By Dan Nosowitz
Posting in Energy
It's a head-scratching puzzle: how could a battery-less wind-powered car possibly travel faster than the speed of the wind, going downwind? Yet this one appears to do just that.
Conventional wisdom says that a wind-powered vehicle without stored energy, when moving downwind, can only move as fast as the wind itself. Though some racing boats and yachts can move faster than the wind, they're actually sailing into the wind at an angle, thus being driven by the apparent wind, not the actual wind.
When going directly downwind, it would seem to be impossible to move faster than the wind. As you approach the wind's speed, the wind's power lessens, and by the time you reach the wind's precise speed, you'll be receiving no help at all.
Rick Cavallero had a different idea, and with the help of the San Jose State University aero department, Google, and Joby Energy, he built this windmill-type car made mostly of foam.
The idea is that the wheels and the propeller are attached, but in the opposite way you'd expect--the wheels drive the propeller, not the other way around. So the wind pushes the car forward, which turns the wheels, which turn the propeller, which pushes the car forward, which turns the wheels, and so on.
That sounds suspiciously like the impossible perpetual motion machine, but in my understanding of the vehicle, it's not. The car is able to accelerate past the wind's speed thanks to the propeller (being turned by the wheels), and since the car still has some speed, it'll keep accelerating for a little while. But since the wind's power on the car has now dropped to zero, the car will eventually succumb to friction and slow down until it moves slower than the wind's speed, at which point the whole process will start up again.
That's my understanding, and from the comments this vehicle has attracted on other blogs, everyone seems to feel differently. The car may turn out to have little value to the auto or alternative energy industries, and remain an intellectual curiosity--but then again, it may not.
Jun 2, 2010
I understand the idea why the wind-powered car moves at speed even if the wind is slow, thanks for the quality information you have provided. http://www.tyre-shopper.co.uk/branches/formby
Two days of NALSA testing complete. ~16-18 speed runs in ~12-13 distinct passes -- wind was lengthwise the lake on the first day giving us room run throught 2 sequential 'traps' on one pass. Every run well over 2x. Most runs over 2.5x. Best runs near 3x or perhaps above. We won't know details of which runs qualify against NALSA rules until they go through the large amount of data and figure out which runs are best documented. There is a fairly comprehensive list of requirements to be met for a run to be NALSA valid and I'm certain that some runs will be disqualified if all the sensor info was not to their liking (wind switching direction too much during the run, etc). There are reams of data for them to go through from more than 20 separate sensors (multiple gps, wind direction and wind speed sensors) on the vehicle itself, chase vehicle and lakebed, plus multiple video cameras. It will take some time for them to go through it all once they get home. To say the least we are quite confident that a record over 2x will be ratified by the NALSA BOD. On the project blog (www.FasterThanTheWind.org) Pictures, video and more information to follow as we recover from the long days and long drive home. A special thanks to the NALSA officials who put in so much time, effort and money (they wouldn't let us pay for their travel expenses) to make this happen. Thanks also to the NALSA BOD who saw something interesting in this crazy little project. JB
@gardoglee, The sailboats on the river brainteaser you present is exactly what inspired us to test the device on a simple treadmill. If the device maintains it's position against the treadmill belt, it is moving downwind at the same speed as the wind. If it moves forward against the belt, it is traveling *faster* than the speed of the wind. http://www.youtube.com/watch?v=Vjt6G8D4x0k&feature=related JB
Initially, drag forces on the car and prop cause the car to move with, but slower than, the terrestrial wind. As the car moves forward the gearing inside the car causes the prop to turn, generating a small wind. The wind generated by the car's prop opposes the terrestrial wind and the disturbance between the two opposing winds acts like a 'virtual sail'. The velocity of the car is due to the sum of the two 'winds' velocities minus friction losses. Once the prop is spinning, the wind no longer pushes the car so much as it pushes against the 'virtual sail'. Terrestrial Wind Wind generated by car's prop |\ # | \ # ---------+ \ # /| ++ + # + ==|| Car's velocity ---------+ / # \| || | / # | \ |/ # [o===o] # ( # boundary layer between opposing winds ) Sorry, this drawing looked better in Notepad - Ken
This is a simple "impedance" transformation. Brilliant idea! I read every comment, many from engineers. Unfortunately, some have reacted based on their "knee jerk" thoughts and failed to recognize the difference between speed and energy. Two completely different things. Of *course* you can go "Faster".. you just have to trade energy around so you can collect enough to add speed and still lose energy in the process. And NO, it doesn't have to cycle faster and slower- there is an equilibrium for any particular design and forces applied, although if there is an instability involved, it could take some dynamic control to force stability into the results. How about a basic example of the same *type* of energy transformation that would seem impossible at first glance: No electricity, no other source of energy than gravity itself, design a pump that can lift water from a pond to a water tower higher that the pond. Sound impossible? Exactly the same basics involved as the concept in the vehicle described above. Throw a lot away to gain a little, thereby getting a little bit to a higher energy state: http://en.wikipedia.org/wiki/Hydraulic_ram Hope this clarifies things a bit. -dra
