TylerDurden
100 GW
Discussion continues here:
http://endless-sphere.com/forums/viewtopic.php?f=3&t=47528
http://endless-sphere.com/forums/viewtopic.php?f=3&t=47528
Car Goes Downwind 2.5 Times Faster Than the Wind
- Thread starter Papa
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busted_bike
100 mW
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- Jun 24, 2010
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- 42
I haven't read the whole thread, but I think I get it - maybe - so I'll try my own twist on an explanation. Forgive me if someone has already taken the same tact on an explanation.
First, it seems obvious that a tailwind turning a prop geared to the wheels would make the thing move forward, at least to the point of zero apparent wind. The question I kept coming back to and the thing that puzzled me was: why doesn't the prop start turning in the opposite direction once zero apparent wind speed is exceeded?
I believe the answer is something along these lines: At the point the vehicle reaches zero apparent wind, the prop is turning at a fairly high rate - effectively pulling air through it and pushing it back against the tailwind. What that means is that the point of zero apparent wind relative to the vehicle isn't zero apparent wind relative to the prop. Stating another way, the tailwind is effectively pushing on a cushion of air behind the prop that's moving at some speed in between the apparent wind relative to the vehicle and the wind relative to the ground, allowing further energy harvesting. It's only when that cushion of air reaches the same velocity as the tailwind that the vehicle stops accelerating.
First, it seems obvious that a tailwind turning a prop geared to the wheels would make the thing move forward, at least to the point of zero apparent wind. The question I kept coming back to and the thing that puzzled me was: why doesn't the prop start turning in the opposite direction once zero apparent wind speed is exceeded?
I believe the answer is something along these lines: At the point the vehicle reaches zero apparent wind, the prop is turning at a fairly high rate - effectively pulling air through it and pushing it back against the tailwind. What that means is that the point of zero apparent wind relative to the vehicle isn't zero apparent wind relative to the prop. Stating another way, the tailwind is effectively pushing on a cushion of air behind the prop that's moving at some speed in between the apparent wind relative to the vehicle and the wind relative to the ground, allowing further energy harvesting. It's only when that cushion of air reaches the same velocity as the tailwind that the vehicle stops accelerating.
Chalo
100 TW
busted_bike said:
From the reading I did on it a while back, the prop is pitch-adjustable. As the vehicle crosses through the range of zero apparent wind, the prop pitch is changed so that instead of the prop driving the wheels, the movement of the wheels drives the prop, which in turn pulls the contraption through the air.
The energy source is the velocity gradient between the wind and the ground, not between the wind and the sail
(because in a wheeled vehicle, the contact patches of the wheels are coupled to the ground).
The DWFTTW car works because its combined mechanical friction and aerodynamic drag are so low, and its prop efficiency is so high, that the exploitable energy difference between moving air and stationary ground is not wiped out by efficiency losses. Because the net energy return of such a system is very small, it's unlikely to have any practical applications (we'll never see "uphill faster than the wind"), but it is a cool proof of concept. I certainly didn't think it was possible when I first heard folks talking about DWFTTW may year ago.
John in CR
100 TW
Sorry Chalo. The prop pitch is only set like a wind turbine to self start. Crossing through 0 relative wind is irrelevant. The wheels are turning the prop. The prop only turns the wheels to get started. The prop is pushing against an oncoming wind.
The vehicle works straight into the wind too, though much slower. For that, yes the wind drives the prop, which drives the wheels to move the vehicle.
Traveling with the wind the spinning prop is blowing air against the wind. The entire prop system is like a sail running downwind, but the wind is blowing against a cushion of air coming off the prop instead of a sail. Wind pushes against that system causing the wheels to turn the prop faster and capture more of the wind energy of the swept area of the prop, maybe even more than the swept area of the prop if the cushion of air it blows becomes larger than its swept area. Those wheels are a drag at the ground, like a hubmotor in regen, so if it was shifted to neutral at speed, there would likely be a momentary surge forward until aerodynamic drag of the prop slows the prop.
