[supahfly_remix]

  Helicopters can't fly over it—the downward force of the air would pull them in.

Can someone explain the physics of this? The air above the hold is at the same pressure as that beside the hole, otherwise there would be a constant wind. Also, above a certain altitude helicopters don't rely on ground effects.

[georgecmu]

If a hole is deep enough — and a half-kilometer deep hole qualifies — the earth will warm the air inside it. The deeper the hole, the warmer the air. Warm air rises, and cool air sinks, so with a big temperature difference between in-hole air and aboveground air, you get quite a bit of air movement.

Thus, two things are happening. First, the warm air rising from the hole is less dense and gives less lift to helicopter rotors than the cooler air it had been flying through. Since the temperature change is extremely abrupt as the helicopter flies over the hole, the pilot may lose a bunch of altitude before managing to adjust the speed enough (read: increase the spin rate of the rotors) to compensate for the loss of lift.

At the same time, the cool air pouring into that hole from all sides is going to create quite a wind shear. If a helicopter loses enough lift to hit the stream of cold air, it could easily be slammed into the side of the borehole before it ever developed enough lift or power to recover.

https://oregonexpat.wordpress.com/2012/09/20/the-helicopter-...

[trevyn]

Nit: most helis have an automatic governor that keeps rotor RPM relatively constant; the pilot adjusts the blade pitch to control lift, and the throttle is automatically adjusted to compensate for the change in engine load.

[quakeguy]

One comment from that link made me curious.

"They should install wind turbines around the perimeter of the hole and take advantage of it! New form of renewable energy – hole in the ground energy!"

Clearly a form of joke, but im intrigued to know why this won't work. (Or why it could work)

[benchaney]

If you are digging really big holes, there are presumably more efficient geothermal schemes.

[flukus]

These typically require large investments to build and suffer from other problems like salt corrosion, whereas putting wind turbines in seems like a very quick win.

[gambiting]

Sure, absolutely - but that hole is already there. Is there any way to use it for generating energy somehow?

[nonbel]

Are they sure it is the earth warming it and not just the continuation of the lapse rate[1]? How much warmer is it at the bottom? If it is 5-10 C warmer that could just be the atmosphere getting thicker.

[1] https://en.wikipedia.org/wiki/Lapse_rate

[davidwihl]

Given the odd local microclimate, I would not want to be on a low and slow approach to runway 6 at the nearby Mirny airport, especially in a slow-to-spool turbojet. I bet the Soviets never published accurate accident data.

[supahfly_remix]

thank you for the explanation.