Personal air vehicles

In this article, we discuss properties and technical challenges associated with personal air vehicles (PAVs). Specifically, we focus on electric, "next generation" aircraft designed for urban air mobility (UAM). An example PAV made by Volocopter is shown below.

What do we mean by PAV? There’s no precise definition, but a PAV should be usable in many urban environments. It should be able to fly people from home to work with safety, affordability, speed and community acceptance. These requirements give rise to many concepts and technical challenges listed below.

• Safety
  • Control / piloting
  • Weather
  • Collision Avoidance / Traffic Management
  • Cybersecurity
• Affordability
  • Control / piloting
  • Energy consumption
  • Maintenance
  • Ride sharing
• Community acceptance
  • Noise
  • Pollution
  • Infrastructure costs

At the time of this writing, no certified aircraft fits the bill. The closest match—a small helicopter—can perform many of the required tasks. However, a helicopter is too costly, noisy and pollutive for widespread urban use.

Safety

Safety is a primary goal for any vehicle. Organizations like the FAA establish safety standards for air vehicles. The standards will evolve as PAVs approach maturity, but should ensure these vehicles are safer than current automobiles, requiring less than 1 death per 100 million miles travelled. Some points regarding safety follow.

Piloting / Control

One of the first questions when considering safety is what or who will control the vehicle. This also plays a role in affordability—the average citizen can’t afford an expert/dedicated human pilot to fly them to work. Personal air vehicles will likely be either (1) automatically piloted by software (autonomous) and/or (2) easily controllable, so that essentially anyone can fly it safely after minimal training (like a regular car).

Autonomy is the preferred method. An autonomous PAV has many advantages, some of which are listed below.

• The driver is free to do other things—work or play—while commuting.
• In some cases, a person is freed from even being in the aircraft. For example, a busy parent doesn’t need to drive their kid to school or soccer practice.
• Ability to bring your car to you. Got a flat tire on your bike 10 miles from home? No problem, click a button to have your PAV pick you up.
• No need for human training, pilot licenses, eye tests, ….
• When research reveals maneuvers that are dangerous or lead to high maintenance costs, a simple software update could prevent further use of such maneuvers (avoiding the need to retrain all human pilots).

Substantial control system software development will likely be needed whether or not the vehicle is autonomous. The per-user cost of such software is small when millions of people use it. Similarly, the safety of this software should improve with the number of users. Each of us will benefit from lessons learned as millions of other people use the software.

The reliance on software for control comes with a major drawback—cybersecurity.

Cybersecurity

As established in the prior section, software will play a role in PAV control. This software will communicate with external entities to receive updates (improvements, bug fixes), coordinate traffic, and/or provide navigation features. Hence, it will be vulnerable to cyberattacks. A hacker could insert a deadly flaw in the control system or override navigation commands, causing vehicle collisions. Cybersecurity will be an extremely important component of vehicle safety.

Weather

Current personal aircraft are more dangerous to fly in poor weather. In particular, low visibility and fog are associated with many accidents, including the crash that killed Kobe Bryant.

Owing to more responsive, automated controls and connectedness to weather networks, it’s likely that PAVs will handle these threats better than current personal aircraft. However, even autonomous aircraft will not be immune to weather. Like human eyes, cameras, LIDAR and (to a lesser extent) RADAR systems are less useful in fog, rain and snow. Since these technologies may be used for obstacle avoidance and landing, inclement weather will continue to be a challenge. PAVs may still be grounded in certain weather conditions, and may have to fly longer routes to avoid storms.

Noise

Traditional aircraft generate too much noise to fly near neighborhoods with frequency. If a single helicopter flying over a neighborhood can generate a dozen complaints, imagine what would happen if multiple such aircraft were travelling by at all times of the day and night.

Noise pollution is not just an annoyance. Studies indicate that it harms reading comprehension in children and causes cardiovascular disease in adults, among other things. Clearly, strict standards will be required, on top of existing regulations.

While many anticipate that noise levels will be constrained below 65dB (here's a guide to noise levels), the exact criteria could be complicated. This is because the amount that noise annoys us (and causes health problems) is complex. Simply limiting the overall sound pressure level (SPL) would be too naive. For example, the CEO of Joby Aviation said they intended to mimic natural sounds. Traditional aircraft noise, even if reduced to the same SPL as wind or ocean noise, seems to be more annoying and more problematic for health.

Noise constraints may be one of the toughest challenges for PAVs. After all, noise reduction has almost always been a goal in aircraft design, yet we are far from obtaining the required noise levels. The hope is that other technological advances may help.

Electric rotors and advances in battery energy density are enabling radically different, multirotor aircraft designs. Research indicates that this will facilitate greater control of noise, but it’s not yet clear if this will be enough.

