Obviously, the blades must be strong enough to withstand the working loads, including a factor of safety. In designing the blade for sufficient strength, I considered only the spar, on the assumption that the skin would only add strength to the blade. I made no attempt to ascertain whether the skin itself fails.
According to Reference [2], the design limit load factor for military helicopter blades must be at least 4.67 (compared to a load factor of 1 in hover). Coupled with a factor of safety of 1.5, the spar must carry 7 times the hover loads without failing. (The loads encountered in propeller mode are much less than in helicopter mode, and are not considered in strength analysis).