I watched the first two parts, amid some interruptions, so I may have missed it - but I don't recall any mention (even while we watched Dan Snow gently rotating the Upkeep outside the hangar at Scampton) that it had to be spun up to 500 rpm before being released from the aircraft.
That, despite a clip that clearly showed the mechanism for doing so, albeit without any explanation.
Maybe it will be remarked on in tonight's final part?