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Down wind faster than the wind

jjcote said:
Most human-powered airplanes in the past 35(?) years have used Berg chain, as DanO noted. This is a pair of steel cables with plastic buttons molded between them so that it will work with standard bicycle sprockets...
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This seems like the perfect solution. JB and I talked about fabricating something similar - but it would have required machining our own sprockets as well. Do you know if this stuff is available off the shelf?

EDIT:

A bit of googling suggests we can get it from: http://www.wmberg.com/pages/contactus.aspx
I should have known. We got some of our original stuff from them for our first cart.
 
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Here's an analysis of DDWFTTW by Mark Drela:

http://www.boatdesign.net/forums/at...30779009-wind-powered-sail-less-boat-ddw2.pdf

I haven't been able to understand all of it, being hampered by lack of knowledge of certain technical terms. I'll do my best to look for help on the Net, but perhaps for a start the specialists on this thread can help me with these terms:

- air prop induced efficiency
- air prop profile efficiency
- air prop swirl efficiency
 
I can't get either of Mark Drela's pdf files (download blocked by FF3). Is there another way to read these?

Regulatory Agency it is!
 
mender said:
New test: treadmill inclined at 2.7 degrees, check break-even speed with different props.

10.2 mph with 12x8
8.1 mph with 15x7.5

Analysis: larger prop influences a larger mass of air, reducing the speed change needed to develop the same force. Same amount of cart drag is present (tested against a scale at a variety of speeds, is pretty consistent from rest through the speed range at about 14 grams). Area of bigger prop is 58.3% more than smaller prop. Momentum imparted to air because of the higher speed with smaller prop per square inch is 58.6%. F=MA still applies!

Variables not accounted for, difference in prop pitch and efficiency - but the numbers were surprisingly close.

Net force is the same for both props, as indicated by the cart climbing the incline ever so slightly. Distinctly different speeds needed, no wheels sliding.

I'm running my cart with the prop at the front (prop oriented correctly of course) because I have a load cell at the back of the treadmill that the cart gearbox rests against for force measurements. The small wheel on my cart momentarily goes into a wobble every so often and noticeably slows the cart. Other than that, the cart runs quite consistently and readings from the load cell are also consistent. I'll refine the test a little more.
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Great that you are doing independent testing. Any chance of video? Are you still going to test using a turntable as well?
 
Yes, I plan on doing the turntable test but it may be a little while before I get to that. No videos yet. They would look just like the ones already out there.
 
I just found this, by a German guy called Klaus Wiederhoeft:

"Downwind for dummies: how to launch a hot air balloon, kite downwind and catch that balloon again".

It starts with an explanation of how to go downwind faster than the wind by kitesurfing. On pages 32-34 you can see Wiederhoeft's cool design for a "Prop Missile", which in nothing other than a DDWFTTW machine:

166124961297372e45.png


He's imagining it pretty big: the little yellow part is the cockpit! There is some undefined means of power transmission between the two red wheels and the prop.
 
mender said:
Yes, I plan on doing the turntable test but it may be a little while before I get to that. No videos yet. They would look just like the ones already out there.
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Are you planning to do any endurance testing on a turntable to see if the “effect” is sustainable?
 
ynot said:
Are you planning to do any endurance testing on a turntable to see if the “effect” is sustainable?
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I'm quite certain that mender doesn't need a turntable to know the the "effect is sustainable". A simple treadmill will tell you that after all.

JB
 
Ynot, I'm more interested in quantifying the speed above windspeed (the speed counter to the turntable's rotation) to test a few notions of mine.

For some of my testing so far, I ran the cart on an incline for several minutes at a time. No question in my mind that it is a continuous state once achieved.

Actually, before spork and JB posted their video of the cart running against the inclined treadmill, I didn't think it would be possible to get it balanced that well. From my own testing I can tell you that it is trickier than it looks!

Once again, kudos to spork and JB! I can see that a competition with you two will be very hard fought - and I like that!
 
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Michael C said:
I just found this, by a German guy called Klaus Wiederhoeft:

"Downwind for dummies: how to launch a hot air balloon, kite downwind and catch that balloon again".
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Pretty interesting story behind that document. It starts here:

http://tinyurl.com/2h9vjq when I claimed it could be done in June of 2006. On page 4 of that thread I provide a number of ways to do it. By page 16 of that thread Klaus is beginning to soften on his claim that it's impossible.

Klaus then starts a new thread here http://www.kiteforum.com/viewtopic.php?f=1&t=2329828&st=0&sk=t&sd=a to announce this document. Klaus is an intelligent guy, and a good guy, but I have to admit I was a bit tweaked that he "published" a document including most of my ideas and arguments with no credit to me.
 
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mender said:
Actually, before spork and JB posted their video of the cart running against the inclined treadmill, I didn't think it would be possible to get it balanced that well. From my own testing I can tell you that it is trickier than it looks!
Click to expand...

I'm not sure we would have been able to do it at all without hanging the prop off the belt -- this gave us a small window of stability where the prop wanted to generally stay. Still it gradually got out of that narrow band as it oscillated.

JB
 
humber said:
I have demonstrated that I can tell belt world from real world
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Really you can?

Ok please demonstrate how you can tell which is which in the following example:

Alien Space Bats transport you from your home in the middle of the night whilst you are asleep.

You wake up in the "morning" in an exact replica of your house, the only difference as far as you can see is that you've landed in what appears to be a grass savannah.

The sky is bluer than you've ever seen before, and there are no clouds in the sky nor can you see the sun, yet strangely the area is illuminated.
Every time you try to walk further than 1 mile from your house, no matter the direction, you are instantly transported back to your house and you're still unable to see anything other than grassland.
Barring these minor differences the world is exactly the same as the Earth.


All that you can see around you is grass blowing in the wind and a windsock at the front of your house that is standing perpendicular to its pole and pointing to the right parallel to your house as you face your house.
You have a wind gauge that is telling you that the wind is moving at the rate of 10mph.

Is it possible that this house is actually:

a) Stuck to a giant grassed treadmill, moving towards the left side of your house as you face it?
/ \ >
___|__|__|__
<- Belt travelling this way

b) Stationary with a 10mph wind blowing from the left side of your house towards the right side?

Wind travelling this way
-> / \ >
___|__|__|__


c) either of the above.

If you've answered b) please provide proof of how you can determine this with the tools at your disposal (ie a Windsock, a featureless savannah and a wind gauge).
 
ThinAirDesigns said:
I'm quite certain that mender doesn't need a turntable to know the the "effect is sustainable". A simple treadmill will tell you that after all.

JB
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The only “endurance test” I’ve seen is the video where the cart hovers towards the back end of the treadmill and ends up falling off that end. Spork said something to the effect that a constant hover isn’t possible to hold indefinitely and that eventually the cart will go off either end. He said it was a “coin toss” as to which end it went off. I have never seen a video of a cart going off the top end from a hover however. The advantage of a turntable is there is no ends to fall off and any variance in “expected” speed can be easily observed and monitored.
 
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mender said:
For some of my testing so far, I ran the cart on an incline for several minutes at a time. No question in my mind that it is a continuous state once achieved.
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Hope I get the same clear result when I get the time to rebuild my cart and start testing.
 
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