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
So, basically there’s IGE and OGE (In ground and out of ground effect) where the rotor downwash (the air being pushed downwards from the rotor blades) is forming a cushion (IGE), or the aircraft is higher than 1 rotor disc diameter from the ground and not operating on said air cushion and requires more power (torque) to maintain sufficient lift. Generally, IGE exists less than one rotor disc diameter in altitude, and OGE is anything higher. For example, in the helicopter I fly, it’s defined as lower than 50 feet, as the rotor diameter is 49 feet.
There’s less of this effect over water, so a helicopter would have to produce more lift, therefore torque, to maintain the same altitude as it would over land. Even more so to take off.
Also think a rescue helicopter may be operating close to its max gross weight (the heaviest it can be to take off) after picking up multiple people, or if it has a lot of equipment or fuel on board.
Rotors cone due to the conservation of angular momentum, think of the classic ice skater who spins faster when they pull their arms close to their body, and more slowly when they extend them out. Same exact concept in the rotor system. As more lift is demanded, the rotor will tend to slow, reducing centrifugal force, but the tips of the blades will be traveling faster than the rest of the blade, producing more lift at the tips, and rotating at a higher plane than the rest of the blade. This is typically visible during take-off, landing, autorotation, but is more pronounced when the rotor system is operating lower than its designed rotational speed while still trying to produce the same amount of lift.
Source: am helo pilot, but I don’t fly over water, I just blow stuff up.
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