Pollution

In addition to being too loud, current helicopters pollute too much to be used by a substantial portion of the population. PAVs need a dramatic reduction in CO2 emission to be widely used. Let’s take a look at some numbers.

The EPA estimates that the average car emits about 400 grams of CO2 per mile. Electric cars emit about 122 grams of CO2 per mile (while the car doesn’t emit directly, the electric grid emits this CO2 to charge the car). Presumably the standard for future PAVs will be under 150 grams per mile, as the world aims to get greener.

According to Bell Helicopter, its 4-passenger 505 model consumes about 740 grams of fuel per mile (4 MPG) while flying 100kts (85kg/hr at 100kts= 115mph). The EESI estimates that 3.16 grams of CO2 are emitted per gram of jet fuel consumed. (This may sound strange that emitted CO2 mass exceeds fuel mass, but realize that each atom of carbon in the fuel binds to 2 atoms of oxygen in the air to create CO2 emissions.) Thus, this small helicopter emits about 2340 grams per mile (20x more than an electric car).

To make matters worse, other factors have driven PAV designs toward multiple, small rotors/propellers. When hovering, taking off and landing, such designs are fundamentally less energy efficient than the single, large rotor in conventional helicopters. They consume more energy to provide the same amount of propulsion. Hence, even more drastic reductions are needed elsewhere to compensate for this loss of efficiency.

Of course, personal air vehicles will probably not use jet engines. You may wonder what the numbers would look like for a PAV powered by lithium-ion batteries, if such a thing is feasible. We’ve provided a CO2 emission calculator to answer this question. With seemingly reasonable assumptions, we estimate well over 500 grams of CO2 per mile—over 3x the target. Of course, this number is proportional to the CO2 emissions (per unit energy) of the energy source that charges the batteries. As coal plants are replaced with cleaner sources on electric grids, these emissions will reduce proportionally.

If you’re curious about the assumptions behind these calculations, here are some other sources. This article says electric motors (cars here) use 60% of grid energy for propulsion. This article indicates a Unfortunately, aircraft will realize an additional loss due to propeller efficiency—approximately 15% of the power delivered to the propeller won't provide propulsive power.

Speed

Although traditional cars can reach speeds well over 100mph, they almost never do. In many congested areas, the average driving speed to get from A to B is under 10mph, slower than a horse-drawn carriage! To me, the fact that travel times have not improved over the last 50+ years is a major disappointment.

Of course, PAVs should easily surpass the point-to-point speed of traditional cars. With the ability to escape the earth's surface and fly at different altitudes, cross traffic including the resulting stop signs and stop lights would not be needed. Highway congestion would be obsolete as well.

Depending on the takeoff/landing mechanism and traffic management techniques, there may be bottlenecks when many people arrive at a certain location around the same time (e.g. downtown skyscrapers at 8AM). However, this too may be avoided if a traffic management system is able to coordinate flights to arrive at the destination at slightly offset times. Fortunately, even if traffic jams occur, autopilot technology could free riders to focus on other things (work or play) rather than stress about it.

Affordability

Like many technologies, affordability may be the last goal achieved. The earliest designs will probably be too expensive for most of us. Some companies may specifically follow Elon Musk's master plan for Tesla, where cash flow from early expensive models funds the development of more affordable models later.

Energy consumption

Of course, energy costs money. The average new car today consumes a gallon of gas in about 25 miles (25 MPG). If a gallon is priced at 3 USD, then energy costs for an average new car are about 0.12 USD per mile or 1,620 USD per year for the average driver (13,500 miles per year).

With seemingly reasonable assumptions used in our electric aircraft range calculator, a PAV could consume about 1.55 kWh per mile. For someone that pays 0.10 USD per kWh, that's 0.155 USD per mile or 2,092 USD per year (again assuming 13,500 miles travelled per year). However, realize that hovering and climbing consume substantially more energy.

Other considerations

Ride sharing. It's possible that PAVs would work better as a subscription service. Rather than owning a vehicle that sits parked over 90% of the time, PAVs may be shared and brought to you on demand. Of course, autonomy makes such a model more attractive, eliminating the need for a chauffeur to bring a vehicle to you.

Infrastructure. The USA alone spends over 150 billion USD per year on roads. Even so, most citizens would argue that the quality of roads is decreasing, traffic is increasing, and that substantially more money is needed just to maintain current conditions. Personal air vehicles have the potential to substantially reduce road construction and maintenance, saving taxpayers trillions over the coming years.

Real Designs

Historical Aircraft

Flying cars are not new, of course. Many designs were proposed, built and even flown in the 20th century.

The Fulton Airphibian was certified by the CAA (predecessor to the FAA) in 1950. The wing and aft fuselage section detached, allowing it to transform from a typical fixed-wing airplane to a car. It could fly 2 people 350 miles at 120 mph. The concept failed to receive significant investment and was abandoned for financial reasons.

