The Natural Philosopher wrote:

The arc drawn looks clearly DC...

You sure? If it were DC the helicopter would just charge up to a
constant voltage and the arc would cease - no further charge to
transfer, unless there's corona discharge from the rotor tips, or unless
it's very ripply DC (unlikely with polyphase rectification).

That arc seems fairly sustained, as you'd expect with AC , and a 120 Hz
buzzing seems to accompany it.

--
Andy