Aspects of Airframe Design 109
MECHANICAL MODULE
ELECTRONICS MODULE
PAYLOAD MODULE
BODY MODULE
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The volume of the circular body is essentially divided into four segments by four vertical diaphragms which act as ‘brackets’ to carry the electronics and payload modules, and to provide insulation from the other two segments which are occupied by each power-plant. There is no ‘front’ of the aircraft as it is symmetric in plan view and can fly equally readily in any direction. However, to simplify the description, we will refer to the segment which carries the detachable payload as the ‘front’.
Opposite the payload module, and balancing it, is the electronics module, whilst the mechanical module gear-box straddles the central box section. The rotor drive shafts rise centrally from the gear-box and the two power-plants occupy each of the lateral segments. The body is completed by four removable covers which form the upper surface of the body and provide access to the modules for attention in situ or removal.
This arrangement enables the mechanical module to carry the weight of the body and its contents from the top of the box-pillars in flight, whilst the weight of the whole aircraft in landing is transmitted directly down through the pillars to the undercarriage. The design concept provides a lightweight, cheap structure and a very compact aircraft with the desirably high density of packaging. These are the features which give the helicopter UAV its small size, for a given payload and performance, compared with the equivalent small aeroplane UAV. It is a trend which is likely to continue into the future as electronic components become ever smaller (for further details of the aircraft see Figure 4.19 in Chapter 4).
Another advantage of the coaxial rotor configuration of helicopter is that the complete power and lift provision can be invested in one module independently of the remainder of the components of the aircraft. This enables a range of sub-configurations to be provided and is exampled in Figure 6.11.
On the right-hand side of Figure 6.11 is shown a two-view drawing of the version of the Sprite UAV discussed above. The configuration offers the ability readily to fly in any direction and with great stealth.
However, that compromises its ability to fly at higher speed because of its higher-drag body shape.
E
F F
F F
E
E= ELECTRONICS
F= FUEL
P= PAYLOAD
F E F
Rotor Diameter 1600 mm
Body Diam.
600 mm Body Length
1 metre
300 mm
P P
70 knot Max Speed 120/150 kt Max Speed
Figure 6.11 Directional and symmetric Sprite UAV
Aspects of Airframe Design 111
C of M C
C of M
PLAN VIEW -- TRI-WHEEL PLAN VIEW -- QUADRI-WHEEL C of M
C = VERTEX ANGLE of CONE C > 90° desirable for off-ship C > 60° essential for off-ship C > 40° essential for off-land
Figure 6.12 Undercarriage stability
Making the change to a more streamlined body, whilst retaining the same mechanical, electronic and payload modules, confers higher speed and range to the aircraft, but at reduced stealth and sideways flight ability. Some reprogramming of the AFCS would be necessary.
Another configuration considered was the installation of the modules in a tilt-wing airframe somewhat similar to the Aerovironment Sky Tote of Figure 4.27, but with a delta-wing. The advantage of this ability of the aircraft to emerge in different configurations is that an operator can operate more than one type and choose the variant which provides the best capability for a particular role. The airframe body is by far the cheapest of the modules and the others can literally be moved into whichever airframe is to be operated.
600
500
400
300
200
100
00 2000 4000 6000 8000 10000 12000 14000 16000 Aircraft All-Up-Mass [kg]
Rotor Disc Loading [N/m2]
Manned Aircraft UAV
Figure 6.13 Rotor scale effect
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This flexibility did, at first, introduce some complexity in operating the Sprite UAV, a ‘problem’ which could apply to other modular UAV. To meet airworthiness regulations it is necessary to define the ‘build standard’ of each aircraft before flight, and for a UAV system, probably of the whole system. Usually in an aircraft the airframe is the primary element and is allocated a series number which is ‘attached’ to the airframe. Components, such as power-plants, are replaced as required and logged as sub-units.
