The Netto Vario

Understanding what the Netto vario is, how it is calculated, and how to use it on the Vector Vario.

1 – What is the Netto vario?

The Netto vario represents the vertical speed of the air mass, the vertical component of the wind.

2 – How is the Netto vario calculated?

To obtain the Netto vario, the method is similar to the horizontal wind calculation : we compare the vertical speed in the air reference frame with the vertical speed in the ground reference frame.. 

The vertical speed in the ground reference frame is computed from altitude variations measured by GPS or a barometric sensor. 

The vertical speed air reference frame corresponds to the sink rate of the paraglider. It is typically between -1 m/s and -3 m/s depending on the flight regime (hands-up, accelerated, or brake input). However, it is very difficult to measure directly. On the Vector Vario, it is estimated using the speed polar combined with airspeed measurements .

3 – The speed polar

3.1 – Definition of the speed polar

The speed polar is the curve that characterizes the  aerodynamic performance of a paraglider in the air mass reference frame..
It describes how the sink rate degrades as a function of flight speed.

The speed polar of a paraglider can be schematized as follows:

We can distinguish 3 flight zones:

  • 1: the flight range from hands-up position to maximum speed (theoretically limited by front collapse, but in practice limited by the speed-bar travel). Maximum glide ratio is usually achieved with the arm raised. The more efficient the paraglider, the flatter this range is. In other words, its sink rate is only slightly degraded even when flying accelerated.
  • 2: the low-speed range when using brake inputs. The wing profile is modified by the camber of the trailing edge. Lift increases, which reduces speed, but profile drag increases significantly, which reduces glide ratio. On modern wings, the sink rate is broadly similar to the hands-up sink rate, before dropping sharply as stall is approached.
  • 3: the low-speed range when slowing down using rear risers. In this case the airfoil shape is not significantly altered, and a better sink rate can be achieved. However, since the profile is less cambered than with brake input, stall occurs at a higher airspeed.

3.2 – The Vector Vario speed polar model

Knowing the speed polar is somewhat the “holy grail” for paragliding pilots. It is extremely valuable information, but also quite difficult to obtain in practice. 

To simplify this task for pilots, the Vector Vario integrates a generic speed polar model, constrained by only four parameters:

  • the wing flat aspect ratio ;
  • the wing projected surface area ;
  • the total flying weight ;
  • the type of harness used.

The Vector Vario then uses the measured airspeed together with this theoretical speed polar to estimate the sink rate. This sink rate is then subtracted from the compensated vario to obtain the netto vario.

It is also possible to visualize the theoretical speed polar model of the Vector Vario using the Vector Vario Analyzer.

After loading an IGC+ file, the model can be displayed in the “Polar Analysis” tab. If the flight was performed in very calm conditions, it is possible to compare the measured data with the theoretical model and potentially adjust the model parameters. However, it should be noted that measuring an accurate speed polar remains a difficult task, even with the Vector Vario.

3.3 – How was the theoretical speed polar model of the Vector Vario determined?

The speed polar model used by the Vector Vario is based on extensive measurements carried out with the Vector Probe, across a wide range of different wings. 

The Vector Probe is a high-precision probe that enables direct measurement of glide ratio (accuracy of 0.1 glide ratio point) and airspeed (accuracy of 0.3 km/h). In a single flight, it is possible to fully characterize a paraglider and obtain its complete speed polar.

However, it is a demanding tool: it requires very precise calibration and must be mounted within the suspension lines. For these reasons, it is reserved for professionals, particularly for wing and harness development.

Below is a sample of measured speed polars obtained with the Vector Probe, which were used to build the model implemented in the Vector Vario.

4 – How is the Netto vario used on the Vector Vario?

The Netto vario provides information about the air mass that is independent of our flight regime.
This means it is possible to know that we are in a rising air mass even before we actually start climbing. 

4.1 – Direct display

The Netto vario is a variable transmitted via Bluetooth. It can therefore be displayed directly on a screen, for example in XCTrack.

4.2 – Audio option

As an option, the Netto vario can be converted into double or triple beeps, which indicate the behaviour of the air mass while descending:

  • a double beep indicates that we are in a rising air mass;
  • a triple beep indicates that we are in a sinking air mass.

The tone of the double and triple beeps follows the frequency curve programmed in the tone simulator (Configurator), allowing a smooth transition when entering positive lift conditions. 

For the Netto functions to work correctly, it is essential to enter the paraglider parameters used to generate the speed polar. These parameters must be configured directly in the Configurator:

The audio options can then be enabled by editing the audio profile:

These options allow (in order): 

  • activation of double beeps when flying in a rising air mass;
  • activation of triple beeps when flying in a sinking air mass;
  • activation of double and triple beeps when flying accelerated, when the vario detects speed-bar use. By default, these are disabled;
  • the activation threshold for double and triple beeps. By default, there is a silent zone when the Netto vario is below this threshold;
  • the cycle rate of the double and triple beeps. They do not follow the conventional vario cycle in order to avoid an overly aggressive audio signal. 

It is possible to simulate the Netto audio settings in the Configurator tone simulator. Once the Netto option is enabled, the activation zones for double and triple beeps appear at the bottom of the graph.

We can also simulate the audio behaviour when using the speed bar by moving the slider at the top of the graph: