Hang in There: Why Hovering Matters in eVTOL Design
At the recent Uber Elevate conference, several high-profile architecture firms presented the latest conceptual designs for Uber skyports, the urban terminals that will handle thousands of passengers arriving from the suburbs each day. While these transportation mega-hubs will be specifically designed to meet the broad needs of Uber across all of its transportation modes, realistically, the crucial need for a network of air taxi takeoff and landing locations will require the use of existing airports as an essential element in this new era of electric aviation.
There are over 5,000 public use airports in the US, and more than 14,000 private airports. Many of the smaller regional airports are located close to where people live, work, and go for recreation. It suggests a multi-modal journey, with ground transportation covering the first mile, air taxi services for the middle segment, and ground transportation for the last mile. Provided that time is not wasted at the transition points, the overall journey time can be much reduced compared with all forms of pure ground transportation.
Unlike the situation in which a single service provider controls all of the landing locations on a vertically integrated basis, operating eVTOLs from existing airports and heliports will pose a hovering challenge that Uber, for example, may not believe it has. First, in an airport setting, helicopters and future eVTOLs will often be treated like airplanes by air traffic controllers. On the ground, helicopter pilots are often asked to taxi like airplanes, and take off over runways like airplanes. Unless an eVTOL has wheels with differential braking, behaving like an aircraft could require hovering for an indeterminate number of minutes before take-off. On the terminating end of a flight, an eVTOL may encounter an emergency situation where the airport landing pads are occupied, forcing it either to land in a temporary location, or hover until ground personnel locate a free spot, eating into whatever battery reserve remains.
eVTOL designs with high-disk-loading typically consume battery power at least four times faster in hover compared to forward flight. This is a serious consideration using existing Lithium Ion batteries. The weight fraction of batteries to gross aircraft weight is usually in the range of 30% to 40%. Going through the math, an eVTOL with disk-loading of 20 lbs/ft^2 will completely drain its batteries after hovering for 15 minutes. By 30 lbs/ft^2, the hovering endurance drops to 12 minutes, and by 50 lbs/ft^2 it’s only 10 minutes. An eVTOL with 50 lbs/ft^2 disk-loading, for example, cannot hover for even two minutes longer than initially intended, consuming a 20% greater fraction of the battery capacity and pushing the endurance of the aircraft to potentially unsafe levels.
The judgement of whether a flight can proceed, considering fuel reserves, is specifically dictated by aviation regulations. In the US, air taxi services operate under either 14 CFR Part 91 or Part 135 rules. For aircraft certified as airplanes (e.g. Part 23), the reserve requirement under both Part 91.151 and Part 135.209 is 30 minutes during daylight operations, and 45 minutes at night. For rotorcraft (e.g. Part 27), the reserve requirement is 20 minutes for both day and night. Many of the airframe companies have stated their intent to certify new tilt-rotor eVTOLs under Part 23, and yet most current designs have less than 45 minutes of total flight time on a full charge. Without significant improvements in battery capacity, the FAA will almost certainly need to readdress the reserve requirement for eVTOLs.
Urban air mobility flights are shorter than traditional airplane flights, though the ratio of eVTOL hover power-to-cruise power is often 4:1 or greater, whereas helicopters operate at about 1.2:1. Discussing minutes of flight time isn’t particularly meaningful for eVTOLs, given the large disparity between hover and cruise power. Perhaps the new requirement should be at least 25% battery capacity at the end of the flight, considering the state of charge after hovering from the takeoff location? Even with a hypothetical 25% reserve requirement, it means that a 50 lb/ft^2 eVTOL will have only 2.5 minutes of unintended hovering at the destination before hitting zero battery. For eVTOLs with high-disk-loading, the margin of error is therefore quite slim.
What are the arguments in favor of the high-disk-loading designs, which many believe are the right answer? One advantage is certainly low drag in forward flight, enabling cruise speeds of 180kts or more. Reducing the time of urban air taxi trips improves the economics and makes the experience more valuable to passengers. In reality, however, I question how viable speeds greater than 120kts will be in urban areas, considering the need to share the sky with traditional helicopters cruising at modest speeds. Consider for example the industry discussion about increasing the number of VFR corridors through Class B and Class C airspace around major airports. The carved-out “tunnels” eliminate the need for helicopter and eVTOL pilots to request and wait for clearance through otherwise controlled airspace. This allows for increased air traffic, reduced delays, and reduced air-traffic controller workload. However, the VFR corridors, which will be critical for urban air mobility, will almost certainly be shared with legacy helicopters like the Bell 206, Eurocopter EC-120B, and Robinson R44. All of these popular aircraft have cruise speeds of 110kts. Sharing VFR corridors creates a situation much like being on a highway with bumper-to-bumper traffic. If there is no room to pass, then you’ll only be going as fast as the slowest aircraft in the VFR corridor. In fact, the FAA recognizes the increased risk of mid-air collisions in congested areas, for example in Los Angeles, imposing a 140kts speed limit through the LAX VFR corridors.
