The mention of 212s and bubble windows brought back a suppressed memory:
Tasked to carry some VIPs late at night in mid-winter over a mountain range. I knew it was gonna be bloody freezing with the big opening in the bottom of the bubble, and I didn't want the blast of 7000' cold air coming in the hole. I cut a section of 6" thick rubber foam to jam in the hole to prevent the blast coming in.
After takeoff, at about 30kt, there was a "fwup" noise and the foam was gone! The pressure differential worked opposite to what i expected, and the cabin pressure pushed the foam out instead of the outside pressure shoving it in.
It was subsequently a very cold flight, but at least nobody complained about being bopped on the head by a foam blank.
