The scenario requires a helicopter with Westland Whirlwind-like capability in 1939. Reaching that point needs a plausible earlier development path. A change in technology must still fit the surrounding historical circumstances. Once it changes the aircraft and ships available, the course of World War II can no longer be assumed to follow the familiar sequence.
Chapter · 4:36-10:10
Building a plausible 1930s helicopter divergence
A Whirlwind-equivalent helicopter in 1939 needs a plausible historical development path, with consequences for the ships and forces available during the war.