Vz???
 

Ok, so I cam across this website ( Index [See How It Flies]) and it mentions Vz as the minimum speed of level flight, I don't think I've came across this before and if I have I don't remember.

So can any one here confirm for me whether this is correct or not as google does not seem to be helping me?

And it would be useful for me to point when pointing out that Vs is not the minimum level flight speed.

No such terminology is used in the certification requirements CS 25 for large
(civil) aircraft.

PBL

Having looked at the site, it seems to be a typo. He is probably referring to Vs, the stall speed in the clean configuration.

Its the far left of the drag curve (less than Vmd) where it meets the power available curve.

:8

Reference?

PBL

The reference is the book mentioned in the original post... Here is a more specific link:
More About Energy and Power [Ch. 7 of See How It Flies]

That book is to a degree an original work, not just parroting what others have written, so Vz is defined just as it is in that book. It's a definition, so it can not be wrong... ;)

That said, I know have come across the concept elsewhere, for instance in some text relating to the Concorde, where the concept of thrust limited lowest level flying speed is actually relevant. For many aircraft in many situations Vz either does not exist (i.e. the aircraft can climb at Vs) or the margin between Vs and Vz is so small that Vz is of no practical interest. That is why the concept is rarely used.

It does, I believe, add some insight into what happens when you are unable to climb out of ground effect. That happens when the airspeed is higher than Vz in ground effect but lower than Vz out of ground effect.

In the US wrt to Light sport planes ---the term is called Vh in KCAS:)

As far as I can see it is an internal definition - saves a lot of writing.

It is also used in aeronautics as vertical speed ie speed in the z axis, which is where I think this author derives it, as in (Vz=zero) speed but more correctly Vzrc.