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Contra-rotating propeller and other concepts. - Hobby Squawk RC Forum for Airplanes, Boats, Cars, Helicopters, Tanks, and Trucks
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File:Torpedos with contra-rotating propellers to minimize heeling moment - Museum of Great Patriotic War (8601790818).jpg - Wikimedia Commons
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piston engine - Why don't contra-rotating propeller configurations with more than two propellers exist? - Aviation Stack Exchange
