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51
Small Aircraft Infrared Radiation Measurements 2019 47 1
Abstract
The large, EU Supported ESPOSA (Efcient Systems and pro-
pulsion for Small Aircraft) project has developed new small
gas turbines for small aircraft. One of the important tasks was
the engine - airframe aero-thermal radiation integration that
included task of minimizing the infrared radiation of the small
aircraft, too. This paper discusses the factors inuencing on the
aircraft infrared radiation, its possible simulation and measure-
ments and introduces the results of small aircraft infrared radia-
tion measurements. The temperature of aircraft hot parts heated
by engines were determined for validation of methodology devel-
oped and applied to engine - aircraft thermal integration.
Keywords
aircraft, infrared radiation, radiation measurements, engine -
airframe thermal integration
1 Introduction
The infrared is an invisible radiant energy. This electromag-
netic radiation has longer wavelength (starting from nominal
red visible line wavelength 700 nanometers up to 1 mm), than
infrared radiation were developed prior the World War I. During
World War II, the infrared detection was used for tracking, too
(thermal) radiation had been developed for beginning of 1950s,
while, because the success of radar systems, its wide applica-
tion was started about 10 - 15 years later. Titterton (2006) found
from statistics that, the heat-seeking missiles had been respon-
sible for more than 80 % of all the combat losses over 40 years
were responsible for 76% of the neutralized aircrafts, while
during the Kosovo war, the NATO aircraft were not allowed to
et al., 2007). On the same time, since 1987, for 26 years 35
that caused more than 500 deaths (Bolkcom et al., 2004). (It is
true, this large number might be less, because some lost civilian
aircraft were used for military purposes, some others or bet-
they may have been perceived as being used for military pur-
-
ian project for detailing the implementation of an anti-missile
system on a single commercial aircraft (Bolkcom et al., 2004).
The aircraft infrared radiation has investigated by many
researchers, developers for the last 50 - 60 years as it summa-
rized by Mahulikar et al. (2007). The most of research were and
-
mental studies had been focused on the military aircraft infrared
signature measurement and aircraft type recognition, too. Later,
the practical measurement of the civilian large aircraft infrared
radiation signatures (Coiro et al., 2012), the radiation modeling
and countermeasure became to set of important tasks. Parallel,
2012) has been developed rapidly, too. The information about
1
Budapest University of Technology and Economics,
H-1521 Budapest, P.O.B. 91, Hungary
2 Aviation Training Centre,
3
* Corresponding author, e-mail: jrohacs@vrht.bme.hu
47(1), pp. 51-63, 2019
https://doi.org/10.3311/PPtr.11514
Creative Commons Attribution b
P
P
Periodica Polytechnica
Transportation Engineering
1*, Istvan Jankovics1, Istvan Gal1, Jerzy Bakunowicz2,
Giuseppe Mingione3, Antonio Carozza3
52 Period. Polytech. Transp. Eng. J. Rohacs et al.
in Minkina et al. (2009).
The research can be divided into two major groups: inves-
tigation of the aircraft infrared signature (including the theo-
retical and practical studies) and sensing the infrared radiation.
list the codes using in investigation of the aircraft infrared radi-
ation (Coiro et al., 2012; McGlynn, Auerbach, 1997; Mahulikar
can be applied to compute the aircraft infrared signature, espe-
Belgium Defence establishment has developed an open-source
can be applied to the Arial vehicles, too.
Nowadays, the small and personal air transportation system
developed special small gas turbines for small aircraft. During
optimization of the engine components and airframe integra-
tion the thermal conditions and the infrared radiation had been
taken into account (Buonomo et al., 2013; Carozza et al., 2013;
This paper shows and discusses the results of the infrared
radiation measurements.
2 The sources of the aircraft infrared radiation
The infrared radiation of aircraft is composed from the dif-
ferent sources (Fig. 1) including the radiation from the aircraft
sun-, sky- and earthshines.
The infrared radiation like all electromagnetic radiation
interacts with matter in a variety of ways (White, 2012):
•
the angle of incidence),
• refracts - when the direction of a wave bends when pass-
ing between two transparent media with different propa-
• scatters - that occurs (in blue sky) upon interaction with
particles whose size approaches the length of the wave,
• diffracts - interaction occurring around the edges of
an obstruction,
• interferes - interaction in both a constructive and destruc-
tive manner,
• absorbs - when radiation is converted into another form
of energy (mostly to heat),
• emits - from matter by conversion from another form
of energy,
• transmits - propagation of the infrared radiation through
a transparent medium (or vacuum),
•
Fig. 1
(Mahulikar et al., 2007)).
