Uploaded by request.
Helicopter quartet. A string quartet recorded with each musician in a separate running helicopter mid air, with the noise of the engines and the blades. Not Stockhausen's best but does build up a tension, an anxious atmosphere. Might be a good starting point if you're looking into Stockhausen, it depends on where you're coming from, though I'd anyway recommend Elektronische Musik over this.
The Youtube clip above is from the film about Helikopter Quartett which I can more heartily recommend. I'm considering uploading an encode of it here later. Stay tuned.