AK 96

December 3 2024

Air Intakes

Air intakes are required to capture free stream air, sometimes change its direction and then supply this to the engine. The inlet design should be such that it has minimal loss in energy, decelerate the air entering the engine so that the Mach number at the engine entrance is ≈ 0.4-0.5, provide low drag to the aircraft as a whole, and also improve the stealth characteristics incase of advanced fighter aircrafts.

The basic types of intakes are:-

Normal shock diffuser or pitot intake,
Oblique shock diffuser (conical or spike inlet),
Two-dimensional intake,
Diverterless intake.

Shock wave

Shock waves are very small regions in the gas where the gas properties change by a large amount. The Mach number and speed of air flow decrease across a shock wave. If the shock wave is perpendicular to the flow direction, it is called a normal shock. When a shock wave is inclined to the flow direction it is called an oblique shock.

Diagram depicting the flow condition in Normal and Oblique shock

Normal shock diffuser or Pitot intake

The intakes work well subsonically and fairly well at low supersonic speeds. However, this design is not suitable for prolonged operation above M=1.6 (i.e. – 1.6 times the speed of sound). In case of subsonic flow the airflow speed decreases to the required value due to the divergent design, (i.e. the area increases as we move further from the intake opening). At supersonic speeds a shock wave forms in front of the intake which decelerates the incoming flow from supersonic to subsonic speed. Thereafter the airflow speed is reduced by the divergent profile of the intake.

Pitot Intake in F-16 aircraft

Oblique-shock diffuser (Conical or Spike intake)

Oblique shock diffuser, spike or conical inlet: exploits shock patterns created by the supersonic flow over a cone. The spike inlet is lighter and has lower loss but has higher drag and is mechanically more complex. It decelerates the flow over one or several oblique shocks. The last shock in the intake is a normal shock, which decelerates the flow to subsonic speed. An oblique shock diffuser is optimum is the most efficient during steady level flight but suffers from a huge loss of efficiency with angle of attack and sideslip, i.e.- when the flow is not symmetrical to the axis of the cone but is at an angle to it. This occurs during maneuvering flight.

Conical Intake of a Mig-21 aircraft

Two-dimensional or Ramp intake

The ramp inlet is one of the type of the supersonic inlets which is rectangular in shape and is designed to generate a number of shock waves to aid the inlet compression process at supersonic speeds. The deflection of the air stream creates a number of oblique shock waves at each change of gradient along the ramp. Air crossing each shock wave suddenly slows to a lower Mach number. The advantages of this intake is its adaptability over a wide range of flow conditions, angles of attack and mass flows, and tailoring of the shock system for optimum efficiency over a wide range of supersonic speeds. However, due to its adaptability it is heavy and complex in construction.

Ramp positions and flow at various flight conditions: (a) Subsonic flow; (b) Transonic flow; (c) Supersonic flow

Ramp Intake in a F-15 aircraft

Diverterless supersonic inlet (DSI)

A Diverterless supersonic inlet (DSI) consists of a “bump” and a forward-swept inlet cowl, which work together to divert boundary-layer away from the aircraft’s engine. This eliminates the need for a splitter plate, while compressing the air to slow it down from supersonic to subsonic speeds.

The DSI bump functions as a compression surface and creates a pressure distribution that prevents the majority of the boundary layer air from entering the inlet at speeds up to Mach 2. Traditional aircraft inlets contain many heavy moving parts. In comparison, DSI completely eliminates all moving parts, which makes it far less complex and more reliable than earlier diverter-plate inlets. The removal of moving parts also reduces the overall weight of the aircraft.

Features of the F-35 DSI air intakes

DSI’s also crucially improve the aircraft’s low-observable characteristics (by eliminating radar reflections between the diverter and the aircraft’s skin). Additionally, the “bump” surface reduces the engine’s exposure to radar, significantly reducing a strong source of radar reflection because they provide an additional shielding of engine fans against radar waves. However, a diverterless intake reacts considerably to oblique flow, which is a disadvantage in maneuvering flight especially for two-engine aircraft.

November 10 2024

Luneburg Reflector

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Stealth performance is critical in stealth aircrafts but it is good that the aircrafts be visible at appropriate times. To be able to appear at will on radars, these aircrafts are equipped with a very special device: the Luneburg Lens.

What is a Luneburg Lens?

The Luneburg Lens is an ellipsoidal multi-refractive lens, partially coated with a metallic reflective layer. This enables it to focus incident electromagnetic waves of pre-determined wavelengths and reflect them back in the direction of the original ray wave. When any radar wave passes through the lens, the focusing characteristics make the radar cross-sectional area (RCS) reflected by the lens much larger than its physical cross section. This feature makes the Luneburg Lens extremely useful as a radar reflector to manipulate RCS.

Working principle of Luneburg lenses

RCS can be increased with Corner reflectors constructed from several perpendicularly placed metal strips which are simple in construction and cheaper in cost. Why use complex Luneburg Lens?

