Originally Posted by
SeenItAll
While I can't speak to the issue of whether computer testing should replace physical testing, what about the issue of design/test specifications? For example, wings are required to handle loads that are 1.5 times their typical expected load. Who chose the 1.5 figure? The fact that it is such a round number makes it suspect by itself. Indeed, over the past 60 years, how many planes have had their wings snap off in situations and attitudes that were otherwise recoverable? I think the answer is few to none. Thus, my hypothetical question: why should the number be 1.5? Maybe 1.4361 is more appropriate and economical? And while computer modeling may be less exact than physical testing, perhaps its margin of error is less than the difference between 1.5 and 1.4361 -- so using computer modeling and keeping 1.5 still results in appropriate safety.
Just my two cents.
You're right. In engineering there are lots of "rule of thumb"s based on past experience. In the early days of flying, planes with 1.5x margin seemed to survive. Those with lower margins seemed to fail. And 1.5x margin is still economical from cost/weight perspective. And yes, it's a nice round number. Thus 1.5 became the standard safety factor -- not based on strict mathematical analysis but from engineering experience / observations.
Over many decades, newer analysis using more sophisticated methods seem to support this ~ 1.5 margin, at least for airplanes. Yes you can save weight by using a lower safety factor, but there are probably other ways to save weight -- and at the end of the day, the cost difference is marginal, so the industry just sticks to 1.5. There's very little incentive to change this.
(But for spacecraft, any weight savings equal big (big!) bucks, so they tend to use lower safety margins when building rockets, etc.)