Moving faster than the wind using only wind power

a reply to: beyondknowledge

As I said, the energy must be coming from the treadmill. Instead of wind power turning the wheels, it's the treadmill motor turning the wheels through the treadmill track.

Consider this: if you are perfectly aligned with the direction of the wind, each bit of 'wind' is only in contact with you for a brief moment. It exerts whatever force it can in that moment and the event ends. If you are tacking at an angle to the wind, you are in contact with each bit of wind longer, giving it more time to exert force on you.

Or look at it this way, if you are in direct opposition to the wind there is a pressure boundary around you created by the wind itself. Sort of a shell that deflects part of the wind before it ever touches you. If you are at an angle to the wind the pressure boundary is weaker allowing more contact, more force, to be exerted on you by the same amount of wind.

Put your hand out the window of a moving car in a "stop" position. You can feel the force of the air on your hand. Now turn your hand at an angle to the direction of travel and it can feel like it will take your arm off.

Angular momentum.

Cool beans.
That contraption would be right on in a "Mad Max" movie.

a reply to: ChaoticOrder

You are missing the point that the wheels power the fan. If you get the full size one going at a certain speed, the wheels will power it. It may take 10 miles per hour or higher but when that speed is passed, it will pull away from a pusher vehicle on its own power. If you do this on an oval track, it will run until the driver gets board or something breaks down.

I am actually thinking of how to take the design of the treadmill model and make it a teather racer. It could be set up and ran indoors and just left to run until it breaks.

It would be much slower than that and I am thinking about 25 foot line would work. There would most definitely have to be a watermelon put in a vice nearby.

originally posted by: ChaoticOrder
a reply to: beyondknowledge

The large version is definitely powered by the wind, where else is the energy coming from? The small version seems to work because the wheels turn the fan, which provides thrust, so it is also wind powered in a sense, but the energy must really be coming from the treadmill. The large version seems to work a similar way because the propeller is connected to the wheels. The vehicle wont keep accelerating forever, there is a limit we can calculate.

Not exactly the wind is used to start the vehicle moving but it's the rotation of the wheels that provide the thrust. I'm betting they have to get this moving by pushing it to start however unless they have the propeller free spin at first. In the beginning, our fan will work just like a sail but eventually, the force applied to the wheels will multiply pushing the fan faster.

originally posted by: dragonridr
Not exactly the wind is used to start the vehicle moving but it's the rotation of the wheels that provide the thrust. I'm betting they have to get this moving by pushing it to start however unless they have the propeller free spin at first.

If the wind is blowing 15mph like it was in trial 4, I doubt they have to push it, that's a rather stiff wind for something on wheels to stand still. They don't show the full start of trial 4 but at 15:50 in the first video in the OP it's not long after the start and I don't see anybody pushing it. When the wind isn't as high they might push it at first to save some time getting it moving, they edited the starts out of the first three trials.

I think some here are looking at this problem incorrectly. You can't really look at velocities of anything to resolve the underpinning question of whether the car is truly moving at a greater rate than the wind, regardless of how the car is motivated. In order to truly understand what is happening (in any of the examples) you have to look at the units of work being input versus the units of work being output.

The Laws of Physics decree that the units of work output from a system can never equal or exceed the units of work input to the same system. At least on Earth anyway. If they did, you would create "prepetual motion" or free energy, and there is no such thing as a "free lunch" in physics and engineering. Things like gravity, friction, resistance (in whatever form, be it heat or otherwise) are all real concepts, not some fantasy.

Given this, I'm not entirely sure what the point of the experiment is. All of the vehicles require some amount of energy to be input to the system for them to move. If this source of input energy is removed the vehicle will slow and ultimately stop. The rate at which the vehicles move in comparison to the air surrounding them is almost irrelevant.

Consequently, if the vehicle is powered by the wheels, as some here are alleging, and the wheels are turning a geared propeller to propel the vehicle forward, then if the input energy to move the wheels is removed the vehicle will slow and stop because the propulsion system will continually lose efficiency owing to air resistance, rolling resistance and friction.

