Originally Posted by
B2N2
This would be an ideal cruise altitude but in reality many more factors need to be considered.
This is all true. What an electrically powered aircraft do is to remove the constraint of combustion performance on the selection of the most efficient altitude. Where a traditional jet engine core might exhibit some efficiency loss as air density (and partial pressure of oxygen) are reduced, I think other trade-offs dictate ideal cruise altitudes. Engines can be specified to cover the altitude envelope that the aerodynamics require without much problem. Given this, it doesn't matter whether the fan or prop is spun by a gas turbine or electric motor.
High altitude engines have been designed and built for planes like the U-2, Concorde and other special cases. The aerodynamics run into efficiency limitations before the powerplants do. One major input into the overall efficiency equation is fuel weight. Jet fuel burns off over the course of a flight, reducing the lift required and changing the most efficient altitude. Electric airplanes will be carrying the same load of batteries throughout a flight (aside from some heretofore unassumed fuel cell technology).