I assume its a function of altitude? Not much air at 60, 000 feet. Although the same goes for the propeller I guess. The altitude would certainly be where the e

I assume its a function of altitude? Not much air at 60, 000 feet. Although the same goes for the propeller I guess. The altitude would certainly be where the efficency comes from, but it's such a simple solution, there must be very good reasons commercial don't already fly that high.