John
The vehicle works straight into the wind too, though much slower. For that, yes the wind drives the prop, which drives the wheels to move the vehicle.
Traveling with the wind the spinning prop is blowing air against the wind. The entire prop system is like a sail running downwind, but the wind is blowing against a cushion of air coming off the prop instead of a sail. Wind pushes against that system causing the wheels to turn the prop faster and capture more of the wind energy of the swept area of the prop, maybe even more than the swept area of the prop if the cushion of air it blows becomes larger than its swept area. Those wheels are a drag at the ground, like a hubmotor in regen, so if it was shifted to neutral at speed, there would likely be a momentary surge forward until aerodynamic drag of the prop slows the prop.
John
That's pretty cool, but perhaps not so practical. I think it would be great for a boat though.
Chalo
100 TW
John in CR said:
I don't think you're making yourself very clear. Or else you are mistaken.
Now you're sounding much more mistaken than unclear.
The prop is making thrust (pulling, like an airplane's prop), driven by the wheels. That's what I'm saying. Is that what you're saying?
itchynackers
100 kW
I think John is correct here. If you pretend for a second that the prop is not spinning... During the initial acceleration, the prop is a sail (even though it is spinning the wrong way). Once apparent wind is reached, then the prop acts like a prop.
busted_bike
100 mW
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- Jun 24, 2010
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Unfortunately, I neglected to look at the date when I read this post - I didn't realize I was complicit in thread necromancy. Nevertheless, I missed it the first time around and it is something interesting to ponder.
But I agree with John - pitch-adjustable blades on the prop may be helpful, but aren't entirely necessary. With a headwind, the wind drives the prop which in turn drives the wheels. With a tailwind, the wind pushes the car which turns the wheels which in turn drives the prop (opposite the direction I had initially expected) which generates the air cushion for the tailwind to push against.
The best way to grapple with this (at least for me) is to think about what's going on when the car is at a standstill and again at zero apparent wind.
But I agree with John - pitch-adjustable blades on the prop may be helpful, but aren't entirely necessary. With a headwind, the wind drives the prop which in turn drives the wheels. With a tailwind, the wind pushes the car which turns the wheels which in turn drives the prop (opposite the direction I had initially expected) which generates the air cushion for the tailwind to push against.
The best way to grapple with this (at least for me) is to think about what's going on when the car is at a standstill and again at zero apparent wind.
John in CR
100 TW
Chalo,
With the wheels being a drag to spin the prop, I'm not sure you can look at it as the prop pulling the vehicle thru the air. That's what would happen for an instant if the drive was put in neutral. It could be semantics, but maybe not. It may depend on what is the true maximum energy that can be harvested. Is it just the wind velocity relative to the ground over the swept area of the prop, or is it something more with a component somewhat similar to a ground effect?
Sure I may not have explained it well, but explaining how the thing works isn't easy, and it doesn't click into focus until you look at it in the right way. Your mention of prop pitch and 0 relative air speed meant that you didn't understand the machine, so don't try playing word games to insinuate that I don't. It came into focus for me way back when it was just a small unmanned wooden structure with a prop driven by the wheels in a grainy youtube clip that proved the concept to the world in less than 100ft before it drifted off the road.
John
With the wheels being a drag to spin the prop, I'm not sure you can look at it as the prop pulling the vehicle thru the air. That's what would happen for an instant if the drive was put in neutral. It could be semantics, but maybe not. It may depend on what is the true maximum energy that can be harvested. Is it just the wind velocity relative to the ground over the swept area of the prop, or is it something more with a component somewhat similar to a ground effect?
Sure I may not have explained it well, but explaining how the thing works isn't easy, and it doesn't click into focus until you look at it in the right way. Your mention of prop pitch and 0 relative air speed meant that you didn't understand the machine, so don't try playing word games to insinuate that I don't. It came into focus for me way back when it was just a small unmanned wooden structure with a prop driven by the wheels in a grainy youtube clip that proved the concept to the world in less than 100ft before it drifted off the road.
