For much of aviation history, the main way for military aircraft to avoid radar detection was brutally simple: stay out of the radar’s line of sight. In the early days of air defense, that usually meant flying low—hugging the terrain so closely that ground clutter masked the aircraft’s radar return—or, at other times, flying extremely high, beyond the reach of fighters and missiles. That era is now decisively over. The development of look-down radars capable of picking aircraft out of terrain clutter has largely killed the idea that simply flying “below the radar” is enough to survive.
Today, modern fighter jets like the Lockheed Martin F-35 Lightning II avoid detection not by altitude tricks, but by using a broad spectrum of stealth technologies designed to delay, confuse, and complicate detection across multiple sensor types. Looking ahead, sixth-generation aircraft such as the U.S. Air Force’s planned F-47 are being built around the concept of all-aspect stealth, optimized not just against radar, but against an entire ecosystem of sensors and networks.
The evolution from flying under the radar to hiding in plain sight reflects a deeper truth about modern air combat: invisibility is impossible. The real objective is not to be unseen forever, but to remain undetected long enough to complete the mission—and to ensure that when detection does occur, it happens too late or with too much uncertainty to allow an effective response.
“Flying under the radar” is one of the rare military aviation expressions that has entered everyday English as a metaphor for avoiding attention. Its roots lie in the Second World War and the early Cold War, when radar systems were crude by modern standards and struggled badly with clutter from terrain, buildings, and weather.
Early radars relied on analog signal processing and had limited ability to discriminate between moving aircraft and reflections from the ground. An aircraft flying low and fast could often disappear into the noise. For a time, this made low-level penetration a viable tactic, particularly for strike aircraft.
During the Cold War, however, the pendulum swung back and forth. In the 1950s and early 1960s, strategic bombers sought safety at extreme altitude. The North American XB-70 Valkyrie was designed to cruise at Mach 3 at altitudes above 70,000 feet, theoretically outrunning and outclimbing enemy interceptors. But by the time the XB-70 flew, surface-to-air missiles had advanced so rapidly that altitude alone no longer guaranteed survival. The concept was rendered obsolete before it ever entered service.
The United States then returned to low-level penetration with aircraft like the B-1B Lancer. The B-1B was designed to fly “map-of-the-earth” profiles, following terrain contours at high speed to stay below radar coverage. It was conceived as an interim solution between the aging B-52 Stratofortress and the still-secret stealth bomber program that would eventually produce the B-2 Spirit.
Flying low, however, came at a steep cost. Terrain-following flight places enormous structural stress on an airframe, especially at high speed. Decades later, that stress is catching up with the B-1B fleet, which is wearing out far faster than the far older but more gently flown B-52s. Low-level flight also imposes operational penalties: higher drag reduces range, sensor horizons shrink, and the aircraft becomes vulnerable to man-portable air defense systems (MANPADS) and short-range missiles.
Most decisively, advances in radar technology undermined the entire premise. The development of look-down, shoot-down radars—capable of filtering out ground clutter and tracking moving targets near terrain—negated the advantage of hiding in the noise. Once that capability matured, flying low was no longer a reliable way to evade detection.
Instead of trying to stay above or below radar coverage, modern stealth fighters are designed to exist within it without standing out. The poster child for this approach is the F-35 Lightning II, along with its older sibling, the F-22 Raptor.
What distinguishes the F-35 and F-22 from other modern fighters is not that they have some stealth features, but that stealth was a foundational requirement from the very beginning of their design. Aircraft like the F-15EX incorporate limited signature-reduction measures, but they were not built from the ground up as stealth aircraft. As a result, they can never achieve the same level of low observability.
It is crucial to stress a point often lost in public discussions: no aircraft, no matter how stealthy, is invisible. Every aircraft can be detected, targeted, and shot down under the right conditions. Stealth is about probability and timing. The aim is to delay detection, reduce tracking quality, and complicate targeting enough that enemy systems cannot generate a reliable firing solution.
The most famous element of stealth is the reduction of radar cross section (RCS). RCS is a measure of how large an object appears on radar, and it depends not only on size but also on shape, materials, and viewing angle.
The most important factor in reducing RCS is shape. Stealth aircraft are carefully sculpted so that radar energy is reflected away from the emitter rather than back toward it. This requires extraordinarily precise engineering. Visible rivets, panel gaps, and sharp discontinuities—once common on older aircraft—are now carefully minimized or aligned to avoid creating strong radar reflections.
Radar-absorbent materials (RAM) further reduce reflections by absorbing some of the radar energy instead of reflecting it. These materials are one reason why modern stealth aircraft tend to wear the same dull gray finish. While gray also helps visually blend the aircraft into the sky, the paint itself is part of the stealth system, optimized for radar performance rather than aesthetics.
Engine integration is another critical factor. Engine faces are highly reflective on radar, so stealth aircraft go to great lengths to hide them. Serpentine intakes block direct line-of-sight to the compressor blades, and exhaust nozzles are shaped to minimize radar returns.
The cumulative effect of these measures is dramatic. Select frontal RCS figures cited by GlobalSecurity.org illustrate the difference:
- F-15 Eagle: 25 m²
- Su-27 Flanker: 15 m²
- F/A-18 or Rafale: 1 m²
- Eurofighter Typhoon: 0.5 m²
- F-35 Lightning II: 0.005 m²
- F-22 Raptor: 0.0001 m²
These figures vary by aspect angle and operating conditions, but they underscore the orders-of-magnitude gap between true stealth aircraft and even the most advanced fourth-generation fighters.
