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Comment by caditinpiscinam | Hacker News Reader

Anyone have a sense of how the energy needed to fly something like this compares to driving an electric car the same distance?

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I think (not an expert though) that a multicopter like this is fundamentally less efficient than a helicopter. It has to do with the surface of the rotary wing (many small props cover less surface than a single big one).

I know that's not answering the question about the car (btw electric or not doesn't fundamentally change the energy question, except that electric cars like Tesla tend to be heavy, I would say), but still I find it interesting.

> many small props cover less surface than a single big one

Well, things change when you make a lot of small props.

This one has an entire ring of them, and they fit a larger outer diameter than a helicopter would. That's quite possibly the reason they are quiet, because they have more propeller area, operating on a smaller speed.

Very likely, here the amount of noise is a better guideline for efficiency than the usual characteristics of the basic design.

(There is no mention of the cost. It is the first thing to suffer when increasing the propeller area.)

Anyway, about the car question, the article says it is as noisy as a small grass mower... I guess an electrical car is much more silent than this.

A friend working on this type of technology mentions that altering the frequency at which sound is emitted to those outside the human hearing range can make a big difference.

One small propeller is less efficient than one big propeller. What drives the efficiency really is the ratio of propeller surface area to mass. So a lot of small propellers are roughly as efficient as a big one. Actually bonus point for the propellers being twisted, which is not possible on helicopters.

A small plane can carry 1-4 people at 120 MPH consuming about 11 gallons an hour, for a "MPG" of about 11 (miles in the air don't exactly translate to ground because air flows but it evens out over time).

Even with modern efficiencies available, I doubt they'd do more than double that, so a car would still easily win, unless the car had one person in it and the plane 4.

Time to resurrect the zeppelin, of course with better safety measures.

Airships never really went away, but "hydrogen go bang" is hardly their only problem sadly.

Hydrogen doesn't "go bang" unless mixed with the proper amount of oxygen. Usually, it just burns nice and smooth. Not much of a difference for anyone riding the airship though.

>>Usually, it just burns nice

Great choice of words to calm the passengers as we lose all our air worthiness.

wouldn't you recover the energy back while landing in same sense with car breaks that recharges the battery

There is constant energy expenditure to stay in air. Unlike with ground or sea vehicles that don't need it.

Yes, the technical term for that is "lift induced drag".

No, it's just drag. Extended landing gear is also a constantly expenditure but generates no lift.

> Extended landing gear is also a constantly expenditure but generates no lift

Negligible lift. Fluid flowing around an object generates lift. Drag is more intuitive; if there is drag, there is lift. (They're just components of the aerodynamic force relative to the direction of relative fluid flow.)

That's about the MPG of a hummer, so it's at least the same order of magnitude of cars.

Cars can't cross over lakes and mountains so there are advantages to having an airplane inefficient or not.

A Long-EZ carries 2 people and goes 1600 miles on 52 gallons of gas. So 31 MPG. That's at a cruise of 144 MPH.

So more like 3 times better performance is available with a modern design... Kind of makes you wonder...

https://en.wikipedia.org/wiki/Rutan_Long-EZ

I'd assume that for very short trips lift off and landing consume a large fraction of the total fuel.

Not for a typical small plane. Takeoff fuel burn isn't that much higher than cruise, and descend and approach fuel burn is similarly lower due to reduced power.

There are people who hypermile on planes and they have all sorts of tricks, including using updrafts when possible.

The most extreme ones fly without an engine at all (gliders).

I doubt that those techniques work very well in areas where you'd be able to run a successful air-taxi business.

The wind component averages out over time or is slightly in favor of the aircraft because you would aim for optimal wind by selecting a preferred altitude.

But air miles and car miles are not equal. The car route is nearly always longer, even in flat terrain roads don't usually go in a straight line to the destination. Adding in mountains or water makes that worse for the car.

For example driving Amsterdam to Barcelona is 1554km according to Google Maps. The flightplan route is 1236km and usually the really flown route is a bit shorter if ATC allows. That's 20% less miles for the airplane.

There are videos online of a cyclist pedaling a craft with enough energy to lift off the ground. Mind you, it was a purpose built craft in a very staged environment with an Olympic level cyclist, but you can still use that as a baseline for how much energy you need to lift a human aloft.

We would have hover cars like we predicted decades ago if it is energy expensive. Lifting a vehicle and passengers is always costly in energy.

It is really a basic physics question, no? Air has little mass, and so provides poor thrust under Newton's third law of motion compared to a tire on the earth.

A car on the ground will always be more efficient than something which dumps its energy into air.

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