With the twist, a smaller angle of attack results at the tip than at the hub. By making the rotor blades as smooth as possible, the parasite drag reduces. Dirt, grease, or abrasions on the rotor blades cause increased drag, which decreases the lifting power of the helicopter. In formulas for lift and drag, the density of the air is an important factor. The mass or density of the air reacting in a downward direction causes the lift that supports the helicopter.
Density is dependent on two factors.
One factor is altitude, since density varies from a maximum at sea level to a minimum at high altitude. The other factor is atmospheric changes. Because of the atmospheric changes in temperature, pressure, or humidity, density of the air may be different, even at the same altitude. Although torque is not unique to helicopters, it does present some special problems. As the rotor turns in one direction, the fuselage rotates in the opposite direction.