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# Infrared Signature Suppression Systems in Modern Military Helicopters

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## Abstract and Figures

Helicopters play an important role in air-to-ground fire covering and the short-distance air-to-air fights, as well as the anti-tank missions and battlefield force transferring. The detection and survivability of helicopters on a battlefield significantly depends on their infrared emissions level, as well as the methods, equipment and systems used by potential enemy. The automatic detection systems, recognition and identification of flying objects use among other the thermo-detection methods, which rely on detecting the infrared radiation emitted by the tracked object. Furthermore, due to low-altitude and relatively low flight speed, today’s combat assets like missile weapons equipped with infrared guidance systems are one of the most important threats to the helicopters performing combat missions. Especially meaningful in a helicopter aviation is infrared emission by exhaust gases, egressed to the surroundings. Due to high temperature, exhaust gases are a major factor in detectability of a helicopter performing air combat operations. In order to increase the combat effectiveness and survivability of military helicopters, several different types of the infrared suppressor (IRS) have been developed. This paper reviews contemporary developments in this discipline, with particular examples of the infrared signature suppression systems.
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64 Mateusz Paszko
radiation of solids – including heat exchange inside gearbox, engine components, heat shields
and nacelles,
radiation of aerodynamically heated surfaces e.g. main rotor and tail rotor,
radiation of exhaust gases – emitted into spherical area of helicopter environment,
reected radiation from exhaust stream, sunshine, skyshine and earthshine.
The arms race creates ever newer and more technically advanced detection, observation, tracking
and infrared homing missile systems. This forces aircraft designers to design and adapt counter
solutions to reduce the risks and consequences of a potential strike. Generally, the infrared sensing
devices can be divided into two groups, which can complement each other:
IRST (Infra Red Search and Track) – the infrared scanner primarily developed for object
detection, localization and tracking and thermal mapping of land surface,
FLIR (Forward Looking Infra Red) – the tracking and aiming devices, mainly to ground–air applications.
Modern military helicopters have a variety of the construction solutions aimed to reduce their
potential detection in infrared and reduce the possibility of destruction by infrared-guided missiles.
Helicopter protection systems can be divided into active and passive. The active protection serves
only to throw infrared-guided missiles o the scent and is divided into two types: electrical equipment,
sending a strong surge of electromagnetic radiation in the infrared spectrum that causes blindness to
the missiles and heat devices like rocket ares – the heat traps hindering the missile nding the aim
true position. A signicant disadvantage of this type of protection is high cost. The electrical devices
are fairly unreliable and consume high amounts of power. On the other hand, depending on their
amount in the tray, ares always have a nite number. The passive infrared protection is a group
of components integrated into the helicopter structure that include:
1. Fuselage surface shaping and coating with radar absorbent materials
2. Exhaust diusers modications
3. Engines shielding and nacelles modications
4. Cooling of exhaust gases in special heat exchangers
Fig. 1. Sources of IR signature of helicopter in ight, [2]
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Infrared Signature Suppression Systems in Modern Military Helicopters
Radiation energy emitted on specied body surface, can be absorbed, reected or transmitted
through the body. In most of solid bodies e.g. metals, the radiation transmissivity is equal to zero.
Earthshine reected by the rear-fuselage skin is signicant in dictating aircraft susceptibility to
infrared-guided SAMs in the 8–12 lm band, but the role in the 3–5 lm band is insignicant. In the
case against a SAM, the lower portion of the rear-fuselage skin is visible to the SAM. In the case
of an AAM, the side portion of the rear-fuselage skin is visible. Therefore, dierent parts
of the rear-fuselage skin should have dierent emissivity, for minimizing its infrared signature
level against infrared-guided SAMs and AAMs, [3].
One feasible technique for reducing the infrared signature level from a metallic surface is the use
of special Radar Absorbent Materials (RAM) which absorb part of the received electromagnetic
energy and dissipate it to heat, thus reducing the reected energy, [5]. Aircraft EM absorbers take
several forms:
structural materials and coatings specially designed for reduced radar reectivity,
coatings, including paints, specially designed for reduced or tailored reectivity or emissivity
in the microwave, infrared or ultraviolet spectra.
Most of the materials used for signature control were originally developed for military
aircraft and are found on both xed- and rotary-wing systems. Modied versions of the materials
and treatment techniques are found on some ships, submarines, and ground combat and tactical
vehicles[5,6]. This approach has been followed since WWII, where special paints containing carbon
(an imperfect conductor) have been used to reduce the radar return of the snorkels of German
submarines. Even though carbon is still being used for such purposes, today magnetic absorbers,
based on compounds of iron, are preferred for operational systems, [7].
