Tutor Lesson

Regular Reflections and Corners

• Specular return, or regular reflection, behaves like light on a mirror: the reflection angle equals the incidence angle. Large flat areas are avoided when they could send energy straight back to the radar.
• Two surfaces meeting at 90 degrees form a corner reflector. A wave can bounce from both surfaces and travel back toward its source.
• A conventional combination of vertical and horizontal stabilizers can create exactly this kind of radar-reflecting corner.

Tail Geometry and Flight Control

• Removing the tail, as on the B-2, helps eliminate corner reflections but greatly reduces maneuverability.
• A V-tail, used by the F-117 Nighthawk, replaces separate horizontal and vertical stabilizers.
• The same V-tail surfaces can supply rudder-like yaw control and elevator-like pitch control.
• Deflecting the two surfaces in opposite directions produces a horizontal resultant force for yaw.
• Moving them together in the same direction produces pitch control.
• The Cirrus SF50 uses this arrangement to place one small jet engine above the fuselage with its exhaust passing through the V-tail. The aircraft is light enough to deploy a parachute in an emergency.

A combined rudder-and-elevator mechanism is less suitable for a high-performance fighter. Control authority depends on surface area, so a larger elevator can exert more pitch control. If one surface must provide yaw and pitch simultaneously, the available authority is reduced.

The F-35 and F-22 instead use large, separate elevators. Their spacing and angles prevent corner reflections while retaining strong maneuvering control.

Choosing an Intake Location

• The F-117 and B-2 place their intakes on top. This keeps ground-based radar energy from entering the duct and returning to its receiver, while also helping reduce the infrared heat signature.
• At a high angle of attack, an upper intake receives lower-pressure air. Engine performance therefore falls during the kinds of combat maneuvers when it may be most needed.
• An under-fuselage intake like the F-16's produces too much radar return for the F-35 design.
• The compromise selected for the F-35 is a pair of intakes positioned on the fuselage sides.

Boundary-Layer Control with a DSI

• When boundary-layer air enters an engine, it reduces performance and can harm the turbine.
• A rotating blade alternately meets slow boundary-layer air and fast free-stream air. The repeated force changes cause cyclic bending that can lead to fatigue failure.
• The F-16's traditional solution is a small gap that separates the inlet from the fuselage. This boundary-layer diverter adds drag and radar cross-section.
• The diverterless supersonic inlet, or DSI, uses a large bump instead of that gap.
• The bump forms a compressed region that moves the boundary layer away from the engine inlet.
• It also scatters incoming radar energy and reduces aerodynamic drag.
• After later F-16 variants demonstrated that the DSI could meet performance needs, it was introduced on the F-35.
• The resulting system cuts weight by 30% and reduces production and maintenance expense.

Breaking Up Long Edges

• Long sharp edges scatter radar in many directions. Energy may also travel along a surface and then scatter strongly when it reaches the trailing edge.
• Because the returned strength depends on edge length, serrations divide a long edge into shorter portions.
• The B-2 displays this technique clearly along its trailing edges.
• On the F-35, serrations appear more subtly around nearly every unavoidable surface gap.
• One serrated hatch conceals the pilot's telescoping access ladder.
• The landing-gear doors also use serrated boundaries.
• Internal weapons-bay doors help conceal missiles that would otherwise reflect radar.
• Smaller serrated openings cover the flare dispensers.

Decoys for Heat and Radar Guidance

• Flares can divert heat-seeking missiles.
• They do not protect against radar-guided weapons, whose growing use and sophistication prompted development of another decoy.
• An access panel opens and a transmitter unreels behind the aircraft to a safe distance on a fiber-optic tow line.

1. First, the aircraft computer calculates signals that the emitter uses to jam a missile while it tries to acquire the F-35.
2. If the missile obtains a radar lock, the system attempts to disrupt its tracking algorithms and break that lock.
3. If the missile continues toward the aircraft, the towed emitter imitates the F-35's radar signature and draws the weapon toward the decoy.

Weakening Surface Waves

Special tape around serrated hatches adds another defense against edge scattering. Its electrical conductivity decreases gradually, so a traveling radar wave scatters in small amounts at successive changes. The surface wave loses intensity before reaching the edge, avoiding one strong return there.

The aircraft's surface also uses specialized radar-absorbing material. Lockheed Martin filed a 2010 patent for a carbon-nanotube composite that absorbs frequencies from 0.1 megahertz through 60 gigahertz, including frequencies used by Russian surface-to-air systems such as the S-400.

Interrupting Waves Around the Nose

A cylindrical shape can still return radar energy. If a wave arrives tangentially and its wavelength is at least one tenth of the cylinder's circumference, it can follow the exterior and head back toward the receiver.

Unlike the F-16's nearly circular nose cone, the F-35 has a sharp ridge that interrupts the circular path and prevents a wave from traveling smoothly around it.

Feeding the Lift Fan

• The aircraft's largest hatch conceals the lift fan, and unlike most other openings, its edge is not serrated.
• The fan behaves like a compact helicopter rotor and produces 85 kN of upward thrust.
• When operating, it creates low pressure above the aircraft and pulls air inward through a 90-degree change in direction.
• Air must cross the inlet hood smoothly; excessive turbulence would decrease fan output.
• A serrated door would introduce sharp-edge flow disturbances, so this hatch remains smooth.
• Several hood designs were tested to improve the airflow.
• The X-35 demonstrator used two doors that opened sideways. The production F-35B instead received one rear-hinged door, which guides air inward and improves pressure recovery during short take-offs.

