North Korea deploys canvas-on-wood Antonov An-2 biplanes as "low observable" stealth aircraft. Most of the radar energy goes right through it, there's very little metal to generate a return.

(Similarly, during the Battle of Britain the Hawker Hurricane was notable for the way German 20mm cannon shells passed through its doped cloth-on-tube airframe. No impact to trigger the fuzes.)

Another interesting property of the An-2 is the incredibly high cross-section of the aircraft means slow flight speeds and very high lift, the aircraft really doesn't have a stall speed due to spring-deployed slats that snap out as airspeed drops... but whatever you want to call the stall speed it's lower than 30 kts. So in a stiff headwind you can fly the aircraft onto the ground in full control at zero ground velocity, or actually while being pushed backwards by the wind.

Wouldn't be enough to deter a dedicated attacker at short range especially with a sufficiently high radar frequency (the engine and other metal parts become a bigger multiple of the wavelength), but would you be able to pick it up at 50-100 miles with longband search radar? especially at low altitude? Probably not. Especially random ex-Soviet crap like a lot of countries have.

Don't knock primitive tech, sometimes it works. The An-2 is archaic but it's a very competent bushplane/light cargo aircraft for squad-level paratroop or liason operations. And it's well-adapted for rustic operations like Russian/Canadian bush, or Korea.

https://en.wikipedia.org/wiki/Antonov_An-2

https://www.youtube.com/watch?v=A3tpV9vUTPo

http://www.smithsonianmag.com/smart-news/why-north-korea-usi...

Probably not, since modern air defenses are effective against small missiles and even mortar rounds. Here's a CIWS in Afghanistan shooting down incoming mortar rounds.[1] (Read the comments for more explanation from people who have been there.) The CIWS has radar which can track not just the target, but its own bullets. It makes quick adjustments until bullets and target are in the same place at the same time.

On one occasion, a US ally shot down a hobbyist-sized drone with a Patriot missile. Very expensive, but it hit the target.

[1] https://youtu.be/KsVUISS8oHs

Not just with airplanes! See the Millennium Challenge 2002 naval exercise:

https://en.wikipedia.org/wiki/Millennium_Challenge_2002

TL;DR: the Red team general, with Iran-equivalent forces at disposal, employed asymmetric tactics and WW2 equipment to sink an aircraft carrier, ten cruisers and five amphibious landing ships, estimated loss of 20 000 US military personell. Then they stopped the exercise because it was too embarrasing, refloated the Blue team ships and instructed the Red team to play by the instructions designed to make them lose.

There was a team from a Large Defense Contractor at my local airfield recently with thermal-imaging equipment that seemed to be doing a good job of tracking light piston-engined aircraft, many of which are tube-and-fabric construction like WW1 machines.

They wouldn't say what they were testing but it seemed fairly obvious. The whole engine block glowed hot as did the uppersides exposed to the Sun, let alone the exhaust, at what I guessed to be several kilometres distance. Before we were shooed away :)

There were some incidents doing WWII where both the British and soviet ran successful raids using low and slow flying biplanes that was simply too low and slow for the motors guiding the anti aircraft guns of the day.

Doing the Taranto raid the low altitude of the Swordfish bombers caused the Italian navy to not just miss the planes but inflict collateral damage on themselves.

Today it's much harder to pull a similar stunt mostly due to night vision goggles/binoculars. But especially the US have a vulnerability as they have replaced proven effective systems based around cheap ammo and smart people, with systems based on dumb computers and expensive missiles.

I remember someone expressing surprise, years ago, that glass fiber gliders were detected by air traffic control radar. The most massive metal parts would be the undercarriage (single-wheel), but my guess is that the control cables, laid out in a cross at 90 degrees, would be the most reflective feature.