Fig. 2. Eects of rear-fuselage skin emissivity on aircraft lock-on range and missile infrared irradiance, [4]
66 Mateusz Paszko
3. EXHAUST DIFFUSERS MODIFICATIONS
Temperature distributions on the fuselage skin and in the exhaust plume have
a direct impact on infrared signature of helicopters [2]. Exhaust gases outflowing from
the helicopter’s engines to the surroundings, firstly create compact streams geometries
but as a result of the rotor downwash impact they are developed in distinctive clouds
around the back and bottom parts of the helicopter. It is also important to point that exhaust
distribution in the surrounding of the fuselage is strictly depended on the helicopter
flight maneuvering and weather conditions e.g. presence of the wind. The theoretical
considerations on the spread in the environment and changes in the speed, temperature
and concentrations of exhaust, outflowing from a helicopter turbine engine during chosen
experimental flight maneuvers, were presented in [8]. Fig. 3 presents the chosen frame from
video representing the PZL W-3 Falcon (with standard exhaust system) flight with visible
zones of outflowing exhaust gases.
Fig. 4-6 present chosen results of the analysis of distribution of exhaust in various phases of PZL
W-3 helicopter ight.
The results of the analysis on these phenomena were the basis for developing a simulation
computer program, presented in [9]. The paper presents the 3D model and simulation of exhaust
gases expansion in various vertical maneuvering of PZL W-3 helicopter in different speeds
of side wind. Chosen results of a numerical simulation were presented in Fig. 7 and Fig. 8
below.
Fig. 3. PZL W-3 helicopter progressive ight in mountainous terrain with visible exhaust gases areas of both
propulsion engines: A - windward side, B - leeward side, [8]
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Infrared Signature Suppression Systems in Modern Military Helicopters
1. 2.
1 – left-side engine: d = 3 ∙ 10-5ξ2 + 0,1876 ∙ ξ + 520,78mm
2 – right-side engine d = 1 ∙ 10-5ξ2 + 0,235 ∙ ξ + 518,16mm
Fig. 4. Distributions and approximations of transverse dimensions of exhaust streams along the natural
coordinates in level ight -VL=130km/h, [2]
left-side engine: d = -5 ∙ 10-5η2 + 0,7114 ∙ η + 387,35mm
Fig. 5. Distribution and approximation of transverse dimensions of exhaust streams along the natural coordinate
in hover - VL=0 km/h, H=15m, [2]
68 Mateusz Paszko
1. 2.
1 – VL = 100km/h, d = -3 ∙ 10-5η2 + 0,3603 ∙ η + 362,51mm
2 – VL = 130km/h, d = -6 ∙ 10-5η2 + 0,2498 ∙ η + 519,65mm
Fig. 6. Distributions and approximations of transverse dimensions of exhaust streams along the natural coordinates
in level ight, 1- VL=100 km/h, 2- VL=130 km/h, [2]
Fig. 7. Stream lines of exhaust outowing from engine’s diusers for helicopter in hover without wind, a – front view,
b – plan view, [9]
b.
a.
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Infrared Signature Suppression Systems in Modern Military Helicopters
According to [9], simulated streamlines and velocity distributions of the exhaust outowing
from the helicopter engines indicate presence of non-deected (from the directions of the collector
axes) dense parts of the streams at certain distances from the collectors outlet cross-sections. Such
a conguration of exhaust streams is a result of the existence of a „well of ow” in the downwash
stream (exhaust manifolds are located in the zone of this well). Simultaneously, momenta and kinetic
energies of the exhaust at the aforementioned distances are not yet subject to signicant dislocations.
Based on the visualization of the obtained streamlines distributions and ow velocities, signicant
changes in the behavior of the exhaust streams are observed beyond the dense areas. Under
the inuence of the downwash and the wind, the streams are deviated from the initial directions
and deforms, which consequently leads to the scattering of exhaust in form of clouds in the distal
area of the rotor downwash.
In order to limit turbine blades visibility, reduce emission from exhaust gases and also to avoid
heating the tail boom, outlet diusers are set under specic pitch to the longitudinal engine axis.
Results of the simulation of the plume ow elds under downwash were presented in [10].
According to [3], the exhaust plume takes on strong downwards deection to the rear fuselage,
as well as deection to the rotor’s rotational direction, under the action of rotor downwash. These
deections are especially obvious under higher rotor downwash. When the exhaust is ejected
upward, the exhaust plume could come into collision with the rear fuselage, and pumping capacity
of the exhaust system is weakened a little. While the exhaust is ejected in oblique or lateral
directions, the exhaust plumes do not come into collision with the rear fuselage, and pumping
capacities of the exhaust system are somewhat enhanced. The exhaust direction shows signicant
inuence on the infrared radiation distribution, as seen in Fig. 9. In this gure, downwash denotes
the downwash velocity. When the exhaust is ejected in the oblique direction, the infrared radiation
intensity detected from the top direction is almost the same as that from the lateral direction.
