I am willing to bet there are some huge misconceptions about the 139 and its VminI of 50 Knots.
Which one of you cares to prove you know what you are talking about?
I'm not qualified on the 139, however I'm fairly sure the term used by the manufacturer
is Vmini,
not VminI. At least that's what it says in my RFM for a different type, published by the same manufacturer. It's defined alongside the relevant entry (aircraft limitations section) as "Minimum IFR speed", which should be self explanatory.
In general terms, this may be for reasons of AP capabilities and/or because of lack of reliable airspeed sensing/indications; which are directly related. If you have no forward airspeed indication, and no visual references, whilst in a hover attitude the aircraft may be traveling forwards, backwards, or sideways, you can't reliably tell, and neither can the aircraft.
To fly on instruments at very low speeds (i.e. where IAS indications become unreliable) requires doppler equipment. This can be tied in to some autopilots to provide auto hover/auto transition & climb capability. But even so, that does not provide an obstruction avoidance capability, hence the regulatory inclusion of helicopter RVR limits at airfields.
The transition to forward speed and climb is a critical stage of flight, both from an aircraft point of view and for the pilot himself, if he is changing from outside visual cues to instruments only. The trick is not to have both occurring at the same time.
If you have no outside visual references you are by definition relying entirely on aircraft instruments, so you cannot be operating under VFR. If an aircraft is going to depart in conditions of low vis/low cloud and climb under IFR, Vmini by definition
must be achieved with reference to visual cues. Which is why attempting a Class A takeoff profile may not be a good idea at all; sufficient forward visibility and room to safely accelerate to Vmini along a clear path is required.