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In a book I'm writing they plan on having a biplane, similar to the RAF se5 but equipped with a radio, as their means of checking in on them. They are on an island where the enemy has WW2 era RADAR but they don't know that. My question is how would that really interact? Would the biplane be almost invisible because it's small, wooden and out at sea?

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    $\begingroup$ As this is a question about Aviation this is the right place. $\endgroup$ Commented May 2 at 3:45

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WW-II-era radar technology actually spans a very wide range of technologies, from British 20 MHz Chain Home with dipole antennas to German 24 GHz prototypes with parabolic disks that surprised its designers by being unusable because nobody until then had figured out that atmospheric moisture dampens the signal at such frequencies.

An SE-5a type design has three items which will interact strongly with electromagnetic waves: The engine, the bracing wires and the control cables. Their radar response depends much on the radar frequency - that is why I started with the frequencies which were used back then. Wires act as dipole antennas and will resonate when radiated with a wavelength double their length (or corresponding harmonics). For a 5m bracing wire that would be 60 MHz, for example.

The size of the engine would require higher frequencies for best detection range. Today, the X-band (8-12 GHz) is most commonly used.

WW-II radars already used the Doppler frequency shift for determining the speed of discovered objects, most famously the British H2S ground mapping radar, but also the German FuG217 Neptun which, in combination with the Elfe-Gerät, would measure distance to the target and even automatically fire the guns when in range. While the Neptun range of radars operated at frequencies between 158 and 187 MHz, the most advanced developments of that era (like the FuG244 Bremen) already used the S-band (3.3 GHz).

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    $\begingroup$ @DanMašek Of course it should! Thank you! $\endgroup$ Commented May 2 at 19:35
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To answer this question I am going to start on the plane itself. Being wooden does not mean the plane is invisible to radar because radar relies on bouncing a wavelength off of a object and then determining how far away the object is. Since wood is not a radar absorbent material it can still be detected by radar. Secondly in WW2 radars were used out at sea and that was one of their primary uses - so yes they work on objects that are out to sea. And thirdly in WW2 biplanes were still used and the newer 2-winged planes were very similar in size to the older biplanes so likely the radar would still pick up the biplane.

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    $\begingroup$ "Radars where used out at see and that was one of there ..." should that be "radars were used out at sea and that was one of their ..."? Homonyms are so difficult... $\endgroup$
    – FreeMan
    Commented May 2 at 13:47
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    $\begingroup$ There are three things a material can do to radio: absorb, transmit, and reflect. Out of those, "reflect" is the one that wood does least. At lower frequencies it's mostly transparent, up around 5-6 GHz it starts to absorb significantly. $\endgroup$
    – hobbs
    Commented May 2 at 22:56
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    $\begingroup$ I wonder if there's anything written about the Mosquito (while a sophisticated monoplane, it was largely wooden) and its visibility on radar $\endgroup$
    – Chris H
    Commented May 3 at 7:34
  • $\begingroup$ @ChrisH: The Mosquito had enough metal to be thoroughly visible on radar. $\endgroup$ Commented May 3 at 21:04
  • $\begingroup$ I recall a fellow glider pilot saying years ago that he was surprised to find out that ATC was often able to detect glass-fiber gliders with radar. I don't think glass fiber composites are good radar reflectors, but gliders also contain metal parts such as control rods and cables. $\endgroup$
    – sdenham
    Commented May 5 at 1:49

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