While the exhaust is ejected upward or sideward, strong infrared radiation occurs at some viewing
directions.
b.
a.
Fig. 8. Structures of exhaust streams for helicopter in hover with: a – head-wind at speed of 10m/s, b – crosswind
at speed of 10m/s, [9]
70 Mateusz Paszko
Upward – turned exhaust
Oblique – turned exhaust
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Infrared Signature Suppression Systems in Modern Military Helicopters
An example of exhaust system modernization was OH - 58D Kiowa Warrior helicopter. Exhaust
system has been located on the right side of the hull’s upper surface and exhaust gases were directed
upwardly. Modied exhaust outlet with exhaust cooler didn’t result in an increase of the construction’s
mass and also didn’t signicantly aect the performance of the helicopter’s engine. Mixing hot
exhaust with rotor downwash resulted in a signicantly lowered infrared emission. Fig. 10 shows
the results of comparing the infrared emissions before and after modication of helicopter exhaust
system.
Lateral – turned exhaust
Fig. 9. Plume ow elds under downwash of 10m/s for dierent congurations of exhaust diusers, [10]
a. b.
Fig. 10. OH-58D Kiowa Warrior in infrared, a – before modication, b – with modied exhaust system, [11]
72 Mateusz Paszko
4. ENGINE SHIELDING AND NACELLES MODIFICATIONS
Because of high power output with relatively low weight, turbine engines revolutionized the aviation
industry. Nowadays turboshaft engines, remain an almost exclusive propulsion type in military
helicopters. Transient temperature elds and temperature gradients that occur in all sections of the turbine
engine, i.e. compressor, combustion chamber, turbine assembly and diuser cause signicant emissions
of infrared radiation. Its presence results from the temperature dierence between engine surface
and the environment. The combustion process of fuel and air mixture continuously heats the surrounding
surfaces and components of the engine. The most thermal loaded parts of a turbine engine are its rst stage
turbine blades. As a result of high temperature processes inside engine duct, large amounts of heat need to
be discharged outside of the engine structure. Otherwise, the engine parts will overheat and get damaged.
On the other hand, hot exhaust gases and heat emission from engines parts to the surroundings exposes
an aircraft to high risk on a battleeld. Heat-seeking missiles and other modern combat assets are guided
into the infrared glow of aircraft hot parts. Infrared stealth, requires that aircraft parts and emissions,
particularly those associated with engines, have to be kept as cool as possible.
Fig. 11. Typical placement of turbine engine in actual helicopters: a – in longitudinal planes , b – in transverse planes, [12]
a.
b.
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Infrared Signature Suppression Systems in Modern Military Helicopters
Placement of engine in hull structure relative to the main rotor axis is important not only
for cooling process but also engine smooth operation.
Fig. 13 presents the temperature distribution along the engine inlet, surface of the gondola cover
and exhaust gas diuser at the helicopter take-o maneuver – the engine in take-o mode.
Heat shields and nacelles are cooperating systems. Heat shields are used on most engines to
protect components and bodywork from heat damage and nacelles from heating. Heat shields can
be divided into rigid, exible and textile. The rigid heat shields are usually made from aluminum,
aluminum sheet or other composites, with a ceramic thermal barrier coating to improve the heat
insulation. The exible heat shield is normally made from thin aluminum sheeting though high
performance exible heat shields sometimes include extras, such as ceramic insulation applied
via plasma spraying. There are also textile heat shields used for protection of various components
a. b.
Fig. 12. PZL-10W turbine engine mounted on PZL W3 Falcon helicopter, a – with heat shield, b – without
heat shield, [2]
Fig. 13. Temperature distribution along the engine inlet, surface of the gondola cover and exhaust gas diuser, [2]
74 Mateusz Paszko
such as electric wires, fuel system or other engine supporting systems. Usually the space between
engine and nacelle creates a ow duct for engine and exhaust system cooling air. Cooling streams
in engine internal parts can be generated in two ways:
1. By an additional fan driven from the engine shaft – the air ow is introduced inside by a system
of channels
2. Through the use of uneven distribution of velocity and pressure in the rotor wake vortex
Fig. 14. presents cooling process of engine duct, supported by surrounding air ow and rotor downwash.
