Quote:
Originally Posted by Matthew.Mc
I want to make circular lightening holes because they are easier to make meaning less time. But my team-mates, want to use the "118 iso-grid", because they say it obviously better but in reality it takes longer to CAD and I'm not sure that its a whole lot better in terms of a drivetrain. Maybe though?
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Nice design Matthew. "Obviously better"? I guess my answer is that it could be but I wouldn't say it's obvious. I've done a little research in
isogrid myself, but the details and the heavy math on how and why it works are found in the
NASA Isogrid Design Handbook. Chapter 4.8 discusses the open shear webs that you have drawn. A few things to point out without getting too technical:
1) The belly pan isn't truly isogrid - it's skewed plate versus equilateral triangle so it transforms the loads and stresses differently. Regardless, it's function is to transfer loads normal to the plate and not in-plane shear so the advantages of the isogrid don't really help you that much. I would consider perforated plate stock or a simple pattern of smaller holes to give you more flexibility in mounting components to the pan.
2) The advantages to isogrid come from tailored thicknesses relative to in-plane compressive stresses and buckling limits. Consider it this way: a .063 thick solid plate wall has pretty much the same theoretical stiffness in plane and the same weight as a .125 thick 50% open area plate because it has the same average shear flow top to bottom. However, the thinner plate has 1/2 the buckling capability (1/2 the radius of gyration) so it deflects more out of plane (even without buckling) and is effectively much less stiff.
3) Weight advantages of isogrid are usually quoted relative to the equivalent thickness solid plate, with both optimized for buckling. In your case, you are pretty much selecting a common thickness and then taking additional material out of it either by an isogrid pattern or a hole pattern, so you are reducing weight at some rate and stiffness at some other (hopefully lower) rate and the question is where is it optimum and is it anywhere near buckling limited? With so many load variables and unknowns that you have to estimate, my gut would say leave in more material and make the simpler cutout.
3) You gotta admit, isogrid looks more "aerospacy"; that must count for something.