It is because, compared to corner reflectors, Luneburg lenses are smaller and lighter, inducing less drag and have no impact on aerodynamics. They are also better reflectors with better directional pattern, lower side and back lobes.

Luneburg reflectors on a F-35

While stealth capability improves survivability of stealth fighters, there is always the danger of accidents such as collision when the aircraft cannot be detected by air or ground radar, especially while flying in friendly airspace. Adding a Luneburg lens allows monitoring by air and ground radars whenever needed and alleviates such potential incidents.

Furthermore, stealth fighters with Luneburg lens gives added capability of deception. By manipulating RCS, the true radar signature of the aircraft can be denied. This is particularly useful in information warfare. Deploying Luneburg lenses not only conceals stealth fighter radar signature, but also prevents anti-stealth experiments by adversaries. The Luneburg Lenses are removable during wartime when full stealth mode is required.

October 15 2024

J-20: China’s “Mighty Dragon”

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The Chengdu J-20 “Mighty Dragon” is a twinjet all-weather, stealth, fifth-generation fighter aircraft developed by China’s Chengdu Aerospace Corporation for the People’s Liberation Army Air Force (PLAAF). The J-20 is designed as an air superiority fighter with precision strike capability. The aircraft has three notable variants: the initial production model, the revised airframe variant with new engines and thrust-vectoring control, and the aircraft-teaming capable twin-seat variant.

Configuration

The aircraft is a twin engine, delta canard, long and blended fuselage, chiseled nose section, frameless canopy, diverterless supersonic inlet (DSI) intakes.

All-moving canard surfaces with pronounced dihedral are placed behind the intakes, followed by leading edge extensions (LERX) merging into the delta wing with forward-swept trailing edges. Leading edge extensions and body lift are incorporated to enhance performance in a canard layout. This combination generates more lift compared to an ordinary canard delta or an equivalent-sized pure delta configuration. This combination allows the use of a smaller wing, reducing supersonic drag without compromising transonic lift-to-drag characteristics that are crucial to the aircraft’s turn performance.

Leading edge extension (LERX) placed behind the Canard and merging into the delta wing

The aft section has twin outward canted all-moving fins, short but deep ventral strakes, and conventional low-observable engine exhausts.

Avionics

Primary radar

Although official information on the type of radar that J-20s use has not yet been released publicly, J-20s is believed to use Type 1475 (KLJ-5) active electronically scanned array (AESA) radar with 1856 transmit/receive modules. However based on the nose cross-section of the J-20 and known data about a single transmit/receive module, J-20s likely contain 2000–2200 transmit/receive modules giving greater power and performance.

As of now there is no information regarding capabilities such as Range, Low probability of intercept (LPI), differing modes available, peak power, electronic counter counter measure (ECCM) capabilities, material used, i.e., GaN or GaAs.

EOTS

All airframes of the J-20 from the 2011 prototype have featured a chin mounted, faceted window for an electro-optical/infra-red sensor capable of search and track functions. The configuration of this sensor is similar to the Electro Optic Targeting System fairing on the F-35’s chin, however J-20’s appears to be less “deep” and “longer” than the F-35’s EOTS. The configuration of the faceted windows on J-20’s sensor also appears different to F-35’s EOTS.

J-20 Electro Optic Search and Track

The F-35 EOTS is capable of air to air tracking and targeting as well as air to ground tracking and targeting, with the air to ground function being a key part of F-35’s overall strike fighter capability. The air to ground tracking and targeting function includes laser spot tracking and laser designation to assist in the guidance of precision guided munitions. F-35’s EOTS features many air to ground capabilities such as laser spot tracking and laser designation, which J-20’s EOTS may lack since it seems to be designed for air superiority role.

Different roles of the F-35 EOTS

A private Chinese aerospace company called Beijing A-Star has offered a range of “EOTS” products, with specifications and performance details, as well as illustrations depicting their products equipped aboard both J-20s and FC-31s, however at present there is no evidence to suggest that the equipment has been installed on the aircraft.

Brochure showcasing Beijing A-Star Science and Technology’s EOTS-86 electro-optical targeting system (EOTS) and the EORD-31 infrared search and track (IRST)

Electro Optic Passive Detection System

The J-20 is equipped with six distinct, window apertures around the aircraft; these house the electro optic sensors. The apertures are arranged in the aircraft in such a way so as to provide three hundred and sixty degree spherical coverage around the aircraft, and the placement and configuration of the apertures are similar to the placement of the six apertures for the F-35’s AN/AAQ-37 Distributed Aperture System (DAS).

Locations of J-20’s six Electro Optic Passive Detection System apertures

The F-35’s DAS provides three basic categories of functions in every direction simultaneously:

Missile detection and tracking (including launch point detection and countermeasure cueing)
Aircraft detection and tracking (situational awareness, infrared search and track (IRST) and air-to-air missile cueing)
Imagery for cockpit displays and pilot night vision (imagery displayed onto the helmet mounted display, i.e., the DAS allows the pilot to essentially “see” through the DAS apertures seamlessly, such that they can even “look through” the floor of the aircraft to what is occurring immediately below the aircraft.)