The nacelle structure has special holes for the purpose of capturing the rotor wake vortex
and cooling engine parts. Such structured cooling process eliminates the possibility of deterioration
of physical properties and durability of materials, reduce thermal loads of engine and its components
and allows to obtain engine performance provided by the manufacturer. The ow of cooling air inside
nacelle duct is caused by the pressure dierences in the inlet and outlet vents, spread over the nacelle
surface. Flow conditions of cooling air inside nacelle duct are mainly formed by ow velocities
and static pressures in nacelle surrounding and inuence of rotor downwash. The weather conditions
under which the helicopter is operated create an additional impact on cooling process eciency.
Often, these conditions inhibit cooling process e.g. due to the high temperatures of atmospheric air.
5. EXHAUST DESUPERHEATERS
When designing new helicopter types, especially for combat applications, it is essential to pay enormous
attention to infrared emissions of the solid parts composing the helicopter’s structure, as well as to exhaust
gases egressing from the engine’s exhaust system. Due to their high temperature and signicant amounts
of carbon dioxide and water vapor, exhaust gases egressed to the surroundings are a major factor in infrared
radiation emission and, in consequence, detectability of a helicopter performing air combat operations.
One of protection methods of a helicopter in ight is cooling the exhaust gasses, emitting
from the engines to the atmosphere, in special heat exchangers. Nowadays this process is realized
in non-diaphragm coolers, where strong heat and momentum exchange between hot exhaust gases
Fig. 14. Schematic ow around the nacelle zone with heat screen in PZL W-3 Falcon helicopter in level ight:
a – cooling air ow inside nacelle duct, b – pressure distribution diagram on the nacelle surface, [12]
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Infrared Signature Suppression Systems in Modern Military Helicopters
and cold air injected from atmosphere takes place. While mixing, agents exchange energy on the heat
tract, changing their enthalpy, [2,13]. Also the chemical composition of the exhaust changes.
In aviation there are two types of non-diaphragm exchangers, namely:
non-diaphragm exchangers in which both agents are introduced into a common mixing chamber
with a conveying equipment like pumps, fans, or compressors,
non-diaphragm exchangers in which one of agents (usually cooling) is introduced into the mixing
chamber due to ejection process caused by ow of a second agent (cooled).
The main advantage of this type of coolers is high intensity of heat transfer that allows to reduce
their mass and dimensions.
An example of a non-diaphragm exhaust cooler, adapted to co-operate with the PZL 10W engine
of the PZL W-3 helicopter was presented by Fijałkowski in [2]. Fig. 15 and Fig. 16 present general
project of a desuperheater adapted to work on a board of PZL W-3 helicopter.
Fig. 15. Cross-section of side view of the cooler adapted to cooperate with PZL 10W engine on board of PZL W-3
helicopter: 1 – PZL 10W engine, 2 – modied exhaust diuser (engine side), 3 – exhaust nozzle, 4 – desuperheater
diuser, 5 – heat shield of engine diuser, 6 – desuperheater internal heat shield, 7 – desuperheater external heat shield,
8 – main swirler in mixing chamber, [2]
Fig. 16. Conceptional conguration of PZL W-3 helicopter equipped with exhaust desuperheater according to design
from Fig. 15, [2]
76 Mateusz Paszko
Fig. 17. General scheme of turbine engine – exhaust desuperheater system, [2]
Fig. 18. Hypothetical distributions of pressures and temperatures of exhaust and atmospheric air in various
sections of exhaust desuperheater, [1]
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Infrared Signature Suppression Systems in Modern Military Helicopters
In general, conceptional helicopter exhaust cooler is formed by a modied turbine engine
exhaust system, ejection chamber, mixing chamber and outow diusor (Fig. 17). Exhaust gases
outowing from power turbine are directed through diusor to nozzle where their static pressure is
lowered below the atmospheric pressure. As a result of exhaust static pressure decrease and velocity
increase, suction of cold air stream into cooler ow channel succeed. In the mixing chamber,
two streams undergo intensive mixing where strong heat and momentum exchange takes place.
As a result of mixing hot exhaust with cold air injected from surrounding new stream is generated
with signicantly lowered temperature and changed chemical composition (Fig. 18).
Process of cold atmospheric air ejection into the cooler duct signicantly depends on the static
pressure level in engine exhaust system section. Application of nozzle in diuser outlet section leads
to static pressure decrease in engine outlet collector and consequently in cooler mixing chamber
below the atmospheric pressure level. The lower static pressure in cooler duct the greater volume
of external air ejected and consequently increase in eciency of cooling.
