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Comment by elevensies | Hacker News Reader

I'm still a bit confused about how the forces line up for the flight path. If I'm understanding:

- the wing flies in a circle which is perpendicular to the wind (oriented the same way an ordinary wind turbine would be)

- the tether is up-wind from the wing, so the force from the tether pulls the wing into the wind which generates lift, which is used to propel the wing along the circular path

- the propellers are driven by the circular movement of the wing, and generate electricity

So the tether and the wing generate movement, the movement spins the propellers, and the propellers drive the generator. Except for take-off and positioning, where they are used as motors. Am I understanding this?

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Here's a Ted talk from 2009 about the early version of the tech.

http://www.ted.com/talks/saul_griffith_on_kites_as_the_futur...

The tethered airframe converts wind into high-speed lateral motion, sort of like a sailboat would if it were sailing into the wind. These higher speeds can drive the blades faster than if they were simply pointed at a wind source.

Wow 6+ years.

hardware takes a long time

Thanks this was my biggest question. I was wondering why it doesn't just stay tethered to the ground in one single position (or as close as possible) flying into the wind.

I guess it makes sense if it rotates perpendicular to the wind - like the tip of a rotor blade, to achieve these higher speeds.

Would be curious to understand more of the math/physics behind that. But at least intuitively it makes some sense now.

I think that's more or less it.

I think what they're doing is a milder version of dynamic soaring, which is a popular technique used among RC glider pilots (and some birds).

The speeds attainable are pretty insane, the current RC record is 505 MPH, which in a rather fascinating twist is faster than the fastest POWERED RC aircraft.

This a video that shows a pretty good idea of what it looks like, at compartively slower speeds.

https://www.youtube.com/watch?v=DegX_we1Mic

It's also basically the same idea as used by the people flying trick kites on the beach. Once an aero-astro friend of mine explained that, this whole thing made so much more sense.

https://www.youtube.com/watch?v=xlPI0AOj5MI a video of how birds do this in nature. Ever notice hawks circling in high winds?

This video explained dynamic soaring for me: https://www.youtube.com/watch?v=SVN-oF6tPLc

The key thing seems to be having different wind speeds available at different heights so you can do the upwind part through slower moving air.

Cool. I never knew that stuff. There's a vid here of a guy getting his glider to 320kph with wind speeds of only 25-30kph before crashing it into a rock face. Reminds me of my own efforts with a RC glider which crashed into a concrete pillar at ~50mph quite promptly due to my lack of pilot skills.

https://www.youtube.com/watch?v=QjWFcYqdgA4

That video helped a lot. I was getting hopelessly lost with the image. In one part the object is moving in still or slow air where the resistance isn't enough to nullify the gain gained at the higher speed upper layer if I'm understanding this correctly.

And finally learned the difference between airspeed vs. groundspeed :D

My understanding is that dynamic soaring takes advantage of a cliffside or a ridge to provide upward air current. This project seems to be working on flat ground, so as far as I understand, it's not really using dynamic soaring.

I think it works more like regular stunt and power kites, just relying on the "trim" of the airfoil to produce lift and thrust (and the tether's pull) to make it go upwards and then "fall" down.

We need a free-body diagram here, talking about the balance of forces in English is madness.

One way to describe it in words is that it's like a normal wind turbine, but instead of 3 blades, you have one wing that moves in a circle like a blade. And instead of the energy being extracted by a generator where the 3 blades meet, the energy is extracted at the wing itself, by many small turbines.

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