In forward flight, the rotor system is subject to various forms of differential loading. Imagine a rotor system where the tips of the blades rotate at 300 km/h relative to still air. When that helicopter is hovering, the blades see the same 300 km/h relative wind throughout their rotation. However, when the helicopter starts to move forward its speed is added to the speed of the blades as they advance towards the front of the aircraft, and subtracted as they retreat. For instance, if the helicopter is flying forward at 100 km/h, the advancing blades see 300 + 100 km/h = 400 km/h, and for the retreating ones its 300 – 100 km/h = 200 km/h.