Helicopters use a large main rotor for both hover, and for forward flight. Helicopters are very good at vertical take off and landing and hover, however, in forward flight they are slower and less efficient as well as less safe than aircraft, for a variety of reasons:

  1. Helicopters do not benefit from the 'nearly free'  lift that the wing of an aircraft generates as it moves through the air at speed. In contrast, helicopters (and other pure rotor craft, such a multicopters) have to actively create lift with their rotors which costs a lot of power. In addition they have to push themselves forward. Overall it is about twice as energy intensive for a rotorcraft to stay aloft than it is for an airplane.
  2. The large rotor causes drag while in forward flight, meaning less speed (typically only up to 150 mph / 250 kph) and more fuel consumption.
  3. The large rotor is subject to asymmetric forces while in forward flight because the air hitting the blade advancing into it, hits this blade with larger speed than the blade which is moving away from the oncoming air. This produces a larger lift on the advancing blade than on the retreating blade, limiting speed, increasing drag and stressing the machine.
  4. The constant high powered rotational motion and pitch changes of the main rotor are taxing on materials and machines, promoting failure and requiring more maintenance than needed for an airplane.