eVTOL - Electric VTOL / Advanced Air Mobility (AAM)
The Air Force launched Agility Prime, a non-traditional program seeking to accelerate the commercial market for advanced air mobility vehicles (ie, "flying cars"). Leveraging unique testing resources and revenue generating government use cases for distributed logistics and disaster response, the government plans to mitigate current commercial market and regulatory risks. Agility Prime also aims to bring together industry, investor, and government communities to establish safety and security standards while accelerating commercialization of this revolutionary technology. The Innovative Capabilities Opening established a rapid contracting mechanism beginning in 2020 with a “Race to Certification” series to drive government procurement of operational capability by 2023.
The NASA Aeronautics Research Institute (NARI) is a part of NASA’s Aeronautics Research Mission Directorate (ARMD). NARI is supporting NASA’s Advanced Air Mobility (AAM) mission to promote the development of a strong and resilient AAM supply chain that can scale as the market matures. NARI and the US Air Force’s Agility Prime initiative are collaborating to better understand and develop the AAM supply chain.
| Area of Interest | AOI 1 | AOI 2 |
| Payload (equivalent) | 3-8 personnel | 1-2 persons |
| Range | Greater than 100 miles | Greater than 10 miles |
| Speed | Greater than 100 mph | Greater than 45 mph |
| Endurance | Greater than 60 minutes | Greater than 15 minutes |
| First Full-Scale Flight Prior to | 17 Dec 2020 | 17 Dec 2020 |
These vehicles are not drones, helicopters, airplanes, cars, trucks, motorcycles, SUVs, and some would adamantly say they are not flying cars. However, they might support similar missions. They could act as an organic resupply bus for disaster relief teams, an operational readiness bus for improved aircraft availability, and an open requirements bus for a growing diversity of missions. They could enable distributed logistics, sustainment, and maneuver, with particular utility in medical evacuation, firefighting, civil and military disaster relief, installation and border security, search and rescue, and humanitarian operations.
There is a growing demand for electric powered transportation. Due to the current limitations of rechargeable batteries (energy density -- 220 Wh/Kg-300 Wh/Kg, depth of discharge, charge/discharge rates, and cycle-life issues), the general sequence of battery-powered vehicles' market entry (discounting slow short-range vehicles such as golf carts) is as follows. Automobiles are the easiest vehicles to adopt electric power. Automobiles can accept heavy batteries, there is a relatively low battery drain rate, and operation can be safely stopped with depletion of the battery. Powered sailplane powerplant used for launching an otherwise safe glider. Fixed-wing training aircraft are useful for flights of short duration, operated from established airports with professional instructors, maintenance, and management. Privately owned fixed-wing have a rolling take-off and landing with a high wingborne lift to drag ratio.
Electric VTOL (eVTOL) are more challenging due the high power required for hover, especially if high-speed efficient cruise is also required. eVTOL has become even more challenging as the market shift from "specialized transport" aircraft making shorter trips (25-60 miles) between well-equipped terminals, to "urban mobility" aircraft making longer trips with at least one poorly equipped landing spot.
The need is exemplified by information published by Uber of a conceptual image of an upcoming urban transportation market for Uber's proposed hybrid-electric vertical takeoff and landing (eVTOL) aircraft. Established in 2016, Uber Elevate played an important role in laying the groundwork for the aerial ridesharing market by bringing together regulators, civic leaders, real estate developers, and technology companies around a shared vision for the future of air travel. Their software tools enabling market selection, demand simulation, and multi-modal operations are at the center of their work, and form the basis of this future-focused deal.
On 08 December 2020 Uber offloaded its air taxi enterprise Elevate to Joby Aviation, the last of several moonshots to be sold by the ride-hailing company in a pursuit to stick to its core business and reach profitability. Joby Aviation, a transportation company developing an all-electric, vertical take-off and landing passenger aircraft, which it intends to operate as early as 2023, announced that Uber Technologies had agreed to invest a further $75 million in Joby as part of a broader transaction involving the acquisition of Uber Elevate by Joby and an expanded partnership between the two parent companies. This investment comes in addition to a previously undisclosed $50 million investment made as part of Joby’s Series C financing round in January 2020. Uber’s new $75M investment brought its all-time total investment in Joby to $125 million and Joby Aviation’s total funding, including previous rounds, to $820 million. Under the terms of this week’s deal, Joby Aviation acquired Uber Elevate, while the two parent companies agreed to integrate their respective services.
NASA's Aeronautics Research Mission Directorate (ARMD) laid out a Strategic Implementation Plan for aeronautical research aimed at the next 25 years and beyond. The documentation includes a set of Strategic Thrusts that are research areas that NASA will invest in and guide. It encompasses a broad range of technologies to meet future needs of the aviation community, the Nation, and the world for safe, efficient, flexible, and environmentally sustainable air transportation. Furthermore, the convergence of various technologies will also enable highly integrated electric air vehicles to be operated in domestic or international airspace. In response to the recently updated Strategic Thrust #4 (Safe, Quiet, and Affordable Vertical Lift Air Vehicles), a new subtopic titled “Full-Scale (2+ Passenger) Electric Vertical Takeoff and Landing (eVTOL) Scaling, Performance, Aerodynamics, and Acoustics Investigations” was introduced in 2021.
