Airspeed Measurement

Everything you need to know

1. Vector Vario sensor type

The Vector Vario measures airspeed using a differential pressure sensor based on the same principle as a Pitot/Prandtl tube.

The Vector Vario is equipped with two ports measuring:

  • Total pressure through the port facing directly into the airflow 
  • Static pressure through the ports positioned perpendicular to the airflow.

A sensor measures the pressure difference between these two ports. The airspeed can then be derived by applying Bernoulli’s principle.

The different airspeeds

Il existe 4 types de mesures de vitesse

IAS – Indicated Airspeed

Indicated Airspeed (IAS) is derived from the pressure differential measured by the Pitot tube, assuming an air density corresponding to ICAO standard sea-level conditions.

For analyzing paraglider performance, IAS is the reference speed, as it does not depend on altitude or meteorological conditions.

CAS – Calibrated Airspeed

CAS corresponds to IAS corrected for errors caused by aerodynamic disturbances around the airspeed sensor.

With careful placement of the Pitot tube, CAS generally remains very close to IAS. This is the case with the Vector Vario mounting on the riser. However, residual disturbances may still exist (pilot’s arms, harness, etc.; see the following section).

The Vector Vario includes a dedicated correction that allows CAS to be estimated in certain scenarios. This speed is then used to determine the pilot’s position on the wing’s speed polar.

TAS – True Airspeed

TAS is obtained from CAS by correcting for the influence of the actual air density, which decreases with altitude.
Air density is calculated from static pressure, temperature, and humidity.

On the Vector Vario, TAS is used for compensated variometer calculations and for wind estimation.

Ground Speed

Ground speed is the pilot’s speed relative to the ground, measured directly by GPS. The difference between TAS and ground speed represents the influence of the wind.

The following diagram provides a simplified visualization of the different airspeeds and how they evolve with altitude.

Limitations of measurements close to the harness

In paragliding, accurately measuring airspeed is not as straightforward as it may seem. The main challenge arises when converting Indicated Airspeed (IAS) to Calibrated Airspeed (CAS). This correction requires a proper assessment of how the pilot’s aerodynamics and equipment influence the Vector Vario.

However, this influence is not universal—it varies from one pilot to another. It depends in particular on the type of harness used, the flying posture, the presence of a cockpit, and even the pilot’s flying technique.

The images below show a sample of flight configurations and the bias that each may introduce into the IAS measurement.

“Constant” disturbances caused by the harness:
a/ Open harness with a fairly upright position: The body presents a large frontal surface that strongly disturbs the airflow. The vario is usually located in an area where the airflow speed is reduced by about 10%. This configuration is also very sensitive to body movements during turns (harness support).
b/ Classic pod harness: The airflow around the vario is only slightly disturbed, and the measurement generally remains close to the true airspeed.
c/ High-performance harness: The airflow is slightly accelerated along the edges of the harness, which can increase the measured speed.

“Dynamic” disturbances caused by piloting:
d/ Bringing the arms in: This adds a lateral obstacle near the vario, locally accelerating the airflow (as seen in the simulation below). The measured speed can increase by about 10%.
e/ Keeping the arm along the riser: Depending on the alignment of the arm with the vario, the airflow can be either accelerated or slowed. Most often, it is accelerated, and the increase can reach 10%.
f/ When the vario is very close to the body: The body acts as a barrier, causing a strong slowdown of the airflow and thus a significant underestimation of airspeed.

To better understand the origin of these disturbances, the image below shows the results of a numerical simulation around a streamlined harness. It is a vertical cross-section illustrating the airflow speed variations near the pilot. 

 

The actual speed used in the calculation is 10 m/s (green areas). Variations of up to ±2 m/s (20%) are common (blue or red areas).

The placement of the Vector Vario on the riser is specifically designed to position the sensor in an area where the airflow is naturally minimally disturbed. The following figure shows a zoom in the horizontal plane, centered on the immediate environment of the vario:

In this configuration, the vario is located in an area where the speed is slightly underestimated (around –0.5 m/s in light blue, about 5% lower than the actual speed).

It is also noticeable that the influence of the arms extends over a fairly large area. Strong airflow accelerations appear on the sides (yellow areas), while a slowdown is visible at the front, partially extending up to the vario. A wake is also naturally present at the rear.

4. Sensor Calibration

The airspeed sensor has an overall stable offset over time. However, this offset can vary slightly with temperature or be altered if the device experiences a shock.

When the vario is not in flight, it continuously adjusts its offset. If the ambient temperature differs from previously encountered conditions, the device gradually updates its thermal compensation.

Note: At takeoff and landing, it is recommended to cover one side of the tube to facilitate this auto-calibration. This precaution is particularly useful in strong winds.

