Mess with your minds: A wind powered craft going faster than a tail wind speed. - Page 1
Forum section: electronics / general

author: BrianHG
author_rank: Super Contributor
timestamp: May 30, 2021, 12:12:01 am
content: A wind powered craft going faster thanA TAILwind speed.It's for real...Anyone here thinks it's impossible?If so, watch the video again...

author: ataradov
author_rank: Super Contributor
timestamp: May 30, 2021, 12:20:01 am
content: I don't get all the apparent resistance from the people once the principle is explained. I somehow have no issues with it and don't really need a full scale replica to believe it.Now, airplane taking off from a treadmill still takes an effort to figure out.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 30, 2021, 12:44:19 am
content: Quote from: ataradov on May 30, 2021, 12:20:01 am

author: IanB
author_rank: Super Contributor
timestamp: May 30, 2021, 12:51:04 am
content: Quote from: BrianHG on May 30, 2021, 12:12:01 amI'm not sure why I would think it's impossible?For an EE analogy: I plug an inverter into a 12 V socket and get a 120 V output voltage which is higher than the input voltage. Anyone here think that's impossible?

author: Red Squirrel
author_rank: Super Contributor
timestamp: May 30, 2021, 12:55:34 am
content: I've always been intrigued by a similar idea myself: Could a wind powered craft go AGAINST the wind? Basically you're still harnessing the wind's energy so it's not really overunity but at same time the idea of going against the wind by using the wind feels like it breaks laws of thermodynamics. I think it would be equivalant to using a hydro electric generator to pump water back up the dam.

author: BrianHG
author_rank: Super Contributor
timestamp: May 30, 2021, 12:59:02 am
content: I only needed 1 important fact in advance to know that it was possible. That sail-boats who tack at an angle achieve a relative forward velocity greater than the wind speed forward velocity because of the air-foil effect on curve bulge of the sail. Then I knew it could be done.I'm not convinced the inverter analogy made by 'IanB' is correct as you even loose power/wattage on the higher voltage side due to efficiency, but I guess I can go with it.

author: Rerouter
author_rank: Super Contributor
timestamp: May 30, 2021, 01:01:07 am
content: Against the wind already would be possible with a craft like this, as long as the drag force of the blades is higher than the craft, it would be working like a windmill,

author: BrianHG
author_rank: Super Contributor
timestamp: May 30, 2021, 01:05:32 am
content: Quote from: Rerouter on May 30, 2021, 01:01:07 amAnd that comment sort of clinches the fact if you are doing it in one direction completely against the wind, then you can do it in the other direction to go faster than the wind...A far better illustrative explanation that what has been demonstrated in the video...

author: IanB
author_rank: Super Contributor
timestamp: May 30, 2021, 01:12:00 am
content: Quote from: BrianHG on May 30, 2021, 12:59:02 amLike, what? You think sailboats achieve 100% efficiency?The analogy is straightforward: A large mass of wind moving at slow speed can lead to a smaller object moving at a higher speed. As long as the kinetic energy (and momentum) of the craft is lower than the kinetic energy (and momentum) of the wind that it harnessed, then there is no violation of the conservation laws.In electricity, a larger current at a lower voltage can lead to a smaller current at a higher voltage, minus some losses.By analogy:Speed of wind/object == VoltageMass of wind/object == CurrentThe physics of this is all about levers and force multipliers.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 30, 2021, 01:25:53 am
content: Quote from: Red Squirrel on May 30, 2021, 12:55:34 amSure, put a wind turbine on a car and use the power generated to turn the wheels moving the car forwards into the wind. I would guess that only losses would limit how fast it can go.

author: sandalcandal
author_rank: Unknown
timestamp: May 30, 2021, 03:27:34 am
content: I think a key point is the wind still must have velocity/energy relative to the "ground" much like a converter must have voltage relative to a ground. You can go faster than wind speed but if windspeed relative to the ground is zero, you're still going have zero velocity.Quote from: NiHaoMike on May 30, 2021, 01:25:53 amNo need to go to that sort of complexity at all. You can already tack into the wind on a normal sailboat.https://en.wikipedia.org/wiki/Tacking_(sailing)Edit: unless you want to go directly against the wind. Some thing similar to the craft in the video would probably work too but as a windmill rather than a propeller. Edit2: which rerouter has already pointed out.

author: james_s
author_rank: Super Contributor
timestamp: May 30, 2021, 05:07:56 am
content: Quote from: ataradov on May 30, 2021, 12:20:01 amThat's one where I don't get why people have trouble with it, especially why some will continue to argue even after having it explained. From the first time someone posed that question, it took me about 2 seconds to come up with the right answer and it is completely obvious to me and totally intuitive. An airplane isn't propelled via the wheels, it doesn't matter what the ground is doing under the wheels and the only effect a treadmill can have is through the rolling resistance of the wheels. Until the treadmill is going so fast that the wheels fly apart of the bearings fail from spinning so fast it isn't going to matter at all.

