Complete Specification of
INFRA-RED SUPPRESSION SYSTEM (IRSS) USING TO REDUCE THE IR
SIGNATURE OF THE HELICOPTER EXHAUST

FIELD
The present invention is to reduce the IR signature of the helicopter by means of blocking the visibility of hot engine exhaust parts with insulating duct.
BACKROUND
The passive nature of infrared detectors makes them possible to acquire a target at sufficient ranges and provide sufficient engagement boundaries. The survivability of the helicopter is its ability to withstand man made hostile conditions without aborting its mission in between. During helicopter operations, the difference between the emission of the hot parts of the helicopter and the surrounding atmosphere produces IR signatures, which can be detected by heat-seeking missiles.
Most of the metals can be approximated as grey bodies, which emit radiation at all wavelengths with the variation of emissivity with wavelength being insignificant. The exhaust plume on the other hand exhibits band emissivity and emits selectively in some discrete bands through out the whole electromagnetic spectrum. Helicopters use turbo shaft engines, and hence the velocity, mass flow and temperature of the exhaust plume are significantly lower than that of an aircraft engine. The exhaust plume from the helicopter is immediately dissipated into the ambient due to main rotor wake, and hence the contribution of exhaust plume in total IR signature of a helicopter is not significant. The major sources of IR radiation in a helicopter are the hot powerplant parts, exhaust duct and associated helicopter structure that gets heated by the exhaust plume. Hence the IR signature of a helicopter depends mainly on the visibility of hot parts, their area, temperature and emissivity. The objective of the IRSS system is to minimise the visibility of hot parts and reduce the temperature of the visible parts so that the IR radiation in contrast to the background is insignificant.
SUMMARY
The present invention is an insulated type Infra red Signature Suppression which is installed to cover (optical blockage) of engine exhaust. This has been conceptualised in co-ordination with I IT Bombay, Mumbai.
To increase the effectiveness of countermeasures, the IR signatures of the helicopter should be much lower than that of the countermeasure used. To bring the IR emission level close to the surroundings, the IR emission from power plant installation has to be reduced. Engines are operated at the highest permissible temperature when maximum power is required, and the reduction in temperature from the point of IR radiation will reduce the power available to the helicopter. Increase in weight due to the addition of the IR signature suppression system will decrease the helicopter performance. Any modification in the exhaust system configuration may increase the back pressure, thereby reducing the engine power available to the helicopter. Hence an improved IRSS system has been conceptualized for a typical 4-5 ton class Military helicopter aiming at minimum back pressure and maximum blockage of hot parts.

BRIEF DESCRIPTION OF THE DRAWINGS:-
FIG. 1 shows the IRSS front and side view on the helicopter. IRSS covers the engine exhaust duct such that the hot exhaust are not visible from large view angle. It is an add-on type system, fastners are used to install IRSS on the helicopter to facilitate the mission requirements (if it is not required it can be removed from the helicopter).
FIG. 2 shows the cross sectional view of IR suppression system. It consists of insulated duct, which covers the engine exhaust duct such that the hot exhaust system parts are not visible from large view angle and the visible external surfaces of the IR suppressor is at a temperature close to the atmospheric temperature, thus reducing the IR signature.

DETAILED DESCRIPTION:-
IRSS consists of two concentric titanium ducts filled with insulating material in between to reduce the outer surface temperature. The gap between the two titanium ducts is calculated depending on the exhaust gas flow, its temperature, out side temperature and required temperature on the outer surface. A gap is maintained all around between engine exhaust and IRSS for secondary air flow. Overlap has been provided to reduce the visibility of hot engine exhaust system parts from large view angles. IRSS attached to the helicopter with composite flange to suit the helicopter outer contour.
IRSS has been installed on helicopter and tested. The results show that the outer surface temperature of the IRSS is close to atmospheric temperature, thus reducing the IR signature.

CLAIM
1. Concept of infra-red suppression system (IRSS) for camouflaging the hot helicopter engine exhaust duct to reduce the IR signature from the large view angle.
2. It is an add-on type system, fastners are used to install IRSS on the helicopter to facilitate easy removal and installation to meet the mission requirements (if it is not required it can be removed from the helicopter).
3.IRSS manufacturing is very easy. It contains of two cylindrical concentric titanium ducts filled with insulating material in between and both the titanium sheet stitch welded at both the edges.