Vd = Vdestruct (ie, when catastrophic failure occurs)
Vne = Vd - 10%
Vmax = The manufacturer (or operators) limit; often the upper limit to which the acft was flight tested.
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It is my understanding that the acronym 'Vne', to which we most often refer, is actually Vmax. (The ins & outs of which have been suitably detailed)
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At a pure (PofF) level the Vmax of a helicopter is limited by:
1. RBS or,
2. Advancing blade compressibilty or,
3. Engine power or,
4. Physical fwd limit of cyclic movement