In the case of Sprite, apart from the payload modules being changed, often between missions to best suit the requirements of the mission, an electronics or mechanical module might be replaced at any time to allow it to be checked or serviced. Thus, it was questioned, which was the primary module and how did one define the build standard of an aircraft at any one time?
The solution which was adopted was to give each module a letter and serial number and these were grouped on the build standard document as Exxx-Myyy-Bzzz and the payload A, C, etc., depending upon type, followed by its own serial number nnnn. The airframe still carries its serial number (Bzzz) externally.
6.7 Ancillary Equipment
UAV need a similar range of ancillary equipment as manned aircraft except for those items relevant to aircrew accommodation and functioning. For the largest UAV this is seldom a problem as they are in a similar mass domain as the lighter manned aircraft. For the smaller UAV problems may arise in sourcing appropriately sized electrical alternators, actuators, air-data systems, attitude and altitude sensors, batteries, fasteners, external lighting, antennae, etc. Existing aircraft-approved and certified equipments, available for even the lightest manned aircraft, are too large and heavy to appropriately meet the requirements of the smaller UAV. Some of these equipments may be available to a limited extent from model aircraft suppliers, but they seldom meet the airworthiness standards required for UAV.
This has certainly been a problem in the past and UAV manufacturers have often had to resort to developing the equipment, and achieving its certification, themselves. Fortunately, following the demand from a burgeoning UAV industry, specialist equipment suppliers have appeared on the scene and, even if they do not have suitable equipment available off-the-shelf for a specific requirement, they are usually able to develop suitable equipment under a sub-contract arrangement. It is not appropriate to list these suppliers in this book but readers are referred again to Reference 6.3.
References
6.1. J. H. Faupel and F. E. Fisher, Engineering Design. Wiley-Interscience, 1981.
6.2. J. Bannantine, J. Comer and J. Hanrock, Fundamentals of Metal Fatigue Analysis. Prentice Hall, 1990.
6.3. Unmanned Vehicles Handbook. Published annually by the Shephard Press.
6.4. A. R. S. Bramwell, Edward Arnold, Helicopter Aerodynamics, 1976.
6.5. B. Hoskin, Composite Materials for Aircraft Structures. AIAA Education Series.
6.6. M.C Y. Niu, Airframe Structural Design. Conmilit Press Ltd, ISBN 962-7128-04-X, 1988.
6.7. C. J. Burgh and J. L. Pritchard., Component Design: Handbook of Aeronautics No. 2. 1954. Sir Isaac Pitman and Sons Ltd.
7
Design for Stealth
There are three main reasons why it is desirable that a UAV system remains undetected in operation.
They apply principally to the air vehicle although other components of the system may be involved.
(a) It is desirable that the air vehicle remain undetected whilst on a reconnaissance/surveillance mission in order not to alert the enemy (military) or criminals (policing) to the forthcoming operation.
(b) Principally in military use, it is necessary to protect the air vehicle from loss due to enemy counter-measures.
(c) Mostly applicable to civilian operations, low-detectable signatures will result in minimising envi-ronmental disturbance.
However, the other side of the coin is the need for the UAV to be readily visible when operating in civil airspace. In some operations, therefore, it may be necessary to have the UAV offering ‘stealth’
which can be changed to overtness, at perhaps ‘the flick of a switch’! This will be taken further in Part 3, Deployment.
The principal means of detecting an air vehicle are through its ‘signatures’, i.e. its acoustic or electromagnetic emissions at the following wavelengths:
a) noise (acoustic) [16 m–2 cm, or 20–16000 Hz]
b) optical (visible) [0.4–0.7 µm]
c) infrared (thermal) [0.75 µm–1 mm]
d) radar (radio) [3 mm–3 cm]
Hence to reduce the air vehicle detectability to an acceptable level, it is necessary to reduce the received emission or reflection of the above frequencies below a threshold value which, itself, is often a function of the operation – principally the operating height of the air vehicle.
