For users of this forum, it should be noted that it is very unlikely that Lockheed’s lawyers will be tracking anyone down over the use of this stress memo. In their text they haven’t even given credit to the original creators of this work, i.e. North American, if my sources serve me correctly.
An old friend of mine, who worked for many years as Head of Methods at a large airframer in the Dallas area, once told me that every US aircraft company has a copy of their competitor’s structures manuals. Where there were gaps in their own manual, the competitor’s data were included and a few “confirmation tests” done to verify the “foreign” method. Thus you’ll find a very large amount of uniformity among stress manuals emanating from US companies.
Differences do appear in SM material and fastener allowables however, where very often different suppliers are able to achieve statistically better results than those published in the public domain literature like the MMPDS. Similar materials (e.g. 2024) are given additional identifiers relating them to the supplier.
Much of this work has also found its way across the pond to Europe, with some sloppy disguise work done, in particular, on commercial aircraft fatigue analysis methods.
In particular the L-SM 88 method under discussion here, on Tension Fittings, has found its way into European manuals, then migrated to Ireland and has finally wound up back in Canada, with the European “tweaks” still evident in the fine-print. At least the European company did some substantial mathematical and FE analysis work on the empirical methods to confirm their validity. They have their own internal substantiating reports to back-up their use of the original.
Most of today’s manuals still rely on the ground-breaking work done by the NACA researchers before during and after WW2. The works of Kuhn and Gerard et al are enshrined in each one.
Besides the inter-company “flow” of manuals, ex-company SM authors have shared their company methods and test results with us in the form of public domain books and supplements that are now widely used by stress engineers around the globe.
IMHO, the L-M lawyers would have an extremely tough time convincing a jury that they were the sole owners of the text contained in L-SM 88. But who knows, in order to turn a buck, maybe they’ll try anyway.
