Electric flying unfeasible for longer distances

In recent years, there has been increasing interest in the use of electricity as a source of energy for aircraft. Is ‘sustainable flying’ within reach? An expert from the aviation industry explains.

Photo: Thomas Kelley via Unsplash

“In order to answer this question, I will first discuss the energy consumption of modern passenger aircraft in this article, and continue with a historical overview of the development to arrive at electric flying.

As will become clear, electric flying is (unfortunately) not possible with conceivable means, except in a few niches. Ultimately, it must therefore be concluded that the discussion about electric flying is in serious need of ‘reframing’.

Passenger aircraft energy use
Airplanes come in all shapes and sizes, with takeoff weights ranging from a few hundred kilograms to several hundred tons. Commercial aircraft are large, especially the aircraft that take care of the lion’s share of air transport.

A modern aircraft like the Boeing 747-8 has an empty weight of 215 tons and a maximum take-off weight (MOTW) of 442 tons. The payload is usually 20 to 25 percent of the latter. I’m counting 88 tons here, which is equivalent to 460 passengers plus cargo. For this example, 120 tons of fuel go into the tanks.

It does not matter for the time being how far this takes us, but it does matter that the fuel has almost run out at the end of the flight. The average weight in flight thus comes to 215 + 88 + 120/2 = 363 tons.

The average lift force to be generated is therefore equal to 363,000 x 9.81 = 3,561 kilonewton (mass x gravity). Now the modern Boeing 747-8 has a glide ratio of 17.7. That slip number is the ratio between the lift force and the air resistance. Dividing gives us 3,561 / 17.7 = 201 kilonewton to overcome resistance.

We also know that this aircraft has a cruising speed of 912 km / h or 253 meters per second. From this we can calculate the power: after all, that is force times speed. We then arrive at 201 x 253 = 50.9 megawatts. In addition, there is some extra power for the on-board systems (electricity, air conditioning), but we leave that out of consideration for the sake of convenience.

Smaller planes obviously consume less. Power scales with weight at a comparable speed and at a similar glide ratio. With current aircraft, speed and glide ratio are quite close together. So what it comes down to is the ratio of curb weight, payload and fuel. And the latter of course strongly depends on the desired flight distance.

However you look at it: flying is very energy intensive.

The fuel consumption is correspondingly. Kerosene has a calorific value of 43 megajoules per kilogram. The jet engines have an efficiency of approximately 45 percent and thus generate 43 x 0.45 = 19.4 megajoules of propulsion power per kilogram of fuel.

This means that the power of 50.9 megawatts requires a fuel consumption of 50.9 / 19.4 = 2.6 kilograms per second. With the aforementioned 120 tons of kerosene, the B747-8 can fly 120,000 / 2.6 = 45,800 seconds. That is 12.7 hours or about 11,000 kilometers.

This is not an unusual flight duration and distance for this B747-8. In practice, we do not make it, because taking off and accelerating also costs fuel that we do not fully see due to the mentioned efficiency due to lowering and braking. The analysis is therefore only indicative.

We can also express fuel consumption in passenger kilometers per liter. The speed of about 250 meters per second means that we need 2.6 x 4 = 10.4 kilograms or 10.4 kg / 0.8 = 13 liters per km per kilometer (the specific gravity of kerosene is 0, 8 kg per liter). With 460 passengers we arrive at 460/13 = 35 passenger-kilometers per liter.

If you drive at 90 km / h on the highway in a car that consumes 1 liter of petrol per 17.5 km and take a passenger with you, you will arrive at the same consumption. But the B747-8 goes ten (!) Times faster. This is an indication of the high degree of perfection that typifies modern aircraft.

How has this technology developed so far?

The first jet for passenger transport was the De Havilland Comet from 1958. If we put the energy consumption at 100 percent of this, then the first substantial improvement was quickly realized: around 1970, only 12 years later, people already flew about twice as efficiently. .

This was mainly due to the arrival of the ‘bypass fan’, a greatly improved jet engine, an increase in scale and various smaller improvements. The next factor two in efficiency improvement has now also been realized. Airplanes like the B747-8 account for about 25 percent of the fuel consumption of the Comet.

However, that last step took from 1970 to 2010 to be realized, so roughly three times longer than the first step. We can therefore conclude that modern aircraft are very well developed, not only in terms of energy consumption, but also in the field of safety. In fact, aircraft are therefore fully developed.

Even a radically different design, such as the ‘flying wing’, only brings about 20 percent further improvement in terms of passenger-kilometers per liter of kerosene. At the same time, we must note that aviation is innovating diligently, but at a slow pace. Carbon components, for example, have been available since the early 1970s to reduce the unladen weight over conventional aluminum grades. However, they only first aired in 2011 with the arrival of the B787 Dreamliner.

The delayed application of new technology can be explained by the high costs and high risks of such innovations.

Electric flying
Especially thanks to the comet-like rise of Tesla, battery technology – in terms of how much energy can be stored in a kg battery – has improved significantly in a relatively short time. A modern lithium-ion battery, the workhorse in the world of electric cars, achieves 0.9 megajoules per kilogram.

Suppose “our” B747-8 uses this type of battery instead of kerosene. We must then replace the jet engines with electric propellers. As a result, the speed drops from 250 to 165 meters per second because, in practical terms, propellers simply do not go faster.

Of course, the entire aircraft should be redesigned, but we will not consider that for this first sketch.

Empty weight, payload and glide ratio therefore remain the same in this analysis. The take-off weight will be 215 + 88 + 120 = 423 tons and this will remain the same throughout the flight. The resistance is then 423 x 9.81 / 17.7 = 234 kilonewton, the required power is 234 x 165 = 39 megawatt.

Now, at best, a propeller has an efficiency of 75 percent. Assuming 100 percent efficient electric motors, 39 / 0.75 = 51 megawatt power is then required. That drains 51 / 0.9 = 57 kilograms of batteries per second.

With 120 tons of batteries on board, we can therefore fly roughly 120,000 / 57 = 2100 seconds: just over half an hour.

This rough sketch makes it clear that electric flying is possible, but certainly not for long distances. It is an option for short flights, especially if the cruising speed is considerably reduced.

The German company Lilium GmbH, among others, focuses on this segment and expects to be able to supply a five-seater aircraft with a range of 300 kilometers in 2025.

In principle, this technology can be scaled up to larger aircraft, which could enter the current short-haul market by 2030. However, speed and range, and therefore transport performance, will nowhere near aircraft like the B747-8.

For this 747-8, ‘hybrid drive’ is also an option for an incremental improvement: under the name E-Fan X, a consortium led by Airbus has already started the first developments in this area. Here too, however, long distances remain out of the picture.

Yet another niche is that of the ‘air taxi’, a quadcopter for vertical takeoff and landing. Electric propulsion is promising for this, but this type of flight is completely incomparable with, for example, a transatlantic flight.

Reframing required
With the foregoing, I try to make clear how energy-intensive flying actually is, how sophisticated the existing technology is already and that electric flying is only suitable for short distances.

Apart from that, I have not yet discussed the costs of such a development and the time required for the further development of electric flying.

Anyone who claims that electric flying is sustainable does not only underestimate the state of the art, but also the limits of physics.

Instead of stating that ‘technology will solve it further’, these people should thoroughly question how realistic it is that the desired transition will take place, and to what extent.

The burden of proof lies with them. As Aldous Huxley put it beautifully: “Facts don’t cease to exist because they are ignored”.

The author is an aeronautical engineer. For private reasons, this person refrains from actively participating in the discussion.

NOTE TO EDITORS
Originally published in Dutch at SchipholWatch

This translation is made with Google Translate from the original document.

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