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Down wind faster than the wind

Red Baron Farms said:
I have a question then. With no wind why couldn't you just tow the thing 10 MPH with the wheels driving the propeller.
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Yup, you could.

Then release the tow, and have the thing drive itself?
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Because when you release the tow, the pull force drops to zero immediately. So you are left with the forward force of the propeller driven from a 10 MPH wheel and the drag from a 10 MPH headwind and the prop drive. The drag wins. 10MPH drive + 10MPH drag. No acceleration.

Relative wind speed would be similar.
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Nope. With the wind behind you and going a hair over windspeed, you have the propeller driven from an 11 MPH wheel, but the drag from a 1 MPH headwind and the prop drive. The wheel-driven propeller wins.

Or if you're going double wind speed, you have the propeller driven from a 20 MPH wheel, but the drag from a 10 MPH headwind plus the prop drive. If the vehicle is sleek enough, prop still wins.
 
OK I think I got it now. Strange, and counter-intuitive, but it explains what we see. Basically the thrust from the propeller is more than the additional drag caused by moving forward, even though it was the wind that got it moving in the first place.
 
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The transition from slower than the wind to faster than the wind is misleading. There should be a steady accelleration relative to the wind before then.


What would make this easier to understand would be instrumentation. The dangling flag is leaving a lot out. What power is the fan putting out, for example? An accellerometer?
 
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a_unique_person said:
The transition from slower than the wind to faster than the wind is misleading. There should be a steady accelleration relative to the wind before then.


What would make this easier to understand would be instrumentation. The dangling flag is leaving a lot out. What power is the fan putting out, for example? An accellerometer?
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Did you watch ynot's video:
https://www.youtube.com/watch?v=QTF5kw51H60
The acceleration looks pretty "steady" before equilibrium.
I really like this video because the transition from going clockwise to counterclockwise appears much more intuitive than the nonintuitive idea of DDWFTTW even though basically the same thing is happening.
 
Into The Wind Faster Than The Wind

What if you turned the propeller around?

The device starts a zero velocity, the wind hits it turning the propeller,
the propeller then turns the wheels and it picks up speed. The wind would
look stronger which would increase the rotation of the propeller which would
speed up the wheels. This action repeats again and again until it hits light speed.
 
Solitaire said:
What if you turned the propeller around?

The device starts a zero velocity, the wind hits it turning the propeller,
the propeller then turns the wheels and it picks up speed. The wind would
look stronger which would increase the rotation of the propeller which would
speed up the wheels. This action repeats again and again until it hits light speed.
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That's basically how the "directly upwind faster than the wind" configuration of the Blackbird works. Except it can't reach light speed, because once it reaches 88 miles an hour, it goes back in time.
 
Inspired by the recent Veritasium video, Steve Mould did a video this week addressing some of the basic principles involved in sailing faster than the wind. (He does also mention the "apparent wind" but doesn't go too much into that aspect.)

"Sailing Faster Than The Wind - How Is That Even Possible?"

Direct link:

https://www.youtube.com/watch?v=u5InZ6iknUM

...or here:

 
Red Baron Farms said:
I have a question then. With no wind why couldn't you just tow the thing 10 MPH with the wheels driving the propeller. Then release the tow, and have the thing drive itself? Relative wind speed would be similar. But I don't see that working. So I am still thinking this thing has something wrong.
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Without wind, what you have is basically a perpetual motion machine. It's akin to an electric motor driving a generator which powers the motor. When running in the wind, the relative motion of air and ground is the actual energy source for the thing. If there were no aerodynamic drag, no rolling resistance, no friction in the bearings, etc. it would keep going forever without a tailwind. Then again, if those losses didn't exist, you wouldn't need the propeller or the gear train to drive it, as it would keep rolling forever anyway. One of the reasons the concept drew some vehement deniers is that, at first glance, it looks like a perpetual motion machine. But, if you have wind, it really is just a clever method of making the wind drive it faster than the wind itself is moving.
 
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smartcooky said:
As someone who has done quite a bit of sailing, the "penny drop" moment for me was section on this video from 6:45 to 8:38
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Back in 2008, when this first hit this forum, I had a hard time wrapping my head around how it worked, until they started doing the treadmill tests. From that frame of reference, it made perfect sense to me: Treadmill drives wheels, propeller moves the cart forward. But it was obvious (to me, anyway) that the wind over ground case was simply a change in frame of reference, and that if one worked, the other had to work.
 
Red Baron Farms said:
I have a question then. With no wind why couldn't you just tow the thing 10 MPH with the wheels driving the propeller. Then release the tow, and have the thing drive itself? Relative wind speed would be similar. But I don't see that working. So I am still thinking this thing has something wrong.
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For a similar reason that you can't tow a yacht to 10MPH in becalmed conditions and let it go, and it expect it to carry on sailing, or take a aeroplane to 10,000 feet and let it go with no forward speed and expect it to climb.

As a yachtie, I understand perfectly that the VMG of a downwind tacking (or more correctly, gybing) yacht can be greater than the wind-speed, but the VMG of a yacht is zero kts in no wind.
 
CORed said:
Without wind, what you have is basically a perpetual motion machine. It's akin to an electric motor driving a generator which powers the motor. When running in the wind, the relative motion of air and ground is the actual energy source for the thing. If there were no aerodynamic drag, no rolling resistance, no friction in the bearings, etc. it would keep going forever without a tailwind. Then again, if those losses didn't exist, you wouldn't need the propeller or the gear train to drive it, as it would keep rolling forever anyway. One of the reasons the concept drew some vehement deniers is that, at first glance, it looks like a perpetual motion machine. But, if you have wind, it really is just a clever method of making the wind drive it faster than the wind itself is moving.
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And for this reason, there will be a low end speed limit, one below which you cannot travel faster than the wind. You need a certain amount of speed to overcome these losses, and its the same in yachting - VMG can only exceed wind speed once the speed reaches a minimum, the value of which is dependant on the yacht's design, and the conditions in which it is sailing.

I imagine it is somewhat akin to stalling speed in an aircraft... you need a certain about of minimum forward speed (~V2) before you generate enough lift to overcome the primary losses... drag and gravity.
 
DDWFTW is easily explained. You push the cart with a truck. 14 mph up wind in a 10 mph wind. The cart's apparent wind speed is now 4 mph , spinning the prop. Geared to the wheels, the wheels drive it forward. Simple.

A variablepitch prop would be like adjusting the main sheet on a boat. Change it to suit the apparent wind speed. A variable transmission would help too. Now you can sail in circles.
 
a_unique_person said:
The transition from slower than the wind to faster than the wind is misleading. There should be a steady accelleration relative to the wind before then.


What would make this easier to understand would be instrumentation. The dangling flag is leaving a lot out. What power is the fan putting out, for example? An accellerometer?
Click to expand...

The initial acceleration is caused by simple drag, but once the cart is moving, the wheels and propeller are turning, and, as its speed increases, the propeller starts to generate thrust, and can generate enough thrust to overcome drag above wind speed.
 
While the previous video was more of a demonstration with a little bit of explanation, Derek released another with much more explanation.

 
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