To me, the most dangerous part of formation flying isn't the flying but taxiing and take-off. Planes on the ground are awkward and jerky and you can't track the other plane as easily as you can in the air. If you get close, there isn't an up or down option. Throw in a cross wind and, well, don't even try it then!
Steve & I did an impromptu formation flying demo during a break at the Mid-States Scale Qualifier in Hillsdale, Kansas. It is a real challenge to get both planes in focus... Photo by Don Thun.
Best formation photo taken of us. This was at Warbirds over the Rockies and the photo was taken by RC Universe's Ken Isaac.
The two planes we started flying formation with, our P-47s. Steve's is the camo one and mine is aluminum. His has a Zenoah G-45 and I mine has a Quadra 41. His is faster and mine can slow down a lot more, but we split the difference and it works but is not as easy as the P-51s with the same engines and props.
Whenever we fly formation, we always are keenly aware of seperation. It has to be in a vertical plane or horizontal. The vertical is usually the easiest to keep visually as we fly, just make sure one is above the other. When we come down low, show center is the only time we don't worry about vertical separation.
Steve was flying Larry Well's B-17 and I flew our B-25 for some multi-engine formation flying. This was difficult for the B-17 was a lot slower than the B-25, so I cranked down some flaps to be able to fly a little slower which helped. We also had to fly a little wider turns since these aren't agile fighter planes. It sounded fantastic though!
The trick we developed is the plane in the lead before turning for the show pass, takes the "lane" closest to us. This usually automatically lines the planes up since the faster plane has more area to cover. With this rule, we can perform without being right next to each other, since we know what is going to happen.
This takes quite a bit of practice and coordination. The more we do it, the more comfortable and better we get at it. We anticipate each others smoothness of turns and what we will do in most situations after a while. The spectators really like the show.