https://spaceflightnow.com/2018/05/14/helicopter-to-accompan...
> She said the size of the rotors was restricted to 1.2 meters by the space available on the Mars 2020 rover’s belly pan. In turn, the 1.2-meter rotor diameter limits the mass of the helicopter to 4 pounds — 1.8 kilograms — based on the rarefied density of the Martian atmosphere.
> “The biggest thing is the atmospheric density at Mars is very thin,” Aung said in an interview Friday. “It’s 1 percent of that at Earth. It’s equivalent to about 100,000 feet in altitude here on Earth. The other difference is the gravity at Mars is lower. It’s about 40 percent. That plays into how much we can lift.”
> The rotors on the Mars Helicopter will spin between 2,400 and 2,900 rpm, about 10 times faster than a helicopter flying in Earth’s atmosphere.