Air is a fluid. The blades create a swirling mass that creates a "wall" for the wind to blow against, so they do not run out of "a surface" to put force against. The blowing wind moves that disturbance along with the bike so that the blades can be more efficient. This is why all airplanes fly faster airspeed than ground speed going with the wind. They are asserting thrust in a moving fluid. This allows the vehicle to reach the speed of the wind by just having something for the window to blow against. Now that the vehicle is at the speed of the wind, the blades' force creates additional thrust that propels the vehicle faster. than the wind. 2X has to do more with the weight, drag and propeller to wheel gear ration in the vehicle design than to this "power mechanism".
almost made it through all the comments!! this is great information and technology, although I have seen it done on naval vessels using wind turbines, they look ridiculous as they have to be quite huge; however it is the same concept, turbines spin using the wind generated by the ship moving through the water using its' standard engines (test this by splashing water on your face, then go for a quick run, if you feel "cool" on your face, it's because the wind in front of you is being pushed past your body and blowing across your face to do so. now try it again, but stand still, no cool air, right?) the power generated by the turbines is used then to power the engines that are moving the vessel, which speed the ship up (we are talking destroyer class and above sized vessels), this increased speed turns the turbines faster, which increases the speed of the ship, which turns the turbines faster, which increases the speed of the ship. this technology is used to actually "turn off" the engines to conserve fossil fuels, while allowing the vessel to travel faster than it otherwise could while using said fossil fuels. It is a very efficient system, once you get it moving.
Well, blow me down!! (pun intended). May I suggest that you (controllably) pitch the prop at start of run to initially drive the wheels so that the vehicle gets into motion faster, then as speed increases (due to the wind at your back) pitch the prop into thrust position for the acceleration and cruise. But then you probably thought of that too.
An old freshman physics test question.an engineer and a physicist agree to a sailboat race on the river. On the day of the race there is no wind. The engineer, knowing there is no wind to drive the boat, keeps his sail furled to minimize air resistance and begins drifting downstream. The physicist starts out the same, but as he drifts he feels the air moving against his face from the motion of the boat, so he decides to raise his sail to use this "apparent wind". Who wins the race? And why? The correct short answer is the physicist (it was a physics class question, after all). The explanation is to think of the water as static, and the earth and air as moving by and over the water. The physicist feels the air moving relative to the boat, raises his sail, and uses this relative air motion to tack into the "apparent wind". Now, apply the same approach to the car. The car is drifting in the "wind", s the car and win are still relative to each other. The earth is moving by the car's wheels, and therefore can drive the wheels, supplying some energy to the car. The car uses the energy from the wheels to drive a propeller to move "forward" in the relatively static and still air. I didn't think it would work until I remembered the sailboat race. I am surprised to find that the simple freshman physics analysis says it should work. And yes, many a class has tried the race on a river. The physicist does win, so this car should go "downwind" faster than the wind itself.
@ Dino Ma no Dino! Stai dicendo una generalit?! Non sei mai andato in barca? You are betraying the tradition of italian engineers I'm so proud of! We are mixing-up energy and speed. Any sailor knows that it is easier to go faster than the wind (with a wind boat, of course!) at the right angle with the wind, you don't even need a good boat. Nestor Another Engineer and sailor that has worked-up a bit the aerodynamics equations (not so difficult by the way).
no one can apply the perpetual motion. you can not have more energy than is obtained by wind. if true, when the wind stops, the vehicle is traveling at half the speed as before, namely the wind speed than before. An absurd physical. A true idiocy. sorry my english Italian engineer
Very cool, but I'm not sure what all the fuss is about. It's just a clever way of extracting wind energy and fundamentally not any different than a tacking sailboat, ice boat or land yacht, all of which can exceed wind speed (the latter two by considerable amounts). No laws of physics are being broken. With the proper gearing and propeller pitch, once the wind starts moving the vehicle and the prop begins to turn, it's inevitable the vehicle will exceed wind speed. I do wonder just how much faster you could get with larger prop, variable pitch and gearing, etc. A rotor (see www.rotorboat.com), driven by the wheels in the same way as the prop would presumably give you greater directional freedom...
