I have an answer in regards to the TH noise and recording. I recorded the seaside above the Pacific near Muir Beach from cliff height. The ambient recording of the waves, birds, bugs, and even wind is pretty great. I then flew on the hillside about 100 yards above where I set the tascam recorder. I then flew around the area and purposely flew around and over the recorder. I did this at varying heights based on usefulness of shot. At a medium height, where subjects would be visible at your typical 50' above, the noise of the H was a hum that had the sound of a prop plane engine going over head.
This did not intrude into the ambience of the recording. If you did not not know it's a drone, you would mistake it for any motor sound like a passing car or passing plane.
But you can't make out mouth movement from 50' so if you get down to the level you need to be for that, the H noise obliterates the ambient. I often turn on the mic on the CGO and record the engines and wind during flight. I got used to it with the GoPro. I like hearing the motor rev changes especially that awesome pause when you stop a fast ascent. I'm very familiar with the angry bee sound of the drone and that's what you get plenty of on your microphones. The more sensitive the mic the worse it will be. Since most field recording is done with condensers that's always going to be the case.
As for a silent prop driven craft....You would have to have rotors spinning inversely of each other to stop air movement (sound) which I believe violates the principles of flight. Anything that displaces air produces sound. You can reduce and reflect and absorb but you cannot stop unless it's an anechoic chamber. You could in theory deign a speaker array that plays back the self noise of the drone phase flipped (giant noise canceling headphones) and make it almost silent but the drivers would weigh more than the craft. The miracle of hearing is all about air molecules moving so anything that displaces air makes sound - even that tree no one hears fall. For sure the word drone is appropriate for the sound produced and picked up.