Stealth aircraft

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An F-117 Nighthawk stealth strike aircraft flying over Nevada in August 2002.

 

 

Stealth aircraft are designed to avoid detection using a variety of advanced technologies that reduce reflection/emission of radar, infrared,^[1] visible light, radio-frequency (RF) spectrum, and audio, collectively known as stealth technology.^[2] Development of stealth technology likely began in Germany during World War II, the planned Horten Ho 229 being described as the first stealth aircraft.^[3] Well-known modern examples of stealth of U.S aircraft include the United States' F-117 Nighthawk (1981–2008), the B-2 Spirit, the F-22 Raptor,^[4] and the F-35 Lightning II.^[5]
While no aircraft is totally invisible to radar, stealth aircraft make it difficult for conventional radar to detect or track the aircraft effectively, increasing the odds of a successful attack. Stealth is the combination of passive low observable (LO) features and active emitters such as Low Probability of Intercept Radars, radios and laser designators. These are usually combined with active defenses such as chaff, flares, and ECM.^[6] It 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.^[7] This philosophy also takes into account the heat, sound, and other emissions of the aircraft as these can also be used to locate it.
Full-size stealth combat aircraft demonstrators have been flown by the United States (in 1977), Russia (in 2010) and China (in 2011).^[8] The US military has adopted three stealth designs, and is preparing to adopt the Lockheed Martin F-35 Lightning II.
Most recent fighter designs will claim to have some sort of stealth, low observable, reduced RCS or radar jamming capability, but there has been no air to air combat experience against stealth aircraft.^[9]

Contents

• 1 Background
• 2 General design
• 3 Limitations
  • 3.1 Instability of design
  • 3.2 Aerodynamic limitations
  • 3.3 Electromagnetic emissions
  • 3.4 Vulnerable modes of flight
  • 3.5 Reduced payload
  • 3.6 Sensitive skin
  • 3.7 Cost of operations
  • 3.8 Reflected waves
  • 3.9 Schlieren Signature
  • 3.10 Infrared (heat)
  • 3.11 Longer Wavelength Radar
  • 3.12 OTH radar (over-the-horizon radar)
  • 3.13 Advancements in computational power
• 4 Operational usage of stealth aircraft
• 5 Stealth aircraft lost
• 6 List of stealth aircraft
  • 6.1 Manned
    • 6.1.1 Dedicated reduced cross section designs
    • 6.1.2 Accidental or secondary function reduced cross section designs
  • 6.2 Unmanned reduced RCS designs
• 7 See also
• 8 References

Background

During the First World War, an attempt to reduce the visibility of military aircraft through the experimental use of "Cellon" plastic transparent covering material resulted in single examples of the Fokker E.III Eindecker fighter monoplane, the Albatros C.I two-seat observation biplane, and one German heavy bomber design, the Linke-Hofmann R.I all being covered with the "Cellon" material; the latter two aircraft had all-wooden structures covered with the "Cellon" material, which degraded rapidly in direct sunlight and were not proceeded with any further.
Nearly three decades later, a more serious attempt at "invisibility" was tried with the Horten Ho 229 flying wing fighter-bomber, developed in Germany during the last years of World War II^{[citation needed]}. In addition to the aircraft's shape, which may not have been a deliberate attempt to affect radar deflection, the majority of the Ho 229's wooden skin was bonded together using carbon-impregnated plywood resins designed with the purported intention of absorbing radar waves. Testing performed in early 2009 by the Northrop-Grumman Corporation established that this compound, along with the aircraft's shape, would have rendered the Ho 229 virtually invisible to the top-end HF-band, 20-30 MHz primary signals of Britain's Chain Home early warning radar, provided the aircraft was traveling at high speed (approximately 550 mph (890 km/h)) at extremely low altitude (50–100 feet).^[3]
In the closing weeks of WWII the US military initiated "Operation Paperclip", an effort by the US Army to capture as much advanced German weapons research as possible, and also to deny that research to advancing Soviet troops. A Horten glider and the nearly complete Ho 229 V3 third prototype airframe were secured and sent to Northrop Aviation for evaluation in the United States,^[3] who much later used a flying wing design for the B-2 stealth bomber. During WWII Northrop had been commissioned to develop a large wing-only long-range bomber (XB-35) based on photographs of the Horten's record-setting glider from the 1930s, but their initial designs suffered controllability issues that were not resolved until after the war. Northrop's small one-man prototype (N9M-B) and a Horten wing-only glider are located in the Chino Air Museum in Southern California.
Modern stealth aircraft first became possible when Denys Overholser, a mathematician working for Lockheed Aircraft during the 1970s, adopted a mathematical model developed by Petr Ufimtsev, a Russian scientist, to develop a computer program called Echo 1. Echo made it possible to predict the radar signature an aircraft made with flat panels, called facets. In 1975, engineers at Lockheed Skunk Works found that an aircraft made with faceted surfaces could have a very low radar signature because the surfaces would radiate almost all of the radar energy away from the receiver. Lockheed built a model called "the Hopeless Diamond", so-called because it resembled a squat diamond, and looked too hopeless to ever fly. Because advanced computers were available to control the flight of even a Hopeless Diamond, for the first time designers realized that it might be possible to make an aircraft that was virtually invisible to radar.^[10][11]
Reduced radar cross section is only one of five factors the designers addressed to create a truly stealthy design such as the F-22. The F-22 has also been designed to disguise its infrared emissions to make it harder to detect by infrared homing ("heat seeking") surface-to-air or air-to-air missiles. Designers also addressed making the aircraft less visible to the naked eye, controlling radio transmissions, and noise abatement.^[4]
The first combat use of purpose-designed stealth aircraft was in December 1989 during Operation Just Cause in Panama. On 20 December 1989, two USAF F-117s bombed a Panamanian Defense Force barracks in Rio Hato, Panama. In 1991, F-117s were tasked with attacking the most heavily fortified targets in Iraq in the opening phase of Operation Desert Storm and were the only jets allowed to operate inside Baghdad's city limits.^[12]

General design

The general design of a stealth aircraft is always aimed at reducing radar and thermal detection. It is the designer's top priority to satisfy the following conditions; some of which are listed below, by using their skills, which ultimately decides the success of the aircraft:-

• Reducing thermal emission from thrust
• Reducing radar detection by altering some general configuration (like introducing the split rudder)
• Reducing radar detection when the aircraft opens its weapons bay
• Reducing infra-red and radar detection during adverse weather conditions