Several Taylor Aerocars were built in the mid 20th century. Like the Airphibian, these could be converted between a car and airplane mode. The Aerocar received civil certification in 1956, but also failed to receive significant investments. Ling-Temco-Vought offered a serial production deal if Taylor could obtain 500 orders, but he obtained less than 300.

Now let's look at some modern designs for mass air transit.

Volocopter VoloCity

This image is copyright protected by Volocopter. All rights are reserved including copying, distribution and other use.

Volocopter’s VoloCity may become the first operational electric air taxi. The VoloCity appears to have the simplest design among its competitors, which should reduce certification time and overall time to operation. The primary mission of the VoloCity appears to be short-distance transport in congested areas, e.g. from a major city center to the closest airport. Volocopter aims to operate an air taxi service around this aircraft—they do not plan to sell these aircraft to end users. Let’s look at some of the technical details.

The VoloCity has 18 fixed-pitch, variable-speed rotors, each with a diameter of 2.3 meters. Each rotor is powered by its own electric motor. The motors receive energy from nine lithium-ion battery packs, which can reportedly be swapped out in 5 minutes.

The aircraft is expected to carry two passengers with a maximum payload of 200 kg (900 kg max total takeoff mass). They expect to carry this load up to 35 km at up to 110 km/h. Volocopter claims the smaller rotor tip speeds (relative to a conventional helicopter) spread noise over a broad frequency spectrum and state the VoloCity emits just 65 db(A) hovering at 75 meters.

Volocopter plans for the VoloCity to be autonomous, and indeed that will be required to carry two passengers (a pilot would take one of the two seats).

Joby S4

Image provided by Joby Aviation.

The Joby S4 (generation 2) is another aircraft in development which satisfies many of the PAV goals. The S4 is more ambitious and complicated than the VoloCity. It's intended to carry 4 passengers and a pilot 240 km at 320 km/h, purely on battery power. These specs give rise to a more complicated design with tilting rotors and retractable landing gear which may delay market entrance and increase costs. Let’s discuss this aircraft in more detail.

The aircraft reportedly stores 200 kWh of energy in its four isolated, redundant lithium-nickel-cobalt-manganese-oxide battery packs. The battery cells hold about 300 Wh/kg, and the battery pack overall reportedly holds 235 Wh/kg. (Some experts question this—known aviation battery packs are not this efficient.)

The batteries will power six electric motors and rotors—four in the main wing and two on an aft V-tail. All six rotors tilt. The rotors on the tip of the main wing and tail rotate along with the nacelle and motor, but the two inner rotors on the main wing use a “linkage mechanism” to tilt while the motor remains fixed. The aircraft aims to have an overall lift to drag ratio over 15, and includes a retractable tricycle landing gear. While great for high-speed and long-range flight, complications with tilting rotors, potentially more novel battery packs, and retractable landing gear could make certification and maintenance difficult.

All rotors reportedly run at tip speeds under 113 m/s. This is much lower than conventional helicopters (a Robinson R44 operates around 214 m/s tip speed) and presumably plays a large role in noise reduction. Joby claims the S4 will be 100x quieter than a comparable helicopter.

Joby expects to meet all this criteria while carrying up to 450 kg of payload, with a max total takeoff mass around 2180 kg. Like Volocopter, Joby aims to operate these aircraft in an air taxi network rather than sell units to end users.

CityAirbus NextGen

This image is copyright protected by Airbus. All rights are reserved including copying, distribution and other use.

The last aircraft we comment on here is the NextGen from CityAirbus. The NextGen is somewhere between the VoloCity and the S4 in terms of speed and range. Like Volocopter, they’ve opted to avoid any moving surfaces and tilting rotors which will likely simplify operation, control, and certification. Yet, like the S4, the design lends itself to higher speed flight (in addition to vertical takeoff and landing) carrying four passengers. Let’s look at the details.

The NextGen aims to fly 80 km at 120 km/h. It’s powered by eight four-bladed rotors, each with its own electric motor. Two are located on the leading edge of the main wing, four on the trailing edge of the main wing and two atop the aft v-tail. Although Airbus has commented that the NextGen will use existing lithium-ion batteries, we’ve not been able to get further details. Perhaps they plan to remain flexible as battery technology develops?

Like their competitors, CityAirbus claims impressive noise reduction relative to traditional helicopters. The NextGen reportedly will emit less than 65dB(A) when flying overhead and less than 70 dB(A) landing. The aircraft will be piloted for now, but CityAirbus aims to replace the pilot with software at some point. The aircraft will have a carbon fiber composite fuselage with four landing struts and plans to fly by 2023.