author: jmelson
author_rank: Super Contributor
timestamp: May 30, 2021, 04:41:02 pm
content: Quote from: Red Squirrel on May 30, 2021, 12:55:34 amThey've been doing this since sails were invented. You can't go DIRECTLY into the wind, you have to "tack" to one side or the other.Jon

author: IanB
author_rank: Super Contributor
timestamp: May 30, 2021, 06:08:44 pm
content: Quote from: jmelson on May 30, 2021, 04:41:02 pmFor a sailboat that's true, but if you had a wheeled vehicle on land with a windmill mounted on it, then with suitable gearing the wind could turn the windmill and the mechanical link could drive the wheels forward against the wind.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2021, 06:29:01 pm
content: Even a boad with a wind turbine and some srew or peddel weel drive could more against the wind. The practical realization could still be tricky has the efficiency is limited and the surface water tends to move with the wind too.The problem of moving faster than the wind is more tricky to under stand: At the moment the vehicle dirves as fast as the wind, a sail would not see any apperent wind speed, so no force to drive the boat and also no wind to drive a kind of wind turbine. The trick is to take power from the forward movement of the wheels and from that drive the propeller to drive the vehicle forward. So the propeller is diving the vehicle and the wheels are opposing the movement, slowing the vehicle down. The propeller sees a low wind speed (essentially 0 if at the speed of the wind), while the wheels see the full speed relative to ground. So with the same power the propeller can have more force than the wheels brake.

author: IanB
author_rank: Super Contributor
timestamp: May 30, 2021, 07:37:48 pm
content: Quote from: Kleinstein on May 30, 2021, 06:29:01 pmThere's a good way to visualize this.If the vehicle is moving forward at the same speed as the wind, it does not see any apparent wind speed relative to the vehicle--it is effectively in still air (and has no drag), but the ground is moving beneath it and the wheels are turning.So we can translate the frame of reference, and keep our vehicle stationary in still air, and put a moving belt (treadmill) underneath it to turn the wheels and simulate forward motion over the road. If we now connect the turning wheels to a fan mounted on the vehicle, the wheels can turn the fan, pushing air backward and driving the vehicle forwards against the belt. The fan effectively gets its power from the moving belt. The vehicle is not so much powered by the wind, as by the road moving under the wheels. Since the vehicle is in still air and has little to no drag (see above), there is not much resistance for the fan to overcome, and so the fan can move the vehicle forward easily.This apparent paradox is similar to the paradox of how a jet turbine works. A jet engine has high pressure in its combustion chamber which drives exhaust gases out the back and thrusts the engine forward. But unlike a rocket, there is an open path out the front of the jet engine too--that is where the air comes in. So why are the hot combustion gases not also driven out the front of the engine? Why does the engine move forward instead of standing still with a balanced thrust pushing out both forwards and backwards?The answer is complicated, but a jet engine will not work until it is already running at a certain speed. It won't start from a standstill. You have to spin up a jet turbine with a starter motor before it starts working. Similarly, the wind powered vehicle won't actually generate forward thrust until it also is moving at a certain speed. Since it gets power from its wheels, it won't have enough power to drive its fan until the wheels are turning fast enough. Fortunately the wind powered vehicle has the wind behind it, and so a simple sail can allow the wind to push it forward and start it going. Once going at sufficient speed the sail can be taken down and the vehicle's motion will become self-sustaining.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 30, 2021, 10:06:30 pm
content: Doesn't bother me at all. Only two simple facts are necessary:1. There is wind shear between the air and ground.2. The vehicle has wheels.Wind shear can be extracted via a grounded propeller, or transmitted to wheels. The vehicle needn't have any drag or mass, by itself. The prop also can have arbitrarily low drag, at any relative wind speed, by rotating at the corresponding co-moving speed. And wheels have very low friction on ground, of course.It follows that, while a sailboat might not be able to go upwind by itself, one with a prop geared to a windmill could. It would be less efficient as the water isn't a rigid body, though it could be quite good as water is a good thousand times denser than air.I'm not actually sure how important the relative stiffness or density is. Say you were in a "large" balloon, sailing the shear band between jet streams. Both flows are air, same density, same temperature. ("Large" as in, "large enough" -- including fractional-planetary scale if necessary. If it helps, think of the shear bands on Jupiter, and something big enough to straddle the shear zone -- perhaps the size of a large moon. Just for sake of argument, we'll assume that's plausible.) Certainly, you can track equal wind velocity, in either direction, by moving towards the middle of the respective stream, or anything inbetween in the shear zone, where you get rotation instead. Is there anywhere you can move in this setup, given one or two propellers, and suitable gearing, that allows greater than local airspeed? (Where "local" is measured within proximity of the path of the vehicle, since we do need to take account of its size in this scenario.)I think you can, because you could extend a turbine into the shear zone, and move it relative to the vehicle so it's co-moving with the flow in the shear zone; it contributes no drag. It can then be geared to a propeller, pushing the vehicle forward through the stream. Mind -- how you retract and reset this turbine, is an open question, no small feat by itself; but if it is collapsible and low mass, it should require arbitrarily little work to do this.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: May 31, 2021, 02:55:05 am
content: I took it as the fan not exactly working as a fan. Instead they are using it to have a large high drag surface appearing to move towards the wind. Increasing its apparent relative velocity higher than the craftThe blade appears to be moving towards the wind at a given point. This apparent velocity allows the wind to push it a little faster than the actual wind speed. But not to get it twisted, the wind is pushing on the fan blades. Not the other way around if it makes sense. So the energy is still coming from the wind. And the theoretcial max gets lost in energy to spin the blades.If this assumption of mine is correct. The delta % between wind speed and actual speed should remain pretty constant until the crafts drag begins to dominate

