Originally Posted by
portsharbourflyer
Just trying to think PDR, while the normal level flight 1 g case load case is upper compression, lower tension it isn't a critical case; the negative gust cases could easily see the load reversal meaning upper and lower booms probably need to have mirrored dimensions for the ultimate cases. The 1 g level flight case is more an issue for fatigue; the zero to tension is more damaging than zero to compression.
Whilst true, if you want to add gust cases (I was keeping it simple) you have to add them to both upright and inverted cases so they have the same effect throughout the envelope. Suggesting fatigue is the dominant case makes certain assumptions about the way the structure reacts the stresses, and also that the structure is made from a material which suffers fatigue failure modes.