Can a wind powered vehicle travel down wind, faster than the wind?

[strop45 0]

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>Perhaps you get your colleague to explain why most boats sailing
>downwind will use spinnakers

Because boat sails can't move relative to the boat.

Exactly my point - if the boat has a higher velocity downwind than the wind then the relative wind to the sail is backwards and the sail will luff or a spinnaker would collapse.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[billvon 2,426]

>Exactly my point - if the boat has a higher velocity downwind than the
>wind then the relative wind to the sail is backwards and the sail will luff or
>a spinnaker would collapse.

Correct! Now allow that sail to make an angle to the wind (either by spinning the sail or by turning the boat about 30 degrees from a direct downwind heading) and you _can_ go faster than the wind. Boats do it all the time.

[kallend 1,644]

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>So where is the energy to turn the wheels coming from?

From the motion of the vehicle.

>This must be the true definition of a circular argument?

Because it involves a wheel? Perhaps!

GROAN.

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The only sure way to survive a canopy collision is not to have one.

[kallend 1,644]

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Understood, but where does the energy required to change the momentum of the air come from?

From the power available at the wheels.

So where is the energy to turn the wheels coming from? This must be the true definition of a circular argument?

I understand what you are saying, I just don't agree.

Simplest possible explanation I can think of. The air actuator (to be totally general, probably a propeller or fan) is blowing backwards, into the tailwind. Hence the windspeed is reduced behind the actuator. Hence the kinetic energy of the air is reduced. Conservation of energy says that energy must end up somewhere else. That is the energy available to drive the vehicle.


Want some algebra? let the vehicle be moving at v and the windspeed is V. Let v > V (vehicle moving faster than wind) so the the air actuator (prop) is moving through the air at (v - V). Let the force exerted by the prop on the air be F so the work done is F(v - V). By Newton's 3rd law, F is also the thrust provided by the prop.

The vehicle is moving over the ground at v and has to provide F(v-V) of work to drive the prop. If the force required (by reaction with the ground) to drive the vehicle is f then the work supplied by the wheels is fv.

Assuming no losses for the moment, fv = F (v-V) (work done by wheels = work needed by air actuator). But (v-V) < v on account of the tailwind, hence F > f

So the propulsive force F provided by the prop exceeds the force f needed to drive the vehicle.

In the real world you will get some losses (drag, friction...) which will reduce the excess thrust.

...

The only sure way to survive a canopy collision is not to have one.

[strop45 0]

Thanks for your explanation.

So this vehicle will move faster than the wind - therefore it will carry on even if the wind drops to zero?

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[kallend 1,644]

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Thanks for your explanation.

So this vehicle will move faster than the wind - therefore it will carry on even if the wind drops to zero?

No - apparently you didn't read the explanation.

...

The only sure way to survive a canopy collision is not to have one.

[mdrejhon 8]

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A sailboat cannot, however, move its sail very far before it "runs out of room." A propeller _can_ keep moving its "sail" (airfoil) for as long as you like.

Semi-eureka moment:

In other words: It's as if the propeller is simultaneously tacking the vehicle left and right, because the propeller has the ability to alternate sides rapidly, averaging out to a direct-downwind motion that is faster than the wind itself. It all makes much better sense now.

It is not perpetual motion because energy is being actually being captured from the wind. Just that the propellers are causing the same kind of cross-tack forces in both left/right directions, that cancel out to a direct-downwind motion.

(I think the OP linked article shows a variable pitch wind-capture propeller. It appears you kind of need a variable pitch prop optimized for this, so you can adapt for optimal energy capture for every speed, including slower and faster than the wind. You'd need to pass through the stationary wind factor (going the same speed as wind) and inverse your pitch I think, for proper operation while going faster than wind. If I am not mistaken.)

Still does not explain everything. I need more thought experiments, before it makes more sense. Hmm. And some scientific articles, that relates this, and the forces, to how sailboats are well known to be able to go faster than the wind.

