The Art of the Helicopter John Watkinson - Karatunov.net

104 The Art of the Helicopter
(a)
(b)
Fig. 3.35 (a) A pair of contra-rotating eccentrics produces linear vibration. (b) If the mechanism of (a) is rotated,
one of the eccentrics will turn faster and the other slower. Thus frequencies transmitted to the hull will be the
vibration frequency plus or minus the rotor frequency.
It will be clear that a two-bladed rotor is at a disadvantage because both blades will be
in the lift trough simultaneously at 90 ◦ and 270 ◦ , and in a lift peak simultaneously at 0 ◦
and 180 ◦ . The result at high speed is a vertical vibration known as 2P hop. Figure 3.36(b)
shows the situation for the third flapping harmonic. As the third harmonic flapping
has one and a half cycles in the time between blade passings, it will be clear that the
3P components will be out of phase between the two blades. Following similar logic it
should be clear that in a two-bladed rotor, only even vertical harmonics exist; the odd
ones cancel.
Figure 3.37 shows the case for a three-bladed rotor. At (a) it will be seen that the
second flapping harmonic waveform of each blade is at 120 ◦ to that of the next. The sum
of three sine waves at 120 ◦ is constant and so there can be no 2P hop in a three-bladed