I'm working on a tabletop roleplaying game about witches. I have most of the mechanics of broomsticks worked out, but I'm having trouble deciding on top speeds. Speed limits are not a result of how fast the broomstick itself can be made to go, but are imposed by air resistance ripping the witch off the stick at high speeds.
I'm looking for two speeds; one for witches sitting sidesaddle, and one for witches astride the broom. Novice witches sit sidesaddle, because it's easier to balance that way, whereas more experienced witches sit astride it. Sitting astride the broom allows them to bend forwards and better streamline themselves, as well as giving them a better grip.
Other facts that might be pertinent: The lift occurs entirely in the broomstick, and the witch must rest upon that. The broom are ridden bristles-backwards. For the purposes of this discussion, the witch is performing no magic besides the magic that lifts and moves the broom.
Assume that the witch has just enough practice to balance on the broom, but she is by no means an athlete. Her grip strength, weight, and so on are average for humans. How fast can she go without being ripped off by the oncoming air?