I found this handy calculator -- http://www.calctool.org/CALC/phys/newtonian/centrifugal

Setting it to feet and mph. If flying 30mph, a loop with r > 62' and you'll have less the 1g of centrifugal force at the top so you would be in negative Gs, perhaps for a fraction of a second. At 40mph, you could get r up to around 105'. At 50mph, around r = 170'.

Hop in your F15 at a peppy 500mph you'll need R around 17000 to pull negative Gs at the top of the loop.