A shape can throw a radar echo away
A faceted or smoothly curved form can send a radar reflection somewhere other than back to the transmitter. That, more than a coat of paint, is what ‘stealth’ is.
A flat metal plate returns a radar wave the way a mirror returns a flashlight, in one direction, and that direction is back toward the radar only if the plate faces it. Tip the plate and the glint goes elsewhere. A shape made of such plates, or a curve designed so the glints miss the transmitter, is a smaller target at the wavelengths the shape was built for. Petr Ufimtsev’s 1962 work on how edges diffract waves gave a way to calculate this instead of only shaping by instinct. Coatings that swallow some of the remainder came later, and they help. They are not the main trick.
A different wavelength, especially a much longer one, sees a different object. A radar that is not waiting for the echo to return to itself — a receiver off to the side — asks a question the shape was not built to refuse. Infrared sensors ask about heat. Eyes ask about daylight. None of these are impressed by a facet.
Caveats
- How small a given aircraft’s return is, from which aspect, at which frequency, is mostly classified. The physics of throwing a specular echo aside is not.
- The 1962 theory is about diffraction. The aircraft that used it are a later, and partly secret, engineering history. The claim here is the physics, not a performance number.
Sources
- 1Ufimtsev P. Ya.. Method of Edge Waves in the Physical Theory of Diffraction. Soviet Radio, Moscow, 1962.