[EB66]

> Is flying in a thin atmosphere harder?

Yep. Setting maths and motors aside... it's why helicopters have service ceilings and why no one has built a helicopter that can fly away into the stratosphere.

[londons_explore]

Isn't that just because a helicopter design that can fly in the stratosphere wouldn't be able to fly at sea level?

Just like the mars helicopter cannot fly on earth.