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Commercial aviation is at its centennial. One hundred years ago it was mostly used for freighting or, more particularly, as couriers on behalf of national post offices. Mail then, as emails now, required expeditious transmission. So, in those days, rickety piston engine aircraft criss-crossed the globe generally successfully fulfilling that requirement. Their pilots had a bravado and mystique similar to that of an astronaut today. The true nature of that unique operating environment is well captured by Sainte Exubery in his biography ‘Sand, Sea & Stars’ – now more famed as the author of the ubiquitous ‘The Little Prince’, which itself was devised while he was awaiting uncertain rescue when his plane came down on a remote, high plateau in Morocco. Commercial passenger flights then were more akin to extreme sports than normal travel. That said, at that time, long haul Zeppelins carried almost 100,000 well-heeled passengers across the Atlantic in lavish style, and reasonably safely – until the Hindenburg’s (possibly terrorist instigated) Hydrogen-fuelled flaming destruction in 1936 which bought that era of commercial aviation to a close.

In the hundred years since, aviation technology and safety have steadily improved in incremental steps with revolutionary changes being very rare. In the early 50’s, the jet airliner was likely the first such ’Revolution’ along with the subsequent wide-body jet airliners making cheap (in every sense of that word) air travel available to all. The helicopter in general aviation is surely another. The (almost unequalled) speed of the Mach 2.2 Concorde had too small an impact to be considered revolutionary. Now, the development of AAM (Advance Air Mobility) electrically powered, city-hopping, autonomous flying vehicles has the potential to be the next such revolution. That these air vehicles are to be electrically powered is under the incremental category as is the fact that there will be no pilot which, technically, if not emotionally, is also but an incremental change – this author flew in a fully automatic helicopter in 1968 (a measure of both his maturity and that of automated flight!).

The revolutionary element here is not so much the vehicle but more in how it will be used. Until now commercial flights have, in the main, been over distances of many hundreds of miles in aeroplanes and dozens of miles in helicopters. These flights, in the main, are scheduled and over fixed routes. Even for helicopters in an emergency response context, flight plans are submitted and fixed routes in an (air) traffic controlled environment are generally followed. The AAM environment however, will be over very short distances – typically of less than a dozen miles – be unplanned and ad-hoc and, if the dream is realised, not much above roof-top height and from any door to any door – in short AAM holds the promise of low level bedlam! S o, the revolutionary element here is less one of vehicle technology and more one of the huge levels of anticipated production of these small flying craft and how they will be used, with the associated automated control software – IF such can be satisfactorily developed and certified. This article will give an overview of each of these elements.

First the vehicles themselves. This article will be limited to the passenger carrying, so-called ‘Air-Taxis’. This is because the freighting variants have lower certification thresholds (Aerial Work rather than Transport Category) and will anyway merge into the already established Drone market. As one would expect, all the prospective Air Taxis use VTOL (Vertical take-off/landing) technologies. While largely based on rotary wing principals, there is one significant difference. In place of the very large main rotor(s) of the helicopter, powered by a couple of engines through a large reduction gearbox, the eVTOLs use multiple small rotors each individually and directly powered by an electric motor – much lighter and with a higher level of failure redundancy. Forward flight will be either by tilting the overall rotor disc (à la helicopter) or with a separate pusher propeller. Like the smallest light helicopter types, the power-to-weight ratio of current eVtol types is low. This negatively impacts hover precision and also make them susceptible to gusting winds, thus predicating fairly substantial landing pads – currently referred to as Vertiports (more on which below) – and probably, with low certified weather thresholds.

