AusAnt
05-01-2008, 09:02 PM
So I was right! Smooth flight is one crash away! :thumbup:
Too true! :YeaBaby:
rscamp
05-02-2008, 06:52 AM
When there is a vibration problem in a heli it is (almost) always, always because the natural or resonant frequency of some part of the structure is being excited. Look for this!
The basic solutions are to:
1. Reduce or remove the source of vibration/excitation
2. Shift the frequency of the source of vibration/excitation, or
3. Shift the resonant frequency of the offending structure
If we want to run the heli at a specific head speed, 2. is not an option. 1. would usually be addressed by a good dynamic balance on the head and/or tail rotor.
If the excitation frequency is above the resonant frequency of the structure, the system is "supercritical" and the vibration will be excited (usually for on a brief period) on runup to speed. Above critical the structure will actually vibrate at low amplitude out of phase with the excitation force.
It is entirely possible to address a vibration problem by intentionally inducing a supercritical situation (i.e. say by adding a mass to the middle of a vibrating tail boom support strut).
There are multiple structures in a heli so in reality we likely run at a speed between two structural resonances.
Making things stiffer (or lighter) to avoid a particular resonant condition is better when possible because there is no runup (or rundown) through the resonant frequency.
Rob