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
I cant ELI5 but I do know that a helicopter flying over water does not benefit from ground effect like it does when flying over land.
This might have something to do with it
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Helicopter blades generate lift as they spin. The outside edge travels faster than the more central parts. Because the outside edge moves faster, it can generate more lift. When the helicopter blades are spinning very fast, the high levels of lift from the rotor tips can bend them upward, making a cone shape. The helicopters you’re seeing are apparently running at a higher RPM than they would be in other contexts.
Can you link to a video? It’s not really clear what you are asking about.
Helicopters don’t fly *in* bodies of water. They can fly over water just like they can fly over land. Spraying some water droplets around makes the airflow more visible but it’s the same flow of air you have anywhere else close to the ground.
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.
Helicopters are meant to fly, not be in the water. You could also ask why fish struggle at the altitude of 400 meters in the air
Rotor systems cone based on how much power is applied at the time. Often when you see videos of helicopters flying low over water, it’s because they’re moving slower. At slower speeds, more power is required compared to traveling speeds. Generally anything above 16-24 knots, the rotor operates in “clean air” and is more efficient, and then needs less power to maintain the same altitude.
If the helicopter is hovering higher (greater than 1 rotor disc generally speaking) that also requires more power, which means more coning.
Rotor systems will also cone more if the helicopter is heavy, if it’s hot outside or at higher altitudes.
“In”?
Ah well, there’s your problem. Helicopters are built to fly in the air, not submerge in water.
Because rotor helicopters need something to push up off of. In the sky, the rotor keeps it afloat because the engine power is higher. When hovering above water the engine power is lower, and hence the helicopter has a harder time pushing itself off of water because it’s not as dense as the ground
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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