In general, however, helicopters will need to reach an rpm of around 400 to 500 in order to generate enough lift to take off. This rpm will increase as the helicopter gains altitude and the air becomes thinner.
As a pilot increases the collective lever, more lift is created by changing the pitch of the blades (increasing the angle of attack) and increasing the rotor speed. At the appropriate rotor rpm and pitch, sufficient lift is created to overcome the helicopter's weight and it will begin to ascend.
It is important to note that the rpm required for takeoff is just a starting point. Once the helicopter is in the air, the pilot will need to adjust the rpm in order to maintain a desired level of lift. This will be done by adjusting the collective lever and the cyclic stick.
By carefully controlling the rpm, the pilot can ensure that the helicopter has enough lift to stay in the air while also avoiding excessive drag. This is essential for safe and efficient helicopter flight.
