Often, the back-up structure is stiffened or loostened to move the rocking mode away from the rotor mode, or damping is increased in the main rotor (no luck here for a teetering "semi-rigid" system).
This signal is sent to the SCAS (stability control augmentation system)which sends out control movements to oppose the transmission rocking, and damp them out.įinding and fixing such "Pylon Rock" is a part of all helicopter testing, and keeping these dynamic frequencies under control is a big part of developing helicopters and tilt rotors. The Cobra had (has?) electronic motion transducers on the transmission feet to sense when the transmission starts rocking relative to the fuselage. This is quite similar in concept to a wing flutter condition in an airplane.Īt some flight conditions, especially high speed and increased load factor) the whole rotor transmission system could go to a large limit cycle amplitude (get big and not die away). In early C model hueys (and the Cobra, which shares the same rotor design) there existed a match between the natural transmission rocking mode (the transmission is suspended on rubber mounts for vibration absorbtion) and the rotor's natural frequencies. All rotors have natural frequencies based on the rotational speed (RPM) and the number of blades, and the natural damping (energy absorbing) of the system. Pylon rock is a natural oscillation common to the Bell two bladed systems, where the natural rotor inplane frequency is matched by the fuselage and transmission. I hope nobody has become convinced of this "fact" as it is wrong, and it is not a necessary piece of explaining the pylon rock issue. Anyone who believes otherwise please email me, I have bridge in Brooklyn that is for sale. The blades all lift all the way around the mast. I stumbled into this thread a bit late, but have to chime in: Naturally then, all the forces generated by this movement will be transferred to the transmission mounts, which will absorb and damp it. The exhaust airflow must have an effect too.Īll in all, the blades experience various different states of relative airflow all round the disc, so no wonder they flap up and down like crazy (just look at that footage from the 'Chopper' series, or whatever it was called, where a camera was attached looking outwards from the hub of a huey inflight, makes you wonder how the damn things stay on for 1 hour, let alone thousands!). Lu, I'm inclined to agree with your reasoning regarding the loss of lift when the blade is aligned with the longitudinal axis, although I'd call it a reduction rather than a total loss.Īlso, there is probably a disruption when it passes over the rear cabin and tailboom area, where the downwash would be deflected and the induced flow would, I imagine, be decreased. We're only talking, fellas and girls, not buying and selling sheep stations.
Maybe it's better not to buy into this discussion, but.bugger it, why not!
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