I got close enough once that I could have touched a Redtail.
I was checking the 18 windmills on my ranch (in south central NE) in my 90 hp Super Cub. We had lots of Red Tails and I was always on the lookout for them when flying. I missed the one that passed between the fuselage/wing and strut. I was flying with the door open (naturally) and could have reached out and touched him (although not something you'd want to do at 90 mph.)
To say the "pucker factor" was at the max is an understatement
WyoBob