On Monday, April 27, 2020 at 1:57:39 AM UTC-7, jld wrote:
Even if CS22 only requires approximately 1.5 m/s (~300 ft/mn) at sea level ISA, this is really not comfortable.
I get bearly 250 ft/mn when taking off from high altitude airport (5700ft) with the Ventus 2 CM and it is OK but not a pleasant experience! Especially in mountainous environment.

Anything below 400 ft/mn at sea level will result in uncomfortable performance during hot days and/or high altitude.

Concerning electric performance, the motor itself does not lapses with altitude. Therefore, at given rpm, the thrust loss is pretty much proportional to the effect of air density variation on the prop.

The rule of thumb you propose is very rough and would only approximate sea level static.

For climb performance you really need to model the prop and get an estimate of thrust at typical climb speed.
Because FES and GP have relatively small diameter props, they are highly loaded and the GP has an efficiency advantage with the 3 blades.
If you run the analysis, you find the GP15 should have a sea level ISA climb rate of ~500 ft/mn at 470 kg, which is pretty good since you are taking off at 60 kg/m2!

Another way to look at it is, how does it climb relative to a towplane? They are engine powered and suffer from altitude sickness as well. With a low drag electric, if you can out climb the towplane at high density altitude they you are relatively better off, regardless of the absolute numbers.

Last year I took two tows behind a Pawnee, climb rate to 1000 ft was 353 ft/m on one and 625 on the other, average 451. On the next 6 days I did self launches, averaging 573, the lowest was 439 and highest 796. This is all at Truckee, density altitude usually between 8800 and 9400 ft. A strong self launcher will beat the tow plane.