3 Governing laws
The aircraft infrared radiation that can be measured or sensed
depends on heat sources, and intensity of sources (initiated by
-
3.1 Planck’s radiation law
It describes spectral radiance (L - energy per source unit
parameter-
T ). It can be given
wavelength (λ
LT Lhc
e
ehc kT
λ
λ
λλ
,,
,, /
()
=⋅
−
Ω
21
1
2
5
-23
-34
L(λ,T) is W·sr-1·m-3
3.2 Wien’s displacement law
λ) at which the spec-
tral radiance of black body radiation per unit wavelength is
λ
maxmax
,. ,==
⋅⋅
−
b
TET
1 309 10
75
bmK=×
−
2 8977721 10 3
..
(1)
(2)
53
Small Aircraft Infrared Radiation Measurements 2019 47 1
3.3 Stefan Boltzmann radiation law
wavelengths (λ
black body per unit surface area per unit time.
QETd T=
()
=⋅
∞
∫
0
4
λλσ
,,
where σ
σ
=⋅
()
−
56710
82
4
..
WmK
3.4 Black body
incident radiation to pass into it and absorbs internally all the
angles of incidence. Hence the blackbody is a perfect absorber
Blackbody is an ideal of comparison for a body emitting
radiation. A blackbody has two main properties in thermal
emits as much or more energy than any other body at the same
the energy is radiated isotopically, independent of direction.
The real body radiation can be given by its emittance or
emissivity that is a ratio of real and black body radiations:
ε
=
E
E
real
black
.
The emissivity varies according to the surface properties,
the material and, for some materials, also according to the tem-
perature of the object. It also depends on the wavelength and
angle of incidence.
For rough surfaces the value is close to unity, for polished
surfaces it is around 0.02.
3.5 Kirchhoff’s law of thermal radiation
The electromagnetic infrared radiation as usually hits the
objects, while the objects absorb some of this energy. The value
of absorptivity (α) is between 0 and 1, where 1 means a black
body, 0 means a perfect white body.
The heated body, having higher temperature emits more
infrared radiation than colder objects.
when the temperature of a body is constant, i.e. independent on
this means the body gives off as much energy as much absorbed:
εα
=.
3.6 Conservation of radiation energy
When the incident radiation reaches the surface of a material,
it can act 3 ways depending on the material properties: it can be
α) describes
how much fraction of incidence radiation is absorbed, the trans-
mittance (τ) is a measure of the ability of a material to transmit
σ) is a mea-
By use of energy conservation law the sum of absorbed,
QQ
QQ
=⋅+⋅ +⋅
ατσ
,
or
ατσ
++ =
1.
As transmission rarely plays a role in practice, the transmis-
sion (τ
ασ
+=1
εσ
+=
1.
This short summary of governing rules contains all the
radiation simulation, measurements and countermeasures.
The other models are given by special studies and papers. For
-
-
ing measurement of boundary layer by infrared means might be
found in Boden et al. (2015).
4 Practical aspects
According to the knowledge on (i) material properties, (ii)
(Pocket, 2012)) and (iii) our practice with developing the ther-
mographic evaluation methods the following important practi-
cal aspects must be underlined.
There are some aspects according to the emittance: (i) for
concrete, organic substances) have high emissivity in the long-
wave infrared range that is not dependent on the temperature
-
(iv) ε must be set properly for accurate measurement.
σ
depends on the surface properties, the temperature and the type
more strongly than rough, matt surfaces made of the same
the ambient temperature (mainly in indoor thermography); (iv)
(3)
(4)
(5)
(6)
54 Period. Polytech. Transp. Eng. J. Rohacs et al.
-
Other comments according to the transmittance are (i)
dependence on the type and thickness of the materials and (ii)
the fact, the most materials are not transmissive, i.e. permeable,
to long-wave infrared radiation.
-
-
-
are the same. In thermography it means that the heat source
in the background is clearly visible on the surface because of
background can be optimized.
Fig. 2
roughness of the surface means that each individual ray of inci-
dent radiation reaches a surface which has different orienta-
tion. The normal lines at each point of incidence of rays are
-
ing depending on the surface roughness. It means, with given
accuracy of measurement of incident radiation, the error of
measurement of emitted energy will be proportionately higher
determination of emissivity values.
directly proportional to the cosine of the angle between the sur-
face normal and the direction of observer.
at 25 °C) up to 0.94 (glass - 90 °C).
