Downwind Faster than the Wind - Successful Run by manned cart

RAI said:
Yes.

At the start, the pressure, or drag, of the wind on the entire structure including propeller is trying to push the cart forwards. To move it forwards, either the wheels must turn or slip. As the road to wheel friction is great, the wheels must turn. To turn they must turn the propeller through the sprocket-chain-sprocket connection. The propeller would prefer to turn in the opposite direction because of the lift on its blades. (Only the blades are fairly stalled at this angle of incidence, so lots of drag and not much lift.) As the friction on the wheels has a mechanical advantage over the lift induced torque on the propeller, the cart can move forward with the propeller turning to oppose the wind.
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I admire the persistence of you, Snowleopard, Ubergeekian and others in trying to explain the obvious.

I have less patience. If they can't understand after such full explanation, you are wasting your energy.
If they wont understand, they are dickheads.... :)
 
dk said:
I don't see friction in this schematic! Surely the most important force on the vehicle.
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That schematic is only to show that there is useful force available even at speeds higher than the wind speed when travelling dead down wind. How much higher is another matter. The team designed to get 2 times the wind speed down wind but now think they may get to 3 times the wind speed DDWFTTW. For an analysis, google "DDWFTTW theory".
 
dk said:
I don't see friction in this schematic! Surely the most important force on the vehicle.

Like many others brought up with a traditional scientific education you will have to put me in the sceptics box. I would be more curious if we saw the vehicle continue for a considerable period, when I believe it would settle down to a velocity just below the true wind speed - just my opinion, but I'll stick with it until I see different!
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Interesting shift of opinion:
From: "It will never exceed windspeed"
To: "Well, it did exceed windspeed, but maybe sometime later it won't!":)
 
I'm sorry, but when, on the second run, the vehicle is stationary, as the prop starts to rotate it is definitely not being rotated by the wind- look at the pitch! The overall aerodynamic drag must exceed the power being generated by the prop, so the wheels are turning the prop the wrong way. At what point does this braking effect suddenly become thrust?
 
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tobermoryphil said:
I'm sorry, but when, on the second run, the vehicle is stationary, as the prop starts to rotate it is definitely not being rotated by the wind- look at the pitch!
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Yes. The mechanical advantage of the friction on the wheels drives it against the wind.
 
The whole point is it is supposed to be going WITH the wind! I can see that, once the apparent wind is from ahead, the prop will start to provide forward motion via the wheels. But with it acting as a brake up to the point of no apparent wind, how do you start going faster than the apparent wind?
 
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If you look at the clip posted by Keen Ed, it starts with the vehicle and the prop satationary. The prop then starts windmilling in the wind - apparently in the wriong direction for wind coming from the left.

I am a physics graduate butconfess to still be confused by all this. If it is for real surely the prop would have started in the opposite direction
 
Tommyrot said:
I admire the persistence of you, Snowleopard, Ubergeekian and others in trying to explain the obvious.

I have less patience. If they can't understand after such full explanation, you are wasting your energy.
If they wont understand, they are dickheads.... :)
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+1000000
 
davidej said:
If you look at the clip posted by Keen Ed, it starts with the vehicle and the prop satationary. The prop then starts windmilling in the wind - apparently in the wriong direction for wind coming from the left.

I am a physics graduate butconfess to still be confused by all this. If it is for real surely the prop would have started in the opposite direction
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The prop isn't windmilling. It's connected to the wheels. It's not a turbine/energy collector. It's a propellor - device driver. You can't go DDWFTTW with the prop as turbine - as shown in the clips of small carts, where they race a turbine cart against a propellor cart.

In this clip, they lose drive when something breaks. The prop then starts windmilling as expected.

 
Keen_Ed said:
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Well in the poll I voted that I'd keep my opinion open until I see a manned version actually doing the deed.

Assuming that video is not faked (and I hope not) I am now convinced, and very impressed. My heart (and logic) says it is impossible but I must believe my eyes.
 
On that blowup run, the vehicle appears to be pushed by the car until the apparent wind is from ahead, at which point it works. Now that, I can understand!
 
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tobermoryphil said:
On that last run, the vehicle appears to be pushed by the car until the apparent wind is from ahead, at which point it works. Now that, I can understand!
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That's just saving time. If you look at the second video, it accelerates to DDWFTTW from stationary, with no external help (other than the wind! :-)). It just takes time. In the same way that bicycling up hill in a high gear takes time.

All vids here: http://www.youtube.com/user/TraderTurok
 
Keen_Ed said:
The prop isn't windmilling. It's connected to the wheels.
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I realise that! BUT

with both vehicle and prop stationary, the only horizontal force on the system is the wind. If you have a windmill with brake engaged on the propellor shaft, the blades would 'try' to windmill and exert torque on its shaft . If the brake is then released it would begin to turn in the direction dictated by the sense of the blades.

This appears to do the opposite.

Are you implying that the wind force on the vehicle starts it off downwind and the gearing is such that it can turn the blades in the opposite direction.

That doesn't seem very likely to me given that the 'frontal area' of the blades looks large compared with that of the vehicle.

I fell like Doubting Thomas - I want to beleive but ' until I put my hand..."
 
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