Postby **edt** » Mon Dec 17, 2018 9:18 pm

kiteexpert I'm not sure if that makes sense? Every mechanical action and strength including physical strength muscle strength, tire strength, all increase directly proportional to the cross section. Rope strength too. So when you double the size of the diameter of a rope, let's choose some sampson asmsteel ropes for example, the 1/8th inch rope (3mm) is 1,100 pounds, while the 1/4 inch rope is 3,900 pounds. double the diameter means 4 times the strength not 8 times. Now that's not exact, it should be 4,400 since 2^2 = 4,400, but it's quite close. For cylinders you have to assume it's a thin wall. Then you integrate the pressure out, and I am pretty sure you get the same sort of result that the axial stiffness is proportional to the cross section. The hoop stress by the way is proportional to the radius, not the radius squared so you can gain some stiffness by making the cylinder bigger without enormously increasing the weight. Now if you have something like fiberglass rods (I designed a few kites with collapsible fiberglass rods, just to try to understand why we are inflating kites instead of using hard rods in our kites), then when you start increasing the size of the cylinder you decrease the wall thickness until eventually you can't make the tube any bigger without making it too fragile and the kite would just explode when it hit, or you would just have to make it too heavy. From my experiments I think I came up with an estimate of something like a 1 inch fiberglass rod being about as strong axially as a tube kite but the weight ended up being something like twice as much. Maybe it could work with carbon fiber? But then the expense is so much. I have temporarily given up on it just because sourcing the carbon rods to make a kite like this would be too expensive for a hobbyist like myself. I can definitely see this for a race kite tho, with a 20-30mm carbon rod, and incredibly small drag but it would be murderously expensive. I think your reasoning seems fine from a qualitative point of view, but I don't see where that cube law is coming from. Just seems like it should be a square law to me. I could be wrong of course, it's easy to calculate things wrong for stuff like this, I just don't see how it could be a cube.