During conversion for vertical or short-runway operation, a clutch connects the engine's extended driveshaft to the lift-fan bevel gear and transfers 29,000 horsepower, turning two counter-rotating fan stages as the rear nozzle also reconfigures.

The Three-Bearing Swivel Nozzle

• The nozzle contains three sealed segments cut at relative angles.
• Rotating the angled sections separately or together changes the nozzle geometry and redirects exhaust smoothly downward.
• Turning the middle section moves the outlet from 0 degrees to 45 degrees. This short-take-off setting divides engine power between forward propulsion and lift.
• That mode permits operation from short amphibious assault ships such as the roughly 258-meter USS Makin Island.
• A ski-jump ramp on smaller ships can shorten the required run still further.
• During a short launch, the nozzle rapidly changes its vector in coordination with the rear elevators to keep pitch safe.
• For vertical landing, the last segment supplies the complete 90-degree downward turn.
• The first segment can move the outlet sideways. During transition it compensates for the arc produced by individual segment rotations, preventing sideways thrust from spinning the aircraft.
• The assembly supports smooth changes from cruise to vertical flight and back again.

Stabilizing the Hover

• Bypass bleed air feeds two wing-mounted roll nozzles. Their distance from the center of pressure lets their thrust control roll.
• Guide vanes below the lift fan vary its outlet area to refine fan performance.
• The same vanes can direct lift-fan thrust from 5 degrees forward to 42 degrees backward.
• Computer coordination of these devices makes vertical flight notably steadier than in the AV-8B Harrier.
• The single-engine aircraft can consequently hover on two air columns: one beneath the fan and one beneath the rear nozzle.

Why the F-135 Produces More Thrust

• A larger fan and larger bypass passages raise mass flow, producing more thrust and better cruise efficiency.
• Bypass air does not enter the combustion chamber, so it misses the acceleration created there and lowers total exhaust velocity.
• The lower exhaust velocity limits top speed. Although the F-35 is the first aircraft described here as combining vertical and supersonic flight, it was optimized for airborne loiter time rather than maximum speed.

Saving Weight in the Lift Module

• The lift-fan module weighs 1.2 tonnes. It contributes nothing during ordinary forward flight and demands extra fuel simply to carry it.
• In other F-35 variants, the same internal space holds a fuel tank.
• Keeping this machinery to 1.2 tonnes was essential to the F-35B's viability and represents a major engineering result.
• The fan contains two counter-rotating titanium blisks.
• A blisk forms its blades and disk as one component instead of attaching separate blades with dovetail joints.
• This integrated construction improves efficiency and removes a possible failure location at the blade-to-disk joint.
• Manufacturing goes further in the first fan stage, whose blades are hollow to reduce mass.

Seeing Through the Helmet Display

• In an older aircraft such as the F-16, the head-up display is fixed to a cockpit panel and leaves the pilot's view when the pilot looks elsewhere.
• On the F-35, a central computer processes inputs from sensors around the airframe and projects the results inside the helmet visor.
• Critical flight and tactical information remains visible as the pilot looks in any direction.
• Sensor imagery provides night vision and an X-ray-like ability to look through the aircraft's structure.
• Some of the distributed sensors sit behind transparent, faceted windows beneath the aircraft and at other points on the airframe.
• These windows use costly sapphire rather than ordinary glass because sapphire is hard, durable, and transparent across wavelengths from ultraviolet to infrared.

Electronic Beam Steering with AESA

The key radar antenna in the nose is an Active Electronically Scanned Array, or AESA. Instead of mechanically turning one dish, it uses 1,600 small slot antennas arranged across metal plates.

• When waves leave different antennas with peaks and troughs aligned, constructive interference increases their combined amplitude.
• When waves differ in phase by 180 degrees, peaks meet troughs and destructive interference cancels the signal, like the principle used by noise-canceling earphones.
• By controlling these relationships, the array directs radio energy electronically and avoids behaving like a conspicuous beacon.
• A passive phased array drives all elements through one transmitter and receiver. It points in one direction and struggles to keep tracking two adjacent targets after they separate.
• In the F-35's active array, every one of the 1,600 elements has its own transmitter and receiver, allowing several targets to be followed at once without moving parts.
• The nose covering must pass radar waves and is generally made from glass-fiber composite, but that transparency also exposes the reflective antenna behind it.
• Because an electronic array can track targets without changing its physical position, the F-35 points it upward so incoming radar energy is reflected toward space.

Secure Networking Through MADL

• The AESA also functions as a communications antenna, an important part of F-35 combat doctrine.
• Aircraft in a squadron and allied ground systems relay large amounts of information, which requires high transfer speeds.
• Ordinary broadcasts can reveal a stealth aircraft to listeners, so communication must remain secure.
• MADL, the Multifunction Advanced Data Link, builds on experience from the F-22 and enables rapid, secure exchange among F-35s and ground systems.
• The aircraft sorts the shared data and places the useful result directly on the helmet display.
• If another system detects an adversary beneath the aircraft, the planes can exchange that location without a spoken report. The pilot can look downward and see the indicated position through the helmet imagery.

Together, these systems make the F-35B a networked stealth fighter: it can depart from a ship much smaller than an aircraft carrier, return in a helicopter-like hover, and maintain a shared tactical picture throughout the mission.