The AH-64 Apache tail rotor has four blades. The blades, however, are not oriented 90° (perpendicular) from each other as in most helicopters. This unusual arrangement is required because the two sets of blades use a “Delta-Hinge“ which allows the blades to simultaneously flap and feather
Hypothesis: The four blades appear to behave as two for the tail rotor blade passing frequency. The acoustic pressure pulses are dominated by the pair-passing rate (2×), not the individual blade rate (4×)
The data in this FFT slide package confirms the hypothesis.
The AH-64 Apache Tailor Rotor parameters in this MIL-STD-810H table are correct but misleading. Again, the tail rotor blade passing frequency is 2× rather than 4×.
– Tom Irvine