The accuracy of the thermo radiation measurements depends
on the environmental conditions, too. The clouds, precipitation,
The objectives of the thermo radiation measurements are the
possible optimization for reducing the infrared radiation.
Our practice going back to the end of the last century (Oravecz
et al., 2000), and it showed that even relatively simple infrared
cameras (like AGA Thermovision 750 type working with a spe-
to measurement, analysis and evaluation of the aircraft infrared
a special thermal emittance reducer. The photos were made in
the same military helicopter without (upper photo) and with
-
strate that, the infrared radiations origins from all the heated
from the rotor blades that origins from the helicopter itself.
Fig. 3 Infrared radiation of a military helicopter without (upper) and with the
thermal emittance reducer (lower image)
During preparation for measuring the infrared radiation
realized several measurements with use of a small gas turbine
research stand and there were analyzed the changes in nacelle
surface temperature and engine airframe integration of a mid-
dle size commercial aircraft.
55
Small Aircraft Infrared Radiation Measurements 2019 47 1
radiation of engine nacelles and plumes.
The Fig. 4 and 5 show the changes in temperature of nacelle
surface during the engine tests. As it can be seen, the tempera-
tures measured at the M2 and M3 points are correlated by the
-
Table 1 Engine nacelle and plum radiation
rotation speed - revolution per
thermal picture is short, with
moderate temperature.
smooth while its length has
increased, and relatively high
temperatures can be observed
in the core.
sharp, temperatures in the core
are higher, but not as high as
during the transition.
Fig. 4 Points in which the nacelle surface temperatures are measured (in time
interval with no measured data, the camera was moved to make measurements
from different point of view)
The Fig. 5 shows eight thermo images of the nacelle that
were selected with nearly same time intervals, to show how
changing the temperature of engine nacelle during the 1100
seconds long time period.
Fig. 5 Infrared pictures of an engine nacelle at engine test
(time is sec after starting the test)
5 Radiation simulation
-
available references all these aspects are investigated.
-
data analysis performed over many years. The vehicle fuselage,
facet, model includes radiation due to aerodynamic heating, inter-
combustion gas emissions were calculated for H2O, CO2, CO,
and other gases as well as solid particles. The developed software
56 Period. Polytech. Transp. Eng. J. Rohacs et al.
-
ture. This methodology is well applicable today, too.
The early 1990 several general models were developed.
imaging of targets and scenes in a modeling software could be
applied to model targets of a generic aircraft (Miller, 1993).
Already in 1998, Joyner et al. (1998) and his colleague cre-
-
-
eling using the CFD results as inputs had been developed
(Andersson, 2002).
a) The calculation of the aircraft infrared radiation and sig-
nature simulation depends on the objectives. The most
and Willers, 2012). According to this, there are several
important key considerations and aspects determining
methodologies should be taken into account.
(2012) for such an imaging infrared simulation system
are: (i) radiometric accuracy in all spectral bands, (ii)
accurate emitting source surface temperature behavior,
-
detailed modeling of signatures and backgrounds, (vi))
accurate atmospheric transmittance and path radiance
models, (vii) realistic rendering of the scene image in
radiometric, spatial and temporal terms, and (viii) com-
prehensive sensor modeling to account for primary and
second order imaging effects.
-
ing, (ii) environmental conditions (as humidity) (iii), air-
b)
-
mined for all spectral band.
c) -
θϕ
(see Fig. 6.a).
d) Distance effect. Generally, the receivers sense the radi-
Φe -
ted or received per unit time) in form of radiant inten-
sity (I
area power density at the receiver, E
distance: E = I/R2.
e) Projected area of source. According to the previously
-
dian angles, Ω (Fig. 6.b). That means the projected areas
solid body facet model elements that will be perpendicu-
lar to lines connecting them to the sensor elements. The
angle between the normal to sensor plane and connect-
real and spatial position of the surface elements
Aproj.
f) Aircraft position. The measurable radiance depends on
aircraft elements. On the other hand, it is easy to under-
-
shine really depends on the position of aircraft related to
the sensor position.
g) -
contained in range of wavelength 3 to 5µm. The measure-
emittance in the given range of wavelength is nearly zero
in range between 4.17 - 4.20 µm and 4.32 - 4.70 µm.
h) -
mined by use of absorptivity of gases. The spectral trans-
missivity and absorptivity in a homogenous isothermal gas
K,
τα
ηη
ηη
==
−
−−
ee
Ks Ks
,,
1
where s is a thickness of the gas cell. Because the absorp-
-
elements as CO2, H2-
i) Modeling aspects. Actually, the aircraft infrared signature
measuring is based on the radiant intensity or irradiance
measurements (see sub point c)) that are analogous to radar
-
Perotoni and Andrade, 2011)). However there is a princi-
For this reason, intensity is actually more closely related to
57
Small Aircraft Infrared Radiation Measurements 2019 47 1
-
mitter power with antenna beam (White, 2012).
j)
to determining the infrared signature of aircraft. The com-
long-range air-to-air detection and tracking engagements.