If, on the other hand, the vehicle is truly powered by the wind (and only the wind) then the velocity of the vehicle can never equal or exceed the velocity of the wind propelling it (in the exact same direction). Because if it did the output of the system would be greater than the input energy. In the sail boat example, the boat is indeed moving faster than the wind while tacking, but it is not moving in the same direction as the wind, AND its forward momentum (exactly in the direction of the wind) will never equal or exceed the velocity of the wind relative to a stationary object such as the water because the system capturing the energy (the sails) are not 100% efficient. Not to mention the boat has considerable resistance being applied to it from the water.

Anything else is just an illusion or a trick with some other hidden or otherwise undocumented energy source.

There simply is no Free Lunch. Sorry.

originally posted by: dragonridr

originally posted by: ChaoticOrder
a reply to: beyondknowledge

The large version is definitely powered by the wind, where else is the energy coming from? The small version seems to work because the wheels turn the fan, which provides thrust, so it is also wind powered in a sense, but the energy must really be coming from the treadmill. The large version seems to work a similar way because the propeller is connected to the wheels. The vehicle wont keep accelerating forever, there is a limit we can calculate.

Not exactly the wind is used to start the vehicle moving but it's the rotation of the wheels that provide the thrust. I'm betting they have to get this moving by pushing it to start however unless they have the propeller free spin at first. In the beginning, our fan will work just like a sail but eventually, the force applied to the wheels will multiply pushing the fan faster.

This problem is almost identical to one that was given to us graduate students as a qualifying exam question in the Aero Astro department at Stanford university, many many years ago. Here's the explanation:

The turning blade on a pylon is basically a wind turbine of the kind used to generate electricity by turning an electric generator. Thousands of these have been designed and built worldwide, so their aerodynamic properties are well known. Pretty much the only two parameters you need to know to solve this problem are the rotor disc drag and the rotor disc torque. For a given rotor disc diameter and wind speed there will be a fixed ratio between the rotor drag and rotor torque. I just looked up some real measured numbers from academic papers, and I will use them as an example.

For a particular rotor size operating in a 10 m/s wind, the rotor disc torque is 1350 Newton-meters and the rotor drag is 1600 Newtons. Using those particular numbers, you can see that if you were able to route that amount of torque directly to wheels that had a radius of about 0.85 meters, the wheels would be able to create just enough tractive force relative to the ground to equal to the drag force on the rotor disc. If the wheels were made smaller than that, they would produce MORE tractive force than the drag and the vehicle could actually move forward in the wind. Of course, as the vehicle moves forward at some ground speed, the wind speed against the rotor disc will increase and the drag on it and the torque it produces will also increase. There will be some ideal wheel diameter (or gear ratio, if you want to think of it that way) at which the increased torque supplied to the wheels and the increased drag will exactly balance each other, and the vehicle won't be able to go any faster than that.

Two conditions are necessary for a vehicle like this to work. First, the tires have to have enough traction to actually turn that torque into forward thrust. In the above hypothetical case, 1600 Newtons is about 360 pounds of weight, and tires running on desert pavement might only have a coefficient of friction of ≈ 0.5. That means that the total vehicle weight (including pilot) would have to be at least twice 360 lb, or 720 lb in order for the tires to have enough traction. That's probably not a difficult requirement. Second, the lateral wheel base of the vehicle has to be wide enough so that the vehicle is not overturned sideways by the rotor torque. That's why this vehicle has a very wide triangular wheelbase.

Finally, a rotor like this generates very little starting torque. When it is spinning at zero rpm in a 10 m/s wind, it cannot generate enough starting torque to get the vehicle moving into the wind. That's why they have to push the vehicle to get it started. Once it gets near its optimum rotational speed, it can sustain its optimal ground speed.

You can much more easily visualize this whole process without any vehicles moving at all.

If you take a wind turbine (a generator) and simply connect it to an electric motor the work output of the motor will always be less than the absolute work input at the input shaft of the generator. And, it will be less at each step along the way. So, it will be less at the output side of the generator, and less again at the input side of the electric motor, and even less still at the output shaft of the electric motor.

