Flying Cars: Challenges In Design And Implementation

Whenever people find out that I’m involved in aerospace research, I get asked the question “Hey what do you think about flying cars?”. Invariably, my drawn out utterance of the word “Wellllllllll” serves as a precursor to me depressing the entire group with copious servings of aerodynamic reality.

Most of the current flying car designs are from startup companies with no history or focus in full-scale aviation. This isn’t necessarily a problem as many times a novel solution is discovered by firms outside of a given industry. Several companies have went with the “Let’s scale up a quad/octo/dodecacopter and stick a person inside it” method. Some others have taken the Star Wars pod racer look and gave it a more utilitarian slant. I will admit, several of these designs look pretty awesome. But unfortunately, the laws of physics don’t respond to awesome. It appears that many of the designs were made without consideration of aerodynamics, human factors, the atmosphere or the realities of being in a small flying vehicle.

Established aerospace companies such as Boeing, Cessna and Bell should theoretically be all over the flying car craze. Bell, as a large helicopter manufacturer is well versed in the techniques associated with vertical takeoff and landing. Cessna not only makes some of the fastest business jets available, but they are responsible for the best selling light aircraft type in history, the Cessna 172 Skyhawk. And as if you didn’t know, Boeing dominates the US commercial aircraft industry.

So far Bell has teamed with Uber to help accelerate the technological leaps required to enable the development of point-to-point, electric VTOL vehicles. This doesn’t mean they’re working on a prototype, it just means they’re helping guide a company that has little to no experience with anything that flies. Boeing for their part has had a few limited projects in the past to design flying cars but were canceled before ever getting to a flying prototype. Finally Cessna is not involved at all with any type of flying car construction or collaboration (at least not publicly).

What could be some of the reasons for wide-eyed startups to be chomping at the bit to build these vehicles while grizzled industry veterans keep a wary distance? Could it be that they’ve seen some things that the new companies haven’t yet? Do Bell, Boeing and Cessna have the thousand-regulation stare? In this piece, I’ll highlight some of the issues facing flying car designers and why it will probably take a lot longer than the public thinks to make them a safe and widespread mode of travel (notice I didn’t say impossible…just a longer wait).

Automation

There is a misconception today amongst the non-aviation public that all modern airplanes are flown by computers and the pilots just sit there eating chocolate cake and watching Top Gun on their tablets. The reality is that pilots still fly the airplane, there’s just a computer that stands as a gatekeeper to ensure that the aircraft stays within limitations. Even still there are regions of flight where human skill is faster and more accurate than the computers. Autopilots/FMS still have to have flight plans loaded in advance and these plans invariably change multiple times per flight. Pilots still monitor and adjust systems during flight. They regularly deal with failed systems, changing weather conditions, rerouting by ATC and a multitude of other tasks. The automation is there to help the pilots focus on things that the automation can’t do.

Yet when reading articles on flying cars, the answer to everything seems to be automation. How will these flying cars operate in congested cities with skyscrapers all around them? Automation. How will the vehicle transition from vertical to horizontal flight? Automation. What will happen if an engine fails or a blade gets thrown? Automation. No detailed discussion of thrust vectoring or ducted fans or control surfaces. No mentioning if the omnipotent automation will activate systems via pushrods, cables, hydraulics or electric servos. Just automation. This shows a disconnect with actual flying vehicle design and a mere concept.

In order for automation to work, it needs something to work with. And once it has something to work with, it has to know the static and dynamic stability margins for the vehicle to ensure it remains flyable (better hope the occupants understand weight and balance). It also has to know where all other vehicles are. And all obstructions. And all restricted airspace. And considering that the passenger-not-pilot won’t have any control, the automation also has to identify and handle any conceivable emergency situations. Given the amount of effort being put into self-driving cars that only operate in 2 dimensions, a 3 dimensional activity like flying will require automation that is going to be unlike anything ever coded.

Airspace

A few questions about how this system will integrate with the National Airspace System are probably in order: Will this system work with the upcoming ADS-B mandate in 2020 that most aircraft in the United States will have to conform to? Will there be an international standard to allow sales in other countries? Will the system communicate to local air traffic control? What about operations near airports? Will flying cars be banned within X miles of a runway, or will they be allowed to funnel into the airport parking lot via strictly monitored ingress routes? Will air traffic control need to monitor and separate commercial, private and military traffic from flying cars? What about military training routes? What about prohibited areas? How will flying cars integrate with news and medivac helicopters? What about sightseeing…at some point people are going to want to see the Grand Canyon, Niagara Falls and Yosemite from their own flying car. Will people on the ground have to hear the constant drone of, well, drones? What about Washington D.C.? The entire nation’s capital has a permanent TFR which, short of some landmark change in legislation would rule out any flying cars within its vast boundaries. The same issue arises at football and baseball games since they too are ensconced in TFRs during games. In all honesty, even if every technical issue is surmounted, just figuring out integration and getting FAA approval for the plan is going to take the wind out of the sails of all but the hardiest of companies.

