Originally Posted by
The Sultan
Helispotter,
I mean it is not related to any rotor harmonic or sub-harmonic, so no .5, 1, 2 etc /revs. These are generally at a natural frequency of the rotor, pylon, and/or airframe. For an asynchronous to become a problem the “and” is the operable term. It generally takes a gust exciting the rotor at a frequency close to a fixed system mode which results in a response in such a manner that it can feed back into the rotor to sustain or amplify the vibration.
OK, so who has experienced the so-called "VH hop" and lived to tell the tale? I am still confused as to whether it is concentrated around a defined frequency or is vibrations all over the show (sort of a "white noise"). Would any pilot that has experienced this even be able to characterise it compared to the regular vibrations they are familiar with from their helicopter?
You earlier wrote: "...Bell blades the “hop” at certain torque settings (ground and flight) is at 1/rev and is aggravated...". So this must be different to the 2/rev vibration that Bell has intentionally tried to tune out with the Nodamatic suspension? Is the "Bell hop" mentioned here something that comes and goes?
You also earlier wrote : "The VH “hop” sounds more like it is sourced from a rotor mode coincident with an airframe/pylon mode". Do you mean the various natural frequencies for the rotor and airframe/pylon? Are you also suggesting an initial disturbance such as effect of a gust is perhaps setting off a vibration at such natural frequencies that isn't being naturally damped out, along the lines of flutter?