Helicopters fly by using an engine to spin rotor blades, which create lift and thrust by pushing air away, similarly to a bird flapping its wings or a swimmer moving limbs in water. The spinning blades act as rotating wings, generating low net pressure above and high pressure below to produce lift. A tail rotor or a counter-rotating blade system is essential to cancel torque, preventing the body from spinning in the opposite direction. Forward motion is triggered by tilting the blade plane.
Helicopter propulsion relies on an ambient medium. The blades gain thrust by pushing the surrounding medium in the opposite direction. The typical speed acquired by air around the blades is of order hundreds of meters per second. Momentum conservation implies that the blades can propel the helicopter body to a speed of this magnitude only if they push an air mass comparable to the body’s mass. Since the mass density of air at sea-level is a few hundred times smaller than the average mass per unit volume of a helicopter, the vehicle must push a volume of air that is at least a hundred times larger than the volume of its body in order to move forward at a speed of a hundred meters per second.
NASA had just announced a plan to launch the first nuclear-powered interplanetary spacecraft before the end 2028 on a mission to Mars called Skyfall. This Space Reactor-1 Freedom will carry three small helicopters to explore a possible human landing site on the Martian surface.
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