An unmanned air vehicle has advantages, compared with a manned equivalent, in its inherent ability to achieve low signatures. These virtues should be pursued to best effect by the competent UAV system designer. They are:
(a) The removal of aircrew and their support equipment enables the designer to achieve a more densely packaged and thus dimensionally smaller machine. For a similar mission, the mass of the air vehicle, therefore, will normally also be less than that of the manned equivalent.
Unmanned Aircraft Systems – UAVS Design, Development and Deployment Reg Austin
C 2010 John Wiley & Sons, Ltd
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(b) Without the need to accommodate aircrew and their need for access and external vision, the shape of the air vehicle may be more specifically designed for aerodynamic efficiency and signature reduction, and the resulting shape will often be a compromise between these two.
7.1 Acoustic Signature
The noise emanating from an aircraft may be the first warning of its presence, although not usually so directionally locatable as to immediately lead to its detection. As a means of alerting other means of detection it merits suppression.
Aerodynamic noise from aircraft emanates predominantly from vortices, principally at the tips of wings, rotors or propellers. This increases with wing or blade span loading and speed, so that low values of both enhance acoustic stealth.
Usually, however, noise from the power-plant(s) is of greater concern, and results from noise of combustion in piston engines and compressor noise or wake noise in turbo-jets and to a lesser degree in turbo-shaft and turbo-fans. Electric motors, of course, develop virtually no noise, but their use, with few exceptions, as discussed in Section 6.3 of Chapter 6, is largely limited to short-endurance MAV.
Noise generally increases with power-plant power usage level, so that keeping the mass and aerody-namic drag of the aircraft as low as possible is a good first step to achieving low noise generation.
The human ear is most sensitive to frequencies of around 3500 Hz and can hear sound down to a practical threshold of about 10 dB. For a given sound pressure level, attenuation of sound with distance in air (and in insulating media) varies as the square of the sound frequency. Hence low-frequency sound usually presents the greater problem.
Turbo-jet and turbo-fan engines tend to be used for MALE or HALE systems and the noise from these high-frequency generators is usually well attenuated by the time it has reached earth.
For the larger tactical UAV operating at lower altitudes, a mitigating solution is to mount the turbo engines above the wing to shield the compressor and efflux noise from the ground. The greater noise problem is posed by smaller aircraft using piston engines where no practical turbine engines currently exist.
Water-cooled engines are quieter than air-cooled engines since the coolant serves as an absorptive blanket. However, they are significantly heavier than air-cooled engines and are therefore seldom used in aircraft.
The choice of piston engines is thereby reduced to conventional two-cycle or four-cycle units, rotary (Wankel-type) units being, as yet, inadequately reliable and difficult to cool. Although four-cycle engines offer slightly lower fuel consumption than do two-cycle units – possibly of order 75% – they have a far worse power-to-weight ratio (see Chapter 6, Section 6.5.1).
Sound is emitted principally from the internal combustion and from the exhaust, although intake noise can be an added factor.
The combustion frequency of a small two-cycle engine operating at, typically, 5000 rpm will produce noise predominantly at about 100 Hz. The equivalent four-cycle engine will generate noise at about 50 Hz. Their noise is less attenuated than the two-cycle by a factor of four, since attenuation of sound in media is proportional to the square of the sound frequency. The two-cycle engine is therefore usually favoured but requires silencing if stealth is needed.
Combustion noise can be attenuated by blanketing appropriate areas of the engine in sound-absorptive materials, though this comes at the price of extra weight. The possibility of surrounding the emitting areas by the fuel tanks, ideally of absorptive materials, should be considered. Whilst there is fuel in the tanks, this fuel will attenuate the sound, depending of course, on the thickness of the fuel ‘wall’.
The exhaust noise can be reduced by silencers (or mufflers) and, if possible, both air intake and exhaust should be directed skyward. However, the level of sound that can be detected also depends upon the level and character of the background noise, i.e. sound contrast. The background noise on a battlefield, for example, may readily drown out the noise emanating from a ‘quiet’ UAV.