Anyone who can upset this model better be on their way to the Patent office in short order because they're going to make Bill Gates look like a pauper!

a reply to: 1947boomer

But not indefinitely. And, the second there is even a minor disruption to the optimum balance of that system (i.e. change in wind speed or less than perfect terrain, etc.) then the system will require some outside influence to re-establish that optimum equilibrium.

Great write up though! Very interesting! I'd love to know the margin of error in that optimum balance.

originally posted by: Flyingclaydisk
If, on the other hand, the vehicle is truly powered by the wind (and only the wind) then the velocity of the vehicle can never equal or exceed the velocity of the wind propelling it (in the exact same direction).

The vehicle has a propeller on it, are you taking that into account? It's the wind plus the "wind" from the propeller in the opposite direction that is propelling the vehicle. There's no other "energy input" besides the wind, but the energy output is harnessing the available wind in multiple ways which are somewhat complicated to analyze, which is why the physicist lost his $10,000 bet.

originally posted by: 1947boomer
Finally, a rotor like this generates very little starting torque. When it is spinning at zero rpm in a 10 m/s wind, it cannot generate enough starting torque to get the vehicle moving into the wind. That's why they have to push the vehicle to get it started. Once it gets near its optimum rotational speed, it can sustain its optimal ground speed.

It doesn't need to generate any torque to start moving, the wind can push the vehicle without any help from the propeller, if it's a 15mph wind like the successful trial 4 in the OP video. What the propeller is needed for is to exceed the speed of the wind.

The videos in the OP only show the blackbird in the propeller mode, going the same direction as the wind, faster than the wind in trial 4.
Not shown in the OP of this thread is a different mode where he can replace the propeller blades with turbine blades to go into the wind, so it does have that capability with the turbine setup, but it's outside the scope of the OP videos.

Blackbird Faster than Wind Vehicle

At New Jerusalem airport in California he travelled downwind at a speed of 2.86 times greater than the wind, proving his concept worked.

But not content with this feat, Cavallaro has also recently shown that Blackbird can also run against the wind. To do this, he reverses the whole set-up.

“All I did to convert it is remove the propeller and replace the blades with turbine blades” he explains. “A turbine blade is turned by the wind, whereas a propeller is just the opposite.”

Yes, but the propeller and assembly also have drag associated with them, as do the wheels which engage the 'transmission' which turns the propeller. Seems like all of this (and then some) would need to be overcome by the static wind pressure alone in order for the vehicle to continue moving. Must be a pretty delicate balance.

In some respects this problem kind of reminds me of the old "airplane on a treadmill" question, but with quite a few more variables.

Wasn't this debunked on an episode of Myth Busters ?

originally posted by: Gothmog
Wasn't this debunked on an episode of Myth Busters ?

I'm going to assume it wasn't unless someone posts a link, and even if it was, mythbusters is not exactly strong on science. One thing to note in the OP video, the inventor says 6mph wind is probably too slow for the vehicle to exceed wind speed, so if mythbusters or someone else tested it at 6 mph or less they could conclude it didn't exceed wind speed. The blackbird inventor says at least 8mph appears to be needed for his vehicle (I'm guessing to overcome some effects of drag and friction) up to 15mph to perform its counterintuitive feat of going faster then the wind from wind power.

a reply to: Arbitrageur

I think he was referring to the "Airplane on a Treadmill" question, and yes, Mythbusters did test that.

It was "true". The plane did take off, because it wasn't the wheels propelling it, but rather the prop, so the speed across the treadmill was irrelevant, it was the speed over the ground which counted (even though it was 2x the speed of the normal takeoff speed over the treadmill).

originally posted by: Arbitrageur

originally posted by: Gothmog
Wasn't this debunked on an episode of Myth Busters ?

