Good morning! It’s been a while since we had a Math Mistake featured here, but thanks to an article sent my way (thanks MM!), we have some breaking news. Well, “breaking” might be a stretch – the error happened in 1945 and was revealed in 1989, so it’s been a while. But still, it’s an error I hadn’t heard of before and it involves *Calculus*, and specifically checking if your critical point is a max or a min.
So what happened? In 1945 William R. Sears and Irving L. Ashkenas, who worked for the Northrup Corporation, wrote a report comparing the range of a traditional bomber to one that didn’t have a tail and was essentially all-wing (looking a bit like a boomerang). As part of the justification they used CALCULUS to find the shape that gave the best range: set up a function, take the derivative, and see what the critical points are. In this case, there were two: one where almost all the weight was in the wings, and one where it wasn’t. They stated that the all-wing design gave the maximum range.
The problem? That was actually the minimum. A test like the second derivative test can help with that – if the second derivative is negative, then the critical point is a maximum, like ⋂. But if the second derivative is positive, it’s a minimum, like ⋃.
This mistake was caught within a few years, when Engineer Joseph V. Foa checked the math and told the authors. They admitted it, but also said it didn’t really change things because that was only part of the story. Foa thought that this did too change things, and if this were a couple hundred years earlier there might have been a duel. Instead, the army canceled an all-wing aircraft they’d been working on, supposedly for budget reasons.
So that’s that! Or maybe not – sometimes endings aren’t so clear. Wayne Biddle’s 1989 article below refers to a new B-2 bomber by Northrup that uses the same all-wing design.
Main source: “Skeleton Alleged in the Stealth Bomber’s Closet” by Wayne Biddle, Science, New Series, Vol. 244, No. 4905 (May 12, 1989), pp. 650-651. (There was a follow-up: “B-2 comes up short” by Wayne Biddle, Science, Vol. 246, Issue 4928 (October 20, 1989), p. 322.)