In product development terms we are exactly where we were in aviation a hundred years ago, with scores of start-ups proposing a myriad of different iterations. Aviation authorities (which did not exist 100 years ago) are struggling to keep things under control and have yet to develop a standard. Developers, being entrepreneurial by definition, are promising in-service dates a year-after-next. Given the absence of a formalised standard and the lack of experienced personnel within aviation authorities to produce it, such dates are surely fictional. This author was involved in the commercialisation of modern airships, so has had first-hand experience of this challenge which, in the lighter-than-air case, was resolved by running the development of platforms and regulations in parallel over several years with the OEM typically preparing the regulatory drafts. The same is unlikely to occur in the AAM context due to the high level of competition between protagonists reducing cooperation, and the massive envisaged number of  air vehicles in end-use complicating regulatory controls. That said, the Big Boys in main-line aviation are now becoming deeply involved including DARPA and some defence ministries, some large airlines, Boeing and Airbus and uniquely, two major car manufacturers, Stellatis and Toyota. Also, among the large helicopter operators, Bristow is dipping its toe in the pond with regard to off-shore applications. So buckets of money are being thrown at the problem with investments now measured in billions of dollars (in one’s Airship days, we only ever enjoyed investments in units of millions!). This will likely both speed up certification and reduce numbers of Newco OEMs on the playing field.

Some Leaders of the ‘Air-Taxi’ pack (top left, clockwise) Lilium(Germany), Archer-Midnight (USA), BetaTech.(USA), Joby(USA), Volocopter (Germany), Ehang (China)

Being based on investment pressures rather than technical requirements, all information in the certification context needs to be taken with ‘a bucket of salt’, but there are clear indicators of progress in the ‘real world’ as summarized in the table that follows. This AAM (Advanced Air Mobility) Reality Index (ARI) has been put together by SMG Consulting, a US based business management consultancy working in conjunction with the Vertical Flight Society and Aviation Week. The top half-dozen players are shown in order of their assessed ARI, which generally reflects their level of investment capital. Stated EIS (Entry into Service) dates here are likely based more on financial imperatives than technological fact so, as discussed below, there is little risk of an air-taxi buzzing in your neighbourhood any time soon……


* ARI –
AAM Reliability Index.           **  EIS – Entry Into Service.          *** PP – Production Prototype flying

The Chinese Ehang was one of the first iterations in this field. With capability limited to a single Pax. over 30 km range – a 12 minute flight – it is more reflective of the Wright Brothers’ initial creation than a commercial prospect! That it aspires to claim CAAC certification is surely a factor of not-too-subtle CCP (Chinese Communist Part) influence. Besides anything else, those knee-capping rotors would not pass any serious risk analysis! It is anyway understood that the initial certification is limited to Cavok (ie. perfect weather), within sight of its ground staff and not over built-up areas – thus somewhat limiting its utility as a taxi even if the planned EIS is achieved! The current 1000 unit sales projection, being almost entirely within China, is possibly also a function of gentle arm-twisting.

Volocopter’s ‘Volocity’, also a two-seater and an early iteration, is little better. But at least one has to reach up for an amputation by one of the 18 rotors (although, in fairness, a new 4 seat iteration has one third the number of the rotors and lifts them out of reach). But capacity and capability are marginally less than the Ehang. So, even if a restricted EIS capability was certified by next summer as advertised, in time for the Paris Olympics, the nature of the transportation service it will provide for a single passenger over the spread-out Olympic events, defies imagination. The series has achieved some 2000 test flights in total which, while more than any of its peers, still only equates to a total of only some 350 flight hours. A lot more will surely be required even to achieve a minimum Aerial Work certification to allow it to fly with a (non-paying) passenger over Paris next summer.

Archer & Joby aspire to reasonable aircraft flight parameters. This is largely because their rotors partially / completely tilt (respectively). While  the two early iterations described above achieve forward flight through a marginal tilt of the overall rotor disc – hence their low forward speed (unlike helicopters their rotors have a fixed angle of attack, increasing lift by increasing rpm so limiting said ‘tilt’). These latter tilt rotor types aspire to light helicopter performance (hence the USAF interest) but, due to current battery limitations, not the range.  Based on what the USAF has agreed to pay to acquire a half-dozen test units, the AAM cost-efficiency appears considerably less than light helicopter types, being some 4 times the cost with half the gross disposable load. Of course this is not representative of the envisaged production run which, unlike aircraft, is intended to be more akin to that of the ground vehicles (taxis) it is nominally intended to replace, hence in the hundreds of thousands. But is such a taxi replacement concept feasible?