It is unknown how many of the capabilities of F-35’s DAS will also be present onboard J-20’s EO PDS.

Location of F-35’s six Electro Optic Distributed Aperture System apertures

ESM suite

An advanced Electronic Support Measures suite is an essential part of a modern fighter aircraft’s mission avionics. An ESM suite helps to detect, identify, locate, record, analyze and even geolocate sources of electromagnetic energy, which for a fighter aircraft typically means an opposing force’s radar system. Modern ESM suites have grown in capability and complexity from mere warning systems to alert a pilot when their pilot was possibly being targeted by radar, to being capable of simultaneously geolocating multiple sources of radar emissions in real time and automatically activating counter measures, targeting solutions and cueing weapons.

Such suites are immensely difficult to identify, and may only sometimes be visible as small, low profile conformal antennae around an aircraft. Multiple sites that could hold small conformal antennae have been identified on J-20 through picture analysis, and there are likely multiple other sites with antennae which are not outwardly visible. Either way, it is certain that J-20 will be equipped with an ESM suite of some kind, and if trends for other fifth generation ESM suites are anything to go by, it is likely the Chinese Air Force would have similarly high requirements for J-20’s ESM suite.

Datalinks

High bandwidth datalinks to provide communication and coordination and transference of information between friendly aircraft, ships and other combat forces are a vital feature for fifth generation fighter aircraft. The J-20 will likely be equipped with a datalink that could at least receive the Chinese military’s Joint Service Integrated Datalink System (JSILDS), a system which is said to be similar to the Link 16. It is possible that J-20 may also feature advanced iterations of other datalinks, such as an equivalent to the Multifunction Advanced Datalink of the F-35, or the F-22’s Intra Flight Datalink, to provide effective stealthy and secure communications between aircraft which JSILDS may be incapable of.

Cockpit

The aircraft features a fully-digital glass cockpit with one primary
large color liquid-crystal display (LCD) touchscreen, three smaller auxiliary
displays, and a wide-angle holographic heads-up display (HUD). The aircraft is
fitted with a helmet-mounted display (HMD) system, which displays combat
information inside the pilots’ helmet visor and facilities firing missiles at
high off-boresight angle.

Engine

The prototype and initial aircrafts were powered by an improved Lyulka-Saturn AL-31 variant, the AL-31FM2 developed by Salyut. The engine has a maximum thrust of 142.2 kN (31,970 lbf).

The J-20 is powered by Shenyang WS-10C, an afterburner turbofan engine manufactured by Shenyang Liming Aircraft Engine Company. The WS-10C will serve as an interim engine before the adoption of the more powerful Shenyang WS-15. WS-10C has a thrust of 142 kN, a thrust-to-weight ratio of 9.5 providing necessary propulsion for J-20 to supercruise, and is fitted with serrated afterburner nozzles for enhanced rear-aspect stealth.

WS-10C Engine on J-20

It has been reported that prototypes of the WS-15 engine were capable of producing at least 36,000 pounds of thrust with afterburner, with the ultimate aim of achieving a thrust rating of 40,000 pounds. The WS-15 in its current form features a standard nozzle without thrust vectoring nozzles. The key advantage it brings is the increased thrust, which will propel the J-20 into the elite group of current fighters that can supercruise well above Mach 1. This would confer it an ability to get places faster, as well as likely increase its ceiling.

J-20A with WS-15 engines

Armament

The J-20 has three internal concealed weapons bays, one in the underbelly and one each on the side of the engine. The main weapon bay is capable of housing both long-range air-to-air missiles (AAM; PL-15 – PL-21) and precision-guided munitions (PGM; LS-6/50, LS-6/100,FT-7). The main weapon bay can house four medium/long-range AAMs while the side bay houses one short-range missile in each.

J-20 carrying four PL-15s in its main weapon bay and PL-10 in the side internal bays.

The two smaller lateral weapon bays behind the air inlets are intended for short-range AAMs (PL-10). The bay has a mechanism where rounded semi-circular launcher rails roll out carrying a missile. These side bays allow closure of the bay doors prior to firing the missile, enhancing stealth and allowing the missile to be fired in the shortest possible time.

A diagram depicting the working of side rail

Closed side bay with PL-10 missile in a ready to fire postion to fire position.

The J-20 lacks an internal autocannon or rotary cannon.

The wings include four hardpoints for auxiliary fuel tanks to extend the ferrying range. External fuel tanks negatively impact the J-20’s low observable (stealth) qualities and maneuvering performance, it is likely that they can be jettisoned along with their pylons in a similar manner as the F-22. This allows the aircraft to recapture a large degree of its low observability.

J-20 with four external drop tanks

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