I'm going to assume it wasn't unless someone posts a link, and even if it was, mythbusters is not exactly strong on science. One thing to note in the OP video, the inventor says 6mph wind is probably too slow for the vehicle to exceed wind speed, so if mythbusters or someone else tested it at 6 mph or less they could conclude it didn't exceed wind speed. The blackbird inventor says at least 8mph appears to be needed for his vehicle (I'm guessing to overcome some effects of drag and friction) up to 15mph to perform its counterintuitive feat of going faster then the wind from wind power.

It was the same concept .
An airplane on a treadmill .
Debunked .

originally posted by: Gothmog
It was the same concept .
An airplane on a treadmill .

An airplane doesn't have any chain between the propeller and the wheels like the blackbird does, so no, it's not the same concept without the chain. The wheels on a plane don't do much, but the wheels on the blackbird perform an important function, it wouldn't work without them. A plane on the other hand doesn't even need wheels, floatplanes can take off from floating pontoons, and planes in the arctic can take off from skis.

Debunked .

There were two theories about the plane, one theory was the plane would take off on a moving runway and the other theory is the plane wouldn't take off on a moving runway.

So unless you say which theory was "debunked", saying "debunked" is completely ambiguous and it's again not the same concept since the airplane doesn't have a chain linking the propeller to the wheels.

a reply to: Arbitrageur

I saw that episode the argument was if the runway was moving under the plane would the plane be able to take off. The simple answer is yes because unlike a car a plane doesn't gain its momentum from wheels. In theory, if the wind was strong enough any plane could act like a hovercraft. Also why sometimes planes seem to hover when landing.

ca.sports.yahoo.com...

originally posted by: dragonridr
a reply to: Arbitrageur

I saw that episode the argument was if the runway was moving under the plane would the plane be able to take off. The simple answer is yes because unlike a car a plane doesn't gain its momentum from wheels.

This is why I explained to Gothmog it's not the same concept with a plane as with a car. With a car, you can put the car on a treadmill, and run the treadmill at a speed that would keep the car stationary with respect to the ground. So far that's entirely possible to do.
Now let's say the wind isn't blowing and the car has a spoiler on it. Whatever the spoiler is supposed to do with air flowing over it won't happen because with no wind, if the car is motionless with respect to the ground, there's no air flowing over the spoiler.

The problem with the airplane version is that it's usually not clearly defined exactly what you're supposed to do with the treadmill, to the point where thr airplane treadmill is a banned topic on xkcd forums. Some people think of the car example and that if you could make the plane motionless with respect to the ground like the car, there would be no air flowing over the wings and so the plane wouldn't take off. The problem with that is the plane doesn't work the same way, you can't keep it motionless with respect to the ground like you did with the car, again assuming there's no wind.

If there's enough headwind, maybe you could keep the plane motionless with respect to the ground, and as your video shows you might not even need a runway. Anyway the airplane on a treadmill and ground vehicles on a treadmill are really completely different situations.

a reply to: Arbitrageur

The vehicle has a propeller on it, are you taking that into account? It's the wind plus the "wind" from the propeller in the opposite direction that is propelling the vehicle.

Think if it in terms of lift rather than "wind." That "wind" is actually lift which is directed toward the front of the vehicle.

A sailing vehicle can exceed the speed of the wind because the sail is an airfoil. Lift is created by the difference in airflow between the leeward and windward sides of the sail and the craft is accelerated by that lift. It's sort of like a glider, except that the force of gravity is replaced by the drag of the keel. The faster the craft goes with a given angle of attack, the more lift it produces. On the other hand, when sailing directly downwind, no lift is created because wind is not moving across the surface of the sail (though there is some "leakage" around the edges), it's working like a parachute (that's why spinnakers are referred to as "'chutes").

But propeller blades are also airfoils. Airfoils which move. As soon as the blades start to move they produce thrust which acts additively with thrust provided by the wind pushing the other parts of the machine.

Unlike the foil riding, rigid sailed, windjammers that we're going to be seeing in the Americas Cup (scary fast), this is not a terribly efficient system. But it was good enough to win $10k. Perhaps a variable transmission or variable pitch prop could be used to increase efficiency.