The flying buttress is an iconic architectural element, used most famously on Notre Dame and other large, historic churches. But what exactly does a flying buttress do?
Ancient buildings, the sort that were designed to last anyway, were almost always built out of masonry: solid blocks of clay or stone fitted together. Masonry has high strength in compression, but almost no strength in tension. To get around this limitation, builders had basically one engineering trick: the arch.
So how does an arch work? Well, the easiest way of spanning between two points is with a simple beam. A beam works by having the top portion in compression in and the bottom portion in tension. An arch cheats this by switching out internal tensile forces for external compressive forces: instead of bending like a beam does, an arch pushes on its supports. This makes it perfect for using with materials with low tensile strength. The drawback is that using them requires a strong, stable surface that the arch can push against.
Being the only play in the playbook, arches tended to get a lot of use – lots of ancient architectural elements are variations on it. A dome is essentially an arch wrapped around 360 degrees*. If a flat surface was needed, a flat arch could be used. And the “flying” part of a flying buttress is just an arch tilted on its side.
Churches tended to have ceilings made of large, ornate masonry vaults. A vault is, once again, just a slightly different flavor of arch, and these would create outward thrusts at the top of the wall. Handling these forces is a difficult task for a comparatively thin wall – something larger is needed to resist them.
One way is just a regular old buttress – a thick heavy section that is big enough to contain the outward thrust (i.e.: keep the member entirely in compression).
You could also move the buttress outside and connect them back to the support walls. This is perhaps a more elegant architectural choice, but it doesn’t improve your actual design much – you’ll need just as large of a buttress wall as you did before.
But if you slope your arch downward, the outside wall will be at a lower elevation. This lowers the overturning moment (force wanting to topple the wall over), and allows for a smaller buttress, and a lighter, more airy church design.
So, to sum up: a flying buttress was an arch that transferred lateral thrust from the roof vault. Transferring the force like this allowed the use of smaller, shorter buttresses on the outside and produced a lighter, more elegant design.
The flying buttress, like every other flavor of arch, isn’t used much anymore. With the development of steel it became economical to design structures that could carry large tensile forces, making the arch unnecessary. In some ways this is the main difference between ancient and modern (post 1850 or so) construction – the former are designed to only be in compression, while the latter can handle both tension and compression with ease. For this reason, it’s unlikely we’ll see a revival of the flying buttress any time soon.