Haha, cool, I didn't know you did that! I enjoyed the back and forth between design and stress. I worked with a lot of really good people in both departments. On one program we were told to target 11% MS, only reason I could figure it wasn't written down was so that when revised loads inevitably knocked the margins down a bit, we wouldn't have to constantly be changing the fudge factor in the reports to avoid showing a negative margin that was effectively positive.

Humor me with a tangent: I think we should go back to design doing the stress work, for the most part, with some graybeards in stress and their anointed apprentices providing guidance and verification. AFAIK we really don't have the tech tools to do that efficiently, though, and nobody wants to invest the time. Design software needs access to all of the loads, temps, material properties from mil handbook, knockdowns, everything needs to be versioned and traceable, it needs to let you choose from a large catalog of analysis methods with good FEA as a fallback, show you the updated MS and critical load cases in realtime, always move your analysis nodes with the model as you edit it, export 3D models into the stress reports that remain live when the part/loads change so it changes in the next version of the report, etc. You mentioned git elsewhere. It would be so much better to validate stress report updates by viewing a diff. I could go on. Plenty to disrupt in this market, but the incentives aren't there for more than slow incremental change. I think only somebody who's willing to sink the money and open source the result has a chance at creating a revolution in how we design airplanes.

The 757 was the last bird drawn by hand on vellum, and the last with a wooden mockup. We mostly used calculators on formulae pulled from the Boeing Design Manual (a treasure), or derived formulae as needed. Some of the draftsmen did calculations by drawing geometric shapes and measuring them, which I'm sure is a lost art now.

We were mentored by greybeards who worked on the B-47 and 747, and sometimes would still use a slide rule. I received quite a wonderful education from those guys.

I was the first flight controls engineer, that I could tell, who stole time on a PDP-11 to much more efficiently do my work beyond what was practical with a calculator. (I was called on the carpet for that, the manager of the computer center wanted me censured for it, but my boss's boss's boss went to bat for me and told him to fork off.)

That's surprising from the Stress Guy. The ultimate load by definition should already have the safety factor + everything else factored in. Having a requirement go over the ultimate load, means its not really the ultimate load.

There's always some concern that the math models correctly reflect reality. That's why there's a 50% safety factor, and it's backed up by the engineers' experience that things "look right", bench testing, and flight testing. (In early flight test, they'll tape strain gauges all over the place to see if the math predictions of the stress matches reality.)

Did you ever experience any times when a part passed the math model, but didn't "look right"?

BTW, this happens all the time when using structural optimization programs, where you tell the computer what material you're using, how it's constrained and loaded, and then it figures out what it thinks is the optimal shape for the part.

It means somebody left out a load or a constraint, or messed up the material properties, which is all another way of saying the math model is wrong.

So when a part doesn't look right but seems to pass the math, be very suspicious of the math! :-)

Sort of. I spent much time arguing with Stress over how to calculate the torsional rigidity of the elevator structure.

I don't mean to be too hard on Stress. They're good guys, we both had the best interests of the design at heart. Their manager paid me a great compliment by wanting me to join their team. But I liked design work too much.

I believe that Boeing pushes for a little margin so they can revise the design without having to redesign a whole bunch of parts.

Geez Walter, you are full of surprises... When did you manage to do THAT?

(wikipedia does not have any other info, so I assumed you went to computer industry pretty much immediately after your aerospace eng degree...)

I spent 3 years on 757 flight controls.