Since planes could fly, military powers have been looking for ways to hide them. Stealth planes were once the biggest secret in the world. Today, they’re famous icons of engineering and military might. But how did they come to be, and how do they work?

Colour, Light and Shape

Camouflage is one of the oldest defence strategies in the world – and planes were adopting visual camouflage as soon as they went to war. This was the first stealth tech ever to make it into aviation, and it’s still used today.

French camoufleurs were among the first to develop aircraft camouflage patterns during World War I – painting disruptive green/brown patterns on top and beige countershading on the underside of their prop planes. This made them harder to spot from above and below. By WWII, all Allied and German air forces were using some kind of camouflage to make their planes harder to spot from the ground and air.

During WWII, German U-boats were evading bombers by spotting them as they appeared over the horizon as black dots. The Yehudi Lights project used counter illumination – lights along the wings and engine mounting were matched to the brightness of the sky, making the plane much harder to see. In fact, Allied planes could come within 2 miles of a ship on the water’s’ surface before being spotted.

It was one of the first uses of active camouflage – certainly the first in aviation – but advances in radar made it redundant soon after it was trialled.

The early attempts at stealth focused on visual camouflage, which remains in use today. Radar detection, heat signatures and sound presented the next challenges

The Nighthawk

In 1964, Soviet mathematician Pyotr Ufimtsev discovered that the strength of the radar return from an object is related to its edge configuration, not its size. The technology to make use of this phenomenon didn’t yet exist – but by the 1970s, and in complete secrecy, the United States government and Lockheed Skunk Works began developing a radar-invisible plane, based on Ufimtsev’s work. The result was the F-117 Nighthawk – the world’s first true stealth aircraft.

The Nighthawk has a radar cross section of 0.001m2 – a about the size of a digital camera’s SD card. This is achieved with its unorthodox shape which wasn’t actually a design choice – it’s a symptom of the underpowered computers used to calculate the required shape for radar stealth.

So, how does it work?

First we need to know how radar works.

A radar station sends out a specific frequency range of radio waves. These bounce off an object and are reflected back to the radar station. The longer it takes for the radio waves to bounce back, the further away the object is. It works exactly like an echo, but with radio instead of sound waves.

If an object has a shape that diffuses or scatters radio waves, the waves won’t be reflected back at the radar station. This can work on a macroscopic scale (the shape of the whole plane) or microscopic – right down to the formations of particles in the paint.

The shape of the F-117 is peculiar to say the least, but the huge flat slabs bounce radar off in every direction except back to its source, making it highly effective for radar cloaking.

That iconic, flat shape made manual flight impossible. The 50º sweep angle of the wings was necessary for stealth, but rendered the plane unstable in flight. Complex fly by wire systems borrowed and adapted from other aircraft were put in place to make small adjustments constantly during flight, keeping the plane airborne.

Stealth aircraft produced since the F-117 have used advanced supercomputers to calculate more aerodynamic, curved shapes that reflect radar even more effectively  – but they definitely don’t look as cool.

A slit shaped engine exhaust reduces the F-117’s heat output. The matte black paint absorbs radar and makes the vehicle harder to see at night, giving an incremental boost to the plane’s overall stealth capability.

The Next Generation of Stealth

What’s the next evolutionary step in stealth aircraft? It’s likely that we’ll never know, at least until the tech is leaked or made public. The F-117 was a black project, shrouded in secrecy and unknown to almost everyone in the world for decades. Next gen stealth planes, with active camouflage, silent running engines and completely neutral heat signatures could be flying over us right now without us even knowing. It might sound crazy, but if you told someone in 1987 that a radar-invisible diamond-shaped flying machine was doing secret missions all over the world, they’d have a hard time believing you too!

What’s in the Sky?

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