Testing high altitude flying is very important to us, therefore we have tested drone in up to 10.25km altitude (33628ft in America).
Efficiency testing results are the following:
In 1km altitude: 22,4A at 14,8V = 331,5W for 13,1m/s climbrate
efficiency: 25,3W / m/s
In 5km altitude: 23,6A at 14,3V = 337,5W for 12,3m/s climbrate
efficiency: 27,4W / m/s
In 8km altitude: 22,5A at 14,1V = 317,25W for 6,3m/s climbrate
efficiency: 50,4W / m/s
In 10km altitude: 20,2A at 13,0V = 262,6W for 3,5m/s climbrate
efficiency: 75,0W / m/s
As you can see, efficiency drops exponentially as the altitude increases.
Efficiency testing results are the following:
In 1km altitude: 22,4A at 14,8V = 331,5W for 13,1m/s climbrate
efficiency: 25,3W / m/s
In 5km altitude: 23,6A at 14,3V = 337,5W for 12,3m/s climbrate
efficiency: 27,4W / m/s
In 8km altitude: 22,5A at 14,1V = 317,25W for 6,3m/s climbrate
efficiency: 50,4W / m/s
In 10km altitude: 20,2A at 13,0V = 262,6W for 3,5m/s climbrate
efficiency: 75,0W / m/s
As you can see, efficiency drops exponentially as the altitude increases.