The code integrated eight standalone modules (Iannarilli,
-
ing module), (ii) CLOUD (sky background imaging mod-
-
face module).
k)
(Davis and Thomson, 2002) (i) background information
(geography, date and time, meteorological inputs), (ii) plat-
form characteristics (size, shape, materials, coating, pro-
-
length band, spatial resolution as scanner, imaging, etc.,
and sensitivity including detector noise). Here the platform
might be the critical elements, because the applied mate-
rials and especial their coating and painting (Mahulikar et
al., 2006) and fuselage skin heated by engine thermal pro-
cesses. The last one can be determined by use of model
developed by Mahulikar and his colleague (Mahulikar et
al., 2001; 2005) that incorporates the radiation interchange
in engine layout, to compute the temperature distribution
of rear-fuselage skin for given engine operating conditions.
plume that is well analyzed by (Mahulikar et al., 2007). Of
fuselage play deterministic role in case of military aircraft
and sensing from the real side. During operating the after
burner system, the plume might be longer then the fuselage.
l) Methodology. Most of the simulation methods com-
(Mahikular et al., 2001; Anderssen, 2002; Jianwei and
Qiang, 2009; Pan et al., 2011).
of small aircraft by use of two major methodologies. The theo-
retical investigations had objectives aero-thermal analysis and
evaluation of the engine airframe integration and reducing the
temperature distribution (by this infrared radiation) of the air-
frame elements heated by working engines (Buonomo et al.,
The other program was the practical measurements of the
The aircraft infrared simulation is based on the coupled
-
fer simulation) and / or multi-disciplinary models (including
-
lation methodology.
nacelle wall (conjugate heat transfer analysis) due to the pres-
ence of the engine hot components and taking into account for
nacelle cooling/ventilation system (Buonomo et al., 2013). The
CFD analysis with proper boundary conditions at engine surface
and nacelle wall considers the both convective and radiative heat
transfer. Three-dimensional CFD calculations were performed by
-
and nacelle was also taken into account, by using the Discrete
Ordinate model (Buonomo, 2013).
Fig. 6θϕ,
b. (right side) - the sterian angle, Ω and measured surface, Asurf and projection area, Aproj)
a.) b.)
58 Period. Polytech. Transp. Eng. J. Rohacs et al.
Fig. 7 Aircraft infrared radiation simulation methodology
6 Theoretical investigations
the developed small gas turbines on several small aircraft dem-
originally designed and built with two piston engine in pusher
retractable tricycle landing gear arrangement, and pusher pro-
The another aircraft was a low wing, 4 seated I-31T aircraft
(Fig. 8) designed by the Institute of Aviation in Warsaw and
Fig. 8
allowed operational temperatures for the TP-100 are given
in Fig. 9. The Figure shows the hot sections that heat the
nacelle elements.
The Figs. 10 and 11 demonstrate the results of the
theoretical investigations.
The Fig. 10 shows the static temperature distribution on
with cruise speed 87.445 m/s at 2750 m. At this level the air
-
enough from engine and where forced convection effectiveness
prevails, as the combustion air intake, at the lower front face
K) is
reached close to the engine pipes and gas generator zone. This
component materials of nacelle skin (Buonomo et al., 2013).
Fig. 9
(Buonomo et al., 2013)
Fig. 10
(Buonomo et al., 2013)
59
Small Aircraft Infrared Radiation Measurements 2019 47 1
The Fig. 11 shows the wall temperature contours of nacelle
(Carozza et al., 2015). For easier evaluation of the results the idle
condition was chosen as 101325 Pa air pressure and 300 K air
temperature. The propeller effects had been taken into account.
(Carozza et al., 2015). Generally, the propeller may reduce the
skin temperature up to 35 degree (Carozza et al., 2015). This
As it can be recognized from Figs. 10 and 11 difference in
-
temperatures are very small.