Weather

So far most of the animations show the flying cars operating in clear skies or silhouetted against a fiery sunset. While it looks really nice, it is not the full story of where these vehicles are going to be operated. They have to consider that people are going to expect that they can jump in their flying car and head home even if it starts raining, snowing or blowing 30mph out of the northwest. Are these vehicles going to be certified for known ice? The fact that even the largest commercial airliners have to be serviced with de-icing fluid before takeoff demonstrates the seriousness of the threat. After takeoff, hot bleed air from their jet engines can be routed along the leading edge of the wing to melt ice off. Just as critical is the engine anti-ice (different than de-ice) system that keeps ice from collecting on the nacelle lip. Needless to say, a chunk of ice down the inlet of a jet engine spinning at several thousand RPM is not an ideal situation so they take steps to prevent it from happening in the first place.

Now imagine a small flying car with uninformed passengers riding in it on a cold night. They have no knowledge of the environment in which they’re in, no ability to affect the flight and no way to save themselves other than perhaps a land-now button or a ballistic recovery parachute. A flying car encountering ice will rapidly collect it on the windshield, flying surfaces and engine nacelles, robbing the rotors of thrust, disrupting smooth airflow over the wings and weighing the aircraft down. Since these aircraft are intended to be electric, its only means of protection is to draw a lot of current to heat the critical surfaces, or carry the extra weight of a fluid known as TKS that is pumped out of tiny holes to retard ice accumulation.

There are other weather threats besides ice. Most of the designs presented are not touting the use of aviation aluminum as the primary construction material. Thus we can assume that a composite such as carbon fiber will be used instead. While very strong for its weight, carbon fiber does not react well with electricity. In the event of lightning striking a flying car…and it will happen…the manufacturers must prove that no strike will damage the structure beyond airworthiness limits and just as importantly, that it will not disable any of the automatic flight systems. Exhaustive testing and shielding will be required before any design is certified. Some existing aircraft use lighting diverter bars on composite parts to give the current a path to follow. It is then able to follow these routes and discharge through static wicks rather than blow holes in structure. All parts of the airplane must have a conductive path to these wicks in order for them to function properly. Again these are the realities of the atmosphere in which we fly and why airplanes cost so much to design.

Passenger Comfort

I make it a point to ask people who have mentioned wanting to have a flying car if they have ever flown in a small plane before. The answer is invariably “No.” This is very telling as the fantasy of flight is sometimes more attractive than the reality. However, most of those same people have flown in commercial aircraft before and remember experiencing turbulence of some kind.

Turbulence can range from gentle rocking to quite literally bouncing people off the ceiling. Mind you, these are heavy aircraft with high wing loadings. They have a lot of inertia and they can still be thrown around like ragdolls in the right conditions. Small aircraft by obvious virtue of being lighter are affected to a greater degree than large aircraft. Imagine the effect of 5 foot waves on a cruise ship and then picture an inflatable raft in the same size waves and you get an idea of the large plane vs small plane dilemma.

I have personally been bounced around rather brusquely in small aircraft before. Friends of mine have hit their heads on the ceilings while wearing their seatbelt due to the severity of turbulence they’ve encountered. Since flying cars are going to be lighter than many general aviation aircraft, this does not bode well for the dyspeptic among travelers. Hot air, strong winds, wind tunnel effects near skyscrapers, outflow from rain showers, and even the wake from other vehicles can all be nauseating for the uninitiated. While manufacturers will probably try to say that computerized flight controls will smooth out the bumps, that’s going to be limited by the vehicle’s lack of inertia. All I can say is these flying cars better have a really good air conditioning system and a place to store used barf bags.

Preflight and General Safety

Before any aircraft takes to the sky, the flightcrew will perform a preflight inspection commonly referred to as a walkaround. This activity is standard for every airplane from Piper Cub all the way up to Airbus A380s. During this inspection, they are looking over the condition of the aircraft, verifying fluid levels and quantities and ensuring that there are no blatantly obvious deformations. So far, flying car concepts have intimated that a user just hops in, pushes a few buttons and flies away. While it may seem like a yeah-so-what detail, this has very important ramifications for who will actually have their butts in these machines. Are the sensors that detect buildings and other aircraft obscured by dirt or dead insects? Are the thrust vectoring vanes able to move freely? Did some idiot ram a shopping cart into my rear stabilizer?

