A tilt-rotor is an aircraft which can hover like an helicopter while cruise like an airplane. The first applicable tilt-rotor is the Bell-NASA XV-15. A tilt-rotor can lift-off ground vertically and hover like a helicopter, however, tilt-rotors have no inherent draw-backs like helicopters.
Noise is a major problem of helicopters. The noise is produced by the high tip speed of the tail-rotor asnd the mini sonic boom of the main blades. Helicopters are also limited in the forward flight speed. Because the relative velocity of the air is different in the advancing blade and retreating blade. The difference is increasing with the flight speed. This results a rolling tendency of the helicopter. The other problem of helicopters is low efficiency in hovering. The main baldes of helicopter are usually traded off between hove and forward flight, so the blade twist and tapper are not optimized for hovering.
The tail-rotor is disappeared in tilt-rotors. Because the counter-rotate rotors cancel out the torque to each other. The rotors are rotating in a relatively low speed than helicopter's rotor, so the noise problem is reduced. The top speed of the tilt-rotor can reach 300 knots. The blades are optimized for the axial direction flight; that means they have optimized efficiency in both hovering and forward flight.
The tilt-rotor have prominent future in the role of military use, civil transportation, fire fighting and rescue. The tilt-rotor will make a revolution in the sky of the 21th Centry!
