What you're describing is the required change in vertical velocity. Because most roller coasters are moving forwards as well, the actual speed will be quite a bit higher. (An exception is a S:ROS LIM type ride. In that case, your numbers are correct, in fact, you can double the airtime because you feel it in both directions!) For 5 seconds of airtime(free-fall) you would need to go at least 110 MPH at the bottom of the hill (actually this is more, because you're not going straight down on a frictionless track.) When cresting a hill, that same frictionless, perfectly designed coaster would give you 10 seconds of airtime (half on the way up, half on the way down). Being shot straight up at 220 MPH would give you 10 seconds of airtime on the way up, and 10 on the way down!
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A bad day at CP is better than a good day at work.