What are the mu ratio and the mu-1 barrier?
, as the aircraft speed increases, the air velocity initially decreases over the outboard section of the retreating rotor blade and at the same time increases the reverse flow over the inboard blade section. From mu of 0 to 0.75, this results in the average magnitude of the air velocity decreasing. However, above mu 0.75, the increasing reverse velocity on the inboard section overtakes the decreasing velocity on the outboard section, and the average magnitude of the air velocity begins increasing, albeit in the opposite direction from what it was before. Even though most of the air is now flowing over the blade in the reverse direction, and this isn’t the most efficient way to produce lift, it does provide a greater margin for flapping without causing retreating blade stall.