If a significant offset is detected, the app will automatically alert the user to perform a calibration in a well-controlled environment.

Calibration is also accessible at any time in the configurator, though normally this action is not necessary.

If you are seeking maximum accuracy and are flying in temperatures unusual for your device, it is possible to speed up the thermal calibration as follows:

  1. Turn on the vario.

  2. Place it in its storage case.

  3. Put the whole setup in the freezer for about 30 minutes.

  4. Remove it and then turn it off.

The calibration is fully automatic and does not generate any specific indicator.

Note: The Vector Vario Pro already includes factory thermal calibration between –20 °C and 40 °C.

5. Vector Vario Accuracy

To ensure reliable measurements even in highly turbulent conditions, the Vector Vario features an aerodynamic design specifically engineered to tolerate large variations in angle of attack.

The graph below illustrates the sensor’s response in a laminar wind tunnel flow at 10 m/s.

It shows remarkable measurement stability, maintained up to approximately ±40° angle of attack.

The airspeed calibration depends on two parameters:

Offset: This is a constant error in the differential pressure measurement.

  • Without thermal calibration: up to 10 Pa error (highly variable, depending on the temperature deviation from the initial calibration).

  • With thermal calibration: less than 2 Pa error.

Sensitivity :
This is the vario’s response to airflow, influenced by its geometry.

  • Vector Vario Pro units benefit from individual calibration, reducing the error to approximately 0.5%.

  • On standard versions, the uncertainty is below 2%.

The table below summarizes the uncertainties in km/h for different flight speeds (IAS). For comparison, the accuracy of the Vector Probe is also provided.

6. In-Flight Airspeed Correction

As previously explained, the airflow around the pilot is not homogeneous. These variations cause disturbances in the local static pressure measured by the Vector Vario. The image below illustrates these variations, showing the deviations in static pressure (in Pascals) relative to atmospheric pressure.
L’image suivante illustre ces variations, en représentant les écarts de pression statique (en pascals) par rapport à la pression atmosphérique.

Near the pilot, static pressure deviations can reach several tens of Pascals.

To address this issue, the Vector Vario uses a Kalman filter that fuses all sensor data to estimate the level of static pressure disturbance.
Currently, this correction is applied only during turning phases. This means that posture changes during a transition cannot be properly evaluated.

Improvements are underway to make this correction more robust and responsive.

A Note on the static pressure bias:
This bias also affects vertical speed estimation. Indeed, any change in airspeed produces an artificial variation in static pressure, which can be mistakenly interpreted by the barometer as a change in altitude.

This phenomenon affects all variometers, and depending on the sensor’s placement, it can be significant (for example, on a helmet). In general, a Kalman filter combining pressure and acceleration data helps reduce the effect of this disturbance.

On the Vector Vario, the estimated static pressure bias used for airspeed correction is also applied to correct vertical speed measurements.

7. Measuring Airspeed Accurately

Although the Vector Vario was not originally designed for high-precision airspeed measurements, it can still be used for this purpose if some precautions are taken. In practice, the corrected airspeed (CAS) provided by the Vector Vario is not sufficiently reliable for this type of analysis. Therefore, it is necessary to work from the Indicated Airspeed (IAS), keeping in mind that it is influenced by the pilot’s position and equipment.

Two main approaches can be considered:

  1. Relative measurements :

    This approach involves comparing measurements while ensuring the pilot’s position is exactly the same for each reading, particularly the position of the arms near the vario. The absolute value of the airspeed may be biased, but reliable relative comparisons are still possible. For example, one can determine whether a wing has gained or lost speed after a trim adjustment.

    In this context, it is recommended to use a pod harness or a performance-oriented harness, and to keep the arms as extended as possible during measurement to minimize the pilot’s influence on airflow around the sensor.

  2. Absolute measurements :

    This approach involves placing the vario in the clearest possible area to reduce the influence of the pilot and equipment on the measurement. Tests are currently underway to identify the most effective mounting solutions (for example, above the accelerator pulley).  
    Caution : Moving the vario on the risers can pose safety risks, particularly during takeoff. This solution should only be attempted by experienced users who are fully aware of the risks and capable of managing them.

Note : Dans le log du Vector Vario Pro, il y a deux colonnes  A0_cor_DP and A1_cor_DP : In the Vector Vario Pro log, there are two columns, A0_cor_DP and A1_cor_DP, calculated at the end of the flight. These represent an offset refinement depending on the sensor’s temperature. The residuals in the differential pressure can then be determined to improve airspeed measurement accuracy. The following relation is used:
DP_cor = DP + A0_cor_DP + A1_cor_DP * T_sensor
Then
IAS = sqrt(2/1.225*abs(DP_cor))/1.28
To allow this correction, it is recommended to briefly cover the Pitot tube at takeoff and landing.