Sunday, January 2, 2022

Wind measurement - Observing earth boundary layer while landing

 With XCVario, several speeds are measured :

  • True AirSpeed (TAS) can be reconstructed from dynamic pressure & temperature measurements.
  • GPS measurement (from FLARM at this stage) gives direct access to Ground Speed (GS)
 --> Difference between both TAS & GS is then an estimate of instantaneous wind in direction of flight.

As an illustration for this, the difference between TAS & GS is plotted during final landing phase, representing earth boundary layer above the airfield.

A few observations can be made

  • On the 8th of may :
    • Both landings show a reclined boundary layer profile, with weaker wind at landing altitude than @ 20m above ground
    • On the 2nd landing, wind is stronger & deeper present on the ground
    • Gust content is rather low for both landings
  • On the 13th of september in St Auban :
    • Wind is present almost down to landing altitude, with little reduction of wind component before the last 5m of altitude
    • @t~10-12sec, a sensible gust can be observed ~70m above landing altitude, with a "gap" of TAS not visible on GS, and roll activity.
    • Another gust is visible @t~30sec, 10m above ground
    • A further gust is visible during ground roll -that has been filtered out of wind speed vertical profile


Landing on the 8th of may 2021 against "vent d'Autan" with no lift



Landing again on the 8th of may 2021 against "vent d'Autan" after a flight in weak distorded lift




Landing on the 13th of september 2021 in St Auban