I’m watching videos of rescue helicopters and noticed that some have their blades form a cone when flying low in bodies of water. This doesn’t seem to happen often on land so why in water?
In: Physics
To properly answer your question, I think further information would be beneficial to be able to give you the right answer. What do you consider as low altitude? (10 ft above water or 150ft? Both low, but will have different effect) and also, is the helicopter moving or hovering? Without any references to a situation you are describing, I’ll do my best to explain why this phenomenon might occur.
Helicopters benefit from something called ground effect. When a helicopter is hovering close to the ground (15ft ish or less, depending on the size of the helicopter) the induced airflow downwards from the main rotor hits the ground, slows down and creates a sort of a cushion underneath the aircraft. This effect allows the helicopter to stay hovering with more efficient lift ratio.
When hovering over water, the ground effect is not nearly as efficient since that induced downwards draft is dissipated due to the water beneath moving and giving away since its not solid – no cushion underneath the aircraft.
So the helicopter needs to apply more power to create more lift. Higher amounts of lifts caused by the rotors will make them cone since the blades do not create equal amounts of lift throughout the span of the blades. The tips of the blade cause more lift than the root, so they will rise higher, causing the coning.
In short, more lift is needed over water and the more lift you have the more the blades cone.
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