Design requirement fron FAR's
§ 27.337 Limit maneuvering load factor.
The rotorcraft must be designed for—
(a) A limit maneuvering load factor ranging from a positive limit of 3.5 to a negative limit of −1.0; or
(b) Any positive limit maneuvering load factor not less than 2.0 and any negative limit maneuvering load factor of not less than −0.5 for which—
(1) The probability of being exceeded is shown by analysis and flight tests to be extremely remote; and
(2) The selected values are appropriate to each weight condition between the design maximum and design minimum weights.
[Amdt. 27–26, 55 FR 7999, Mar. 6, 1990]
Due to Vne limit and control response it it difficult to exceed even +3 g in your average helicopter. Negative G especially in a twp blade system can result in a much more critical condition and should be avoided like the plague