Not quite correct
Go smoke.
Vd is the design limit, which is the lowest speed generated by a huge list of issues, which might include:-
- (that's the speed at which the wings twist off)
- Structural limit of the tailplane
- Predicted canopy buckling limit
- Predicted flutter onset.
- Predicted wing drag structural limit.
All modified by various safety factors.
Then, when a new aircraft type is flight tested the speeds are slowly increased, until either something goes wrong (actual flutter, actual canopy bucking, any unexpected handling deficiency), OR Vd itself is reached. This speed is now set at Vdf (the diving limit).
The flight test evaluation is then carried out up to that speed, and everything confirmed satisfactory (there are a whole stack of things that are investigated).
It may be that things are then done to modify Vdf - in either direction, for example it may be too low and a particular design feature is modified to stop it distorting/fluttering/etc.
Then, finally, Vne is set to a value selected by the design team, but it's not allowed to exceed 90% of Vdf. The odds are, that they'll probably actually set it to 90%Vdf.
The other factor that's relevant is Vh - the maximum achievable speed in level flight. There is a margin that this may not exceed below Vd (the design structural limit). The margin will vary depending upon the size and shape of aeroplane, but as a general rule it's probably not allowed for Vh to be greater than Vd/1.25.
Either as a Test Pilot or Flight Test Engineer, I've exceeded Vne more times than I'd care to count - it's particularly interesting when you have the builder of a kitplane in the right hand seat
. What's really scarey however, is finding the value of Vdf!
G