To judge that one needs to consider AAM operating parameters and particularly landing sites and ‘refuelling’ (ie. recharging) requirements. The latter first. A wheeled taxi, if used 24/7, typically will refuel once a day. A light chopper shuttling, maybe after every 2½ hours of flight. But even the most capable AAM birds will need to recharge every half-hour and likely, sooner. While to recharge car fuel takes a minute and a light chopper maybe 5 minutes, a full AAM recharge will be more than five times that. As we all know from our computers and cell phones, constant topping up destroys battery life. So a regular half-hour full charging is likely an essential requirement. OEMs suggest a typical transit distance for an AAM of some 50km, which at typical stated cruise speeds will take approximately 15 minutes. On landing, the stated battery recharging time is about 12 minutes. So it appears that, in general, flight times and recharging times will be similar – while not an impossible situation, it would seem to be a less-than-optimum state of affairs.

Then there is the small matter of parking. From this diagram it is clear that this air taxi will not be able to pick up passengers in the same way as a standard (4.5x2x1.9m (l-w-h)) London Cab. So the door-to-door ethos of taxis will be inapplicable to the so-called air-taxi.  And, in aviation safety terms, the low power-to-weight ratio of AAMs will predicate a fairly substantial landing pad. And indeed, from glossies provided by prospective, so-called, Vertiport service providers, it would appear that CAP-437 heliport criteria will remain generally relevant for AAMs (see overleaf). So hailing an air taxi will be less like hailing a Cab or ‘Apping’ an Uber and more like hailing a helicopter – so not done on a whim! And then there is the likely weather limitations to consider. Since the developed AAM end-product will be fully automated, low visibility should not be an issue, but wind will. Low power-to-weight ratios and lack of inertia will make AAMs sensitive to updrafts & general turbulence caused by wind – and in the city-scape environment, there are plenty of sources of both in high winds. One would therefore expect a (say) 25knot wind limitation to be imposed – hence, essentially limiting AAMs to fair weather flying. So it would appear that, just when a taxi is most required, these air taxis will be unavailable – not a good prospect!

Costs similarly reflect this. The London Cab illustrated above can be bought for 150-160,000 pounds – say $200k. The USAF has just agreed to purchase a half dozen each Archer and Joby AAMs at a unit cost just short of $25m. United Airlines are expecting a unit cost of an early production AAMs of $2m. Let us assume an established production run is half that. Given the thrill and high transit speeds, a factor of 5 in unit cost seems not unreasonable but again, unlike taxis, one would not use it on a whim. This is made the more so because, to ride in said AAM ‘Taxi’, one will need to first hail a cab to get to the nearest Vertiport atop a multi-level parking lot, take the lift to the upper level (in such salubrious areas, with risk of a mugging) and at the destination Vertiport reverse the process again – in short, such seems low in convenience and high in cost!

Vertiports will likely be of similar dimensions as existing Heliports

And then there is operating cost. The USAF is budgeting AAMs at $1500 per flight hour – roughly twice that of a 4-seat helicopter (which has twice the payload). This is similarly reflected in the Volocity’s suggested seat price for a 10 minute transit (jaunt?) during the Paris Olympics next year which is currently pitched at $385.  Again, for the well-heeled, paying five times the normal taxi fare to cover a distance in one fifth of the time is not unreasonable – but again, for us normal folk, not on a whim! (…..and then one needs to also take into account the cost of that ground taxi (and the time lost) to get to/from the Vertiports at each end…..).

Finally there is the issue of control. Based on outline specifications, AAMs will be buzzing around between 1000-3000ft. So interfacing will national ATC systems should not be a significant issue. But given the high numbers of AAMs envisaged in any city, and the very large number of destination Vertiports required, to operate reliably and safety will require an automated control system of enormous complexity (and cost), the more so because it will also need to cater for massive inter-vehicle 4D positional/vector communication and with passenger reservations in time-frames measured in minutes rather than a day or two as now with existing commercial aircraft types.

