitudinal axis of the duct with their second ends pivotally connected to the underside of the wing. A second pair of struts are pivotally connected to the sides of the transition section by their first ends and extend upward and rearwards from the inlet slightly outward and having second ends pivotally coupled to the underside of the wing.

FIELD OF THE INVENTION 
The invention relates to the field of infrared suppression exhaust systems 
for aircraft mounted turboprop propulsion systems and, in particular, to 
an infrared suppression exhaust duct system for an aircraft having the 
propulsion system installed in wing mounted nacelles. 
DESCRIPTION OF RELATED ART 
Infrared suppression exhaust duct systems for aircraft are usually designed 
into the basic propulsion system. Typically, they use a center body in the 
exhaust nozzle to prevent direct viewing of the turbine section of the 
engine. An additional method used is to transform the generally round 
inlet of the exhaust duct to a two dimensional exhaust nozzle. This has 
the effect of ejecting the hot exhaust gases out in a horizontal "sheet" 
reducing the detection angle on or about the horizontal plane of the 
aircraft. An example of an infrared suppression exhaust duct can be found 
on the US Air Force F117A fighter bomber. In this aircraft, an internally 
mounted exhaust duct is coupled directly to the jet engine aft of the 
turbine. The initially round cross-section expands into a two-dimensional 
(rectangular shape) exhaust nozzle and in a side view has a very shallow S 
shape. The unusual aspect of the design is the incorporation of a coanda 
surface in the rectangular section that shields the turbine section of the 
engine when looking into the nozzle. Because the exhaust duct is 
internally mounted within the fuselage of the aircraft, its length can be 
very long and it can be semi-rigidly mounted therein. However, the use of 
such a design on a wing mounted turbo prop engine would impractical. 
On conventional reciprocating or turboprop powered propulsion systems for 
aircraft, the exhaust ducts are generally as short as possible to keep 
weight to a minimum and noise reduction is not taken into consideration. 
However, with the advent of infrared seeking missiles, it has become 
necessary to retrofit infrared suppression exhaust ducts on existing 
military aircraft. More recently, they even being incorporated on 
commercial aircraft. Needless to say, this minimum weight view is 
identical when considering infrared suppression. Thus such systems are 
kept as small as possible. 
Typical examples of a infrared signature suppression exhaust ducts are 
illustrated in Design Patent Nos. 357,722 "Exhaust Duct" by L. Figueroa 
and 357,665 "Exhaust Duct" by D. Creyts. The exhaust duct in Design Patent 
No. 357,722 "Exhaust Duct" by L. Figueroa was designed for use on the US 
Air Force C-130 aircraft manufactured by the Lockheed Martin Corporation. 
The "S" duct design prevents direct viewing of the hot turbine of the 
turboprop engine and the transition from a circular cross-section to a 
rectangular cross-section further reduces the exhaust gas temperature. 
However, retrofitting such an exhaust duct to the propulsion systems not 
originally designed to include it, can be a difficult task. 
Thus it is a primary object of the subject invention to provide an infrared 
exhaust duct system for such an aircraft. 
It is another primary object of the subject invention to provide an 
infrared exhaust duct system for an aircraft that requires minimum changes 
to the aircraft. 
It is a further object of the subject invention to provide an infrared 
exhaust duct system for an aircraft that accommodates the relative 
movement between the propulsion system and airframe. 
It is a still further object of the subject invention to provide an 
infrared exhaust duct system for an aircraft that further includes 
infrared signature reduction for the actual exhaust duct. 
SUMMARY OF THE INVENTION 
The invention is an infrared suppression exhaust duct system for an 
aircraft, the aircraft having at least one turboprop propulsion system 
mounted on the wing within a nacelle, the propulsion system having a 
circular shaped exhaust port generally aligned with the longitudinal axis 
of the aircraft. In detail, the invention includes a generally shallow S 
shaped duct having a longitudinal axis aligned with the longitudinal axis 
of the exhaust port. The duct includes a circular shaped inlet section, a 
rectangular exhaust port section and a transition section therebetween. 
The inlet has a larger diameter than the exhaust port of the propulsion 
system and is positioned thereabouts so that their centerlines are aligned 
The inlet includes a plurality of flexible finger like members extending 
inward from the inner wall thereof such that they are in slidable 
engagement with the external surface of the exhaust port. Thus the exhaust 
duct is in flexible engagement with the exhaust port being slidable fore 
and aft therealong, rotatable in relationship thereto, as well as able to 
displace its centerline from that of the exhaust port. 
Mounted about the exhaust duct is a conformal shroud joined thereto by a 
plurality of fastener assemblies. Because of the high temperatures of the 
exhaust gases from the engine, the exhaust duct is made of temperature 
resistant material such as titanium, nickel or steel alloy. However, the 
shroud is made of composite material and acts as an insulator to prevent 
heat from radiating from the exhaust duct. Of course, the interior of the 
exhaust nozzle and external surfaces of the shroud are coated with low 
emissivity materials to further reduce the infrared signature of the 
aircraft. 
A first pair of struts are pivotally connected at the center of the 
rectangular exhaust port section by their first ends and extend upward and 
outward from the longitudinal axis of the duct with their second ends 
pivotally connected to the underside of the wing. A second pair of struts 
are pivotally connected to the sides of the transition section by their 
first ends and extend upward and rearwards from the inlet and slightly 
outward and having second ends pivotally coupled to the underside of the 
wing. 
