You think the real gunners of WWI were all suicidal idiots? - They strapped themselves in for combat, so they wouldn't fall out. They trained hard to be able to defend their plane in turning combat, because, ehhh, their life actually depended on it, you know?
This kind of appeal to common sense is not a valid form of argument, especially when dealing with events from 100 years ago during the dawn of aviation. The remark about "training hard" also belies the reality of WW1 air crew training, where boys were thrown into combat with less than 20 hours of flight time.
Anyway, there was a huge discussion about this at the RoF forum. Historical evidence is that gunners were not always strapped in, and some were thrown from the aircraft. One anecdote in particular describes the gunner aiming with one hand and holding onto the aircraft with the other when the aircraft was in a turn.
If this seems crazy, remember that these guys weren't wearing parachutes, and their aircraft were doped with highly flammable resin.
