# What happens if a helicopter's propeller spins in reverse?

I know airplanes and helicopters fly thanks to the same principle, the Bernoulli Principle. The wings and blades are designed so that the air travels faster across the up side than it does on the bottom side, generating an upward pressure, or lift. My question is... if the propeller rotates in reverse, does the pressure becomes downward? The distance the air has to travel from point A to B of the wing/blade is still the same so I would think that the way air moves around it do not matter. From the airplane perspective, I think this reverse rotation of the propeller would be equivalent to the airplane flying backward. So, what is the case here?

• The Bernoulli principle isn't why planes fly. That's a common misconception. The majority of the lift comes from the angle of the wings/propellor blades. Run a helicopter propeller in reverse and it will accelerate downwards. – John Rennie Aug 29 '17 at 16:13
• It can be done, but as John says, not by reversing the rotation: youtu.be/SIOGuidZuew – user167453 Aug 29 '17 at 17:50
• When a helicopter loses engine power, the pilot must autorotate the propeller to speed it up, by "feathering" the propeller blades properly. In a sense, the propeller is spinning "backwards" at this point. Very close to the ground, the pilot feathers the blades such that they provide the lift which allows the helicopter to land safely. For more info, see en.wikipedia.org/wiki/Autorotation – David White Feb 5 at 22:01