Model Forum / Radio Controlled / Helicopters / August 2008

FWIW, while I haven't flown a helicopter at altitude I have flown
electric planes at almost 9,000 feet. Other than a noticeable increase
in groundspeed there wasn't much difference. The electric motor develops
just as much power at high altitude as it does at lower ones. If the
rotor head speed is governed then you might have to increase collective
pitch to compensate for the thinner air. And/or, increase the head speed
by 10 to 15 %.

Here is an interesting article about Electric R/C at high altitudes.
While it doesn't address helicopters specifically, it does show that
there is not as much difference as most people think and that electric
motors don't lose power like IC engines do.
http://www.stefanv.com/rcstuff/qf200101.html