John
Hillhater
100 TW
Errr? .. I think the WHEELS are pretty important to make this work ! :wink:fechter said:
Hillhater said:
Underwater propeller instead of wheels. Of course there is waaaay more loss, but the same principle should apply.
John in CR
100 TW
It's been done, though I don't think they've tried the same method as the car for running downwind faster. Props have a lot of "slippage" in the water anyway, and the hull has a lot of friction through the water.. The selling point is being able to sail in any direction, including directly into the wind. Me I couldn't get past that spinning prop above deck.
http://www.youtube.com/watch?v=IzGCYaJbf0A
http://www.youtube.com/watch?v=IzGCYaJbf0A
Ah, it's been done.
Yes I can see how having a deadly spinning blade would be a downside.
Yes I can see how having a deadly spinning blade would be a downside.
doctorGONZO
100 W
This is still an enigma to me. Don't have the car figured out yet so it is still a tentative hoax in spite of the mountain of testimonials. I know a little about physics and it just doesn't add up for me yet.
The guy in the boat with the prop an inch over his head: NO A INHALE. Either has a deathwish or is f'ing goofy. Maybe the boat is fast, build mine with a moved prop.
The guy in the boat with the prop an inch over his head: NO A INHALE. Either has a deathwish or is f'ing goofy. Maybe the boat is fast, build mine with a moved prop.
itchynackers
100 kW
It's not that hard. A balloon is powered by the wind, and can only be as fast as the wind. A wind vehicle in contact with the ground is powered by the wind AND the ground, and can move faster than the wind.
busted_bike
100 mW
- Joined
- Jun 24, 2010
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- 42
itchynackers said:
And yet a vehicle powered only by the ground goes nowhere... :wink:
Perhaps you're oversimplifying?
itchynackers
100 kW
busted_bike said:
Perhaps I need to. But it ISNT powered only by the ground. Nor is it powered only by the wind That is the point. It is the differential between the ground and the air that is the energy source. Just like your food isn't just Carbon, Hydrogen, or Oxygen.
doctorGONZO
100 W
itchynackers said:
I figure you are meaning to be helpful, but such abstract talk is just word salad. Trace the path one step at a time from wherever the energy originates all the way to the outcome of accelerating the car. Otherwise you're just chanting a mantra and talking of magic.
Beachcruzer
100 W
This is brilliant concept and execution, but I'm surprised at how many people refuse to believe it's real. As LFP said way back on the first speed of this thread, people have been sailing faster than the wind for thousands of years. The same aerodynamic principles are in play with Blackbird. The difference is the Blackbird developers have found an ingenious way to apply those principles so that they work when traveling dead downwind as opposed to across the wind.
A sail creates aerodynamic lift to drive a boat (landsailer, windsurfer) forward, and that lift increases with the speed of the vessel. That in turn makes the vessel go even faster, make more lift, etc. The efficiency of this relationship varies depending on the point of sail (the boat's angle of travel relative to the wind). The fastest point of sail for most modern sailplans is close-hauled, or basically as close to the wind as you can get, because that angle produces the most airflow past the sail for a given speed. The slowest point of sail is dead downwind, because the sail can't cut through the wind and create aerodynamic lift. It's just a bedsheet.
With a traditional sail rig it's impossible to sail dead downwind at greater than windspeed. This isn't really a problem in real life, because by then you're headed back to the dock and you can still jibe back and forth and have fun but without all the heeling that makes it hard to hold a beer.
Anyway, the Blackbird guys have simply developed a mechanism to go from bedsheet mode to aerodynamic mode. The prop's pitch and rotation is the same as if it were driven by a motor. Instead of a motor, though, it's driven by the rotation of the wheels. The wheels get started rotating when the vehicle is in bedsheet mode--getting blown very slowly and inefficiently downwind. As that wheel rotation is translated into prop rotation, the prop gains aerodynamic efficiency. You can see this point in the video. You're just about to nod off watching this carbon fiber albatross blowing slowly across the lakebed when all of a sudden it comes alive. The prop begins to push the vehicle, which in turn makes the wheels spin faster, which makes the prop spin faster and develop more power. Pretty soon it's hauling ass.
