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Christopher Greaves

The Ups and Downs

The Fokker-Friendship aircraft lifted of from Perth, WA, around 7 p.m. on a Friday night; I settled into the seat with a glass of orange juice and a magazine. My first flight in a commercial aircraft. After 30 minutes the captain announced that we had not yet reached cruising altitude, twenty-thousand feet, and were beginning our descent into Gerald ton.

"Disappointment" doesn't describe my feelings; I'd thrilled at the phallic-symbolism of the roar down the runway and the steep take-off. I wanted several hours in the stratosphere, where the jet-set flew. What a let-down. I certainly hadn't got my money's-worth, even though my parents had forked-out for the ticket.

Here's what the flight looked like, in cross-section:

Christopher Greaves PerthGeraldton.png

Commercial aircraft are designed to cruise at high altitudes, and this flight didn't approach that. Let's see what it was doing.

Imagine that you are flying behind the aircraft; what do you see?

You see the cross-section of the fuselage and wings, schematically:

Christopher Greaves CrossSection.png

Greatly simplified, to be sure, but the wings and tail are quite thin compared to the fuselage, so we'll consider just the fuselage.

Let's take the diameter of the fuselage as twenty feet, radius ten feet; it's an approximation.

The cross-sectional area of the plane is then (Pi times the radius squared) about 314 square feet; let's call it 300.

For each foot the aircraft travels forward it has to shoulder 300 cubic feet of air out of the way.

That's quite a lot of air on a 300-mile flight; about 500 million cubic feet, if I did the sums right.

And to make things worse, it doesn't get to push the really thin air up at 20,000 feet; it's doing this pumping business down in the relatively thick air closer to the ground.

On top of all that, the engines have to lift the aircraft, all 20 tons of it, up as far as it can go - that takes a lot of energy, only to shed that energy as it descends. There's no way to reclaim the kinetic energy of an aircraft; it just has to be dissipated into thin air.

If you go back and look at the first diagram, you'll see that all the energy used in the flight disappears into the air, and does so in the most inefficient manner imaginable.

Now why does my neighbor's gas consumption vary between 30 mpg to 90 mpg?

My neighbour drives from our apartment building to The Montreal Deli for breakfast:

Christopher Greaves Neilson.png

I have extended the Google maps directions at both ends with a thick blue line, to represent the part from the parking lot to Bloor Street, and from Dundas Street into the parking lot at The Montreal Deli .

My neighbour drives from the parking spot to Bloor Street (stop!), then to the intersection of Bloor & Mill Road (stop lights), Forest View Road (stop lights), Markland Drive (stop lights), Neilson Drive (stop lights), then the sharp left turn on to Rickshaw avenue, the sharp right turn back onto Neilson Drive, down the steep hill to Dundas Street (stop lights), to neighbor's Drive ((stop lights)), turns into the driveway at The Montreal Deli , and pulls into the parking spot there.

Now if you do the sums you'll see that my neighbour, on a really bad day, brakes to a stop eleven times.

Christopher Greaves MontrealDeli.png

You can see where I'm going with this, can't you!

A car is little different from a plane, except that it travels in 2 dimensions, not 3, and so enjoys friction with the ground, which allows us to make use of brakes.

Car brakes convert kinetic energy into heat, which (heat energy) then dissipates into the air.

And in eleven short hops totaling 3 kilometers, my neighbour never gets up to cruising speed. So the energy that is pumped into the car, each time she starts from a standing position, has to be dissipated into thin air.

Go back and read the story of my first flight, and you'll see that it is so.

If my neighbour drives in the fashion of accelerating as heavily as possible, then braking to a complete stop at each of 11 points along the trip, then all that gasoline is dissipated into thin air.

30 mpg

But if my neighbour accelerates gently, with an eye on the sequence of lights at the next intersection, then there is significantly less need for braking, which is, of course, significantly less need for dissipating energy into thin air, which translates directly into significantly less need for dissipating gasoline into thin air, which translates directly into significantly less need for dissipating money from her wallet, into thin air.

90 mpg

Here's my neighbor's ideal trip:

Christopher Greaves MontrealDeli90.png

It really is prudent to come to a complete stop when approaching a sidewalk; that way you'll be going no faster than walking-pace when you come into contact with a pedestrian.

Apart from that, a steady, smooth trip, looking ahead and anticipating lights can save you a lot of gas.


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416-993-4953 CPRGreaves@gmail.com

Toronto, Friday, August 14, 2015 12:54 PM

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