Fig. 19. Comparison of ow processes in section between power turbine and outlet section of engine exhaust
system for two variants of helicopter engine exhaust system conguration, [14]
78 Mateusz Paszko
Simulation of PZL W-3 helicopter equipped with the cooler in selected NOE ight maneuvers
was presented in [15]. The developed calculation program included calculations of the thermal
and gas-dynamic parameters of the exhaust gases at the outlet section of the collector and calculating
the process of cooling the exhaust gas in the cooler. As a result of the simulation, the values
of the temperature and partial pressures of the mixture of exhaust gases and air outowing to
the environment were obtained. According to [15], use of exhaust gas coolers in helicopter turbine
engine exhaust systems signicantly reduces the cause of excessive infrared emissions and also changes
the conguration of the exhaust streams in the environment. Thus, the range of static temperature
variations in the outlet crossings of the exhaust systems depending on the helicopter maneuver is as
follows: in the braking and returning maneuver - in the case of the classic exhaust system 607K-690K,
when using a cooler 344K-372K, in the jump up maneuver - in the case of the classic exhaust system
641K-720K, when using a cooler 355K-383K, in the, the fast acceleration from hovering maneuver
- in the case of the classic exhaust system 646K-731K, when using a exhaust cooler 359K-389K.
Another example of a non-diaphragm exhaust cooler is HIRSS (Hover InfraRed Suppression
System) that has been developed to reduce the infrared emissions of the exhaust from helicopter
engines, such as the General Electric T-700 engine employed in helicopter designs such as the Black
Hawk UH-60, the Apache AH-64 and the AH-1. Example of HIRSS used on AH -1 Super Cobra
version was shown below, where reected thermal signature of tailboom was signicantly decreased.
a. b.
Fig. 20. AH-1 Super Cobra: a – without exhaust system modication , b – with HIRSS system, [21]
a. b.
Fig. 21. AH-1 Super Cobra tailboom in infrared, a without exhaust system modication, b with HIRSS
system: 1 – exhaust diuser, 2 – exhaust gases, 3 – tail boom, [11]
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Infrared Signature Suppression Systems in Modern Military Helicopters
The suppression system reduces infrared emission by recirculating hot engine exhaust gases
within the suppressor core and mixing the heated gases with ambient air before discharging into
the atmosphere.
Exhaust cooling system in AH- 64 Apache helicopter called Black Hole also works on
the principle of atmospheric air ejection. The BH is a low-cost IR suppression system without any
moving parts [16], as seen in Fig. 22.
The principle of this system’s operation is directing the engine exhaust through special ducts
that connect the engine exhaust stream with a stream of a cold air passing over the helicopter.
These channels ensure uniform outow of exhaust stream which gives a better heat dissipation
eect by the air stream. The fresh air-stream dissipates the hot exhaust that emerges from
the vents evenly, rather than allowing hot spots to appear. Also, before leaving the Black Hole
system, air-exhaust mixture is directed through a special insert made from heat absorbing
material.
Fig. 22. Exhaust cooling system in AH-64 Apache, [17]
Fig. 23. AH-64 Apache rear view, 1 – engine, 2 – exhaust cooler, [21]
80 Mateusz Paszko
In order to additionally improve the helicopter’s thermal signature reduction, ejected fresh air is
used to cool both the engines and the transmission. The engine exhaust nozzles are angled outward
from the airframe to better direct the output into the air-stream and they are cooled with outside air
from rotor downwash (while hovering in place) or with turbulent stream during progressive ight.
Mi -24 Hind is a Russian heavy attack helicopter. During the Afghan war, helicopters were
modernized and they received complex self-defense systems. In the rst phase, the thermal traps
launchers (ares and metallic foil strips ASO- 2W) were added. Helicopters were also equipped
with distracting shields EWU, designed to cool the exhaust gases. EWU reduced the exhaust gas
temperature from 500-600 oC to 150-200 oC and helicopter thermal signature up to 60 %, [18].
The third system was the infrared active interference station Ł166W 1A.
The applied counter-measures of the Russian helicopters utilized in the described devices
reduced losses in man and equipment to a signicant low during ghting in Afghanistan. Before
the modications, the average accuracy of surface-to-air missiles red was a hit of one out of ten
red, after the modications just one out of a hundred red hit the target.
The most advanced low-detectability helicopter is the prototype of RAH 66 Comanche (Fig. 25).
It is mainly made from composites and it is covered with radar absorbent materials. To reduce
the radar cross-section, the all-composite fuselage sides are at and canted, and rounded surfaces
are avoided by use of faceted turret and engine covers. The Comanche helicopter was equipped
with an innovative method of infrared emission reduction, wherein the exhaust gases cooler is an
integral part of the tail boom. In this solution, the exhaust outlet is disposed over the entire length
of the tail boom providing fast, full and eective mixing of the hot exhaust gases and cold rotor
downwash. The mixed exhaust is discharged through slots built into an inverted shelf on the sides
of the tail-boom.
