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I heard somewhere that modern glider made of plastic and composite materials are less detectable than other aircraft by primary radar. I wonder how much.

What is the radar cross section of a typical modern glider such as the Schempp-Hirth Duo Discus? How does it compare to a typical GA aircraft such as a Cessna C172 and to a typical airliner such as an jet airliner such as an Embraer E190 and to a typical turboprop such as a De Havilland Canada Dash 8?

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  • $\begingroup$ Depends on the material for the primary structure: In glass fiber gliders the radar return is mostly from things like metal control rods while in carbon fiber gliders the conductivity of carbon fiber makes them much more visible. $\endgroup$ Apr 15 '20 at 21:15
  • $\begingroup$ @PeterKämpf If the question is restricted to the duo discus, is there still multiple possible materials? (otherwise I'll restrict it another glider) $\endgroup$
    – Manu H
    Apr 15 '20 at 21:27
  • $\begingroup$ @PeterKämpf don't hesitate to tell me if there is another glider for which it is easier to answer and I'll modify the question accordingly. $\endgroup$
    – Manu H
    Apr 15 '20 at 21:49
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    $\begingroup$ When flying glass fiber gliders in the ADIZ back in the Eighties, we thought that we are invisible for radar - until some of us were invited by the military and they could tell exactly where we had been. They said a glider is about as visible as a large bird, but faster, so it can readily be identified on radar. $\endgroup$ Apr 15 '20 at 22:05
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    $\begingroup$ Obviously. The DH Mosquito had the radar signature, to the Germans, of a small light aircraft since the only reflective components were the engines, armour plate and other bits. $\endgroup$
    – John K
    Apr 16 '20 at 12:58
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Smaller than that of a normal aircraft, but very far away from a VLO/stealth one.

The RCS of various light aircraft has been measured and published here: https://apps.dtic.mil/dtic/tr/fulltext/u2/a029262.pdf
The RCS of the human body is on the order of -20 to -5 dBsm: https://ieeexplore.ieee.org/document/7027834

The glider can't hide the human inside. Extrapolating light aircraft RCS down to a glider, my best guess for the lowest-RCS gliders would be on the order of -10 to -5 dBsm, and no less than -12 dBsm or 0.05 m² due to the pilot.

The reason is that a glider still contains a lot of non-radar-transparent parts, which will reflect radar frequencies and produce a sizable RCS.

It doesn't take much to produce a radar reflection. -40 dBsm corresponds to a sphere just 1.4 cm in diameter. Anything that contains metal or water will reflect radar. Even small metal or flesh objects will produce a significant return.

To get seriously low RCS, such as -20 to -30 dBsm, below a bare human, where "real stealth" is considered to begin, you need to specifically work for it, intentionally reflecting the waves in the wrong direction, not avoiding reflections.

In particular, you need to ensure the reflection is directed into specific lobes. This isn't best achieved by radar transparency - in fact, both the F-22 and even non-VLO fighters like the F-16 intentionally coat their canopies with gold to make them radar-reflective, so as to avoid random reflections from the objects inside.

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  • $\begingroup$ is your "guess" valid for a duo discus? $\endgroup$
    – Manu H
    Jun 14 '20 at 16:44
  • $\begingroup$ @ManuH It would be for something like the Duo Discus - for really light gliders with a minimum of metal. $\endgroup$
    – Therac
    Jun 20 '20 at 11:25
  • $\begingroup$ and how does it compares to typical airliners, typical GA and typical turboprop? $\endgroup$
    – Manu H
    Jun 21 '20 at 23:14
  • $\begingroup$ @ManuH Way way lower. You can expect 10-20 m^2 from GA and 100-1000 m^2 from airliners. Turboprops are easily 2x the RCS of a corresponding turbofan. $\endgroup$
    – Therac
    Jun 22 '20 at 2:49

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