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
It is probably related to the speed of forward motion . A helicopter flying faster than about 15 to 20kts gains translational lift, and requires less power to fly. Think of the rotor disk acting as a wing .
Slower than that needs more power. Out of ground effect, even more power.
The more power required, the more the blades will flap, and you see the cone shape .
Missions over water probably involved hovering, at least the parts you’re watching. If it’s a high hover, out of what’s called “ground effect”, it takes even more power (than hovering in ground effect). Also, it’s likely the chopper is quite heavy at the time, for example, if it’s hovering to pick up a big bag of water (Bamby bucket) to dump in a forest fire. A gallon of water weighs a bit over 8 lbs, so imagine picking up a quick 500 gallons or more. That’s A LOT of extra power needed which makes the rotors cone.
Ground effect over water and land, when there isn’t many waves, shouldn’t be all that different. Power required out of GE should be the same because the ground doesn’t affect the power at that point.
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