Hello,

I am trying to loft a sheet metal skin on an aircraft fuselage.

I have generated the fuselage cross section at the front and rear of the skin using conics. At each section there are two conics, one for the top half and one for the bottom. The K factor for the top conics is 0.55 at both sections and 0.75 for the bottom.

I generated a sheet metal lofted bend between the two sections. i expect this skin to be developable or flat wrappable (it is sheet metal afterall).

The fuselage has a straight taper so I can easy draw in two more intermediate sections using addition conics with the same K factor for the top and bottom. Those sections should lie against a developable skin that joins the front and rear sections. That is lofting 101. 

Looking at the pictures below, you can see that the skin is clearly warped and hence not developable. It is also nowhere near the additional sections I sketched.

For a skin to be developable the Gaussian curvature must be equal to zero. This implies that the skin can only have curvature in one direction and must be rolled around straight lines in the other direction. For a developable skin, those straight lines must be drawn between points on the front and rear conics that have the same tangent slope. Again, lofting 101. If the straight lines do not join points of similar tangency then the surface will be warped (ie have curvature in two directions) and is not developable (ie flat wrappable). Solidworks will flatten this skin but the skin must be stretched in some areas and shrunk in others. 

When I study the model closely I believe that Solidworks is simply dividing the two section curves into X number of equal length segments and effectively using lines between those points to define the surface. This approach can only generate a flat wrap developable surface if the two cross sections are the same with one simply being a linear scaled down version of the other. In my example this is not the case and hence the surface ends up warped. The skin in my example if definitely developable but using the tangent method I described. To illustrate what I am saying I generated a surface between the same curves and turned on the surface ruling and manually drew in a line between two points of tangency for comparison (Line 1). The line at maximum breadth with another such line (Line 2). Clearly the ruled lines are no where near the lines joining points of similar tangency.

So the bottom line is that this warped sheet metal loft that Solidworks produces is useless. I need to generate additional canted frames by taking intersections to this skin. I need a proper developable flat wrap surface .... the question is how to I force Solidworks to generate a proper developable surface ?  Any ideas or is this something that Solidworks cannot do ?  If so is there any work around ?