Not a huge event... and it's all over within about one hour. I'm the official photog but it's open to the public and it's free. I'll even buy lunch!
They fly fairly low, so if you live somewhere between St Louis and Cape Gerardeau MO you may be able to get a shot as they fly over. The map is on their website. 40 entries this year.
http://bigmuddyairrace.blogspot.com/
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