They come ready-made, like objects chosen by Marcel Duchamp. They are actual transportation. A musician can actually ride in such a machine. They produce a sound more powerful than any synthesizer, with a deeper bass. And they actually fly.
Rotation is one thing, the texture of helicopter sound is another.
What is important in this case is a source of vibration that demonstrates an unequivocal continuum between pulsating rhythm and pulsatile sound and pitch. A helicopter produces a sound of such a wide range as to manifest both rhythm and pitch simultaneously, the engine generating the pitch component, the rotors the rhythm component, both locked together so that they rise and fall synchronously. A distinctive feature of the rotating sounds in Kontakte is the audible 'flap' as the sound passes from one speaker to the next, which sounds very like the beat of a helicopter rotor. Naturally Stockhausen would have preferred a nice smooth rotation without any discontinuity - as for example in the original 1941 Fantasia movie 'Waltz of the flowers' sequence. For sounds to move in space realistically it is not enough to vary the output from speaker to speaker, you have to incorporate phase changes in the spectrum of sound that convey movement relative to a listener, and this is a difficult task that Stockhausen comes back to again and again.