Fig. 24. Mi-24 Hind helicopter after modernization, 1 – engine,
2 – exhaust cooler air intake, 3 – exhaust cooler, [21]
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Infrared Signature Suppression Systems in Modern Military Helicopters
Comanche’s RCS was 360 times smaller than that of the AH-64 Apache attack helicopter, [19,20].
Because of that, the Comanche’s chances of detection, tracking and destruction by heat-seeking
missiles are negligible. Because of such an eective exhaust cooling system, the helicopter doesn’t
need an infrared band active jammer or interference generators.
CONCLUSIONS
The detection of helicopters on the battleeld signicantly depends of their emission of infrared
radiation, as well as the methods, equipment and systems enabling their detection by the enemy.
Modern missiles equipped with guidance systems seeking infrared radiation sources are one of the most
important threats to helicopters performing combat missions. The infrared suppression systems increase
the aircraft’s survivability by reducing the opportunity for an infrared signature seeking system to
acquire, lock onto, track, and destroy the helicopter. Because of a high temperature, cooling of exhaust
gases is a major tactic employed in aim to decrease infrared emissions by a helicopter in ight, but
the most eective protection is a combination of all techniques, both passive and active.
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82 Mateusz Paszko
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Infrared Signature Suppression Systems in Modern Military Helicopters
METODY OGRANICZANIA EMISJI PODCZERWIENI
we współczesnych śmigłowcach wojskowych
Abstrakt
Śmigłowce odgrywają niezwykle istotną rolę w krótkodystansowych walkach powietrznych
jak również jako wsparcie ogniowe na cele naziemne, szczególnie w trakcie operacji przeciw
pojazdom opancerzonym. Wykrywalność oraz żywotność śmigłowców na polu walki istotnie
zależy od poziomu emisji podczerwieni emitowanej z ich pokładu jak również od metod, urządzeń
i systemów detekcji wykorzystywanych przez przeciwnika. Współczesne systemy wykrywania,
rozpoznawania i identykacji obiektów latających wykorzystują szereg metod termolokacyjnych,
które polegają na wykrywaniu promieniowania podczerwonego emitowanego przez śledzony
obiekt. W śmigłowcowej technice lotniczej, szczególnie istotna jest emisja podczerwieni z gazów
spalinowych odprowadzanych do otoczenia. Ze względu na ich wysoką temperaturę, gazy
spalinowe stanowią główne źródło emisji podczerwieni przez śmigłowiec w locie co w połączeniu
ze stosunkowo niską prędkością oraz wysokością wykonywanych manewrów, czyni śmigłowce
łatwymi celami dla współczesnych rakiet wyposażonych w głowice samonaprowadzające się
na podczerwień. W celu zwiększenia bezpieczeństwa, efektywności oraz żywotności śmigłowców
wojskowych wykonujących lotne zadania bojowe, opracowany został szereg metod redukcji poziomu
emisji promieniowania podczerwonego. W niniejszej pracy przedstawiono przegląd współczesnych
osiągnięć w tej dziedzinie wraz z prezentacją systemów maskowania podczerwieni stosowanych
we współczesnych śmigłowcach wojskowych.
Słowa kluczowe: podczerwień, śmigłowiec wojskowy, stealth, maskowanie promieniowania.
... Turboshaft engines are usually installed in one of three zones: I, II, or III [7]. The intensity of the impact of this complex flow on the engine will depend on the mounting position (radius r) below the MR with the radius R [ Figure 1(a)]. ...
... Due to the importance of the MR in helicopter design, most of the studies in this field have focused on rotor modelling to be able to use these models for performance optimization [8,9,10,11,12,13,14,15]. On the other hand, the modelled rotors have been assembled into the overall helicopter body, and the interactions between the rotor and helicopter's other components have led to further research and developments in the field of aerodynamic and structural design [1,5,7]. ...
... When the tip of the blade is at the speed of sound, it can cause a sudden and large decrease in the performance along with higher power requirements, higher blade loads (mechanical damage), vibration, and noise (stall) [11]. The average blade tip speed, tangent to the rotor disk with radius R [ Figure 1(a)], can be calculated using Equation 7. ...
... This paper reviews the various optical blocking methods of IR signature suppression (mainly of aircraft and helicopters from rear aspect) and the penalties associated with them. Advancements and developments in IRSS techniques in helicopters have already been discussed by Zhang et al. [53] and Paszko [35]. Cooling of exhaust gases is a major technique employed to reduce the IR emissions of a helicopter in flight [35]. ...