Another argument in favor of the high-disk-loading eVTOL designs is the smaller footprint compared with helicopters, in an attempt to deliver the “flying car” that we’ve been promised for so many years. If an eVTOL is small enough, and quiet enough, it’s argued, it will find much greater adoption in urban settings. Again though, I don’t believe this argument holds up to inspection. First of all, high-disk-loading designs are inherently loud. If we as an industry cannot reduce the noise of eVTOLs by 20dB relative to helicopters, there won’t be community acceptance of eVTOLS or air taxi services. Second, the numerous skyport designs presented to date show TLOF (Touchdown and Lift-Off) and FATO (Final Approach and Take-Off) areas comparable to existing heliports, scaled for the largest aircraft envisioned to utilize the facility. This is a logical design tradeoff to maximize the value of the new skyport real estate. If existing heliports and future skyports must have room for a Sikorsky-style helicopter with a 35-foot rotor, what’s the point of trying to shrink an eVTOL to fit within a fraction of this size, given the detrimental noise and efficiency tradeoffs?
The compromise in battery capacity, and consequently flight endurance, is another consideration with high-disk-loading designs. Typical Lithium Ion batteries have a maximum discharge rate of “2C,” meaning that the battery can be fully discharged in 30 minutes. Several manufacturers make their highest capacity batteries with the maximum 2C discharge rate, while offering a higher-power version, albeit with lower energy density, for performance critical applications. The high-power batteries allow a discharge rate of 10C or greater, and are popular for use in drones and electric vehicle racing. While the 2C-rated batteries have an energy density of about 120Wh/lb., the 10C-rated batteries top-out at about 100Wh/lb. Translated to the eVTOL application, a disk-loading of less than 5 lbs/ft^2 allows the use of 120Wh/lb batteries, while high-disk-loading eVTOLs will unfortunately use the 100Wh/lb batteries. The compromise in battery energy density for high-disk loading eVTOLs is an additional impact to hovering endurance, beyond the efficiency issue of moving air at a high velocity.
Battery safety is yet another concern with high-disk loading designs. It’s well understood that Lithium Ion batteries can undergo thermal runaway when operated at high temperatures. Electric aircraft battery modules will certainly employ cooling techniques, such as a pumped glycol loop, to maintain the cell temperature well below the danger zone. However, if the cooling system were to fail, batteries operating at a 10C discharge rate heat much more quickly than batteries operating at a 2C rate. The rate of temperature rise may be so fast for the 10C aircraft, that once a failure has been detected, the high-disk-loading eVTOLs are at an increased risk of not landing quickly enough to avoid battery combustion.
A myth persists that low-disk-loading translates to a low lift-to-drag ratio at respectable cruise velocities. While it’s true that traditional helicopters have an L/D of about 5:1 at 110kts cruise speeds, this doesn’t necessarily translate to the efficiency we can expect from low-disk-loading eVTOLs. Consider, for example, the opportunity for compounding. Adding one or more electric motors for thrust, separate from the electric motors for lift, allows for an eVTOL design that reduces the drag from the lift rotors, while aligning the fuselage with forward flight. Another design opportunity is using the connecting structures from the fuselage to the rotors as augmented lifting surfaces. If half the lift comes from the “wings,” and half the lift comes from the rotors, for example, the need for airplane-like control surfaces can be avoided, while the L/D can grow to a respectable 9:1.
In summary, eVTOLs with low-disk-loading have several key advantages relative to those with high-disk-loading, including flight endurance, reserve safety, battery safety, and reduced noise. The advantages of high-disk-loading designs are clearly speed and size, though the importance of each may be less clear in real world applications. Considering the expanded range of missions made possible with eVTOLs that can both hover well and fly well, it’s time for eVTOL engineers to creatively design new aircraft to improve hovering efficiency without materially impairing the forward flight performance.