All-electric and hybrid-electric aerial vehicles, typically driven by multiple rotors/propellers have become increasingly popular over the past decade. Such aerial vehicles, however, are prone to generate noise; a primary source of which are the rotors themselves. There are many existing strategies for reducing the propeller noise, a majority of which address the rotor blade design itself. The noise coming off of a rotor can roughly be broken down into two main types of noise -- Broadband and Periodic noise. Broadband noise includes vortex noise and turbulence-induced noise, while Periodic noise includes rotational noise and interaction/distortion effects. The tonal noise is heavily determined as a function of the propeller blade count and the revolutions per minute (RPM) at which the propeller blade is spinning (for a given operating point). Having multiple rotors operating at the same operating point has numerous negative effects on the perception of noise -- one being constructive interference of the noise signatures that creates higher sound pressure level (SPL) peaks in the tonal noise components.
Safety and efficiency are perhaps the two most critical factors for developing eVTOL aircraft to satisfy this market. To achieve high safety, much of the prior art is focusing on aircraft that use six, eight, or even more, independently operated rotors. If any single rotor fails in such aircraft, the other rotors are likely to be capable of making a safe landing. Even quad rotor aircraft are not considered to be particularly fault-tolerant, because failure of a single rotor can crash the aircraft.
Over 100 companies are pursuing various alternative designs. Known solutions, however, suffer the disadvantages of the mismatched and sometimes competing requirements for VTOL and cruise operation. Aircraft typically must carry heavy batteries and extra motors that are only used for VTOL operations. These systems produce high levels of drag during horizontal, fixed wing flight cruise operations due to the use of vertical rotors and their accompanying support and control structures. These mismatched requirements for VTOL and cruise reduce the speed and range of these alternatives. Consequently, most designs do not come close to their published values for speed and range in operation and are limited in scope and mission.
A limiting factor for traditional VTOL designs is that take-off/landing is a high-power event. Current battery systems typically do not have high enough power to discharge energy rapidly enough, particularly at lower energy levels (state of charge), resulting in an effective energy capacity of approximately 50% the theoretical value in flight.
Several proposed and prototype aircraft are being designed using this many-rotor strategy. Examples include the 16-rotor Volocopter, 8-rotor Ehang, and 8-rotor CityAirbus. All these designs are, however, problematic because the rotors do not tilt from vertical lift to forward propulsion positions, and there are no wings. That combination is extremely inefficient in forward flight, which limits the aircraft to relatively short ranges.
Some eVTOL aircraft are being developed that continue to use the many-rotor strategy, but add a wing to improve forward flight efficiency. For example, with the 36-rotor Lilium eVTOL, the rotors tilt about the forward and aft wings. The manufacturer claims a 300 km range, and 300 km/hr speed. This aircraft is, however, still problematic because the high disc loading results in low power loading (high installed power per weight), which reduces efficiency and range, and produces high noise levels.
Instead of having the rotors tilt about the wings, it is possible to have the rotors disposed in fixed position with respect to the wings, and tilt the wings. An example of that strategy is the 8-rotor Airbus A3 Vahana. This aircraft is problematic because it trades off higher efficiency in forward flight for very high power requirements during transition from vertical lift to forward flight. In such transition, the wings act as huge airbrakes.
It is also possible to have the rotors tilt about one or more fixed wings. The Joby 6-rotor eVTOL concept aircraft resolves some of the problems cited above, but the use of many-rotor strategy means the rotors are relatively small. This necessarily means high disc loading, which results in low power loading (high installed power per weight) and high noise level.
Another solution that the prior art seems to have contemplated is to separate the vertical lift rotors from the forward propulsion rotors/propellers. The idea is that use of different lift and cruise propulsion systems allows each system to be is optimized for its particular function. The Aurora eVTOL concept uses eight lifting rotors and an aft facing propeller. This design is problematic because the duplicate propulsion systems require heavier and more expensive hardware, have marginal climb rate in wing borne cruise due to sizing the cruise powerplant for level cruise, and potentially have a smaller wingborne stall speed margin--gust entry and recovery, due to design optimization for higher cruise lift coefficient (smaller wing). Example: if flying at 130 mph, a vertical gust of 20 Ft/sec will increase the angle of attack by 6 degrees, may stall a small wing at its efficient lift coefficient of 0.9, but not stall a bigger wing at CL=0.5.
The similar design of the Terrafugia eVTOL has the drawbacks mentioned above with respect to duplicate propulsion systems, and in addition, the twin tilt rotor configuration does not provide a method for pitch control in rotor borne flight.
Motor installations in the prior art are also directed towards small rotors having small torque requirements. For example, the motor installation of the Airbus A3 Vahana is arranged in a direct-drive configuration where motor and propeller spin at the same rotational speed. This simple propulsion system solution is problematic for large rotors with large torque requirements.
Because of the physics involved, it is relatively straightforward to design a many-rotor eVTOL that carry a small payload (less than 500 pounds) over short distances. For larger payload weights and commercially desirable ranges the strategy of using many rotors becomes increasingly problematic. Using a larger number of smaller rotors provides less disc area than fewer big rotors, requires more power per aircraft weight, is difficult to hover at low noise because the lower total rotor disc area results in higher blade tip Mach numbers, or in larger number of wide-chord blades, or both, and makes autorotation flight after loss of power more dangerous, because the autorotation descent rate increases in proportion to the square root of rotor disc loading and recovery from high descent rate is risky.
Using a small number of bigger rotors (two, three or four) could solve some of the problems discussed above, but that approach is completely contrary to the prevailing wisdom. Among other things, the characteristics needed to optimize vertical lift are very different from the characteristics needed to optimize forward flight. Still further, these problems cannot adequately be resolved by having separate lift and forward propulsion systems. In addition, those of ordinary skill in the art would dismiss the idea of having fewer rotors on the grounds that doing so would unacceptably sacrifice safety in the event of motor failure of any of the rotors, and would introduce unacceptable inefficiencies for an eVTOL.
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