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7.2 Visual Signature
The most common means of visibly detecting the aircraft is initially by the unaided eye. The human eye operates within a small range of the electromagnetic spectrum, i.e. between about 0.4 and 0.7 µm, peaking at maximum efficiency at about 0.55 µm wavelength.
Criteria which determine the ability of the eye to see an airborne object against an open sky or cloud background include:
a) the size and shape of the object,
b) its contrast against the background and the sharpness of the edges of that contrast, c) the effect of the atmosphere,
d) any movement of the object, e) exposure time,
f) the stability and diligence of the observer, g) glint.
This is a complex subject, on which many volumes have been written, and research continues to refine the current understanding. Even so, the physics of ground-to-air observation are simpler and somewhat better understood than that of air-to-ground. An introduction to the physics is given in Reference 7.1.
(a) The Size and Shape of the Object
These combine to determine the threshold of detectability of the airborne object.
(b) Contrast
Contrast C is defined as the ratio of the difference in luminance between an object and its background to the luminance of its background,
C= (B − B1)/B1= B/B1
where B is the luminance of the object and B1is the luminance of the background in units of cd/m2. For example, if the luminance of object and background is the same,B = 0 and C = 0. Thus the object would be undetectable.
In ground-to-air observation, the luminance of the object is generally less than that of the background andB is negative.
The threshold of contrast C, where the average observer has a 50% chance of detecting the object is given the symbolε. However, in real operations, predicting the background luminance and that of the object is problematic. The background luminance will depend upon the atmospheric conditions at the time, and the position of the object relative to the position and height of the sun.
The luminance of the object, although dependent upon its shape and surface texture, is also dependent upon atmosphere-reflected illumination which is unpredictable.
Although absolute prediction of visual detection of an object is not possible, some approximation is possible and relative detectability can be determined for various aircraft from the size, shape and angular velocity of the object as presented to a ground observer. The larger the object, other things being equal, so the object becomes easier to see – that is, the value ofε reduces.
Laboratory tests have shown that for a black, sharp-edged, circular disc against a well-lit background, and within a limited field, there is a 50% probability of its detection by the average unaided human eye if it subtends an angle of aboutα = 0.15 mrad. For such circular objects, which subtend a small angle α (of the order of 1 mrad or less), the simple lawεα2= constant applies.
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Such a condition is unlikely to be realised in practice and, for realistic contrast values, a large field of view and atmospheric attenuation, it may be assumed that there is little probability of detection for a stationary object in a large field ifα is less than 0.5 mrad.
(c) Atmospheric Effects
The atmospheric water content or pollution, apart from affecting object visibility to a ground observer through attenuation, may also increase the level of luminance of the object through reflecting the sun’s rays upwards. Although subject to variation, a general conclusion from observation is that an object at 2500 m altitude will have a contrast typically reduced to 40% of its contrast at 200 m due to increased luminance.
(d and e) Movement of the Object and Its Exposure Time
Field experiments have shown that movement of the object at angular speeds of about 40 mrad/s offer the greatest stimulus to the eye to aid detection. The stimulus reduces rapidly at speeds lower than that, and speeds above that steadily reduce the exposure time, thus reducing the probability of detection.
40 mrad/s equates to about 80 kt at 1000 m height, and the object would take about 80 s to traverse the whole bowl of the sky from horizon to horizon.
(f) Stability, Awareness and Diligence of the Observer
Under field conditions, the observer may not be aware of the impending presence of an aircraft. Even if he is, if no aircraft appears, his attention may lessen. If he is mounted on a vehicle, for example, he may not have a stable platform from which to observe.
All these factors can reduce, significantly, the probability of detection unless the observer is alerted by another indicator, in particular, noise.
(g) Glint
Glint results from the sun being reflected from glossy surfaces, especially from glazed canopies. UAV have an advantage in not requiring canopies for aircrew vision. However, attention should be paid to ensuring that camera windows are as small as possible and that the aircraft exterior has a matt finish, preferably in light grey.
