In our discussion of airframes suitable for Industrial V-Twins, I've noted that the airframe I'd really like to build (aluminum tube and gusset, with aluminum spars and ribs, parasol - I don't weld, can't weld in my current shop) isn't out there as a plans built design. So I start diddling around, trying to come up with equivalencies.
My starting point is the steel tube version of the Pietpenol in the Flying and Gliding Manual.
I also used this calculator from people building roll cages. Tube Calculator - Rogue Fabrication It seems to assume a distributed load in the middle of a tube, and calculates stresses for that loading.
My method was to take the tube sizes for the Piet, look for the maximum span on the plans for that size, load that span of 1020 Drawn Over Mandrel tubing (the closest current commercial equivalent to Depression Era 1025) and to Factor of Safety 1.0 (Tensile Yield in Bending). Next, I kept the load the same, changed to 6061-T6, bumped the tube size up the next commercially equivalent size as a nod to buckling resistance (which is driven in part by tensile yield, lower for aluminum than steel), and played with the wall thickness to get at least 1.0 Factor of Safety. I also tabulated the weights per unit length. Results:
The gussets will weigh something, and there are important detail design issues to solve, like, how to transfer point loads like landing gear and the lift struts cleanly into the truss, how to handle the Cabanes, etc. But it looks do-able, with perhps a 30% weight reduction for equivalent strength.
Plates and cranial donned; start firing ball and tracer.
My starting point is the steel tube version of the Pietpenol in the Flying and Gliding Manual.
I also used this calculator from people building roll cages. Tube Calculator - Rogue Fabrication It seems to assume a distributed load in the middle of a tube, and calculates stresses for that loading.
My method was to take the tube sizes for the Piet, look for the maximum span on the plans for that size, load that span of 1020 Drawn Over Mandrel tubing (the closest current commercial equivalent to Depression Era 1025) and to Factor of Safety 1.0 (Tensile Yield in Bending). Next, I kept the load the same, changed to 6061-T6, bumped the tube size up the next commercially equivalent size as a nod to buckling resistance (which is driven in part by tensile yield, lower for aluminum than steel), and played with the wall thickness to get at least 1.0 Factor of Safety. I also tabulated the weights per unit length. Results:
| 1020 DOM Steel Tubing Example: Pietpenol from FGM (which used 1025) Diameter (in.) | Wall (In.) | Weight (#/ft) | Equivalent 6061-T6 tubing, considering bending ONLY Diameter (In) | Wall (In.) | Weight (#/ft) | Longest span of Piet Fuselage (in.) |
| ½ (.500) | .035 | .174 | .625 | .049 | .104 | 38 |
| ⅝ (.625) | .035 | .221 | .75 | .049 (or .058 to nest the .625 dia ) | .127 (.148) | 34 |
| ¾ (.750) | .035 | .267 | .875 | .058 | .175 | 28 |
| ⅞ (.875) | .035 | .314 | 1.0 | .058 | .202 | 29 |
The gussets will weigh something, and there are important detail design issues to solve, like, how to transfer point loads like landing gear and the lift struts cleanly into the truss, how to handle the Cabanes, etc. But it looks do-able, with perhps a 30% weight reduction for equivalent strength.
Plates and cranial donned; start firing ball and tracer.
