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EagleRJ said:The Vmu test is performed to ensure that the aircraft can still take off if it is rotated too early by the pilot. Some early types (DC-6, I think) would stop accelerating if they were rotated early, so even though you were trying to fly, it was never going to happen.
Certification standards under Part 25 require an aircraft to be able to take off at any speed above that where the elevator physically has the authority to raise the nose off the ground. The high alpha involved in the test means the tail usually scrapes along the runway before liftoff.
Singlecoil said:I think the purpose of this test is to prove the aircraft can become airborne if over-rotated, or rotated too early. I can't remember which plane it was, but an early jet was over-rotated onto its tail and subsequently crashed because it was not able to get airborne after that event. Part of the certification of new jets is to make sure they can power out of that situation.