Netto

On 7/31/2011 2:02 PM, Martin Gregorie wrote:

To me, "netto" means a vario display indicating the actual vertical air
motion, relative to the earth's surface...i.e. 'net air motion' once the
glider's own speed-dependent sink-rate contribution has been
subtracted/eliminated from the picture. In other words, 'my netto
display' always indicates actual air motion, independent of glider speed
(the 'glider speed' bit being the 'compensation' part). No
interpretation needed - that's the beauty of it, so far as my brain is
concerned. And that's also why the speed ring doesn't require the pilot
to iterate in on the speed to fly...because the glider's increasing sink
rate with increasing speed has already been subtracted out of the
display. Hence the vario needle *always* points to 'absolute air
motion,' and in consequence to the whatever speed to fly your ring
setting calls for.

Yes, I agree, but 'super netto' or 'relative netto' are alternative terms
for something different from either TE vario of 'netto'.

A 'super netto' vario shows what a glider would be doing if it was flying
in the current air mass at its thermalling speed, so as well as the TE
input, it also needs the current IAS and the glider's polar.

Snip...

Trying to keep things simple (for my simple brain), here's my understanding of
the following terms...

Unadjectivized "netto" - subtracts out the glider's (presumed known) straignt
line, speed-dependent polar from (the common, everyone-is-comfortable-with-it)
'pure TE vario' display...(theoretically) yielding a speed-independent display
of actual vertical air motion, relative to the earth's surface.

Super/Relative "netto" - *FURTHER* aubtracts out the glider's (also presumed
known) circling-in-lift sink rate *increment* (i.e. the bit exceeding the
straight line sink rate at thermaling speed) *FROM* the 'unadjectivized
"netto"' display...(theoretically) yielding a speed-independent display NOT of
vertical air motion, but of 'theoretical projected climb rate' should Joe
Glider Pilot decide to stop and thermal.

The former subtracts out a speed-dependent, quadratically-approximate polar
curve, while...

the latter subtracts out [the former + a thermaling increment/constant].
- - - - - -

Both quantities subtracted are approximations of reality, though a good
argument can be made that speed-dependent polars are more precisely known (at
least in the public world of shared, measured data) than are circling sink
rates (and by implication, their increment above the non-circling sink rate at
thermaling speed).
- - - - - -

My guess is 'actual vertical air motion' is what every hawk intuitively is
interested in. Certainly an interesting, discussion-worthy topic for those
non-soaring times. Is there a 'best display'? It depends!!!

Regards,
Bob W.