... Advancements and developments in IRSS techniques in helicopters have already been discussed by Zhang et al. [53] and Paszko [35]. Cooling of exhaust gases is a major technique employed to reduce the IR emissions of a helicopter in flight [35]. The airframe, stagnation region of the aircraft nose and the leading edges of the wings are prime sources of IR signature from the front and side aspect. ...
Article
With rapid advancements in Infra-Red (IR) detection techniques, the range from where the IR-guided missiles are able to lock the target aircraft has increased. To avoid the detection and tracking by modern IR-guided missiles, the aircraft and helicopters also demand progress in its stealth techniques. Hence, study of Infra-Red Signature Suppression (IRSS) systems in aircraft and helicopters has become vital even in design stage. Optical blocking (masking) is one of the effective IRSS techniques used to block the Line-Of-Sight (LOS) of the hot engine parts of the exhaust geometry. This paper reviews the various patents on IR signature suppression systems based on the optical blocking method or a combination of IRSS techniques. The performance penalties generated due to installation of various IRSS methods in aircraft and helicopters are also discussed.
... Аналіз останніх досліджень і публікацій. Слід зауважити, що незважаючи на значну кількість наукових праць з захисту вертольотів від ракет з інфрачервоними головками самонаведення [1][2][3][4], питання розробки комплексної методики оцінки впливу ЕВП на льотно-технічні характеристики (далі -ЛТХ) та характеристики інфрачервоної помітності вертольотів не втрачають своєї теоретичної та практичної важливості. ...
Article
Розглянуто порядок створення методики оцінки впливу екранно-вихлопного пристрою на захищеність вертольоту від керованих ракет з інфрачервоними головками самонаведення. Для вирішення задач багатокритеріального вибору застосовано метод аналізу ієрархій. В його межах визначені критерії оцінки зміни помітності та льотно-технічних характеристик вертольоту. В результаті отримані критерії захищеності при застосуванні ЕВП, встановленого на вертольоті, у порівнянні з базовим вертольотом без ЕВП. Розроблену оцінку доцільно використовувати при випробуваннях систем захисту модернізованих і новітніх зразків вертолітної техніки.
Article
To address deeper understandings about the aero-thermal performance of an integrating infrared suppressor under more realistic situations, a numerical investigation is motivated in the current study, concerning the effects of forward-flight speed on exhaust plume flow and infrared radiation of the Infrared Suppressor-integrating (IRS-integrating) helicopter, wherein the forward-flight speed is changed from 0 m/s (hover state) to 100 m/s, while both the engine exhaust parameters and the main-rotor operation parameters remains unchanged during different forward-flight velocities. The results show that the interaction between forward-flight flow and downwash flow alters the exhaust plume development and the internal flow inside the IRS-integrating rear fuselage more complicatedly, tightly dependent on the forward-flight speed. Of particular concern is the situation where the forward-flight flow has nearly the same level as the downwash flow, the hot mixing flow could possibly interacts with the helicopter rear fuselage to play a local heating effect. With the increase of forward-flight speed, the ejection coefficient is generally increased and the average exhaust temperature of mixing flow is decreased, leading to a reduction of the infrared radiation intensity of exhaust plume in 3–5 μm band. However, the influence of forward-flight speed on the overall infrared radiation intensity of IRS-integrating helicopter is conjectured not monotonous due to the complicated interaction between forward-flight flow and downwash flow. Under high-speed forward-flight states, the overall infrared radiation intensity of the IRS-integrating helicopter in 3–5 μm band is reduced with the increase of forward-flight speed. With respect to 3–5 μm band, the forward-flight speed has little effect on the infrared radiation in 8–14 μm band.
Article
In this work, a three-channel infrared photodetector with three independent related electric signals as a single optoelectronic chip has been introduced. The proposed structure has been implemented by exploiting an array of PbSe/ZnSe quantum dots and the PbSe/ZnS selective energy contacts considering solution-processed nanotechnology. The electron transport system has been designed to allow the transmission of electrons stimulated by three wavelengths simultaneously from different paths to the corresponding output. The high efficiency, simplicity and fabrication ease, cost-effectiveness, and high tunability of the photodetector have been accomplished as a consequence of solution-process technology. Moreover, the dark current has been significantly declined as a consequence of resonance tunneling in the proposed selective energy contacts. Last but not least, our proposed multi-wavelength ( $\lambda _{1}=2.9\,\,\mu \text {m}$ , $\lambda _{2}=2.25\,\,\mu \text {m}$ , $\lambda _{3}=1.75\,\,\mu \text {m}$ ) infrared photodetector has been promising candidates for a wideband spectrum analyzer or multi-wavelength infrared camera.