Since a person can’t just pull over if there’s a problem, and more critically, there may not be time to land or deploy a parachute if certain items fail, inspecting a flying car before flight will be just as important as it is for real aircraft. This will require teaching non-pilots the importance of ensuring the airworthiness of their vehicle before every flight. Somehow I have a feeling that there will be a lot of lip service paid but very little attention. The allure of jumping in and flying away is just too strong.

As for general safety, some designs have serious issues with the placement of certain components. For example, when I see a 2 seat quadcopter with unprotected blades, I see a lawsuit because someone walked into a running rotor and was disfigured or killed. Therefore, regardless of the number or orientation of the blades, they would have to be recessed into ducts in order to keep people from getting maimed when entering, exiting or walking around the aircraft. This would also protect the blades from damage in the event of a bird strike (assuming none got sucked down into the duct) and allow for thrust vectoring without tilting the entire motor/rotor combination.

There should be a battery firewall feature since the risk of thermal runaway while remote, is still a possibility. Someone once suggested to me that the offending battery be jettisonable. I then mentioned that the people below who get crushed/burned by a 500lb lithium ion battery falling out of the sky would be pretty upset if they survived and he agreed the idea wouldn’t be socially acceptable.

Likewise, a land-now feature and ballistic recovery parachutes should be standard. If a passenger is not going to be allowed to be a pilot, they should at least have a way to save their own life if an emergency develops. With that in mind, occupant enclosures should have sufficient bracing and structural integrity to protect people in the event of a rollover or hard landing.

Building a flying car to a reduced power level is another good idea to help in the event of an engine failure (assuming multiple engines). Setting MTOW to correlate to 80% of total engine thrust gives the vehicle a margin in case of engine failure. If a powerplant takes an early retirement, the remaining engines can be brought up to 100%. This should be enough to help balance the reduction in power and prevent airborne rollover from the sudden loss of thrust in a given quadrant.

So those are some of the off-the-top-of-my-head observations about flying car design (we didn’t even bother talking about the motors and the energy density of chemical fuels versus batteries as that’s enough for another article). Mind you, these are considerations that anyone who builds an aircraft must incorporate into their design before cutting metal. Some people will complain and say “Well if you know so much why don’t you build one!”. To which I’ll respond “How do you know I’m not?”

In any case, I write this because I love aviation. I grew up involved in aviation and what I see is a collision course of people who have great imaginations and some wonderful ideas, but not enough grounding (punintentional) to know why some of those ideas are not wonderful. If flying cars are going to be commonplace and accepted, they have to be built right. People are not going to tolerate these things taking them to the wrong destinations, to say nothing of what would happen if they start falling out of the sky due to underestimating the laws of aerodynamics.

Next Generation Reentry

A friend of mine and I were talking about the Columbia accident and how a similar situation could be avoided in the future. He said that any new spacecraft should go back to capsules with ablative heat shields that splash down in the ocean beneath a trio of parachutes. His reasoning is that the design is structurally simple, there is a minor ability to control splashdown point through roll maneuvers to adjust entry angle, and the heat shields tend to be more robust than the tiles used on the Space Shuttle.

Of course, I then took the liberty of reminding him of his occupation and the fact that any real pilot would rather fly a spacecraft back to earth rather than dangle beneath nylon chutes. Emotional connection noted, he alerted me that every ounce of aerodynamic concession built into a spacecraft is an ounce not dedicated to the primary mission of being in space. What was I going to do? Lie and say that a wing is useful in an airless vacuum?

After a few more hours of going back and forth with the pros and cons of wings vs capsules, we agreed that there is no single solution that can satisfy all the possible needs of spaceflight. Imagine having one type of automobile to serve all transportation purposes from going to the supermarket to hauling freight cross country. It’d either be really useful for one job and horrible at the others, or mediocre at everything.

Humans are pretty good with building capsules so there’s not much for us to do with regards to their design. Spaceplanes on the other hand have a lot of room for experimentation. The fact that they have wings is the single greatest advantage over a capsule with regards to crew safety, comfort and airframe longevity. After Columbia, many people, (self proclaimed experts and otherwise) said that spaceplanes are not as useful, not as safe and not as efficient as capsules. These statements were reactionary and misguided. One cannot make a blanket conclusion about an entire class of vehicles based on the experiences of the only existing model. It’s not accurate to say that all spaceplanes are going to be problematic just because we flew a very risky system that just happened to have wings for nearly 30 years.

