Ordinarily on a helicopter, the main (top) rotor is mechanically linked to the tail rotor, so the spinning of the former causes the latter to do likewise. An experimental new system replaces the tail rotor with four electric fans, however, potentially making for both safer and quieter flight.
First of all, why do helicopters have tail rotors?
Well, they mainly serve as an anti-torque measure, preventing the aircraft from twirling around in response to the spinning of the main rotor. Additionally, by moving the tail to one side or the other, they determine the direction in which the front of the helicopter faces. They always spin at the same time and the same rate as the main rotor, with the amount of thrust they produce being varied by adjusting the angle of their blades.
Tail rotors are a potential hazard, though – people periodically get lethally injured when they walk into their whirling blades, while the copter is idling on the ground. Additionally, because they're always spinning at high speed, those rotors contribute significantly to the overall operational sound of the helicopter.
That's where the Electrically Distributed Anti-Torque (or EDAT) system comes in.
Currently being developed by the Bell helicopter company, it does away with the tail rotor, substituting four ducted variable-RPM fans. While these are electrically powered by a main-rotor-located generator, they operate independently of that rotor – their thrust is determined not by the angle of their blades, but simply by how fast they're spinning.
For one thing, this means that they don't need to spin at all when the aircraft is securely held in place by its own weight, while sitting on the ground. Additionally, once the chopper takes off, they only spin as fast as is needed in order to produce the necessary amount of thrust, producing less noise overall.
In fact, thanks to a vertical fin on the EDAT that serves as an airfoil, the fans can be shut off entirely while the helicopter is in fast forward flight – the rapid air flow on either side of the fin holds the tail in place all on its own.
And as an added bonus, should one or more of the fans fail for some reason, the thrust of the others can temporarily be adjusted in order to compensate. By contrast, there's no such redundancy possible with a single tail rotor. That said, the main reason that the new system incorporates multiple small fans lies in the fact they're highly responsive, as they can each change speed much quicker than a single large fan.
According to Bell's Program Director of Light Aircraft, Eric Sinusas, the company is still in the process of determining how EDAT compares to tail rotors in regards to weight, cost and fuel usage. He tells us that the system is not, however, as mechanically complex.
"It certainly has less moving mechanical components," he says. "A conventional tail rotor, not only does it have a drive shaft and gear boxes and couplings and bearings and all those components, but it's also changing the pitch of the blades dynamically, while they're rotating. Those inputs go all the way back to the pilot's pedals, through a set of control tubes."
The single existing EDAT demonstrator helicopter made its first flight last May, and can be seen in action in the following video. Sinusas states that the company is encouraged by the results of flight tests conducted so far, and hopes to ultimately integrate the system into a commercial product.
Source: Bell via Popular Science