Stealth
I thought I'd do a quick and dirty article on stealth so that its easier to understand some of the responses people like Aussie-Digger, Gremlin and myself make. The first part I wrote, the last part I got lazy.
Typical stealth features:
The plane does not have any antennas or protruding objects.
Minimal flow design. e.g. The stealth bomber consists of only a wing. Radar signals tend to pass over the aerodynamic flying-wing. (much like laminar flow)
Reduced Radar Cross Section. The measure of how much a platform or object reflects radar is called the radar cross section, (RCS). The lower the RCS, the less visible the platform is. The B-2's RCS has been estimated at one millionth of a square meter (a Toyota Landcruiser sized vehicle’s RCS is about 180-200 square meters).
Platform Shaping. Stealth aircraft design requires careful and complex shaping of both exterior and interior components. The tolerances required to achieve this are enormous, and are typically outside of the skills available in a normal production line. In a real sense stealth aircraft are more “hand built†than general aviation platforms.
Tolerances. Parts machining and other components need to be of exceptional tolerances, both during manfacture and during the assembly of the platform. The sensitivity of construction can be to such a level that even scratching the surface of the structure (eg a wing) will increase the radar cross-section of the plane. Surfaces are typically flush, so as not to interrupt laminar flow. (Laminar flow being a good example of not only flight behavioural characteristics, but also the way that radar signals will travel over the platform.
Electronics. Every platform has a degree of electronic intervention to assist in signal reduction.
Engines.
The engine design and type is critical. Typically non stealth platforms use turbojets or low-bypass turbofans. The design of these engines is to suck in a small amount of air and accelerate it out the back at high speed to produce thrust. It is a very noisy process. OTOH stealth planes and some of the newer aircraft use engines that are called high-bypass turbofans. These engines take in a much larger quantity of air but accelerate it far less. They are able to generate more thrust due to inherent efficiencies. High-bypass turbofans also have a n added side benefit in that the way that they exhaust air. It is much like a mufflered system, because it’s exhaust is sending larger “pockets†or large cushions of slower-moving air around the noisy part of the engine, this effect blocks much of the noise so the engine is significantly quieter.
The fan blades at the front of a jet engine and the turbine blades on the back are very radar reflective. Most stealth aircraft place the engine ducts above the wing or fuselage to help block the engine interiors from radar sources below the aircraft. The F-117 also makes use of special screens on the engine inlets that block radar waves from reaching these surfaces. However, these screens are difficult to design because of their adverse impact on engine performance and have been abandoned in later stealth aircraft. More recent stealth designs use S-ducted inlets that bend off center to hide the blades from being seen. The F-117 also makes use of a special "platypus" nozzle that effectively hides the turbine blades from any radar source behind the aircraft.
The other technique used to make a stealth plane quieter is to place the engines on top of the plane so that their noise is blocked from reaching the ground by the body of the aircraft itself. This also acts as an infra red mask by shielding the hot exhaust tip signature from lower (usually ground based) detectors
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Radar cross section (RCS):
Radar, short for radio detection and ranging, is an instument that sends out radio beams and then picks up any reflected energy from aircraft, ships, or other objects to determine their location and speed. The range at which radar can detect an object is related to the power transmitted by the radar set, the fidelity of the radar antenna, the wavelength of the radar signal, and the radar cross section (RCS) of the vehicle. RCS, a measure of the radar waves scattered in a given direction, is the only variable an aircraft designer has any control over.
At first, designers tried to reduce RCS by placing various radar-absorbant coatings on the aircraft exterior. However, these coatings typically only produced minor reductions in RCS since many parts of the aircraft cannot be covered (cockpit windows and engines, for example). Designers soon realized that the only way to make substantial redcutions in RCS was by carefully designing each portion of the aircraft to scatter radar waves away from their source. Some obvious solutions led to moderate reductions. For example, 90° corners and flat perpendicular surfaces were found to produce high radar returns, so designers rounded corners and used chined noses as well as canted vertical tails to reduce radar signatures. The Lockheed SR-71 Blackbird is an excellent example of these early efforts.
However, these modifications were only made when they did not interfere drastically with the overall performance of the design. Though the final product might have been more stealthy, it was still detectable. The revolution in radar stealth came in the 1970s when computers were powerful enough to solve the Maxwell electromagnetic equations for reasonably complicated shapes. These equations determine how radar waves are reflected and scattered, and by developing the capability to analytically predict the RCS of an entire aircraft from different angles, designers were able to drastically reduce the RCS. The major limitation of this early method was that it could only analyze flat panels. As a result, the F-117 and its Have Blue prototypes were composed of a number of faceted panels. The massive improvements in computer technology over the past two decades have allowed the same basic method to be applied to smooth, contoured surfaces. These improved codes have been instrumental in designing aircraft like the B-2 and F-22.
