Norma,
The Myths list works!
In fact, many helicopters have control phase angles of less or more than 90 degrees, based on the hinge angle (delta 3) and the blade inertia vs its flap damping. I know of one helo that had a lead angle of almost 180 degrees, and the S-76 has a lead angle of 56 degrees, both not at all 90 degrees!
The way the blades flap as they whirl around, and the way the cyclic makes them flap is not gyroscopic at all, but a gyro and a rotor share the same need to make fundamental physics happy. The conservation of angular momentum is the key, so there is a bit of gyro DNA in a rotor, but not enough to make the lead angle precisely 90 degrees. In fact, it s almost never exactly 90 degrees in any helicopter. In fact, it is not even the same angle in one helicopter. let me explain:
The real phase angle of a helo can be easily found, just tilt the swash plate to the north, and watch where the rotor plane dips lowest after all settles out. The typical rotor will dip lowest somewhere around west, so we call that one a 90 degree phase angle (engineers call it gamma, the swash plate phase angle).
If you take a given helo and do that test at three different speeds, you will find three different gammas, because the airspeed has a strong effect. In other words, the "gyro precession" angle of a rotor varies with airspeed!
For a helo with delta 3, the gamma shifts for each degree of delta 3, so the Robbie, with 18 degrees of delta 3 has a gamma of 72 degrees. This has lost Lu a lot of sleep over the last few years, as he ponders the universal significance of that horrid 18 degree shortfall, all the while thousands of Robbies have flown millions of miles.
This has been beaten to death here on pprune over the last few years.
Suffice it to say, the typical rigging angle of 90 degrees for most helos is an approximation, it works, it can be different for any model helos, and it is not due to gyroscopic precession.