I've a video of a day ULLA drop at Keevil of fuel cells. Load prepared and dropped level at about 12 feet but the load nosed in and disintegrated in front of an audience of VIPs. Turned out the c of g calculations were done for Jet A-1 but with the dummy load being water it made them wildly inaccurate. That's my excuse and I'm sticking to it........
From recollection I think 130 knots but it's a while ago ! That's way above stalling speed before release and even further above stalling speed once the 15 tons has vacated the freight bay !