Thus as the propulsion system vibrates or moves under torque loads, etc., 
the exhaust duct can move there along without inducing significant 
structural loads into the exhaust port of the engine or into the aircraft 
structure. Furthermore, even though there is a leakage path about the 
flexible fingers in the inlet of the exhaust duct, the "path of least 
resistance" is along the duct and no exhaust reverses flow direction to 
exit between the inlet and exhaust port. The advantage of the invention is 
that it can be easily retrofitted to exiting aircraft requiring no 
structural reinforcements of the aircraft. 
The novel features which are believed to be characteristic of the 
invention, both as to its organization and method of operation, together 
with further objects and advantages thereof, will be better understood 
from the following description in connection with the accompanying 
drawings in which the presently preferred embodiment of the invention is 
illustrated by way of example. It is to be expressly understood, however, 
that the drawings are for purposes of illustration and description only 
and are not intended as a definition of the limits of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Illustrated in FIG. 1-4 is a C-130 Hercules aircraft manufactured by the 
Lockheed Martin Corporation, generally indicated by numeral 10, while 
illustrated in FIGS. 2-4 are partial views of the aircraft. The aircraft 
10 has a fuselage 12, wings 14A and 14B with four turboprop engines 16 
mounted in nacelles 18. A transition duct 19 is coupled to the exhaust 
port 20 of the engine 16 by means of a clamp 21 and extends out an 
existing opening 22 in the nacelle 18. The subject exhaust duct system, 
indicated by numeral 30, includes a generally S shaped exhaust duct 32 
having a longitudinal axis 33 aligned with the longitudinal axis 23 of the 
transition duct 19 and coupled thereto, supported by a strut assembly 24. 
The exhaust duct 32, having a longitudinal axis includes a circular inlet 
section 38, a rectangular shaped exhaust nozzle section 40 with a 
transition section 42 therebetween. The inlet section 38 includes an 
internally mounted annular ring 44 having a plurality of flat finger like 
members 46 in slidable contact with the transition duct 19. A vertical 
reinforcement 48 extends from the exhaust nozzle section 40 and into the 
transition section 42 and a horizontal reinforcement 50 is mounted in the 
exhaust nozzle section. Mounted about the exhaust duct is a conformal 
shroud 52 joined thereto by a plurality of fastener assemblies 54. Because 
of the high temperatures of the exhaust gases from the engine 16, the 
exhaust duct 32 is made of a temperature resistant material such as 
titanium or steel alloy. However, the shroud 52 is made of non-metallic 
composite material and acts as insulator to prevent heat from radiating 
from the exhaust duct 32 to the ambient environment. Of course, the 
interior of the exhaust nozzle 32 and external surfaces of the shroud 52 
are coated with low emissivity materials to further reduce the infrared 
signature of the aircraft. Sandwiched between the exhaust duct 32 and 
shroud 52 is a thermal blanket 53. 
A fitting 56 is mounted on the top of the exhaust nozzle section 40 along 
the longitudinal axis 33 having two clevises 58A and 58B in close 
proximity to each other, while mounted on each side of the transition 
section 42, equally spaced from the longitudinal axis 33, are clevises 60A 
and 60B. A pair of rear struts 62A and 62B are pivotally mounted by their 
ends 64A and 64B to the clevises 58A and 58B, respectively. Rear struts 
62A and 62B extend upward at an acute angle 62 to each other and generally 
perpendicular to the longitudinal axis 33 and are pivotally connected by 
their second ends 66A and 66B to lugs 68A and 68B, respectively, mounted 
to the underside of the wing 14A. Thus the rear struts 62A and 62B resist 
lateral movement of the exhaust duct 32. A pair of front struts 70A and 
70B are mounted by their first ends 72A and 72B to the clevises 60A and 
60B, respectively. The front struts 70A and 70B extend upward at an acute 
angle 74 to each other and generally upward and rearward and are pivotally 
connected by their second ends 76A and 76B to clevises 78A and 78B, 
respectively, mounted to the underside of the wing 14A in close proximity 
to lugs 68A and 68B, respectively. Thus the front struts 70A and 70B 
resist rearward movement of the exhaust duct 32, but allow lateral 
movement (along with rotation about its longitudinal axis 33 thereof). 
Referring to FIG. 5, it can be seen that the exhaust duct system 30 can 
compensate for movement of the turboprop engine due to torque loads as 
power is increased and, of course, normal engine vibrations. As 
illustrated, the exhaust duct shown in dotted lines and indicated by 
numeral 32' is shown deflected to the left due to engine torqueing. The 
front struts 70A and 70B are rotated to the left, however, the rear struts 
62A and 62B resist lateral movement and the exhaust duct 32 literally 
rotates about the fitting 56 to the position indicated by numeral 32'. 
Thus it can be seen that the exhaust duct system 30 can provide a 
reduction in the infrared signature of the aircraft 10 by blocking off 
direct viewing of the hot sections of the engine 16. The use of low 
emissivity coatings on the interior surfaces of the exhaust duct 30 
further reduces the aircraft's signature from the rear. Finally, the 
shroud 52 and thermal blanket 53 also shield the heated exhaust duct 30 
from infrared detection from directly below the aircraft. 
While the invention has been described with reference to a particular 
embodiment, it should be understood that the embodiment is merely 
illustrative as there are numerous variations and modifications which may 
be made by those skilled in the art. Thus, the invention is to be 
construed as being limited only by the spirit and scope of the appended 
claims. 
INDUSTRIAL APPLICABILITY The invention has application to the commercial 
and military aircraft industry.