300ft above take off would be enough!
That would be enough altitude to successfully deploy a reserve parachute in the event of an asymmetric collapse which would greatly enhance safety. A friend threw his reserve last week below 200ft successfully.
The experiment my brother tried didn't result in any appreciable deviation from actual wind speeds from rotor turbulent and more important are Delta speeds. Temperature is extremely important which indicates inversion hence possible shear as I know after hitting extreme windshear at MTOW and nearly ended up in the Shwarzwald in a MD80.
I have a XCTRACKERMAX computer outputting vario/gps/altimeter/wind calculation with Flarm and a logger, it is the size of a box of swanvestas which uses accelerometers and GPS to calculate wind speed fairly accurately but needs an orbit for initial precision. Could always strap that onto a small drone but doesn't have temperature although some kit does.
Last edited by blind pew; 26th Sep 2021 at 21:18.