Radar is only one way to find an aircraft. Modern fighters can also be detected by infrared search and track (IRST) systems and heat-seeking missiles. From an infrared perspective, a jet engine is a blazing beacon.
To reduce infrared signatures, modern stealth aircraft employ a range of techniques. Hot turbine components are shielded from direct view, exhaust is mixed with cooler ambient air to reduce temperature, and airframe surfaces are designed to manage heat buildup.
The F-35’s top speed of Mach 1.6 is sometimes criticized as modest compared to older fighters, but that limit is partly deliberate. Higher speeds generate more friction heating, increasing infrared and even radar signatures. In this case, performance was traded for survivability.
One of the most important—and most constraining—features of advanced stealth aircraft is their internal weapons bays. External weapons and drop tanks are radar reflectors, plain and simple. No matter how stealthy an aircraft is, hanging stores under its wings dramatically increases its radar signature.
By carrying weapons and fuel internally, aircraft like the F-35 and F-22 preserve their low observability. The F-22’s ability to supercruise—fly supersonically without afterburners—combined with its large internal fuel capacity means it rarely needs external tanks at all.
The downside is obvious: internal space is limited. Stealth fighters can only carry what fits inside their bays. This constraint is driving efforts to miniaturize missiles so more can be carried internally, as well as research into stealthy drop tanks that could be jettisoned before entering contested airspace.
Carrying weapons internally is also technically challenging. The Russian Su-57 Felon has been in service for years, but only recently has it been publicly demonstrated carrying munitions internally, and even then largely in test settings. It remains unclear how routinely it can do so in combat.
The South Korean KF-21, meanwhile, is being built with an internal weapons bay but will initially carry munitions externally. Its transition to a true low-observable configuration is planned as part of a later block upgrade, highlighting how difficult and expensive internal carriage can be.
Stealth is not just about what an aircraft reflects; it is also about what it emits. Modern fighters are equipped with Low Probability of Intercept (LPI) radars designed to be difficult for enemy sensors to detect and classify.
These radars use techniques such as frequency hopping, spread-spectrum waveforms, and highly directional beams. They are paired with passive sensors—IRST, electronic support measures (ESM), and off-board cueing from other platforms—to reduce the need for active emissions.
Active Electronically Scanned Array (AESA) radars are central to this approach. Unlike older mechanically scanned or PESA radars, AESAs can change frequency and beam direction almost instantaneously, making them harder to detect and jam.
By contrast, much of Russia’s modern fighter fleet is built around the Flanker family, equipped with powerful PESA radars such as the Irbis-E. These radars boast impressive detection ranges against non-stealth targets, but they achieve this through very high peak power. The result is a system that is easy to detect, classify, and geolocate.
The difference is often compared to someone searching in the dark with a flashlight. The person with the flashlight can see far, but everyone else can see them too—and in modern air combat, that makes them a target.
Stealth is not static. It is part of a constant competition between sensors and countermeasures. Modern fighters increasingly rely on electronic warfare (EW) to complement physical stealth.
Aircraft use several types of signal generators, including digital radio frequency memory (DRFM), Doppler, and noise generators. These systems can obscure the presence of the aircraft, create false targets, or confuse enemy radars about a fighter’s true location and speed.
Noise jammers flood the battlespace with interference, making it harder for radars to pick out real targets. DRFM systems can capture radar signals, modify them, and rebroadcast them as convincing decoys. Enemy radars may see multiple targets blinking in and out of existence, with no reliable way to tell which one is real.
As Saab notes in describing the Gripen E’s offensive EW capabilities, modern fighters combine electronic attack pods, decoy missiles, and networked sensors to operate in highly contested airspace and counter even advanced threats such as the Su-57 and S-400. The goal is not simply to hide, but to dominate the electromagnetic spectrum.
The F-35 Lightning II is often described as a multirole or strike fighter, but its most strategically significant role may be as a platform for SEAD and DEAD—suppression and destruction of enemy air defenses.
The aircraft is itself a capable electronic warfare platform, able to jam enemy radars and share targeting data across a network. More decisively, it can carry weapons like the AGM-88 HARM, which homes in on radar emissions and destroys the emitter itself.
In this role, enemy air defense systems face a dilemma. If they turn their radars on, they risk being detected and destroyed. If they turn them off, they become blind.
According to a senior British admiral, the effectiveness of this approach was demonstrated in 2024, when Israeli F-35s reportedly dismantled large parts of Iran’s air defense network without losses. Similar operations were reported again in 2025, leaving Iranian skies largely open to follow-on operations.
Stealth fighters do not operate alone. One of their greatest strengths is how they enable less stealthy aircraft to operate more effectively.
In mixed formations, fifth-generation fighters like the F-35 can penetrate defended airspace first, using stealth, electronic warfare, and precision weapons to punch holes in the enemy’s air defense network. Fourth-generation fighters such as the Eurofighter Typhoon or F-15EX can then follow, carrying larger payloads of externally mounted weapons to exploit those gaps.
This concept is reflected in Royal Air Force slang, where the F-35 is sometimes referred to as the “assassin” and the Eurofighter as the “thug.” One slips in quietly and opens the door; the other comes through with brute force.
As air defenses continue to evolve, so too must stealth. Sixth-generation aircraft like the proposed F-47 are being designed with all-aspect stealth, advanced networking, and integration with uncrewed systems from the outset. The emphasis is shifting from individual platforms to entire combat systems operating across domains.
The lesson of the past century of air combat is clear. Whether flying high, low, or somewhere in between, no single trick guarantees survival. Stealth is not about being unseen forever. It is about shaping the battlespace—controlling when, where, and how you are detected—and ensuring that by the time the enemy knows you are there, it is already too late.
