aerodynamics - Number of blades in a helicopter rotor - Physics Stack Exchange
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The Boeing Chinook HC.2 is a tandem rotor helicopter operated by the Royal Air Force, RAF. A series of variants based on the US CH47 helicopter Stock Photo - Alamy
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aircraft design - How can a helicopter be designed without a tail rotor? - Aviation Stack Exchange
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K-MAX Helicopter With Two Crossing Rotors Is Lighter And More Efficient - YouTube
SimplePlanes | Types of helicopters (and how they work)
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auto rotation - Is it possible to fly a tandem-rotor helicopter after a single rotor failure? - Aviation Stack Exchange
Transverse rotors - Wikipedia
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Twin rotor helicopters hi-res stock photography and images - Alamy
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33 Things You Probably Don't Know About the CH-47 Chinook
CH-47F Chinook Block 2 moves toward production, without advanced rotor blades - Breaking Defense