7.3 Thermal Signature
Infrared (IR) radiation is emitted from a heat source and propagates in much the same way as light.
The detectability of the radiant body is similarly determined by its contrast with the background and the radiating area. However IR radiation is more readily absorbed by the atmosphere than is light, particularly by water and CO2 molecules. The major windows for IR radiation to pass through the atmosphere are in the 3–4 and 8–12 µm wavelength bands. Thus IR detectors are designed to receive within one or the other of these bands.
Heat in an aircraft is generated principally as waste heat from the power-plant and to a lesser degree from electronic components. Heat can be generated at the stagnation points at the leading edge of wings, propellers and rotors. However, this is negligible until high-subsonic speeds and above are reached, and
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will always be of lower order than heat energy emitted from power-plants, and so will be ignored in this volume.
Heat is radiated or conducted from the engine carcass and from the exhaust. Dealing with the former first, it is necessary to prevent this being radiated from the aircraft to ground by containing it within the aircraft and emitting it skywards – away from ground-based detectors. Materials of low emissivity such as silver or aluminium may be used to prevent radiation in adverse directions.
The exhausts pose the greater problem as they will be particularly hot and will inevitably be made of high-emissivity materials such as steel or Nimonic alloys. They should be screened as much as possible by other airframe components.
As a background for IR, the sky is very cold, offering a base for an excellent contrast. However, although more advanced detectors are under development, long-range detection of low-emission targets is only achievable with a small (5◦) field of view. So that, unless other indicators show the direction of the target, the detection of an object at high altitude is unlikely. The detection range is a function of temperature (K) to the fourth power. Reducing the target emission temperature is very effective in evading detection by IR.
7.4 Radio/Radar Signature
The radio signature relates to radio frequency emissions from the aircraft (and also from the control station) and these must be minimised to prevent detection. This is discussed in Chapter 9, Communica-tions.
The radar signature, like the visible spectrum signature is a reflected frequency, in this case from radio frequency pulses generated by an enemy emitter which is scanning the sky, from the ground, looking for return (reflected) pulses from a body entering its sector. Therefore the emitted radiation will be approaching the aircraft from the lower hemisphere. Except for when the aircraft is close to overhead the transmitter, the radiation will be arriving at the aircraft from a small angle beneath the horizontal.
The UAV designer’s aim is to prevent the pulses being reflected back to a detector. As with the visible spectrum, the UAV usually offers the advantage, compared with a manned aircraft, of being smaller (less reflective area) and not having its shape constrained by the necessity of accommodating aircrew.
There are basically three methods of minimising the reflection of pulses back to a receptor:
a) To manufacture appropriate areas of the UAV from radar-translucent material such as Kevlar or glass composite as used in radomes which house radar scanners. This method can seldom be effectively employed except in the case of very small MAV where the wing ‘skins’ are very thin since these materials are translucent, and not transparent as is sometimes mistakenly thought. The proportion of the received pulse which is reflected increases with thickness until at a skin thickness of typically 15 mm, depending upon the material matrix composition, a sufficient proportion of the received energy is reflected to be detected by the searching radar. Also, of course, the material cannot be used to house metallic or other components as these will return, and even amplify the pulses back through the translucent cover.
b) To cover the external surfaces of the aircraft with RAM (radar absorptive material). This material absorbs radio energy in much the same way as sound energy is absorbed in anechoic chambers, turning the energy into heat. The material is usually comprised of foam sandwich, is rather bulky, somewhat fragile and can add significant weight if used extensively. Another problem is that it is usually designed to absorb a limited range of frequency. Any frequency outside that range is not absorbed but reflected. It is best used in small amounts in critical areas.
c) To shape the aircraft externally to reflect radar pulses in a direction away from the transmitter. This is probably the most effective method of the three and is particularly suited to UAV where the external