In addition to the overall shaping of the aircraft, a number of other aircraft components can produce high radar returns. These include:
cockpit and other windows: The interior of the cockpit is full of sharp corners and reflective metal objects. Even the pilot's helmet is radar reflective. The F-117 uses flat window panels and radar-absorbing treatments on the cockpit windows. The same methods are also used on the windows housing the bomb laser-guidance systems. These measures block radar waves from entering these areas.
sharp perpendicular edges: Any kind of edge perpendicular to radar waves causes them to to be diffracted and reflected. In particular, the edges of landing gear doors and other access doors as well as the trailing edges of the wings produce strong radar returns. This effect can be minimized by sweeping the edges so they are not perpendicular to the radar waves. Thus, the edges of doors on the F-117 and other stealth aircraft are covered with small saw-tooths, or diamond-shaped edges that dissipate the radar energy in many directions.
Infrared signature:
Along with radar, another major detection system used today is infrared heat sensors, such as night-vision goggles. In addition, most short-range missiles like Sidewinder home-in on heat sources, including aircraft engines. The key to reducing an aircraft's infrared signature is to cool the exhaust air rapidly using long nozzle ducts or mixing the exhaust with cooler air. The F-117 platypus nozzle, one of the more difficult items to construct, does both of these things. The Stealth Fighter also uses a high-bypass turbofan engine that mixes the hot jet exhaust with cooler bypass air. The B-2 and YF-23 also route the hot exhaust through long troughs coated with heat-absorbing material that not only help cool the air but block the hot gases from being seen from below.
Aural signature:
Aircraft are noisy, and all the fancy methods to reduce RCS or infrared-signature mean nothing when someone on the ground hears your plane roaring by. In an aircraft such as the F-117 where it is a high-altitude bomber, this issue isn't so critical, but high-bypass turbofan engines do tend to be much quieter than turbojets. Most stealth aircraft are also subsonic so they do not create sonic booms. (The US is working on reshaping and conformal technology where supersonic flight can be achieved without signaling a “boomâ€)
Visibility:
The ultimate level of stealth is to make the aircraft invisible to the human eye. Obviously, nothing developed to date (that we know about) has achieved this extreme, but experiments to find methods of doing so are ongoing. The most common and least sophisticated approach is through the use of camouflage paint schemes. Light glinting off of canopies can also be reduced using flat panels or special coatings. More sophisticated approaches revolve around using lights or mirrors to bend beams of light around the aircraft making it more difficult to spot. However, little information about this kind of research is publicly available. In addition, care must be taken to avoid contrails, the white trails of condensed water vapor that can be seen in engine exhausts. Cooling the exhaust goes a long way towards achieving this goal, and special fuel additives that help eliminate contrails have also been developed. A number of high altitude aircraft in the USAF use contrail additives to reduce the “plumeâ€.
Canted Tails
Lockheed designers included these canted tails based on early research into stealth technology. As far back as the 1940s, engineers had realized that perpendicular surfaces, like vertical tails, generated strong radar returns. By canting the tails away from 90°, the radar cross-section (RCS) of an aircraft could be considerably reduced. Kelly Johnson of Lockheed included canted tails in the SR-71 design since the original CIA requirements called for an aircraft as difficult to detect as possible, given the limited techniques of the day.
Northrop and McDonnell Douglas included canted tails on the YF-17, F-18, and F-18E/F for similar reasons. Though none of these planes is a true stealth aircraft by any means, simply canting the tails away from the vertical reduces the RCS to the same levels as a smaller aircraft.
The canted tails on these aircraft also provide some aerodynamic benefits as well. Along the sides of the forward fuselage are two long strakes called leading-edge extensions. These surfaces generate strong swirling vortices that allow the aircraft to operate at very high angles of attack, up to 50° on the F-18. As the vortices travel back away from the plane, they create a high-speed airflow past the two vertical tails making them more effective in providing yaw control. In addition, the tails generate some pitch control since the aerodynamic forces acting on the canted tails are broken into vertical and horizontal components. The horizontal component is used most effectively during takeoff. By turning both rudders inboard, the vertical tails produce sufficient downforce to help lift the nose of the plane and allow it to takeoff at a lower speed.
Factors that determine the energy returned by a target
A term used to describe the relationship between these variables is power density, sometimes also called power flux. To understand power density, consider the following diagram. The power transmitted by a radar is dissipated the further it travels because it is spread over an increasingly larger area. The area over which the power is spread is proportional to the square of the distance, or range (R), from the transmitting radar. The amount of power spread over a given area is called the power density, and this quantity decreases by the square of the range. The power density of the transmitted radar wave at the range of the target has a special name called the incident power density
Radar cross section of a cylinder
Data of this form is also routinely collected for more complex shapes, like complete aircraft. However, these RCS signatures from all aspects are seldom declassified until long after the aircraft has been removed from service. The only data I have been able to find so far is for the obsolete T-33 trainer that dates back to the 1940s..
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So, when you look at the complexity of what is required to render an aircraft with lower visibility, you can see that the majority of the requirements are not met by production aircraft like the LCA or FC-1