[ryoder 1,397]

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You'd need to pass through the stationary wind factor (going the same speed as wind) and inverse your pitch I think, for proper operation while going faster than wind)

Incorrect.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[Calvin19 0]

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You'd need to pass through the stationary wind factor (going the same speed as wind) and inverse your pitch I think, for proper operation while going faster than wind)

Incorrect.

no, he got concept, just maybe a bit backwards

with a relative tailwind, the propeller pitch is working against the torque of the attached wheels. this is not a huge issue as the relative wind drag is helping push the vehicle forward. some models have variable pitch propellers or way of disengaging the drive wheels. A variable pitch propeller is ideal for fastest transitions.

with a relative headwind, the propeller is driving the vehicle forward and providing all forward thrust.

-SPACE-

[billvon 2,426]

>therefore it will carry on even if the wind drops to zero?

No. At that point the power required to run the propeller will meet or exceed the power available from the wheels, and the device will coast to a stop.

[strop45 0]

OK, now I can see where my thinking was going wrong.

A wind powered vehicle works on energy transfer from the air to the vehicle.

With a static sail, once you reach wind speed, there are no longer any collisions taking place between the air and the sail and hence you don't get any additional energy. With the wheel driven propeller, the energy for the propeller is taken from the air by slowing the air down more than a static sail would. You can get more energy because the mass of air >> mass of vehicle.

I'm still not convinced that a sail powered vehicle can exceed wind speed in the direction of the wind. The speed which can be reached is a function of 1/cos(angle) and the vehicle efficiency/drag.

Racing boats going downwind tend to travel at about 30 degrees to the wind for a number of reasons - the higher boat speed can give lower drag (boat will plane), most boats are more stable and safer to sail (less likely to gybe) and it can result in tactical advantages.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[billvon 2,426]

>I'm still not convinced that a sail powered vehicle can exceed wind speed
>in the direction of the wind.

A vehicle with stationary sails - no, it cannot exceed that speed going directly downwind. But once you can spin the sail (i.e. use it like a propeller) then you can.

Here's the more mathematical angle:

One horsepower is 550 ft-lbs per second. Put another way, if you are going 1 foot per second, a 1 horsepower engine can give you a push of 550 lbs. (assuming perfect coupling, traction and efficiencies.) If you are going 10 feet per second, a 1 horsepower engine can generate 55 lbs. At 100 feet per second it can generate 5.5 lbs etc.

Likewise, if you extract 1 horsepower from a vehicle doing 55 feet per second, you will create a 'push' (drag in this case) of 10 pounds.

So:

If there is a wind blowing at 50 feet per second, and your vehicle is doing 55 feet per second, you can extract 1 horsepower from the wheels with a drag penalty of 10 pounds. If you use that 1 horsepower to drive the propeller (which is operating at a speed of 5 feet per second relative to the wind) you get 110 pounds of force forward. Result: the vehicle will accelerate beyond wind speed.

All the above numbers assume perfect efficiency; the actual values will be much lower (but still positive.)

[strop45 0]

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>I'm still not convinced that a sail powered vehicle can exceed wind speed
>in the direction of the wind.

A vehicle with stationary sails - no, it cannot exceed that speed going directly downwind. But once you can spin the sail (i.e. use it like a propeller) then you can.

Understood and agreed, but I thought you and Kallend were saying that by travelling at an angle to the wind e.g. 45 degrees, a convectional yacht could exceed the wind speed in the downwind direction. To put some numbers on it, ignoring fricition - if the wind is 10 ft/sec and you travel downwind at 45 degrees to the wind, I believe that the maximum boat speed is (10*2^0.5) = 14.1 ft/sec, i.e. 10 ft/sec in the downwind vector.

Once you go faster than 14.1ft/sec, the wind hits the front of the sail and slows you down.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[kallend 1,644]

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>I'm still not convinced that a sail powered vehicle can exceed wind speed
>in the direction of the wind.

A vehicle with stationary sails - no, it cannot exceed that speed going directly downwind. But once you can spin the sail (i.e. use it like a propeller) then you can.

