Originally Posted by
Flying Bull
Hi
I guess, its Bell approach to incorporate a fenestron type tail rotor - without infringing Airbus patents.
Unfortunately, with fixed blades and variable speeds there might be some high pitch noises - which the original fenestron had also - but was overcome by the unequal spacing of the blades.
I´m also not sure, how well speed changes will cope witch quick demands of power?
And - with different speeds of the fans there will be different speeds of the airflow - while with a "conventional" fenestron/tailrotor (roughly) only the amount of air is altered....e.
Actually, most of Airbus’s Fenstrom patents expired over a decade ago. this includes the unequal blade spacing. That’s why you see some small light sport helicopters with them.
NASA published some articles on electric distributed propulsion and the noise reduction benefits a few years back. In one article they described how operating multiple fans at different speeds reduces the apparent noise by spreading the frequency tones of the noise. Equivalent to playing a cord with multiple soft notes on a piano versus one loud note. I believe this is what Bell is trying to do with their EDAT.
Noise also increases with rotor tip speed, even in a duct. With electric motors driving fans, when thrust is not needed the fans can be reduced in speed, or even stopped. Not something that can be done with a conventional gear driven rotor.
As far as control response, as the Drive article notes, this is the reason for using 4 fans. Reducing individual fan inertia using multiple fans allows for rapid speed changes. If you watch some of the large quad drones maneuver, you will see this concept is already in use.