What is the importance of the number G [lift to drag] in AWE

#1

So, the number G will state how fast an AWE rig if any sort will rotate. How important is maximizing this number?

A fixed wing glider can see G numbers approaching 60 (https://en.m.wikipedia.org/wiki/Lift-to-drag_ratio) while kitesurfing kites could have G numbers in the 5-8 range (source needed…) and paragliders below 11 (http://www.thefullwiki.org/Glide_ratio).

With increasing G, the importance of tether drag seems more important.

At a high level view, drag represents loss and thus reduces the maximum extracted energy of the wing. Still, even a rig with low G should be able to harvest large parts of the Betz limit…

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#2

I am not sure to understand. Please could you explain? And is it “tig” for “rig”?If yes what is “rig”? Thanks.

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#3

Thanks. I reworded it to escape a new discussion on Betz’ limit.

BUT - it seems regardless of G, for a drag mode rig running at 2/3 maximum speed, you will only ever approach 0.5 times Betz’ limit as drag power is equal to useful power (more or less)

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#4

in an idealized (theoretical) environment you can find the equations in the awe book:

power depends on CL^3/CD^2 which seems quiet significant to me, next to v_a^3…

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#5

Let me elaborate a little:

With high G we get:

  • power depends on G^2
  • the above gives us more power with less mass
  • higher speeds lets you cover more air area with the same wing
  • changing effective G is an efficient means of depower giving an immence depower
  • as the kites are already flying fast, dealing with extreme winds is «built in»

But, it also means

  • tether drag becomes a dominant factor
  • larger forces need to be soaked up by a kite on a smaller area
  • a kite can only have a biggest G and still be flyable (pushing to rigid wing vs soft kites)
  • high AR wings stall really easily at a lower AoA
  • the power effect of gravity increases with speed and mass (power = speed x mass x 9.81), leading to uneven power generation

The question I am wondering is where the balance is between pros and cons. At one end of the scale we have «single skins» that fly with low G but have a huge area at close to zero mass. At the other end, Kitemill/Makani/Ampyx/(most other actors) going for rather heavy high G designs. I think this might be perhaps the most important question for AWE, and a question where there seems to be no concensus at the moment

My worry is that focusing only on the first order approximation that high G is better for power, we might be excluding the most useful kite designs…

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#6

I agree.

As example IMHO low G wings have also a low turn rate at almost full speed, harnessing the more powerful central zone, allowing a better power/space use ratio Low radius loop, above all when a kite-farm is implemented.

The low lifetime of soft wings is a major concern, far above their low G.

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