Plywood fuselage design methods

[mcrae0104]

In Stress Without Tears, Tom Rhodes discusses the design of a cone-shaped plywood fuselage tail section (chapters 25-27). I understand his method of analyzing this "beam" for bending, shear, and torsion; however, there seems to be little to no guidance out there in either this simplified book or in Bruhn on:

• Design of the stiffeners (or formers). Just how much moment do they need to be designed for? If we ignore aero loads normal to the fuselage then the only thing to cause bending in the formers would be buckling. Is there a method to determine the forces introduced to the formers by skin buckling? Can you point me to a source I can read? Bruhn does present a method for turning these frames into an equivalent determinate structure. It's a little over my head but I think I can slog through it--but I need to determine what loads to apply to the frames first.

• Cockpit opening. The tail cone is relatively straightforward as a cantilevered beam. Now where I have a cockpit opening, it seems simplest to treat the two side walls of the fueselage as beams with the longerons acting as caps. How does the curved shear web of this beam affect it? (Imagine a venetian blind used as a beam.)

• Firewall to canopy. Like the tail section, this is hollow beam. However, because of the concentrated loads at the motor mount points, longerons will be used to distribute the load into the almost-round beam. Also, it would be nice to have some openings in the side of the fuselage here for a forward baggage compartment--precisely where the structure doesn't want them. For these two reasons, I am thinking this section would most easily be analyzed as either a) a beam, similar to the fuselage sides under the canopy, but with openings, or b) as a truss. How would you approach it?

• Longerons and I of a hollow beam. Am I correct to include the longerons when calculating I of the fuselage? (Rhodes had no longerons in his example and I did not see a similar example in Bruhn.)

Based on what I am learning, I strongly suspect that most stressed-skin plywood aircraft (particularly the former frames) are not fully analyzed, but taken partway and given either a TLAR check or the "well-it-worked-for-that-other-design" test. I am also beginning to think I may need to build a full-scale test article to gain full confidence in the design (although I don't really want to build two airplanes!). Sure does make a steel truss look easy from a designer's perspective.

Thanks for any guidance you can offer on how to analyze this animal.

 

[TFF]

 

[mcrae0104]

 

[Mad MAC]

 

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[BBerson]

 

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[wsimpso1]

 

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[DaveD]

 

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[mcrae0104]

 

[mcrae0104]

 

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