Rotor X's quad-rotor eVTOL promises extreme efficiency and autorotation
Arizona's Rotor X wants to step up from being the world's biggest kit helicopter manufacturer and get into the eVTOL game, and to do so, it's put forth a design it claims is "dramatically more efficient and less expensive than all other eVTOL concepts being proposed or developed today." Its huge blades could also make it one of the safest eVTOLs in the sky, since they give it the capacity to autorotate in the case of motor failure.
Rotor X's design is called the RX eTransporter. It's a relatively simple quad-rotor multicopter, with a helicopter-like cabin that seats up to nine, including pilots, or carries up to 1,600 lb (726 kg) of cargo. These guys are not interested in the complexities of tilt-rotor design or the hover inefficiency of small-diameter rotors; this thing offers four of the biggest rotors you'll see in the eVTOL space, extended out from the cabin on long poles.
Where most transitioning vectored-thrust or lift-and-cruise eVTOLs rock a large wing for efficient forward flight, the eTransporter has a T-tail and a small top wing. This looks to us like a clever way of compensating for some of each rotor's retreating blade stall as airspeed increases, but this still won't be one of the faster air taxis in the sky. Cruise speed is listed at 140 mph (225 km/h), with a max speed "over 160 mph" (257 km/h) – vectored thrust designs are aiming at more like 200 mph (322 km/h).
This is the opposite approach to the ultra-small rotor concept used by Lilium, and its advantages, disadvantages and mission profiles will tend toward the other end of the spectrum. The Lilium jet is horribly inefficient in a hover, but highly efficient on the wing, so Lilium is looking to position itself as an inter-city longer-range transport service.
The eTransporter, on the other hand, will be among the most efficient eVTOLs on the market in a hover – indeed, Rotor X says it'll be able to hover on the spot for more than 45 minutes if necessary on a single charge. Moving through the air at speed will develop enough lift from the small wings and the body design to double its endurance figure to more than 1.5 hours, and the company is claiming a max range up to 230 miles (370 km) running on battery power. That's an incredibly impressive figure for a straight-up multicopter, and a testament to just how efficiently larger rotors like this can produce lift.
There are some possible downsides to this approach. One could be blade tip noise, which could prove a limiting factor for a given eVTOL's ability to operate in densely populated spaces. That said, Rotor X president Don Shaw tells us that the company is currently working on quiet rotor technology, and expects this design to make less whooshy downwash than small-rotor competitors. Another is footprint, although Shaw says the eTransporter's four wide arms will fold in to make it easier to garage.
And then there's redundancy. Lose one of the rotors on a typical quadcopter and you've got yourself a one-way ticket to tumble town. According to Shaw, however, each of the eTransporter's rotors is powered by multiple electric motors, allowing multiple failures before you lose a single prop. And even if you do completely lose power to a rotor, these large blades will also make the eTransporter one of the very few eVTOL market entrants capable of autorotation, a process in which an unpowered rotor can be spun up by the ambient air as the aircraft descends, effectively allowing the pilot to use a dead rotor a bit like a parachute, making for a controlled and cushioned landing.
In forward flight, Shaw says the total failure of a single rotor will be even less of an issue; the pilot will be able to choose to continue the flight or land, whatever's safest. "The eTransporter," he says, "will be one of the safest means of transportation by far in the eVTOL air taxi market."
Still, it'll be interesting to see how the quad-rotor configuration goes when it comes to certification, even if the rest of the design is a lot less complex than most of the bigger players in this field; redundancy upon redundancy is the mantra with which other eVTOLs hope to rock the regulators (and their investors) to sleep.
Either way, an aircraft tailored toward super-efficient hovering could make a lot of sense for quick cross-town transport, deliveries and industrial operations. Rotor X says a number of mining companies, for example, are "already showing serious interest" in pilot-optional cargo lift versions of this thing.
The company is working on this project with Advanced Tactics (AT), makers of the Black Knight VTOL "flying truck" for the US Military. AT will be developing a military version of this aircraft as part of the US Air Forces AFWERX/Agility Prime programs. There will also be a longer-range, heavier-lift version built using either a fully combustion-fueled or hybrid powertrain.
Rotor X says it expects to start testing an experimental-class prototype in Alaska next year. It's expecting commercial FAA air taxi certification by 2024.
Editor's note: This article was amended on 29 June, 2021, to include additional information provided by the Rotor X president Don Shaw.
Source: Rotor X