Since, given the envisaged nature of the service, pilots will be in insufficient numbers and anyway, too expensive, this massively complex system will have to operate fully autonomously with no ‘human in loop’. Such appears to be a mind-boggling challenge but likely, not beyond the capabilities of Artificial Intelligence. That currently, all are piloted, is in order to simplify the certification process. All proposed types have built-in autonomous capability which, it is expected, will be introduced some years downstream once the flight infrastructure / software is in place. This reflects the anticipated reality that AAM will evolve from the existing low level airspace helicopter management infrastructure which, initially, it will use and from which an Urban Air Mobility (UAM) system is expected to evolve.

Swishing quickly and quietly around the world’s megacities in air-taxis is a worthy dream but, from this elemental overview, it is clear that there are a range of killer issues that may prevent its realization. That said, AAMs will almost certainly come into service albeit in timeframes more likely stretching into the next decade and with only a very few of the dozen’s of current OEM players surviving. In this stretched timeframe, Battery technologies will surely improve, thus resolving the currently ridiculous situation of requiring the same charging times as flight times, and also correcting the present low endurance flight hazard. Green electricity will also surely soon be the norm as Nuclear Fusion becomes a reality (given present trends by 2050). So UAM transportation has good environmental credentials, which however, will apply equally to electric ground vehicles (EVs).  Both AAM and EVs are intended to be fully autonomous with similar risk of systems’ failure leading to accidents. The difference here is that, accidents involving the flying vehicles will likely be very much more serious in terms of damage to both human life & infrastructure. Hence, the associated autonomous UAM management software will need to be very much more demanding and byzantine which, one suspects, is at the limit of human capability, so likely be a productive job for Artificial Intelligence. So, in short, notwithstanding all these challenges, with respect to technologies, this next aviation revolution could happen.

However, the EV ground vehicle revolution which is running in parallel, may prove to be the ‘killer threat’ to the UAM concept. It is lower risk, significantly lower cost and far more advanced. Within the same suggested ten-year AAM certification timeframe all production cars will become electrically powered and (likely) autonomously controlled. Already there are autonomous taxi services in the USA and the use of Apps world-wide to rapidly book a lift through a small number of very large ground vehicle service providers such as Uber, is now a routine matter. As such car ownership in cities is already inessential. In addition, the software systems allowing inter-vehicle communication for effective traffic management are already in service with Google Maps and Waze to name but two.  Once ground vehicles are fully autonomous, vehicle ownership will likely become an anachronism.

It is thus very probable that, using adaptations of existing Apps., within the next decade, vehicle sharing will likely replace vehicle ownership. It is a simple fact that privately-owned vehicles spend some 90% of their life parked. But the new generation of autonomous EVs will run 24/7 and never be idly parked except when recharging. Accordingly, within AAM certification timeframes, individual vehicle ownership will have become an anachronism for rich eccentrics and hence, the numbers of vehicles on the road may be expected to reduce dramatically (by something approaching that 90% of previously parked units) thus eliminating traffic jams and hence, the need for those airborne taxis.

Another threat to AAM is a new generation flying cars. Ground-vehicle-to-fixed-wing conversion types have been around for a couple of generations, but the need for an airstrip at each end has limited their use to that of an eccentric plaything rather than serious transportation. However, a new generation of e-VTOL type cars, now in the early stages of certification, will surely negatively impact the current larger AAM market. While their range and payload limitations /control issues are the same as for AAMs, the fact that flying cars are of the same size as road cars, and are to be offered at the same price as an expensive car, and its ability in principal to go door-to-door, will drive nails into the coffins & demise of AAM. The 2 person-on-board limitation will obviate their use as air-taxis but as a play-thing of the rich, it is in every respect most attractive.

Subaru motor company (1/2) & Alef Aeronautics (3) will lift Flying cars form affluent dream to certified Reality

For these reasons Advanced Air Mobility, while a certain future prospect, may not be revolutionary but, similar to Concorde, more likely will be just another plaything for the rich and famous.  So while these small AAM air vehicles, produced a few small niche manufacturers, will surely soon be swishing over cities quietly, smoothly and safely in the sunshine, the currently booming, multi-billion dollar, prospective AAM industry will most likely ‘crash and burn’!