Notes

Why Fast Airplanes are Bad for the Environment
https://replicauhrende.com

Since the dawn of time, man has sought to get there quicker.
Whether it be domesticating wild stallions, bobbing back and forth in a metal tin over deep waters, or sipping ginger ale 35,000 feet above the Earth, it’s evident our need to move — and do so more and more efficiently — has always been a central driver of humanity.
Our desire for adventure, economic gain, and leisure has, quite literally, propelled us forward as a civilization. For better or for worse, we’ve been bred to value speed; engines are compared by horsepower, and supercars are sold based on top speed (“it goes 0–60 in how long?”).
In the airline industry, customers are promised additional legroom and gourmet snacks, though what we all really wish for is a way to spend less time in the air. As a nervous flyer (and an aerospace engineer — yes, I recognize the irony), I never seriously consider investing in teleportation start-ups harder than when I have a flight approaching.
Un-luckily for me (and for all you parents traveling with young children), commercial airline speeds haven’t changed much in the last few decades. Nevertheless, absurd amounts of money are continually poured into airplane research and development (for money spent on safety and those little foot-rests — thank you).
As faster airplanes would lead to less time in the air (leading further to happier customers, less fuel burnt, more flights per year, and, ultimately, more money for the airlines), engine research is always at the top of the to-do list. While faster airplanes seem better for everyone, aviation research has shown some negative effects on our environment.
Faster might be better — but it’s definitely more harmful.
What’s the Deal with Soot? To get an understanding of the relationship between fast planes and environmental impact, we first have to realize the mechanics of engine emissions, and how to quantify what makes these emissions harmful. We know that engines cause emissions, and emissions are bad — but why?
In combustion engines, the burning of fuel creates a number of harmful by-products, primarily consisting of physical carbon particles. Popularly, we call these carbon particles “soot”. Numerous studies have outlined the negative effects of these particles in our environment — soot has been determined to be a major contributor to global warming, and is considered one of the main causes of lung-disease.
While these effects can be mitigated a number of ways through our relationships with automobiles (transitioning to electric/hybrid vehicles, using public transportation to reduce cars on the road, riding bicycles or walking where possible, etc.), we have no true efficient mitigation methods when it comes to flying. I like to think I work hard to reduce my carbon footprint, but the 9-hour direct flight from Toronto to Rome will beat the 10-day boat trip every time.
So, if faster planes would mean less time in the air, wouldn’t that lead to less carbon emissions?
Fire, Fractals, and Pressure In order to improve the efficiency of the engine (i.e. fly faster), modern combustion engines are designed to operate at high pressures. While this is by no means a comprehensive look at combustion engines, this increased efficiency allows for higher thrust to be output from the engine. Essentially, higher engine operating pressure = faster airplanes.
Until recently, due primarily to experimental limitations, there have been very few studies observing if a relationship between higher engine pressure and soot emissions exists (spoiler alert: it does, and it’s bad).
To understand the results gleaned from recent research, we have to return to one of our original questions — what actually makes soot/carbon particles harmful?
The answer can be found in one of the most mesmerizing phenomena in mathematics (and, nature!) — the fractal.
Very simply, a fractal is a geometrical property of a shape, where if you would continually zoom in on a section of that shape, it would continue to repeat (called self-similarity). Don’t linger too long if you value your sanity.
Fractals are difficult to comprehend abstractly, but understanding the mathematics of fractals can help us solve some otherwise impossible problems. While a number of formulae and properties can be used to describe fractals, we’ll be looking at the property most useful in most applications — the fractal dimension (Df).
The fractal dimension simply characterizes how “complicated” a fractal is. During my studies, I visualized the fractal dimension as an indication of an object’s “roughness”.
For example: Imagine our goal is to draw a line from point A to point B.
A straight line from point A to point B (i.e. a one-dimensional line) would have a fractal dimension of 1.
A line with a 90° angle leaving point A and arriving at point B (i.e. a 2-dimensional line) would have a fractal dimension of 2.
While our first two examples wouldn’t require fractal mathematics to determine things like distance, what happens when our line is inherently squiggly? If, when zooming in, the line continues to exhibit twists and turns like the coastal beach gif above, the “roughness” can be described using a fractal dimension between 1 and 2 (in this 2-dimensional case).
Straight lines rarely exist in nature, and this fractal self-similarity can be observed all over the place (go spend a few minutes staring at that head of cauliflower you’ve been ignoring). So, for the purpose of this lesson, fractals allow us to better measure “rough” lines.
What does that have to do with soot?
It may seem like we’ve flown this plane (ha) all over the place, but I promise to bring us towards a landing.
When soot is formed through the combustion process, individual particles stick together to form what we call “soot aggregates”. These aggregates — when collected from flames properly — can be observed under electron microscopes.
As you’ve probably guessed, these soot aggregates are fractal!
So, why does this matter?
Studies have shown that when it comes to soot aggregates, higher fractal dimensions lead to:
An increase in surface area (which leads to the absorption of hydrocarbons).An increase in light extinction efficiency (resulting in an opaquer soot particulate and, consequently, negative global warming impacts).Aerodynamic changes, which are directly related to the probability of being inhaled by humans. Essentially, the higher the fractal dimension (i.e. the “rougher” the geometry), the worse it is for us as humans, and for the environment as a whole.
What Does This Have To Do With Fast Planes? Let me summarize what we’ve learned so far:
Aviation research is pushing for engines capable of operating at higher and higher pressures to (amongst other things) make planes fly faster.Burning fuel in engines creates soot particles, which are harmful to the environment. These soot particles group together to form fractal soot aggregates.A higher fractal dimension leads to more devastating effects on human health and the environment. Very recently, new experimental techniques have surfaced, allowing us to get a better idea of what happens to these soot aggregates as the operating pressure increases. Initial studies have shown us two important results:
1. As pressure increases, the number of soot particles created increases (as an obvious rule of thumb: more soot = bad).
2. As pressure increases, the fractal dimension of soot aggregates increases (which we also learned, is very bad).
Like doctors recommending smoking “fresh” cigarettes in the 1950s, these new findings make our quest for faster airplanes seem misguided. While side-effects often lag far behind innovation, our job should be to recognize when we’re headed in a fatal direction, and correct course.
When dealing with fuel-based fire-burning engines, it’s clear that faster airplanes are bad for the environment. It’s up to the next generation of dreamers and inventors to take the torch (and, ideally, make it electric).
I look forward to the day when human ingenuity allows us to step into a machine and be quickly whisked to our destination. Whether it be time-travel, quantum teleportation, or the boring electric airplane, we’ll find a way to get there quicker.
-Ben

Website: https://replicauhrende.com