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Not sure the SE5a (or any other WWI bird) was structurally capable of having retractable gear, or had the power to carry such a weighty mechanism. Would be interesting to see how the SE5a would work as a monoplane, I think it would need a dramatically improved powerplant to maintain it's performance though...
Slightly OT, but related to Ball and the SE5... Could this be the first a/c with a weapon firing through the prop shaft?
Austin-Ball A.F.B.1 - Wikipedia, the free encyclopedia