DWFTTW

[l33tminion]

Is it possible to make a wind-powered vehicle that moves directly downwind faster than the wind speed? Or is this a hoax? (The wheels of the vehicle in the video are connected by a drive train to the propeller / sail. Any force that accelerates the vehicle keeps the propeller going, too.)

At first glance, the physics seem to work out. It can work trivially at speeds lower than the wind speed, with the wind pushing the vehicle from behind. When the vehicle is at wind speed, it's friction versus the thrust from the propeller. Propeller thrust is unaffected by wind speed (working even in still air), and there's no apparent reason why the thrust from the propeller at that speed can't exceed the friction on the system. If that is the case, the vehicle will continue to accelerate.

Conservation of momentum works out fine at faster than wind speeds, same as thrust generally works (propeller pushes air back, which pushes vehicle forwards).

However, I can't figure out how that works with regards to conservation of energy. What is slowing down (or otherwise losing energy) as the cart speeds up? My first thought would be the propeller, but that doesn't work since it's connected to the wheels. The propeller cannot slow down without the vehicle slowing as well.

What do you think?

Comments

yes it is

[(Anonymous)]

"The wheels of the vehicle in the video are connected by a drive train to the propeller"

If the gear ratio of the propeller to rear drive wheels is greater than 1 then it is possible go faster than the wind speed. Neglecting all sources of friction the vehicle speed to wind speed ratio would be equal to the gear ratio. By designing a vehicle with low air resistance and optimizing the gear ratio one should be able to rather easily achieve faster vehicle speed than wind speed. Playing with the propeller pitch would also be helpful.

Also, if you turned the vehicle around and headed upwind the vehicle speed would equal wind speed assuming no friction or resistance.

-v-

[l33tminion]

I believe it's possible (I don't think the video is a hoax), I'm just still a bit confused about the physics involved.

[(Anonymous)]

This is a very interesting demonstration. I need to think more about how the propeller pitch affects the vehicle speed to wind speed ratio. For me, getting to the equations is always the last part.

I also have a funny feeling that this is really not a true downwind vehicle because of the propeller pitch.

-v-

[chiaki777.livejournal.com]

I'll say the totally crazy and dumb non-engineering skeptic thing to say.

the vehicle could be going downhill, the road just doesn't seem to look so. (Though, yes, I'll say that the odds are highly unlikely that the gradient is steep enough to create a speed so fast.)

but... aside from that, can't say much.

Oh, also, if it were possible, more colleges would have made experiments of the like and be entering competitions... >.>

[peristaltor.livejournal.com]

Yeah, the variables here appear to be not just wind speed but direction. Sailboats can easily exceed wind speed by traveling across the path of the wind.

Also, note in the video that the tattletale fluctuates in direction and intensity. This tells me that gusts may be boosting speed which doesn't appear on the vehicle until a bit later. The prop would be acting as a flywheel, briefly conserving the gust energy and doling it to the wheels. Note that most times after a gust, the vehicle catches up and passes the wind (according to the tattletale), but never sustains these speeds for long. Eventually the wind/road drags have their way.

I would be interested in seeing a vid with a real-time speedometer superimposed. Or maybe I should just build one of these? Yessss. . . .