Stealth aircraft
Stealth aircraft are designed to avoid detection using a variety of technologies that reduce reflection/emission of radar, infrared, visible light, radio frequency spectrum, and audio, collectively known as stealth technology. The F-117 Nighthawk was the first operational aircraft explicitly designed around stealth technology. Other examples of stealth aircraft include the B-2 Spirit, the B-21 Raider, the F-22 Raptor, the F-35 Lightning II, the Chengdu J-20, the Shenyang J-35, the Sukhoi Su-57 and the Bayraktar Kızılelma.
While no aircraft is completely invisible to radar, stealth aircraft make it more difficult for conventional radar to detect or track the aircraft effectively, increasing the odds of an aircraft avoiding detection by enemy radar and/or avoiding being successfully targeted by radar guided weapons. Stealth is a combination of passive low observable features and active emitters such as low-probability-of-intercept radars, radios and laser designators. These are typically combined with operational measures such as carefully planning mission maneuvers to minimize the aircraft's radar cross-section, since common hard turns or opening bomb bay doors can more than double an otherwise stealthy aircraft's radar return. Stealth is accomplished by using a complex design philosophy to reduce the ability of an opponent's sensors to detect, track, or attack the stealth aircraft. This philosophy takes into account the heat, sound, and other emissions of the aircraft which can also be used to locate it. Sensors are made to reduce the impact of low observable technologies and others have been proposed such as IRST systems to detect even reduced heat emissions, long wavelength radars to counter stealth shaping and RAM focused on shorter wavelength radar, or radar setups with multiple emitters to counter stealth shaping. However these have disadvantages compared to traditional radar against non-stealthy aircraft.
Full-size stealth combat aircraft demonstrators have been flown by the United States, Russia and China., the only combat-ready stealth aircraft in service are the Northrop Grumman B-2 Spirit, the Lockheed Martin F-22 Raptor, the Lockheed Martin F-35 Lightning II, the Chengdu J-20, and the Sukhoi Su-57. a number of other countries developing their own designs. In-development aircraft include fighters such as the US' F-47 and China's J-36, as well as strategic bombers, China's H-20 and Russia's PAK DA. There are also various aircraft with reduced detectability, either unintentionally or as a secondary feature.
Stealth aircraft first saw combat when the F-117 was used in the 1989 United States invasion of Panama. Since then US, UK, and Israeli stealth aircraft have seen combat, primarily in the Middle East, while the Russian Su-57 has seen combat in the Russian invasion of Ukraine.
, there has been one confirmed shootdown of a stealth aircraft, during the 1999 NATO bombing of Yugoslavia, of an F-117 by a Serbian Isayev S-125 'Neva-M' missile brigade commanded by Colonel Zoltán Dani, while a second incident damaged an F-117. Russia and allegedly China studied the relatively intact wreckage, which the US military considered too outdated to warrant further action.
Design principles
Besides all the usual demands of flight, the design of a stealth or low-observability aircraft aims to reduce radar and infrared detection, including:- Reduce thermal infra-red emission from the engine and its exhaust wake
- Reduce radar reflection back to a hostile receiver by shaping the airframe
- Reduce radar reflections from the airframe by the use of radar-absorbent materials or radar-transparent materials such as plastics.
- Reduce radar detection from exposed internal surfaces such as the cockpit, weapons bay and engine intake ducting.
Rotorcraft introduce a particular design challenge, due not only to their multiple wing surfaces and articulated joints, but also to the constantly-changing relationship of these to the main airframe surfaces. The Boeing–Sikorsky RAH-66 Comanche was one of the first attempts at a stealth helicopter.
Limitations
Instability of design
Early stealth aircraft were designed with a focus on minimal RCS rather than aerodynamic performance. Highly stealthy aircraft like the F-117 Nighthawk are aerodynamically unstable in all three axes and require constant flight corrections from a fly-by-wire flight system to maintain controlled flight. As for the B-2 Spirit, which was based on the development of the flying wing aircraft by Jack Northrop in 1940, this design allowed for a stable aircraft with sufficient yaw control, even without vertical surfaces such as rudders.Aerodynamic limitations
Earlier stealth aircraft lack afterburners, because the hot exhaust would increase their infrared footprint, and flying faster than the speed of sound would produce an obvious sonic boom, as well as surface heating of the aircraft skin, which also increases the infrared footprint. As a result, their performance in air combat maneuvering required in a dogfight would never match that of a dedicated fighter aircraft. This was unimportant in the case of these two aircraft since both were designed to be bombers. More recent design techniques allow for stealthy designs such as the F-22 without compromising aerodynamic performance. Newer stealth aircraft, like the F-22, F-35 and the Su-57, have performance characteristics that meet or exceed those of front-line jet fighters due to advances in other technologies such as flight control systems, engines, airframe construction and materials.Electromagnetic emissions
The high level of computerization and large amount of electronic equipment found inside stealth aircraft are often claimed to make them vulnerable to passive detection. This is highly unlikely and certainly systems such as Tamara and Kolchuga, which are often described as counter-stealth radars, are not designed to detect stray electromagnetic fields of this type. Such systems are designed to detect intentional, higher power emissions such as radar and communication signals. Stealth aircraft are deliberately operated to avoid or reduce such emissions.Radar Warning Receivers look for regular pings of energy from mechanically swept radars while fifth generation jet fighters use Low Probability of Intercept Radars with no regular repeat pattern.
Vulnerable modes of flight
Stealth aircraft are still vulnerable to detection while and immediately after using their weaponry. Since stealth payload is not yet generally available, and ordnance mount points create a significant radar return, stealth aircraft carry all armaments internally. As soon as weapons bay doors are opened, the plane's RCS will be multiplied and even older generation radar systems will be able to locate the stealth aircraft. While the aircraft will reacquire its stealth as soon as the bay doors are closed, a fast response defensive weapons system has a short opportunity to engage the aircraft.This vulnerability is addressed by operating in a manner that reduces the risk and consequences of temporary acquisition. The B-2's operational altitude imposes a flight time for defensive weapons that makes it virtually impossible to engage the aircraft during its weapons deployment. New stealth aircraft designs such as the F-22 and F-35 can open their bays, release munitions and return to stealthy flight in less than a second.
Some weapons require that the weapon's guidance system acquire the target while the weapon is still attached to the aircraft. This forces relatively extended operations with the bay doors open.
Such aircraft as the F-22 Raptor and F-35 Lightning II Joint Strike Fighter can also carry additional weapons and fuel on hardpoints below their wings. When operating in this mode the planes will not be nearly as stealthy, as the hardpoints and the weapons mounted on those hardpoints will show up on radar systems. This option therefore represents a trade off between stealth or range and payload. External stores allow those aircraft to attack more targets further away, but will not allow for stealth during that mission as compared to a shorter range mission flying on just internal fuel and using only the more limited space of the internal weapon bays for armaments.