The second law of thermodynamics relates to energy in a system, not speed alone. I agree that on a very simplistic level your statement makes sense, but if the thing took a push from the wind, and the wind dropped, it would still travel forward. Because of momentum. So if the wind dropped, the vehicle would still be moving forward. Faster than the force pushing it, which, since the wind dropped, would be nothing. Insofar as the vehicle running entirely off the reciprocal force of wheel-to-prop, that would be where 2lot would come in. But, and it's a fairly big but, we're not talking about the vehicle running entirely off that reciprocal force. There is an input, a constant input, from the wind's energy. Everyone who is citing 2lot here should probably remember that it refers to a closed system. With a constant (if variable) *input* of energy into the system, 2lot arguments just can't apply.
steve_jonesuk Another thing to understand is that speed is not energy, at least not directly. So your point in regard to the energy is good but looking at *rates* of energy (power) you have force times speed and as long as the rotational force times speed gathered from the prop is greater than the linear drag and speed of the entire vehicle it will go forward with energy losses and all. The differences in the forces (prop lift vs vehicle drag) enable a difference in speed between the vehicle and the wind.
Also, it must be understood, that by beating the wind going downwind I mean that iceboats can tack downwind and their "velocity-made-good" is actually greater than the wind. In other words, if someone were to release a balloon at a starting line and at the same time an iceboat started tacking downwind, it would beat the balloon to the finish line and by a good amount, half the time or less!! This is the concept that must be grasped and which is being translated into a rotary mechanism by the vehicle above. Basically, the prop blades are performing as sails that are tacking down wind, close-hauled. Brilliant indeed!!!
I did some research on this while working on for the Naval Research Center in DC before becoming a friar. A sailboat can sail up wind by tacking but can't sail directly into the wind. It has been demonstrated that windmill powered boats (windmill drives an underwater prop) can do what a sailboat can't by going directly into the wind. Hard surface wind powered vehicles (Ice boats and sailing cars) can beat the wind by tacking downwind (with the wind blowing in the pilot's face, sailing close-hauled) because there is much less friction with the ground at speed than with water. But these can't do so going directly downwind. Just as with water vehicles the windmill enables these vehicles to do what their sail driven counterparts cannot, in this case beat the wind going directly downwind.
of thermodynamics. Sorry, folks, but you can't go faster than what's pushing you without adding extra energy to the equation.
When a senior scientist/engineer says something is possible believe him. When he says it's not . . . . . ignore him because he's liable to be wrong. Seem to have heard that somewhere before. ;-)
@steve_jonesuk "... the link between prop and wheels would be taking more energy via the wheels than it will be adding with the increased incident speed of the prop's sails." *************** A: Your above statement IS true (if I understand it correctly). The wheels DO take in more energy than the propeller delivers to the air due to all the losses in the system that you and I would agree on. Now please do the work = force*distance calcs for the wheels and then the propeller and you'll learn that due to the wind, there is plenty of power available and we can afford to waste a ton on the way to (and through) the prop . Remember that the ground and air are moving relative to each other so the "distance" portion of the work = force*distance calculations will be different for the wheels than it will be for the prop. Try it. JB
...but in each of these cases, the wing / sail isn't taking power out of the interface with the supporting medium (water in the case a boat). My entire contention is that ThinAir's stuff doesn't work because the link between prop and wheels would be taking more energy via the wheels than it will be adding with the increased incident speed of the prop's sails.
Of course it's perfectly possible it goes faster than the wind if it's going downhill... I didn't realise that that was the claim.
Sadly it's a bit far away for me. Shame really, as I had a whole turn- around stance planned, based on claiming that donkey poo is underrated as a fuel source. Good luck with the test guys, I look forward to having it explained to me somehow. I still don't believe it, though.
