I always think of steampunk as being able to do magic...with steam. (Magic as in things we can't do/don't have now, such as a strong but very light weight alloy for the flying machines.)
If we could have done everything with steam, the internal combustion engine might never have been invented.
As long as every engine is a steam engine and your flying crafts aren't airplanes, I think it's all good, no matter the era. And I can imagine both Wodehouse and Agatha Christie stories in steampunk. Jeeves disapproving of Wooster's new fashionable flying helmet and Poirot on a blimp, solving a murder. (Plot bunnies, go away. I don't have time for you today.)
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If we could have done everything with steam, the internal combustion engine might never have been invented.
As long as every engine is a steam engine and your flying crafts aren't airplanes, I think it's all good, no matter the era. And I can imagine both Wodehouse and Agatha Christie stories in steampunk. Jeeves disapproving of Wooster's new fashionable flying helmet and Poirot on a blimp, solving a murder. (Plot bunnies, go away. I don't have time for you today.)