We know that drone when flying forward with the help of momentum can be more efficient that hover. Then there is a sweet spot where the speed which the drone is moving is the most efficient, not too fast and not too slow.
The cruising speed.
Any good idea to test this out other than theoretically calculate from motor spec?
Interesting question. I would first do a couple of free flight runs with various speeds so you can check in the flight logs how “slow”, “not so slow”, “fast”, “max” etc. translates into actual m/s values. Then I’d do a first test series with slow - mid - fast speeds, meaning: Three separate flights, all starting with 100% battery, all flying the same autopilot route. Say your drone’s max speed is 20m/s, do one run at 6m/s, one at 12, and one at 18. After each run retrieve the remaining battery charge and the flight time from the logs. Let’s say the 12m/s run was the most efficient one, then you can do another two tests at 10m/s and 14m/s and so on, until you hit the sweet spot.