Infrared suppressor for a gas turbine engine

An IR suppressor produces a thin "ribbon" exhaust plume using a tapered exhaust manifold which has a plurality of discrete exhaust nozzles that are longitudinally aligned with the exhaust manifold. Optionally, the nozzles extend within but are spaced apart from mixing ducts which are open to the ambient air at both ends. The mixing ducts mix ambient air with the exhaust plume. In another aspect of this invention, a single nozzle (which is longitudinally aligned with the manifold) is substituted for the plurality of discrete exhaust nozzles. In this aspect, the nozzle extends within but is spaced apart from a mixing duct which is open at both ends and has a curve sufficient to block a line of sight to the nozzle. A helicopter that has a rotatable IR suppressor so that the exhaust can be directed substantially parallel to the helicopter blades when the blades are not turning to protect them from exhaust heat is also disclosed.

TECHNICAL FIELD 
The field of art to which this invention pertains is suppressor systems for 
gas turbine engines. 
BACKGROUND ART 
The advent of infrared radiation (IR) homing weapons has greatly increased 
the vulnerability of aircraft such as airplanes, missiles, and helicopters 
to air and ground launched missile attack. These aircraft discharge hot 
engine exhaust which has a strong infrared radiation signal. In addition, 
the propulsion means for the aircraft invariably include hot metal parts 
that radiate a strong infrared radiation signal. The hot metal parts are 
normally associated with either the gas turbine or jet engine exhaust 
system. In order to decrease this vulnerability of aircraft, it is 
necessary to reduce or suppress this infrared radiation signal or 
signature. Typically, a reduction in IR signal has been approached in two 
manners. A first approach has been the mixing of cooling air with the 
engine exhaust to lower its temperature and IR signal. A second mechanism 
has been to block a direct line of sight into the core engine. While 
certain advancements have been made in this direction, considerably 
further effort is still required to produce satisfactory IR suppression 
results. 
DISCLOSURE OF INVENTION 
This invention is directed to an infrared suppressor for a gas turbine 
engine that, through an efficient configuration, mixes ambient air with 
the exhaust to lower the IR exhaust signal, reduces the IR signal of hot 
exhaust parts and eliminates a line of sight into the exhaust nozzle. 
The present invention produces a thin "ribbon" exhaust plume using a 
tapered exhaust manifold which has a plurality of discrete exhaust nozzles 
that are longitudinally aligned with the exhaust manifold. Optionally, the 
nozzles extend within but are spaced apart from mixing ducts which are 
open to the ambient air at both ends. The mixing ducts mix ambient air 
with the exhaust plume. 
In another aspect of this invention, a single nozzle (which is 
longitudinally aligned with the manifold) is substituted for the plurality 
of discrete exhaust nozzles. In this aspect, the nozzle extends within but 
is spaced apart from a mixing duct which is open at both ends and has a 
curve sufficient to block a line of sight to the nozzle. 
Yet another aspect of this invention is a helicopter that has a rotatable 
IR suppressor so that the exhaust can be directed substantially parallel 
to the helicopter blades when the blades are not turning to protect them 
from exhaust heat. 
Other features and advantages will be apparent from the specification and 
claims and from the accompanying drawings which illustrate an embodiment 
of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION 
In FIG. 1, the exhaust manifold 3 has an inlet 4 for receiving exhaust 
gases from an engine. The exhaust manifold 3 is tapered to have a 
progressively decreasing cross-sectional area from the inlet 4 (upstream) 
to the closed end 7 (downstream) of the manifold. The exhaust manifold 3 
may have a variety of cross-section shapes such as round, oval, square 
etc. The exhaust manifold 3 has a plurality of nozzles 6 extending from 
the manifold that are disposed along a substantial length of the exhaust 
manifold 3. The nozzles 6 may be integrally formed in the exhaust manifold 
3 or they may be fastened on by conventional means such as fasteners. 
Nozzle openings 9 and the exhaust manifold 3 are longitudinally aligned. 
The nozzles 6 extend substantially normal from the exhaust manifold 3 as 
this reduces the nozzles 6 acting like a turning vane (e.g., the nozzles 6 
increasing back pressure). Thus, each nozzle 6 has a substantially 
unobstructed exhaust flowpath to the ambient air. Preferably, the nozzle 
openings 9 have a length to width ratio that is above about 6 to 1 as this 
provides a long thin plume resembling a gaseous "ribbon". Since the ribbon 
is thin in cross-section, it is less visible to IR detectors. Furthermore, 
the ribbon exhaust is more readily dissipated into the surrounding 
atmosphere further reducing the IR signature. 
The exhaust manifold cross-section area at any preselected point along the 
manifold 3 has a total area that substantially equals the total area of 
the nozzles 6 downstream of the preselected point. In conjunction with 
this equivalence of manifold inlet area and nozzle area, the taper of the 
exhaust manifold 3 is such that there is a substantially constant exhaust 
velocity flow and substantially constant exhaust velocity pressure along 
the length of the exhaust manifold 3. This results in a substantially 
constant exhaust gas turning angle along the length of the exhaust 
manifold 3 through the nozzles 6 without the use of turning vanes which 
may increase back pressure and result in hot metal parts. The particular 
turning angle achieved is dependent on and varies with the particular 
exhaust velocity. An exemplary configuration used for helicopter 
applications results in a constant turning angle of about 45.degree.. 
