Groundspeed vs Airspeed

To safely fly in high winds, it is important to understand the difference between the groundspeed and the airspeed to enable in-flight monitoring of the wind speed at altitude.  

The airspeed is the speed that the drone is ‘seeing’, so the speed of the air which is moving over the drones wings, measured by the Pitot tube.

The groundspeed is the speed that the drone is moving across the ground, so is essentially the survey speed, and is measured by differential GPS measurements.

The effect that the wind has on the difference between these values is shown above. You can see that if the drone is travelling at a constant airspeed of 60km/h, but the wind is directly against the drone at 20km/h, the resulting groundspeed that Marlyn is surveying across the ground is going to be the subtraction of those, so 40km/h. Likewise if the situation is reversed, and the wind is behind the drone, and the airspeed remains 60km/h, but the ground speed is 80km/h. If there was no wind whatsoever the airspeed and groundspeed would be the same.

In is important to note that if you understand the difference and meaning of these two, you can actually get a very accurate reading of the wind speed in flight when the drone is on its directly upwind and directly downwind legs.    

Warning Signs when Monitoring Marlyn’s Telemetry and Flight

During the take-off, transition, and mapping procedure, Marlyn will be on full automatic mode, so the main action for the user is to diligently monitor the flight telemetry.

Despite all prior preparations, it is still possible that in flight Marlyn encounters winds which are above the limits. Therefore, it is important to monitor for the following signs that Marlyn is flying above the wind limits and bring the drone home.    

Warning Signs: Helicopter Mode

If any of these are observed by the pilot;

  • Marlyn appears to be struggling to maintain its position above the home point, or is being blown off course.
  • Marlyn appears to ‘lean’ into the wind at an angle greater than 45°.

It is recommended to use the Manual Flight option to abort the climbing procedure and begin the descent, and if necessary take manual control if it appears that Marlyn cannot hold its position automatically, further information on this is given in the upcoming section ‘Landing in High Winds’.  

Warning Signs: Airplane Mode

If any of these are observed by the pilot, it is recommended to use the ‘Go Home’ function to immediately begin Marlyn’s RTH and landing sequence.

  • When flying upwind, the observed wind speed (Marlyn’s Airspeed - Groundspeed) is greater than 45km/h. (Refer to diagram)
  • Marlyn is unable to maintain a groundspeed above 20km/h when flying into the wind.