How a sky serpent shaft can scale up?

Windy_Skies · February 9, 2019, 1:59pm

I think, like I said earlier, that that does not hold for thin-walled structures.

For a thin-walled structure, most of the extra volume is air, which is of course neutrally buoyant in air.

Related, from the YouTube comments under the video I linked earlier in this thread:

https://www.youtube.com/watch?v=P55HfnGR6kQ
Blade material is V=hA, meaning the blades material doesn’t grow at a cubed rate, larger turbine propellers are hollow not solid. A 10 meter long blade may have a 0.5 meter wall thickness with a diameter of 3 meters, so if we simplify and make it a cylinder the material would be approx. (10*pi*(1.5)^2)-(10*pi)=2.23 cubic meters of whatever material. Whereas a solid 1 meter simplified blade with similar proportions would use 1 *pi*(.17)^2=0.52 cubic meters of the same material. So now we examine length to material use, at 10/2.23 and 1/0.52, and we can easily see the 4.5 meters per cubic material beats the 2 meters per cubic meter. Of course the material used would determine the minimum wall thickness but at a certain point, with any given material, a longer hollow propeller will use proportionally less material.

Secondly, the sweep area is not a square it’s a circle so it’s pi*(r^2)*(blade count). So lets say you have a superturbine with 10, 1 meter long, 2 blade props, and a single 3 blade, 10 meter prop. The sweep area on the first is going to be 10*(2*(pi)) =62.8 square meters. The second will have a 3*pi*10^2=942 square meter sweep area. Then we come back to the original argument about which uses less material per sweep area, so lets do the math. 62.8/(10*(2*.52))=6ish square meters per cubic meter of material used, discounted material for driveshafts to connect the units of course, for the superturbine design. Then we get 942/(3*2.23)=140ish square meters of sweep area per cubic meter of material.

Given the example is simplified and shapes are actually more efficiently designed in reality, it’s quite easy to see that a larger single turbine is more efficient, cheaper, and easier to maintain than a multi-prop design.

None of the above example takes into account the additional friction losses present in a higher speed turbine, nor was the cost of high speed bearings and lubricants accounted for, but, we can plainly see why engineers and physicists have gone for the single larger turbine versus the multi-prop designs. As far as your gearing argument, the larger generators typically produce power at about 1000 rpms, with a 30 rpm input, to get the same output you’d need many times more small direct drive turbine setups, and large direct drive generators are being deployed now negating the argument.(…)