Directly Downwind Faster than the Wind on Discovery Channel

Discovery Channel aired a report on the wind powered vehicle that goes directly downwind faster than the wind.

World record now official : 2.8 x windspeed, directly downwind:
http://www.nalsa.org/DownWind.html

Full Clip:
http://watch.discoverychannel.ca/daily-planet/may-2010/daily-planet---may-04-...

Project blog:
http://www.fasterthanthewind.org/

Category:

Science & Technology

Tags:

• DDWFTTW
• DWFTTW
• directly
• downwind
• faster
• than
• the
• wind
• power
• propeller
• cart
• brain
• teaser
• physics
• puzzle
• riddle
• sailing
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• car

[technolope]

PhD in Aerospace Engineering here, and it took me a little while to grok this, but I can see now how this is possible. The key to figuring it out (for me) was to recognize that this will only work on a vehicle for which there is significant friction with the ground. When the car is moving faster than the wind, the wind isn't pushing the car forward, the wheels are. But the wheels get power from the (relative) headwind.

[piratewoodleg]

@technolope

No, the wheels are not pushing the car forward, but braking, while turning the prop which creates more thrust than they brake.

The wheels cannot get the power from the (relative) headwind (via a turbine). That turbine would push the air forward, and thus accelerate the (true) tailwind. It would generate more true wind. To harvest wind energy , you have to slow down the true wind. Here that means: push the air backward with a propeller, turned by the wheels.

[technolope]

@piratewoodleg

You are totally right. I had my concept correct, but got my result wrong. In all regimes, the prop turns the same direction; but at start-up, power goes from prop to wheels, and at super-wind, power goes from wheels to prop. Top speed should be determined by energy conversion efficiencies and optimized by gearing. This is a wonderfully counterintuitive thought experiment.

[piratewoodleg]

@technolope

Your idea of using turbine-mode below windspeed, and switching to prop-mode later is correct, and would improve the initial acceleration. But they didn't use it (to make it even more counterintuitive).

On this specifical vehicle the power NEVER goes from the prop to the wheels via the transmission. The wheels ALWAYS turn the prop. Even at start up the prop rotates opposite to torque from the air, and according to the torque from wheels:

watch?v=5CcgmpBGSCI

watch?v=EEuAqq8FINw

[honestjohnnfld]

@piratewoodleg

You are wrong. Works in a windmill boat, faster than wind down wind.

This is an old principle and was first brought to light in the late 1960s early 1970s in a Marine Architect paper in Boston, MA.

My efforts were in a senior design paper at UW-Madison in mid 1980s. Got in Capital Times, front page for my scheme, The Prairie Schooner. Built a 10 foot dia 3-blade prop and tested it.

Bravo to Steve Calavarro ( sp. )

John Archibald ( leadminer@qwest.net )

Northfield, MN

[piratewoodleg]

@honestjohnnfld

Hi John,

The wheels turn the prop. The rotor cannot work as a turbine (or windmill) above wind-speed directly DOWNwind for two reasons:

1) It would accelerate the true wind instead of slowing it down to extract wind energy

2) They have ratchet hubs, that prevent the rotor from turning the wheels

Maybe you mean the directly UPwind case? There the rotor is a turbine(windmill) and can go faster than wind too. Rick Cavallaro will try it this summer.

[technolope]

@piratewoodleg I thought I was commenting under your comment. I must be doing something wrong.

[00bean00]

There is something stupider than the Internet?! :P

[piratewoodleg]

@eurogoldexchange

Yes that is the trick: Just the "sails" a tacking here, not the entire yacht.