By Tad McGeer (UAS legend my words because he is ed)
Continually I am told that the age of electric-powered aircraft is nigh. So say magazines to which I subscribe –not just nerdy pilot’s journals but also the weighty Economist and New Yorker, as well as business press keeping tabs on projects in established firms and start-ups.
I see placards touting electric airliners-soon-to-be as I board yet another old-fashioned Airbus or Boeing. I come across an academic discussion of policy issues arising from so-called urban air mobility – i.e. swarms of electric sort-of helicopters – a discussion which I must sadly admit includes even the Department of Aeronautics and Astronautics at Stanford, where I did my PhD a few decades ago. To be sure, swarms of helicopters would indeed raise policy issues – where to put all those heliports, what would the neighbours think, and so forth – but I would expect all of my Stanford colleagues to know that such issues are, well, academic. Electric aircraft can never be more than expensive toys.
Such a flat statement may seem curmudgeonly in the face of such voluminous exuberance. But regrettably, there is precedent for mass fantasy in aviation. Two decades ago, for example, the journals that now hype electric aviation were touting a revolution in air transport with so-called Very Light Jets. These were to be ubiquitous Ubers-of-the-air (albeit before Uber) offering affordable air-taxi service to all comers using every community airport on the continent. But anyone who had been around aviation for a while could see that this was nonsense: otherwise, the mass market would already have existed, served by existing aircraft which might, perhaps, be a little less economical than the proposed VLJs, but not fundamentally so. And so in due course, the hype faded; the sham orders evaporated; and the startups closed shop as their venture capital was exhausted. Electric aviation will follow the same path.
My own work on small vertical-takeoff-and-landing aircraft, boringly powered by plain old gasoline, is technically related to urban air mobility. So a conference at which I was recently asked to speak was well-attended by electric enthusiasts, and I felt as a matter of responsibility that I should point out some quantitative truths. These were well received, at least by those who wanted to listen, and I hope that readers will be similarly interested.
I will start with an example. Then I’ll explain more generally why electric power is practical for cars,
but not for aircraft. Finally, I’ll discuss the prospects for sustainability in aviation.
An example is provided by Harbour Air, which operates floatplanes from my hometown of Vancouver. A few years ago the airline announced that it would switch in due course to an all-electric fleet.
When I saw this I thought back two decades, to Boeing’s announcement of the Sonic Cruiser. It was a supposed revolutionary airliner that would cruise slightly faster than the speed of sound, as opposed to about 80% of sonic speed – in technical parlance, 0.8 Mach – for jet airliners built since the late 1950s. 0.8 Mach had been achieved over the preceding four decades, in which speed and economy had improved in step as airliners became increasingly productive. But speed beyond 0.8 Mach was and is uneconomic because of sharply rising and inescapable wave drag. Wave drag is why we have been cruising at 0.8 Mach through the six decades since the advent of the 707 and DC-8; why 1960s supersonic transport projects failed in the US and USSR; why only a handful of Concordes were built; and why the Sonic Cruiser was an obvious non-starter – particularly so because drag near sonic speed is actually even worse than at Concorde’s uneconomic Mach 2.0.
Boeing’s engineers of the late 1990s must have known this perfectly well. But at the time Boeing also knew that Airbus was on the point of committing to the A380, which would displace the 747 as the world’s largest airliner. In hindsight, the A380 turns out not to have been a profitable venture, but at the time it seemed a threat, and Boeing evidently wanted to keep it out of the limelight. Hence anyone who knew about wave drag saw immediately that the Sonic Cruiser was just a diversion, to be hyped for a while and then quietly forgotten. It seemed to me a cynical ploy unworthy of Boeing’s engineering heritage.
A hardy and venerable de Havilland Otter, ubiquitous on floats along the British Columbia coast, is an unlikely heir to the Sonic Cruiser. Yet it shares the same destiny. Harbour Air converted one to electric power as the harbinger of its coming fleet. But enough batteries to complete one of the airline’s shorter routes, from Vancouver to the nearby Gulf Islands, left it with such a high empty weight that it could only just accommodate a pilot. It had nothing left for paying passengers.
The inescapable quantitative truth is that the best batteries are more than tenfold heavier than gasoline per unit of energy delivered.
Comparison is slightly veiled by convention, whereby battery capacity is usually expressed in energy per unit mass, while fuel efficiency is usually expressed in mass per unit energy: energy density vs Specific Fuel Consumption. For direct comparison let us settle on mass per unit energy. The energy density of the best practical batteries, at about 0.25 kWh/kg, corresponds to SFC of 4 kg/kWh. The SFC of my Cirrus, cruising at say 180 kt, is 0.25 kg/kWh.