Understood and agreed, but I thought you and Kallend were saying that by travelling at an angle to the wind e.g. 45 degrees, a convectional yacht could exceed the wind speed in the downwind direction. To put some numbers on it, ignoring fricition - if the wind is 10 ft/sec and you travel downwind at 45 degrees to the wind, I believe that the maximum boat speed is (10*2^0.5) = 14.1 ft/sec, i.e. 10 ft/sec in the downwind vector.

Once you go faster than 14.1ft/sec, the wind hits the front of the sail and slows you down.

No, that is not correct. Draw the vector triangle.

Here, someone already did.

www.animations.physics.unsw.edu.au/jw/images/sailing_files/petitlargue.gif

Because the boat is sailing close hauled with respect to the relative wind, the wind is still on the correct side of the sail.

...

The only sure way to survive a canopy collision is not to have one.

[billvon 2,426]

>but I thought you and Kallend were saying that by travelling at an angle
>to the wind e.g. 45 degrees, a convectional yacht could exceed the wind
>speed in the downwind direction.

Yes, but that's a different case. In that case, the vector sum of the two lift forces (from the sail and the keel) has a net bow-ward direction, and thus can provide some forward thrust even at speeds much higher than either water or air speed. Needless to say in boats this is opposed by hull drag, and thus maximum speeds aren't all that fast.

>Once you go faster than 14.1ft/sec, the wind hits the front of the sail and
>slows you down.

Even at slower speeds the wind is hitting the "front of the sail." Sails usually don't work like parachutes; they work like wings, and their effects are due to lift rather than drag. Keep in mind that sailplanes can achieve very high speeds (and cover long distances) even when the wind is "hitting the front of the wing."

[wolfriverjoe 1,351]

(Replying to Billvon because it's the post that makes the most sense to me).

Sort of "Eureka" moment- If the force to drive the car faster than the wind is less than the force of the wind, that force can be extracted, and then manipulated to produce a speed greater than the windspeed.

Someone (Billvon or Kallend are giving the explanations I understand best so far) correct me if I'm wrong, but -

The wind pushing on the "propeller disc" pushes the car. Since that force is more than enough to move the car faster than the wind, then some of it is used to spin the prop, generating more forward thrust (the prop is pushing againt the tailwind component).
You are just able to capture the force efficently enough to produce extra velocity.

If you had a "normal" wind powered vehicle, you could run at maximum speed, and still capture that extra force and use it to pump water or generate electricity or something else. You couldn't use it to generate extra speed, but that doesn't mean it isn't available.

"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

[kallend 1,644]

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(Replying to Billvon because it's the post that makes the most sense to me).

Sort of "Eureka" moment- If the force to drive the car faster than the wind is less than the force of the wind, that force can be extracted, and then manipulated to produce a speed greater than the windspeed.

Someone (Billvon or Kallend are giving the explanations I understand best so far) correct me if I'm wrong, but -

The wind pushing on the "propeller disc" pushes the car. Since that force is more than enough to move the car faster than the wind, then some of it is used to spin the prop, generating more forward thrust (the prop is pushing againt the tailwind component).
You are just able to capture the force efficently enough to produce extra velocity.

If you had a "normal" wind powered vehicle, you could run at maximum speed, and still capture that extra force and use it to pump water or generate electricity or something else. You couldn't use it to generate extra speed, but that doesn't mean it isn't available.

By "normal" wind powered vehicle, what exactly do you mean?

...

The only sure way to survive a canopy collision is not to have one.

[wolfriverjoe 1,351]

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By "normal" wind powered vehicle, what exactly do you mean?

I was pondering this while walking my dog.

Something along the lines of a Viking longboat. Square rig, single sail.
Put wheels on it and it's a fairly efficient downwind land vehicle.

It will take a certain size sail to reach maximum speed. Anything bigger won't add any speed, but will be capable of gathering more energy.

You could attach an electric generator to one of the wheels, and gather power that way. You would need a "bigger than minimum" sail, but you could generate power to run lights or something similar while still maintaining the same speed (no faster than the wind). You would need to add sail area to gather the energy to generate the electricity (to overcome the drag added by the generator), but you could do so with little or no loss of speed.

