Consider the origin of this myth. The US Army bought a bunch of UH-1 helicopters. Bell wrote an aerodynamic text which described their 2-bladed rotor system as being subject to gyroscopic precision. The Army needed an aerodynamic text, so it copied the Bell text. The FAA, needed a helicopter handbook, so, it copied the US Army text. And so on... Problem was, the Army substituted "helicopter" in-place of "UH-1." The industry progressed beyond the Bell-Hiller rotor system, but the texts never have.