A plurality of mixing ducts 15 for mixing cooling air with the exhaust to 
lower the intensity of the exhaust IR signal, are disposed along the 
exhaust manifold 3. The mixing ducts have an upstream orifice 18, a 
downstream orifice 21, and sidewalls 24. The mixing ducts extend 
substantially normal to the manifold 3. Thus, the nozzles 6 extend within 
and are aligned with the mixing ducts 15. These mixing ducts may be 
attached directly to the exhaust manifold by conventional means such as 
brackets 25 or may be disposed over the manifold by a suitable airframe 
attachment. As illustrated in FIG. 3, the mixing ducts 15 are also spaced 
apart from the exhaust manifold 3. This provides an inlet space between 
the nozzles 6 and the mixing ducts 15. The mixing ducts 15 also inhibit a 
direct line of sight 27 to the exhaust nozzles 6. Thus, the depth and 
spacing of the mixing duct sidewalls 24 are selected to prevent a direct 
line of sight 27 to the hot exhaust nozzle 6 from preselected angles. 
Thus, the exhaust nozzle is not visible from limited viewing angles such 
as those that are substantially perpendicular to the mixing duct sidewalls 
24. 
As the exhaust is discharged from the nozzles 6 through the mixing ducts 15 
a venturi effect is created drawing ambient air under atmospheric pressure 
into the upstream orifice 18 of mixing ducts 15 through the inlet space 
surrounding the nozzles 6. The air mixes with the exhaust gas in the 
mixing ducts 15 and is discharged at a cooler temperature from the 
downstream orifice 21 of mixing ducts 15 thus lowering the intensity of 
the IR signal. In addition, the cooling air lowers the temperature of the 
mixing ducts sidewalls 24 since a blanket of cooling air is disposed 
between the sidewalls 24 and the hot exhaust gases. Thus, the IR signal of 
hot metal parts is also reduced. 
In FIG. 4, the plurality of longitudinally aligned discrete nozzles have 
been substituted by a single continuous nozzle. A single curved mixing 
duct has been substituted for the plurality of straight mixing ducts. The 
exhaust manifold 53 has an inlet 54 for receiving exhaust gases from an 
engine. As illustrated in FIGS. 4 and 5, the exhaust manifold 53 is 
tapered to have a progressively decreasing cross-sectional area from the 
inlet 54 (upstream) to the closed end 57 (downstream) of the manifold. The 
exhaust manifold 53 has a nozzle 56 extending from the manifold that is 
disposed along a substantial length of the exhaust manifold 53. Nozzle 
opening 59 and the exhaust manifold 53 are longitudinally aligned. As with 
the previous configuration, the nozzle 56 has a substantially unobstructed 
exhaust flowpath to the ambient air. Preferably, the nozzle 56 has a 
length to width ratio that provides the highest practical ratio preferably 
above about 60 to 1 as this provides a long, thin plume resembling a 
gaseous "ribbon". 
A mixing duct 65 is disposed external to the exhaust manifold 53. The 
mixing duct 65 mixes cooling gases with the exhaust to lower the IR signal 
of the exhaust. In FIG. 4, the mixing duct 65 comprises a mixing duct 65 
disposed along the exhaust manifold 53. The mixing duct 65 extends 
substantially normal to the exhaust manifold 53 and then curves transverse 
to the exhaust manifold. The mixing duct 65 has an upstream orifice 68, a 
downstream orifice 71 and sidewalls 74. The mixing duct upstream orifice 
68 is in fluid communication with the nozzle opening 59. The nozzle 56 
extends within and is aligned with the mixing duct 65. The mixing duct 65 
is also spaced apart from the exhaust manifold 53. This provides an inlet 
space between the nozzle 56 and the mixing duct 65. 
As with the previous configuration, this IR suppressor lowers the IR image 
by mixing the exhaust gas with ambient air and discharging the resultant 
mixture through the mixing duct. In addition, a cooling blanket of the 
ambient air coats the mixing duct 65 interior surfaces preventing any 
significant temperature rise, thus lowering the IR signal. Thus, the IR 
signature of the exhaust gas and the hot metal parts is reduced. Finally, 
since the mixing duct 65 curves transverse to the exhaust manifold 53 a 
line of sight is obstructed into the exhaust nozzle as depicted in FIG. 6. 
These IR suppressor systems can be rotatable in contrast to prior art 
systems in order to change the exhaust plume direction. Cross-section 
FIGS. 3 and 6 illustrate this feature for the two IR suppressors shown in 
FIGS. 1 and 3 respectively. As shown in FIG. 2, means for rotating the 
suppressor such as an actuator 2 is utilized. In helicopter 1 applications 
similar to the type in FIG. 1A, direction of the exhaust plume towards the 
helicopter blade results in increased IR suppression through the exhaust 
mixing with the helicopter down wash. In addition, this eliminates ground 
and most air based line of sight viewing angles. When the helicopter is on 
the ground and the blades are not turning, direction of the exhaust plume 
substantially parallel (shown in phantom, FIGS. 3 and 6) to the blades 
protects the blades and any surface combustibles. 
This invention provides an IR suppressor that reduces the IR signal of the 
exhaust gas through mixing ambient air with the exhaust in addition to 
rotor and free air stream interaction. In addition, the suppressor reduces 
the IR signal of the hot exhaust parts and eliminates a line of sight into 
the exhaust nozzle without creating significant back pressure. Finally, it 
increases the pumping action of the exhaust manifold. These suppressors 
use low weight configurations to achieve the above results. Thus, this 
invention makes a significant contribution to the field of IR suppression 
by providing an IR suppressor that significantly reduces the overall IR 
signal in an efficient manner. 
It should be understood that the invention is not limited to the particular 
embodiments shown and described herein, but that various changes and 
modifications may be made without departing from the spirit and scope of 
this novel concept as defined by the following claims.