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Comment by westbywest | Hacker News Reader

Although intriguing, this comparison is apples to oranges. Planes can achieve substantially better fuel economy than land-based vehicles because of 2 factors:

1) More flexibility in body design. Cars must have 2/3/4 wheels with reasonable clearance to maintain safe balance on the road surface, which usually adds to drag. Planes can vary from tri-wing to tailless flying wing.

2) Low air resistance at high altitude. Planes, passenger jets especially, capitalize on the reduced drag at high altitude for substantial fuel economy. Cars are generally stuck at sea level most of the time.

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I'd also like to add in 3) no stop and go traffic. That, by far, is one of the worst contributors to poor fuel economy in cars. If we could all go directly to our destinations in our cars without stopping for traffic/stop signs/stop lights, our fuel economy would look a whole lot better.

More interesting to me is that he was able to realize significant improvements in fuel consumption with the same airframe and same basic engine by upgrading the ignition and fuel injection systems to something that probably wouldn't look out of place on a 20 year old civic.

True, though it seems flexibility in body design is mostly limited by fashion. Things like the Aptera (www.aptera.com) are road legal.

But I disagree it's apples to oranges. If the comparison is how to get from A to B with the highest fuel efficiency, why should cars be protected from their disadvantages? (Disregarding for a moment the chaos that would ensue if everyone driving had their own plane...)

The 100mpg figure seems to apply to cruise flight at 17500ft. Given that you have to climb up there before you enjoy that advantage, it would be interesting to know what the total trip mpg is for trips of different lengths. How far do you have to go before you beat the cars in fuel efficiency?

And then there is the problem of getting to an airfield first, unless you just happen to have one in your back yard.

Also, an engine in a car cannot run all the time at the same load, because there are hills, crossroads, faster and slower roads, etc., while a plane operates in much more constant conditions, so the engine can work at roughly the same load most of the time. Combustion engines doesn't scale, so the engine cannot have the same efficiency for every possible load. (And it's not practical to put an electric generator and engines, which are much, much better, in a plane or car.)

it's not practical to put an electric generator and engines, which are much, much better, in a plane or car Actually, that's exactly how the Chevrolet Volt works.

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