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 31, 2021, 04:36:08 am
content: The explanation took me a bit to follow, but it clearly works. I think the real kudos go to the guy who figured this out. As he explained it there were a couple of pretty inventive steps in the process, first imagining sailboats tacking around a cylindrical ocean and then the translation into a wheeled vehicle. He also had some other advantages. Realizing that a good sailboat can tack downwind fast enough to beat a balloon gets the right mindset, but most sailboats cannot achieve that kind of performance. The vast majority of human experience with sailing doesn't support that observation so the wrong intuition is built up.

author: IanB
author_rank: Super Contributor
timestamp: May 31, 2021, 04:45:34 am
content: Quote from: Rerouter on May 31, 2021, 02:55:05 amWell, drag is resistance to motion, and drag is bad, it will reduce performance. Drag is to be minimized. You do not want a high drag surface.The fan is absolutely working as a fan. It spins and blows air backwards, just as if it had a motor attached.Now the energy is always coming from the wind. The craft doesn't have any onboard power, and it doesn't work on calm days. It needs the wind to power it.Any such craft has a speed up ratio or multiplier relative to the wind speed. The faster the wind, the faster the craft can go. Let's say, for example, that the ratio might be 1.5. If the wind speed is 10 mph, the craft could go at 15 mph. But if the wind speed is zero, the craft can't move, because 1.5 times zero is zero.

author: bdunham7
author_rank: Super Contributor
timestamp: June 01, 2021, 12:59:47 am
content: I thought about how to explain this to someone that doesn't 'get it' after watching the video and I can see how the principle--which boils down to a force and a reaction a la Archimedes--has a result that seems counterintuitive. So if anyone hasn't figured it out yet, especially how it gets past the apparently calm air when the vehicle is at exactly wind speed, consider these problems:First, consider a small test vehicle in a tunnel with a uniform height and smooth, flat roof. The tunnel has a belt, like a moving walkway, in the floor that moves at 1 meter/sec. The test vehicle has 4 rubber wheels that it can roll on, two of which are connected by a rigid axle and are called the 'drive' wheels. There is also a large rubber reaction wheel that can be pressed against the roof. Your job is to design a geartrain using gears and/or chains between the reaction wheel and the rigid axle so as to enable the vehicle to travel in either direction at an arbitrary speed determined by the exact configuration of your drivetrain. Assume infinite traction and no losses due to friction of the moving parts or aerodynamic drag. Can you do that?Second, once you've completed the first design, imagine tunnel's drive belt is in the ceiling, rather than the floor. This should be a trivial rearrangement.Once you have that, replace the big rubber reaction wheel with a propeller and the tunnel ceiling with still air in the first example and you have the test vehicle that they showed on the treadmill. Replace the tunnel ceiling drive belt in the second example with the wind and you have the test vehicle that they drove.

author: tszaboo
author_rank: Super Contributor
timestamp: June 01, 2021, 10:36:52 am
content: Quote from: IanB on May 30, 2021, 01:12:00 amI know where you are coming from, but I have the perfect example to explain, why we shouldn't just rely on the energy conservation law.You have a glass of water on a desk. Suddenly the glass of water freezes and jumps in the air. Thermal energy got converted into motion. Conservation of energy is intact, second law of thermodynamics is broken.These sort of systems, or the Magnussen effect, or helicopter autorotation for example, are not really easy to understand.

author: BrianHG
author_rank: Super Contributor
timestamp: July 01, 2021, 12:23:11 am
content: Who will win the bet?All you need to do to prove it is make a toy with a propeller where when you blow into it, say with a fan, it travels toward the wind. Demonstrate this, moving/accelerating directly into the wind, then it is obvious you can gear the device in the other direction to accelerate faster than the wind directly behind you.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: July 01, 2021, 12:45:01 am
content: Quote from: ataradov on May 30, 2021, 12:20:01 amSpecifically, where to buy a 2 kilometer-long treadmill.

author: EEVblog
author_rank: Unknown
timestamp: July 01, 2021, 12:48:04 am
content: Quote from: BrianHG on July 01, 2021, 12:23:11 amDerek said at the end of the video that he won the bet, and the physics prof paid up the $10k.Settled?