Why is Thunderbird 5's number written upside down?
This is because Thunderbird 5 is the opposing solo, and spends almost all of it's time inverted.
Apparently though, this plane's number was not always painted upside down:
Just guesswork on my part, but maybe the Thunderbirds did not fly inverted maneuvers before the F-16s?
Just for clarification sake, Thunderbird 5 is the LEAD solo, not the Opposing Solo. Opposing Solo is Thunderbird 6. The 5 is upside-down because During the show he spends the most time upside-down. On crowd passes, and show center, he is usually inverted. Also, the F-4 Phantom and T-38 Talon fly perfectly well inverted.
When the Thunderbird's fly the reflection pattern, one plane will be right side up flying level. Number 5 will be upside down and below the first one. They are like reflections in a mirror, bottom to bottom. Number 5's engine number is written upside down, so that it can be read correctly. If they fly the calypso pattern, where one plane is flying right side up and the second will be above it, upside down, the upside down plane will always be number 5.