I fly hang gliders, and we regularly exceed the wind speed both upwind and downwind, due to gravity. In this case the prop is similar to the effect gravity gives us on a glider, i.e., forward motion. Since the wind is creating drag on the frame of the car, this adds to the forward motion as gravity adds to the forward motion of a glider.
Actually, anyone with aviation understanding can see how this car could exceed the speed of the wind. It's just a matter of the difference between ground speed and air speed. Keep in mind that the wind speed is just a measure of how fast the air is traveling over the ground. With a propeller, the prop pushes air relative to the ambient wind, in other words, any speed gain due to the propeller thrust will be added to the wind speed, therefore increasing the ground speed. As to slowing down and speeding up, as long as the air being thrust by the prop is being pushed along with the wind, only ground friction is the limiting factor. The speed of the car being faster than the wind is really irrelevant. Simple math really.
Actually, light displacement boats can travel considerably faster than the wind as long as they are at some angle to it. High speed multihulls as a rule tack downwind for this reason.
Jkirk -- when you're immersed in a moving fluid, it's possible to *outpace* your power source, but it's impossible to "outrun" it as you claim. Try swimming downstream in a 10kt river current and see if you can swim fast enough to cause the moving current to longer increase your relative shore speed by 10kts -- no matter how hard you try, how fast you go or how long you swim, the available power of the current keeps up with you, always pressing you faster than you could go on your own.
Steve and Jkirk, you are both welcome to attend our next test/demo. It's will happen this month on the runways of the NASA Ames Research Center and under the care of their engineers. We just yesterday met with these engineers for almost 2 hours and not one of them holds the same opinion as you two. Really -- we'd love to have you there where you can see for yourself just what the "donkey poo" is all about. :-)
"I confidently proclaim this "wheels drive the prop" thing to be a massive pile of donkey poo." Seconded. With enough gearing, you could use the horsepower from the prop to drive the wheels at higher speeds. Briefly. But you're outrunning your own power supply. So the power to the prop will diminish, and you will slow down again. It's simply impossible without storing the wind energy somehow. A simple set of springs would do it. Use the springs to buffer some of the wind energy and then take off like a rocket.
With four years of Mechanical Engineering behind me - albeit quite a long way behind me - I confidently proclaim this "wheels drive the prop" thing to be a massive pile of donkey poo.
[Though some racing boats and yachts can move faster than the wind, they?re actually sailing into the wind at an angle, thus being driven by the apparent wind, not the actual wind] ,,actually, there's more to it than that. The way a sailboat goes faster than the wind is by "reaching" - that is with the wind coming from the side (though not necessarily directly from the side) and the sail acting as an airfoil, like the wing of an airplane.
@ Dan - Though some racing boats and yachts can move faster than the wind, they?re actually sailing into the wind at an angle, thus being driven by the apparent wind, not the actual wind. Hi Dan, and so are the propeller blades on the car in question. They are cutting into the wind at an angle, rather than being "simply pushed".
I don't believe you. If the prop drove the wheels, then yes. But what you're claiming is equivalent to saying if you put this car on a treadmill that it would move forwards relative to the treadmill. You'll never get more power out of the prop forwards than the backwards force on the wheels.
@attoman. Yes, this particular vehicle was optimized to do one thing and one thing only -- set as fast a record as possible directly with the wind.
The wheels require good frictional contact with the road to drive the propeller so the vehicle can only go before or into the wind to have both wheels on the road. Further the propeller/sail structure is at least twice as tall as the rear wheel to wheel distance making it unlikely to survive upright from variable direction gusts or simply being steered at any substantial angle to the wind.
@Dan: " .... it?ll keep accelerating for a little while. But since the wind?s power on the car has now dropped to zero, the car will eventually succumb to friction and slow down until it moves slower than the wind?s speed, at which point the whole process will start up again." Hi Dan -- I'm one of the two primary designer/builders of the vehicle in question. The vehicle does not cycle up and down through wind speed as you propose -- it will accelerate to well over 2x the speed of the wind and hold that speed steady until the wind stops or we run out of space.
It would be an ACME boat, MEEP MEEP! Seriously, though you could. It would need a paddle-wheel to lock the wind to the water to work like the car but it should work. Great idea, the first improvement to the sail since, well, the sail...