The Space Shuttle was was designed in the 1970s and went through several downgrades in order to keep the price manageable. These changes left the Shuttle that actually flew with a very basic aerodynamic design. The reentry procedures were derived from experience with capsules and computer simulations. Suffice to say, it was a first generation spaceplane and through its entire life was constantly being upgraded, modified and improved. Just as jetliners in the 1960s may physically resemble jetliners of today, the differences in aerodynamics, reliability, avionics and materials make them totally different. The same will hold true for spaceplanes built in the next 50 years when compared with the Shuttle. We should not be discouraged with a system that provided 2012 performance with 1979 technology.

One of the drawbacks to the Shuttle was that it was exposed to very high reentry temperatures due to varying factors including entry angle, angle of attack, entry velocity and wing loading. The Shuttle started entry at an AOA of roughly 40 degrees at Mach 25 and would ramp down to 14 degrees by the time the velocity had bled down to Mach 4. This profile keeps the total reentry time relatively short but drastically increased heating on the vehicle. The shuttle falls to roughly 250,000ft before its entry angle relative to the local horizon begins to shallow out. This is the constant drag portion of the entry. As it slows and descends, the AOA is gradually reduced to more conventional values and it begins to fly like an airplane.

If one were to reverse the angle ratio and have shallower angles early in the sequence, a spaceplane would experience far less severe heating at the expense of time. The Shuttle had APU and radiator limits and could not remain in high speed hypersonic flight indefinitely, so there was a reason to expedite its entry. A shallow initial entry for the Shuttle would have exhausted the APU fuel and potentially overheated its systems due to the cargo bay radiators being shut down. However for a purpose designed spaceplane that could last a longer time in the entry configuration, this would not be a factor.

The shallow entry is really simple as a concept, even easier to practice on a simulator and of course ridiculous to write out in raw math. Enter the atmosphere at a very shallow angle with respect to the local horizon, descend at a very slow rate to maintain a roughly constant temperature and wait for speed to bleed off. For all intents and purposes, AOA will control rate of descent and by association, hull temperature. For this reason, wing loading and 1g stall speed are the most critical aerodynamic considerations.

If a spaceplane has a very high wing loading, it will stall at a higher airspeed (max AOA is tied to wing design and sweep). In order for an spaceplane entering the sky from above to remain at a low temperature, it has to stay high and slow as long as possible. The only way to accomplish this is to utilize a low stall speed so that it skims down into the sky instead of dropping into it. Any new spaceplane will have to have a much lower wing loading than the Shuttle to allow it to “surf” through the upper mesosphere without excessive sink rates.

Once relatively level flight in the upper atmosphere has been established, AOA will vary based on final design but will most likely be less than 25 degrees. A ramp down process will then commence to reduce AOA, thereby maintaining the shallow descent rate and slow deceleration. Once a predetermined speed is reached, AOA is increased to descend at a higher rate to a lower altitude. Once arriving at that lower altitude, AOA will be reduced again to reduce descent rate. The entire process then repeats itself as many times as required to bleed speed from Mach 25 to Mach 4.

The control of AOA at hypersonic speeds will not the difficult part of this exercise. Managing energy properly to arrive at a given point while maintaining docile deceleration rates will be. Advanced planning and real-time monitoring will be the solution to this challenge. Cross-ranging will be enhanced first by virtue of the higher L/D ratio and second by the larger fraction of time spent in high altitude flight. Temperature control will not be an issue provided the craft follows the entry procedures and does not allow extremely high descent rates to develop. Theoretically, the leading edge temperature should not exceed 750 degrees Fahrenheit at the maximal heating stage, while remaining far cooler for the majority of the procedure. This allows for far more flexibility in vehicle construction and much larger safety margins. The lack of vehicle heating will quite literally be a life-saving feature.

The Space Shuttle was infamous for the difficulty experienced in maintaining the heat resistant tiles on its undersurface. Being very delicate and brittle in nature, they would break easily if mishandled. On the first Shuttle (Columbia, before the upgrade) had over 30,000 tiles of various thermal properties protecting its airframe. Later modifications replaced some of the upper surface tiles with thermal blankets and fabrics that reduced maintenance between missions. Any new spaceplane would be able to use similar materials over its entire fuselage while still maintaining a substantial margin for airframe protection in emergency or off-design entry profiles.

So its just an idea, one that deserves a lot more thought and research from various people, companies and agencies. Having the capability to fly back from orbit means the potential to bring back heavy payloads, no need for extensive maritime recovery forces, the ability to land at any runway of adequate length and no requirement for passengers and crew to withstand heavy g forces. Capsules are and will still be useful for certain space activities, most notably lunar or interplanetary travel. But for any flight that is intended to spend most of its time in low earth orbit, a spaceplane will offer far more advantages. Let’s not write off the future because of the past.