Batteries are worse by a factor of 16 – and more, actually, if one accounts for losses in the powertrain, That is, the battery pack required to travel a given distance would be more than 16 times heavier than the equivalent tank of gasoline.
With such low energy density, batteries can be practical for transportation only when the fuel that they would replace is a small fraction of total vehicle mass. Cars, happily, fit the bill. In my Subaru, a full tank weighs only 3% of the total weight, as opposed to 17% in my Cirrus. I can multiply 3% by a factor of 16-ish, i.e. to about half of total weight, and still have some useful load for a practical car. Furthermore, for a car, the switch from gasoline to batteries isn’t quite so bad. Unlike an aircraft, which mostly drones steadily in cruise, a gasoline-powered car usually has part-throttle, transient, and braking losses that can be at least partially avoided on batteries.
Consequently, the battery weight in a Tesla turns out to be about 40% of the total mass, rather than half as indicated by our first approximation. The number is still painfully big, and a battery-powered car consequently has to be heavier than its gasoline equivalent, but it can still be viable.
For an aircraft, however, electric power can be practical only where a gasoline-powered equivalent would have a tiny fuel mass. A self-launching sailplane is a good candidate, since it uses its engine for only a few minutes on each flight, and moreover in some circumstances actually benefits from carrying ballast. But if the job is to carry a load over any distance at all – even Harbour Air’s standard 50-mile hop from Vancouver to Victoria – battery power is out of the question physically, let alone economically, because the battery would be prohibitively heavy.
An elementary formula is helpful here to determine, the weight of the battery as a fraction of the total aircraft weight. It is a function of distance travelled R, specific fuel consumption c, gravitational acceleration g, and lift-to-drag ratio, as tabulated below:
Only a few kilometres of range is enough to whittle into available payload, as our Harbour Air example illustrated. And of course if you halve the payload of an airliner, then you double the fares. Such sensitivity is why airliner manufacturers guarantee maximum empty weights, and pay a substantial penalty if the guarantee is not met by even a little bit. It is also why my beloved doctoral advisor Dick Shevell, who led airliner design at Douglas in the 1960s, would as he put it “shoot your grandmother for 3%.”
How about the much-hyped urban air taxis? Vertical takeoff and landing is essential in order to have an advantage over surface transport, so unpleasant helicopter-like constraints apply in the mass-fraction calculation: helicopters are not efficient travelers. Some of the proposed aircraft may not look like conventional helicopters, but the arithmetic applies nonetheless. The range of such an aircraft must be short enough to maintain an economic payload fraction, but long enough to beat surface transport. The space between these bounds is slim to none. Constraints are all the worse when you consider the need for reserves. Most pilots wouldn’t take off in a fixed-wing aircraft, let alone a helicopter, with fuel for only 10 km or so.
In fact the question of viability has already been answered. Around 1950, when helicopters were new, urban air mobility was similarly hyped. But high costs and limited performance have kept the air-taxi market small even for conventionally-powered helicopters, which are much more capable than any electric (or hybrid) taxi can ever be. Hence my earlier comment that electric aircraft will be, at most, rare toys for the well-heeled.
Aviation was stimulated, and enabled, by the advent of the internal-combustion engine in the late1800s. But imagine a society that had taken a different path, all-electric, with aviation consequently a rare rich man’s amusement rather than a significant industry.
Imagine that you come upon the scene, claiming to have unearthed a mysterious liquid that burns readily, can be pumped around at will, stores in a container of any shape, has unheard-of energy density, and to
top it all off, effectively vanishes when its energy is extracted, so that it no longer needs to be lugged around. The claims would be so outlandish that you would be dismissed as a charlatan.
Today, though, we face not the advent of gasoline but rather its possible disappearance. Indeed while Urban Air Mobility seems to be based on hubris, electric-airliner concepts may be genuine if ill-advised proposals for sustainability. If batteries cannot make aviation sustainable, then what?
The best answer is to electrify things that are not so weight-sensitive. Thus work toward aviation sustainability should not be on electric aircraft, but rather on electric cars, boats, and trains. The hope is that such terrestrial electrification will be cheap enough to occur without scarce hydrocarbon fuel – now fossil; eventually bio–pricing aviation out of the market. A possible alternative for airlines is liquid hydrogen electrolyzed and distributed from a formidable new infrastructure of power plants. Hydrogen’s energy density, more than double that of hydrocarbons, is attractive for aircraft, and it can be burned in more or less standard jet engines.
Since it is cryogenic it cannot practically be carried in high surface-to-volume wing tanks as at present, but would instead have to go in an unusually fat fuselage. Also it must be used as fast as it boils off, which restricts feasibility to intensive airline-like operations. Lockheed studied all of these issues carefully during the first energy crisis in the 1970s, and nearly went ahead with a hydrogen airliner. Perhaps they were not off-course, but merely ahead of their time.