You couldn't use the electricity generated to add speed because doing so would decrease the ability to gather energy (by reducing the relative wind).

The system in this thread seems to have figured out a way to generate additional speed without decreasing the energy gathered.

"There are NO situations which do not call for a French Maid outfit." Lucky McSwervy

"~ya don't GET old by being weak & stupid!" - Airtwardo

[strop45 0]

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Draw the vector triangle.

Here, someone already did.

www.animations.physics.unsw.edu.au/jw/images/sailing_files/petitlargue.gif

Because the boat is sailing close hauled with respect to the relative wind, the wind is still on the correct side of the sail.

Thanks for that, it makes sense to me now.

In practice, I wonder what limits a vehicles capability to sail close hauled downwind and therefore what is the best angle to choose to give best VMG if sailing from point A to point B where B is directly downwind of A. I guess it is related to highest angle the boat can point upwind, albeit that boat speeds, sail efficiency etc etc are different upwind to downwind.

I also wonder if this means that boats which use spinnakers are not capable of this level of performance since when sailing this close to the wind downwind, the relative wind is opposed to the direction of travel.

The difference between stupidity and genius is that genius has its limits." -- Albert Einstein

[kallend 1,644]

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Draw the vector triangle.

Here, someone already did.

www.animations.physics.unsw.edu.au/jw/images/sailing_files/petitlargue.gif

Because the boat is sailing close hauled with respect to the relative wind, the wind is still on the correct side of the sail.

Thanks for that, it makes sense to me now.

In practice, I wonder what limits a vehicles capability to sail close hauled downwind and therefore what is the best angle to choose to give best VMG if sailing from point A to point B where B is directly downwind of A. I guess it is related to highest angle the boat can point upwind, albeit that boat speeds, sail efficiency etc etc are different upwind to downwind.

I also wonder if this means that boats which use spinnakers are not capable of this level of performance since when sailing this close to the wind downwind, the relative wind is opposed to the direction of travel.

I suspect only the very cleanest hulls, or iceboats, can take advantage. I found this about iceboats, I highlighted parts that I though relevant to this discussion:

Because of the very high potential boat speeds, iceboat race courses are established around fixed marks which are to be rounded in a one-way route. The courses are deliberately set in a straight line upwind and downwind, though the boats have to tack back and forth to advance through the course. This course configuration provides a measure of traffic safety between competitors. Also, by directing the iceboat straight upwind or straight downwind, the sailor can best control and prevent the sail from generating excessive horsepower, resulting in the slowest directions of travel. Recorded racing speeds for an International DN in 9 knots (17 km/h; 10 mph) winds are 26 knots (48 km/h; 30 mph) upwind, and 35 knots (65 km/h; 40 mph) downwind. A downwind speed of 49 knots (91 km/h; 56 mph) was recorded at a World Championship in winds of just 13 to 15 knots (24 to 28 km/h; 15 to 17 mph).

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The only sure way to survive a canopy collision is not to have one.

[ryoder 1,397]

"Wired" magazine;
March 2011;
Page 128;
"Faster - One man's quest to outrace the wind";
Author: Adam Fisher.

Previous article: http://www.wired.com/autopia/2010/06/downwind-faster-than-the-wind/

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[FB1609 0]

You can't sail directly down wind faster than the wind no matter how hard you try, on any surface.

[piisfish 135]

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You can't sail directly down wind faster than the wind no matter how hard you try, on any surface.

skeptic ?
www.fasterthanthewind.org/

scissors beat paper, paper beat rock, rock beat wingsuit - KarlM

[ryoder 1,397]

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You can't sail directly down wind faster than the wind no matter how hard you try, on any surface.

Even a sailboat can do that.

See: http://en.wikipedia.org/wiki/USA_17_(yacht)#Racing_results

The phrase you are looking for is velocity made good.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

[ryoder 1,397]

Guess what just showed up on eBay: http://www.ebay.com/itm/Blackbird-Faster-Than-The-Wind-vehicle-/281114481020

That would be a great toy for those windy days at the DZ.

"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.