All I get is Stress!!

We also need to think about what pressure we need to keep the blimp envelope at. We did the calculations for how much stress is allowable based on the weight vs. buoyancy. The envelope pressure keeps the shape we want for minimum drag but it adds to the stress on top of the lift from buoyancy.

Some want to add wings        this takes away from required buoyancy
                                            but adds stress to mount wings

Some want hot air                 different sources of buoyancy
augmentation                        require a frame to collect stress

I did a calculation in the past to determine the pressure needed and hoop stress on the envelope. When I get home I will have to look back at my spreadsheet. The pressure needed for a lenticular shape will be less than for more complex shapes.

In your spreadsheet did you include the hoop stress and the balance of buoyancy forces? When we were talking in a past conference, I don't think we looked at both at that time.

No, I didn't.

Here is the post that explicates the calculations. I can't say it is totally clear, even to me. Of course the post is like 9 months old so my memory is a little fuzzy now. I am working to get the formula integrated with my current spreadsheet.

http://smallblimps.lefora.com/2010/04/23/ultralight-design-pitch/68/ 

What was your calculation for balance of buoyancy forces?

I looked at the perimeter of the gas bag mid section. At this location the buoyancy is pulling up at 452 lbs force and the weight of the craft and occupants are 452 down. ( To be exact, subtract 1/2 of the bag weight above the perimeter or for safety margin just leave it in.) So the perimeter times the bag thickness is the cross-sectional area of the bag wall. The buoyancy force divided by this area is the applied stress. To this stress the hoop stress for inflation must be added.

To be real exact the stress distribution based on the actual bag shape ( non-spherical) can be accounted for.

While we're talking about stress, a standard college book on "strengths of materials" has numerous examples of problems worked for us. Like if you attach a bolt to your gondola, knowing the smallest bolt ( weight savings) that will hold the applied load safely is important to know.