
One Man’s Quest to Outrace Wind (2011) - mds
http://www.wired.com/magazine/2011/02/ff_fasterthanwind/all/1
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newnewpdro
The last time I read about this [1] was while working at a Silicon Valley
startup with a small team of bright software engineers.

Most were perplexed by the problem as the URL spread, and the article, in my
opinion, did more to confuse than explain. The newer article linked here,
which I only skimmed briefly, looked to probably make the situation worse
judging from its visual aids.

I thought it was obvious and simple to understand (and explain) how this
worked:

The wind pushes the vehicle via simple drag forces, this alone can accelerate
it, eventually approaching the speed of the wind.

The vehicle also drives a propeller via its forward motion through a
mechanical drivetrain.

By spinning, the propellor acts as a forward motion compensator, and the wind
will exert force against it even when the vehicle travels at speeds >= the
wind.

Introducing sailboat-based analogies does not help understand any of this in
my opinion, it only serves to confuse the audience. The folks behind
developing this vehicle come from a sailing context, which explains why
they're approaching it from that perspective. For the general public, sailing
is not a natural model for reasoning about this stuff.

[1] [https://www.wired.com/2010/06/downwind-faster-than-the-
wind/](https://www.wired.com/2010/06/downwind-faster-than-the-wind/)

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caconym_
I don't know if your explanation is simple or obvious either, except maybe in
a very hand-wavey sense that may or may not correspond to the truth.

The sailboat thing isn't really an analogy—it's a direct comparison, and the
operating principle is the same. There are a lot of resources explaining the
phenomenon of "apparent wind" and how it's possible for a sailboat's downwind
velocity _component_ to exceed the environmental wind speed. If you understand
vectors at the level of a college freshman, you can understand how that works;
you don't have to be a sailor. From there, the "cylinder Earth" thought
experiment provides the intuitive leap to the operating principle of this
dead-downwind vehicle.

They do touch on the concept of a faster-than-the-wind VMG downwind, but they
probably should have emphasized it more in the explanation.

~~~
newnewpdro
Introducing sailboats doesn't simplify explaining anything in my experience,
they just pollute the mental model with more complexity.

There's value in the ground vehicle model for explaining to someone how
sailboats can travel faster than the wind propelling them. But I don't think
sailboats are a useful teaching aid in the opposite direction; explaining how
the ground vehicle works.

So unless the goal is to teach specifically about sailing, I feel it's best to
leave it out entirely. Except maybe at the end as a passing mention like "BTW,
this is how sailboats manage to sail faster than the wind as well."

~~~
caconym_
I have to disagree, because a sailboat is a pure manifestation of the model
that must be understood to understand how this vehicle works: a foil held at a
fixed angle in a moving fluid, constrained to move only along a straight line
at a fixed angle wrt. the motion of the fluid.

Understanding this vehicle requires heaping more complexity (the spiral
motion) onto that same model, so in fact I think a sailboat is a great
teaching aid here, as it's a clean waypoint on the journey to full
understanding. Sailboats also exist, and have non-controversially demonstrated
the surprising ability to sail with a downwind VMG higher than the
environmental wind speed, which will help skeptics to engage with the problem.

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animatedb
When I was a kid I used an erector set to make a car that moved directly into
the wind using a similar technique. Many people don't think that is possible
either, but with proper gearing, it works fine.

~~~
LeifCarrotson
It's also possible to use a propeller sail to go directly upwind in water. You
don't need gearing in water if you pick the right pitch on your underwater
propeller relative to the pitch of your sail/fan.

[http://www.sailwings.net/windspinner.html](http://www.sailwings.net/windspinner.html)

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caconym_
Classic. I already knew about this, but the "cylindrical Earth" thought
experiment is new to me and as a sailor (lapsed) with a physics degree I have
to say I find it immediately convincing. It's not controversial that you can
sail faster than the wind on some angles, so with a propeller in the mix I
don't understand why so many smart people were willing to make absolute
statements that this is not possible.

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LeifCarrotson
If you can go downwind 3x faster than the wind, can you turn and go
perpendicular to the wind using this mechanism? Can you turn again and go
directly upwind?

At a multiplier of 3x windspeed, it seems like the fact that there's wind at
all is irrelevant other than to get off the starting line. You're generating
your own wind power by pushing the propeller through the air. Is this correct?

Because if this works other than directly downwind, you have a perpetual
motion machine - scale up, drive in a circle, and hook a dynamo to a wheel.
Which makes me think I've neglected something somewhere...

Edit - from the Wired article:

 _" Skeptics think that the wind is turning the prop, and the car is turning
the wheels, and that's what makes the car go," Cavallaro said. "That's not the
case. The wheels are turning the prop. What happens is the prop thrust pushes
the vehicle."_

OK, now I'm really confused. I thought the prop rotation was screwing through
the air slower than the relative airspeed and driving the wheels - now you're
telling me that the wheels are spinning the prop to produce forward thrust?

If he can go from sitting still: wind speed -10, ground speed 0, to W0/G10,
and accelerate up to W20/G30, what happens if the wind drops to 0 mid-run? Can
he keep going at W20/G20, isn't that easier than W20/G30?

~~~
btilly
Here is a simplified picture.

The propellers are being pushed into a spin by the difference between the
airspeed and the groundspeed. (Note that the propellers are locked to the
ground by the wheels.) As they spin, the blades generate lift. That lift then
pushes the craft forward.

Take away the difference between airspeed and groundspeed, and nothing is
keeping the blades spinning. Therefore the system will run to a stop due to
friction.

The connection between spin and lift is mediated by aerodynamics, not a
mechanical linkage. And aerodynamics allows the vehicle to be pushed faster
than the spin. Which is why, if you get friction low enough, it can go faster
than the wind. And not slightly faster, but a lot.

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andrewflnr
So my question is, given the cylindrical Earth thought experiment (which I do
find quite compelling), why bother with the weird drive train involving the
wheels? Putting sails of the right proportion on a propeller shaft should be
enough.

~~~
LeifCarrotson
Because the cylindrical Earth experiment neglects the reaction forces of a
sailboat's keel, or the land-boat's wheels. The propeller shaft would just
rotate, you need that cylindrical Earth to have, essentially, a ballscrew
spline.

You can't generate the differential with clever aileron wings, either - has to
be in a different medium, otherwise the thing would just tumble.

~~~
andrewflnr
I think I get it. I would have thought plain, free-spinning wheels could serve
the role of the keel (I guess they do in a normal sail car), but you need to
transmit that keel action from the wheels on the ground to the sort of virtual
cylindrical Earth the propeller lives in.

~~~
Doxin
In a normal sail car you'd have to tack, but otherwise the same forces apply.
The clever thing about the propellor is you get a sort of cork-screw-tacking
for free allowing your motion to be in-line with the wind instead of having to
be at an angle.

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gibolt
I remember reading this in the physical magazine. Super interesting article
around physics and harnessing the wind.

Spoiler: They went almost 5x faster than the wind propelling the craft

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trhway
i think sailboat based explanation is useful only to the point. Airplane wing
provides better and simpler model outright. It becomes more intuitive when you
think how this craft goes straight into the wind (that is what sailboat can't
do).