This machine doesn't defy the principles of aerodynamics as the DDWFTTW naysayers would have you believe. It harnesses them. Kudos.
A sail creates aerodynamic lift to drive a boat (landsailer, windsurfer) forward, and that lift increases with the speed of the vessel. That in turn makes the vessel go even faster, make more lift, etc. The efficiency of this relationship varies depending on the point of sail (the boat's angle of travel relative to the wind). The fastest point of sail for most modern sailplans is close-hauled, or basically as close to the wind as you can get, because that angle produces the most airflow past the sail for a given speed. The slowest point of sail is dead downwind, because the sail can't cut through the wind and create aerodynamic lift. It's just a bedsheet.
With a traditional sail rig it's impossible to sail dead downwind at greater than windspeed. This isn't really a problem in real life, because by then you're headed back to the dock and you can still jibe back and forth and have fun but without all the heeling that makes it hard to hold a beer.
Anyway, the Blackbird guys have simply developed a mechanism to go from bedsheet mode to aerodynamic mode. The prop's pitch and rotation is the same as if it were driven by a motor. Instead of a motor, though, it's driven by the rotation of the wheels. The wheels get started rotating when the vehicle is in bedsheet mode--getting blown very slowly and inefficiently downwind. As that wheel rotation is translated into prop rotation, the prop gains aerodynamic efficiency. You can see this point in the video. You're just about to nod off watching this carbon fiber albatross blowing slowly across the lakebed when all of a sudden it comes alive. The prop begins to push the vehicle, which in turn makes the wheels spin faster, which makes the prop spin faster and develop more power. Pretty soon it's hauling ass.
This machine doesn't defy the principles of aerodynamics as the DDWFTTW naysayers would have you believe. It harnesses them. Kudos.
itchynackers
100 kW
I tried simplifying it for some people. Now others won't understand the way Beach described it. Apparently, it is too simple and too complex to understand.
Beachcruzer
100 W
itchynackers said:
I enjoy word salad with vinegar and a bit of olive oil. Can't resist a second helping, so . . . The ground doesn't power the car; the wind powers the car. What the ground does, or more accurately what the fixed connection between the wheels and rotor does, is allow the system to harness aerodynamic lift while moving dead downwind. Wind-driven vehicles have been using aerodynamic lift in order to travel faster than the wind for a very long time, but they have not been able to do it while traveling dead downwind until now.
Beachcruzer
100 W
Slightly off-topic, but landsailing at El Mirage (where the Blackbird video was shot) is a total hoot. My buddy built a landsailer out of square steel tubing and a little girl's bike, and I had it up to 45 mph in 20 knots of wind the first day I tried it. Also, dry lakes are great for eBiking.
he's got such a long way to go.
to make it to the border of mexico.
to make it to the border of mexico.
endlessplane
1 W
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- Mar 4, 2013
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- 53
itchynackers said:
This is a rather abstract way to put, but basically correct. Here is an animation which visualizes this. "PASSIVE SAIL" corresponds to a balloon without ground contact drifting downwind at windspeed. With ground contact you can go faster than that.
[youtube]g8bxXRQtcMY[/youtube]
endlessplane
1 W
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- Mar 4, 2013
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- 53
It depends in which frame of reference.busted_bike said:
With contact to air only, you can move relative to the ground at windspeed max, powered by the air.
With contact to ground only, you can move relative to the air at windspeed max, powered by the ground.
With contact to ground and air, you can move at any speed and direction relative to both, powered by the velocity difference between ground and air.
Understanding this symmetry makes it easy to grasp downwind and upwind faster than wind.
[youtube]za_rPKSwiyc[/youtube]
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