Bicester Aviation Services

FAQs on Variometry and Instrumentation Systems

1.  What is the difference between 'Relative Netto' and 'Super Netto'? 

The short answer is nothing; Relative Netto and Super Netto are alternative words for the same thing.   However let us refresh our memories regarding the definitions of Total Energy, Netto, Relative Netto and Super Netto.

Total Energy compensation, sometimes referred to as TEK, is simply the actual sink or climb rate of the glider as measured by a basic variometer, with no alteration being shown if the glider changes speed, other than the change in polar sink   With an uncompensated vario, if a glider does a 'zoomie' in still air from 100 kts to 45 kts, the vario will show a large rate of climb as the glider's kinetic energy is exchanged for potential (or height) energy.   Total energy compensation simply removes this rate of climb caused by the energy exchange, and vice versa.   To translate this to practical figures, let's assume that at 100 kts the polar sink of the glider is 4 kts and at 45 kts it is 1 kt.   Then at 100 kts the vario would show 4 kts sink and as the speed is reduced to 45 kts during the zoomie, the vario reading should slowly show reducing sink until it reads 1 kt sink at 45 kts.

Netto is the vario reading shown when a pneumatic or electronic gizmo removes the glider's polar sink from the TEK vario reading.   In the example above, a netto vario would show zero at 100 kts and as the speed was reduced to 45 kts, remain at zero.   In other words it is telling us what the air mass is doing, in this case it is still air.  For this reason, a netto vario is sometimes referred to as an 'airmass vario', which is much more accurate name for what it shows.   If we now take the same glider at the same speeds but place it in a constant air mass 5 kt thermal, then at 100 kts it will show a 5 kt climb.   However our TEK vario will only be showing a 1 kt climb (5 kts air mass thermal minus 4 kts polar sink).   At 45 kts the netto vario will still be showing a 5 kt climb, but our TEK vario will now be reading 4 kts climb (5 kts air mass minus 1 kt polar sink).    It is important that you understand this so far, because if you are not clear on these definitions, then the next one will totally confuse you!

A Super Netto or Relative Netto vario will show the actual rate of climb that you would achieve if you reduced your speed to thermalling speed.   Mathmatically it is the netto  vario reading less the polar sink rate at the best thermalling speed.  

In summary, lets consider the same glider as discussed above flying through a true, air mass 5 kt thermal at 100 kts and at 45 kts.   At 100 kts, the total energy vario would show 1 kt climb, the netto vario would show 5 kts climb, and the relative vario would show 4 kts climb.   At 45 kts, the total energy vario would show 4 kts climb, the netto vario 5 kts climb and the relative vario 4 kts climb.