A helicopter is a machine that generates its own runway. Instead of accelerating along the ground to make a fixed wing produce lift, it spins a wing in a circle and changes that wing's pitch faster than the eye can follow. This is a from-first-principles walk through rotor aerodynamics, the swashplate, the cyclic and collective, the tail rotor's thankless job, dissymmetry of lift, retreating blade stall, autorotation, and why coaxial and tilt-rotor designs exist at all.
Vertical-Flight
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How a Modern Helicopter Works