Article
Full-text available
Due to their low-attitude and relatively low-speed fight profiles, helicopters are subjected to serious threats from radio, infrared, visual, and aural detection and tracking. Among these threats, infrared detection and tracking are regarded as more crucial for the survivability of helicopters. In order to meet the requirements of infrared stealth, several different types of infrared suppressor (IRS) for helicopters have been developed. This paper reviews contemporary developments in this discipline, with particular emphasis on infrared signature suppression, advances in mixer-ejectors and prediction for helicopters. In addition, several remaining challenges, such as advanced infrared suppressor, emissivity optimization technique, helicopter infrared characterization, etc., are proposed, as an initial guide and stimulation for future research. In the future, the comprehensive infrared suppression in the 3-5μm and 8-14μm bands will doubtfully become the emphasis of helicopter stealth. Multidisciplinary optimization of a complete infrared suppression system deserves further investigation.
Article
Full-text available
Article
Full-text available
This paper analyzes the main contributors of infrared (IR) signature in a typical aircraft on a low-altitude mission. Various computational models are used to predict IR radiation from the aircraft. The bands within IR spectrum in which aircraft are susceptible to a typical IR-guided surface-to-air and air-to-air missile, for typical cases of tactical relevance, are identified. Lock-on range for aircraft against a typical missile is also computed. The feasibility of a low-altitude mission against a ground-based IR-guided threatis analyzed. The technique of emissivity optimization of aircraft rear fuselage skin, for reducing its infrared signature, is introduced and compared with other IR signature suppression techniques. The effectiveness of this technique in enlarging the safe flight envelope of aircraft, with respect to threat from heat-seeking missiles, for both surface-to-air and air-to-air missiles, is demonstrated. It is found that earthshine reflected off the aircraft surface plays a crucial role in the effectiveness of this technique against a surface-to-air missile (SAM) in 8-12 mu m band.
Conference Paper
Full-text available
Article
The effects of rotor downwash and exhaust direction on plume flow field, rear-fuselage temperature distribution and helicopter infrared signature were numerically investigated. The internal flow inside IR suppressor originated from engine exhaust nozzle and the external flow around helicopter airframe originated from rotor downwash were computed in a coupled mode to determine the temperature distributions on the helicopter skin and in the exhaust plume. Based on the skin and plume temperature distributions, a forward–backward ray-tracing method was used to calculate the infrared radiation intensity from the helicopter with a narrow-band model. The results show that the exhaust plume takes on strong downwards deflection to the rear-fuselage, as well as to the rotor rotational direction under the action of rotor downwash. The rotor downwash has a complicated influence on the infrared radiation distribution of helicopter. It is benefit for reducing the infrared radiation intensity when the exhaust is ejected in oblique-turned or lateral-turned mode. While for the up-turned exhaust mode, the exhaust plume could heating the helicopter rear-fuselage and the infrared radiation intensity may be enhanced under the action of downwash.
Article
This paper describes the development of an engine exhaust IR suppressor for the Bell 205 helicopter. The design methodology of combined computer analysis and scale model testing was applied to produce a final aerothermal shape that met performance requirements. Prototypes were later built and tested in an engine test cell, and in flight on a Bell 205. Results from these prototype tests are discussed in the companion paper "Part II -Engine & Flight Testing", submitted to the 11 th CASI Propulsion Symposium.
Stealth Technology Deployed in battlefield
• S Vass
Vass S., 2003, "Stealth Technology Deployed in battlefield", AARMS.
Identifying the Behaviour of a Jet Stream in the Environment After Leaving a Helicopter Engine Diffuser in Flight
• S Fijałkowski
Fijałkowski, S., 2011, "Identifying the Behaviour of a Jet Stream in the Environment After Leaving a Helicopter Engine Diffuser in Flight", Transactions of the Institute of Aviation 219 (in Polish).
Numerical Model of Exhaust Gases Expansion in Rotor Wake Vortex During Vertical Helicopter Flight”, Transactions of the Institute of Aviation 221
• S Fijałkowski
• M Kania
Fijałkowski S., Kania M., 2011, "Numerical Model of Exhaust Gases Expansion in Rotor Wake Vortex During Vertical Helicopter Flight", Transactions of the Institute of Aviation 221 (in Polish).
Development and implementation of the H-1 turned exhaust system
• K Groninga
• Texas
Groninga K., 2005, "Development and implementation of the H-1 turned exhaust system", AHS Texas.