Fig. 11 Wall temperature (K) contours in case of engine idle condition and
taking into the propeller effects (Carozza et al., 2015)
7 Measurements
(see Figs. 1 and 8) were applied in practical measurements of
the infrared radiation of engine - airframe sections. The engine
ground tests were measured. A Testo 885-2 thermal imager was
-
-
peratures under nacelle by use of thermocouples (T1 - put close
to the engine control unit, T2 - near the top of the engine cover,
around the gas generator, near the inner surface of the mask,
fuel oil pump (see Fig. 12)) were registered, too, for supporting
the further evaluation of the theoretical studies and practical
measurements.
-
tion as position of sun, wind direction, humidity, etc., too. The
Fig. 13 shows the position of the investigated aircraft EM-11
Fig. 12 Thermocouples under the nacelle
Fig. 13 Measurement condition
The Fig. 14 presents images recorded during the measure-
ments. The left engine was started at 2:47 PM, i.e. at 46020
included the series of changes in operational condition as it
off, stopping engine regimes.
Table 2 Engine test process
Time Engine operational
condition Time Engine operational
condition
46020 Left engine start 49680 Approach
46200 Left engine warm up 49740 Take-off
46440 49860 Approach
46620 49920
46920 Idling 50220 Engines stop
47520 Take-off 50280 Cooling
47880 50880 End of measurement
48660 Cruising power
60 Period. Polytech. Transp. Eng. J. Rohacs et al.
Fig. 14
In Fig. 15, there is the same image showing the thermal con-
dition of the left engine nacelle after engine test with use of two
different temperature scales. As it can be seen, the hottest part of
C that is well correlat-
ing with the theoretical calculations (see Figure 10). The investi-
pipe has top radiation, because the infrared camera looks into the
pipe (as it can be understood from comparison the pictures of
The recorded images had been used for determining the
temperature distribution on the nacelle surface. The changes
in nacelle surface temperature along the chosen reference
lines in case of middle cruise regime are presented in Fig. 17.
from left end to the right end. P2 and P3 lines are going through
inaccurate. However there is a well detectable zone with heat-
Fig. 15 Left side of left engine nacelle radiation with
different temperature scales
Fig. 16
Fig. 17 Changes in wall temperature along the chosen reference lines
The Fig. 18 demonstrates the real dependence of infrared radi-
ation on engine installation. There are following zones of high
temperature. Upper air intake louvres, which make visible hot
0 5 10 15 20 25 30 35 40 °C
61
Small Aircraft Infrared Radiation Measurements 2019 47 1
and ground around and after nose wheel to the temperatures
well as for asphalt. Prolonged static operations of the engine set
oil cooler, located in the rear bottom of the cowling with tempera-
tures within the scale. Because the applied temperature scale, the
image, in reality. However, the tube temperature is much greater
(in analogy to Fig. 15), but additional temperature measurement
indicated, that the fumes are not so hot to damage the composite
structure of the hood.
Fig. 18
aircraft demonstrator I-31 T
The Fig. 19 shows the thermal image of the aircraft I-31T noise
section after 63 minutes ground test and 10 minutes cooling.
In Fig. 20 the temperature distribution shown at end of
cruise phase along the chosen reference lines on nacelle wall
of engine mounted in aircraft I-31T. This represents the best
I-31T aeroplane included the measurements of temperature
-
ing points concerning engine cooling. The points were located
on the inner and outer surfaces of the cowling in areas of the
generator. The analysis revealed that the most critical points
gas traces. The temperatures on the inner surfaces of upper
-
eral. However, recorded values reached temperatures up to 100
noticed in the Figs. 18 and 19, after engine shut down the tem-
peratures in measuring points increased radically.
Fig. 19 Image after engine ground test and 10 minutes cooling
(aircraft I-31 T)
Fig. 20 Changes in nacelle wall temperature along the chosen reference lines
for engine built into aircraft I-31 T
8 Conclusions
project developing and implementing a multi physical, namely
aero and thermal optimization in integration of engine into the
airframe. The goal was to minimize the aero-thermal losses for
increasing the engines' effectivities and reducing the infrared
radiations origin from engine - airframe structure. The study
and optimization process were started by theoretical investiga-
tions resulting to development of a CFD methods for aero-ther-
mal integration of engine into airframe. After it a special prac-
tical investigations were organized and realized.
The analysis of previous theoretical investigations and prac-
tical measurements of the aircraft infrared radiations as well as
the condition and methodology for infrared radiation mea-
surements of the aircraft demonstrators. The paper introduced
radiation. The full engine ground test was measured. The mea-
the possible improvements for reducing the infrared radiation
origin from engine-airframe structure.
62 Period. Polytech. Transp. Eng. J. Rohacs et al.
Acknowledgement
funded by the 4th call of the FP7 Cooperation Work Programme.
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