Ever since the first controlled flight airplane by the Wright brothers in 1903, and the first practical powered helicopter flight by Igor Sikorsky in 1939, aeronautical engineers have striven to design a composite vehicle featuring all the advantages of each with none of their respective disadvantages. The "magic vehicle" advantages included high speed and economical efficiency, paired with the ability to takeoff and land vertically from virtually any available parcel of real estate. While not perfect, the modern tiltrotor aircraft comes closest to realizing this hypothetical ideal. In helicopter mode, with their rotor rotational axes perpendicular to the ground, they takeoff and land like a helicopter. When their rotor axis is turned parallel to the ground, they fly like a turboprop airplane. The sticky design wicket is maintaining controlled flight while transitioning between these two modes, since numerous aerodynamic forces, gyroscopic effects, and mass moments are all changing simultaneously, and often with destabilizing effects.
The fastest conventional helicopters have a maximum cruise speed usually in the neighborhood of 175 ktas, while the fastest twin turboprop conventional airplanes can typically sustain between 300 and 400 ktas. The civilian AW-609 tiltrotor features a 275 ktas cruise, while the Bell V-280 is designed to cruise at 280 ktas. If vertical takeoff and landing are a mission requirement, the 100 knot speed advantage of a tiltrotor over a conventional helicopter becomes very advantageous, even though the tiltrotor is not as fast as a pure airplane of similar power.
Early tiltrotors, like the military V-22 Osprey and civilian Leonardo AW-609, featured tilting nacelles, in which the entire engine-rotor-nacelle assembly rotated together to transition between helicopter and airplane modes. Given the large mass of the engines and the gyroscopic effects of the high-speed turbomachinery contained within, significant center-of-gravity shifts occur while rotating the nacelles, and substantial gyroscopic moments are induced. New generation tiltrotors--like the military Bell V-280--tilt just the rotors during flight mode transition, with the engines and nacelles remaining fixed in place. This design scheme reduces the center of gravity shifts during flight mode transitions and simplifies the flight control mixing required to deal with gyroscopic effects. Applying my personal corollary to Murphy's Law--"simplicity makes everything better"--the V-280 design approach probably improves safety too.
The complexity of tiltrotors--which for all intents and purposes, are almost all fly-by-wire air vehicles--means that they will be slow to comprise a significant portion of the General Aviation fleet. However, as a point-to-point business aircraft they are hard to beat, so there will likely be a significant commercial market developing in the coming years.