Wind-Powered Car Actually Moves Faster Than Wind Speed
Comments
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Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Then in still air and with a push--the vehicle should also continue to move forward too? The trust of the propeller is greater than the retarding force of the wheels?
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
You mean without true wind? No, of course it won't work. You need a velocity difference (airspeed < ground speed) to create the force difference (thrust > retarding force).Then in still air and with a push--the vehicle should also continue to move forward too? The trust of the propeller is greater than the retarding force of the wheels?
Here again with emphasis:In the reference frame of the vehicle, it works like a gear box: A small input force (braking) coming in at high speed (ground speed) is transformed into a larger output force (propeller thrust) going out at a lower speed (air speed) -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
But Power = Force * Velocity...
So, in a steady state system and ignoring drag (aerodynamic and from bearings, etc.) we know that Power from retarding wheels must equal forward energy provided by the propeller.- Force * Vground = Force * Vair
Basically the energy from the retarding force must be higher than the energy generated by the turning blades--So we have an unbalanced system where we are loosing energy.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
In the vehicle's frame of reference the propeller is not providing energy, it is using energy from the wheels to generate a forward force.But Power = Force * Velocity...
So, in a steady state system and ignoring drag (aerodynamic and from bearings, etc.) we know that Power from retarding wheels must equal forward energy provided by the propeller.
Without losses, the thrust must be greater than wheel braking. With losses, it depends on the total efficiency of the whole thing (including propeller efficiency):- Force * Vground = Force * Vair
F_air * V_air = total_efficiency * F_ground * V_ground
For the required total_efficiency check out equation 7 from this:
http://www.boatdesign.net/forums/attachments/propulsion/28167d1231128492-ddwfttw-directly-downwind-faster-than-wind-ddw2.pdf
In the reference frame of the vehicle:Basically the energy from the retarding force must be higher than the energy generated by the turning blades
- The power input from the wheels is greater than the power output at the propeller
- The force at the prop is greater than the retarding force at the wheels
The true wind is loosing energy, and we are accelerating because the forces are not balanced (until drag equals net thurst)So we have an unbalanced system where we are loosing energy. -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
I don't believe the derivations of equation 6/7 are correct... Just a simple boundary test shows meaningless results.
V-W is the "apparent" air speed of the air propeller. With V = W (i.e., boat/vehicle is moving exactly the speed of the wind), the system develops infinite force:- V/(V-W)
I believe the error in equation (2)--The author talks about the efficiency of a propeller and use it as an equation to generate thrust.
And the problem is that is not the efficiency of a turbine/propeller, that is the formula for extracting energy from a moving object (or air or water)... Basically, Formula (2) states that if you use a mechanical device to extract the energy from the flow--The difference in the flow rate is how much energy you extracted from it...
So, if I have something moving at 10 mph, take some energy out, and it is now moving at 5 mph--then I have extracted 50% of the energy. [actually, I made a mistake here--V^2 is the energy in motion, 1/2 speed reduction would be 3/4's of the energy removed from the system]
If I have something moving at 10mph, extract some energy, and now it moves at 0mph, then I have extracted 100% of the energy.
For example, send at lot of water from a dam down a pen-stock into a turbine that takes the F*MassFlow of the water, spin it into the turbine runner, and dump the water with only a small velocity left into the river at the base of the dam, I have extracted 90% of the available energy--And this has nothing to do with the efficiency of the turbine--If the turbine is also 90% efficient, then I have turned the falling water into 0.9*0.9=0.81 or 81% useful energy...
Vastly different than how they are trying to use the math in the assumptions above.
I agree with the first statement--in reality, they would probably be lucky to get 30-50% energy transfer from the wheels to the propeller (as thrust).In the reference frame of the vehicle:
- The power input from the wheels is greater than the power output at the propeller
- The force at the prop is greater than the retarding force at the wheels
Looking at force in a steady state condition the two have to balance (even giving them the "zero drag" assumption).
Force by itself does not equate into energy usage. One must take velocity into account too... 10 lbs of force at zero miles per hour is ZERO ENERGY. 10 lbs of force at 100 MPH is a lot more zero energy.
There is nothing in the equations that account for the change in mass flow of the wind... There is a mass flow of energy through the propeller, but that is relative wind and energy transfer does not appear to be taken into account.The true wind is loosing energy, and we are accelerating because the forces are not balanced (until drag equals net thrust)
For example, if the entire forward motive force of vehicle is provided by air propeller and the entire energy to drive the propeller comes from the wheels.
And there is a non-zero energy transfer loss between the energy into the wheels and through the propeller--then we have the classic thermodynamic statement is the best you can do is break even. And, you can never break even.
The only way I can understand the vehicle going downwind faster than the wind in a steady state is through wind speed distribution... Wind speed is slower near the ground to goes up the farther away from the ground you get...
If the velocity is measured at driver level, the wind speed would be much higher 30+ feet higher in the air... So at this point the propeller becomes "flat disc" and is dragging the car along at a higher rate of speed than the "surface wind"--or possibly the propeller is being driven by the wind and helping to turn the wheels forward...
According to the rules, the wind speed is supposed to be measured at hub height--so that should not introduce a huge error...
-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
The size of the propeller is rather great compared to the size of the car - especially the profile of the car. -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
No, because the propeller efficiency(as used here) tends toward zero at this point, so the thrust force is still a finite number. It is called "static thrust". The propeller efficiency is not a constant, but depends on the situation.With V = W (i.e., boat/vehicle is moving exactly the speed of the wind), the system develops infinite force:
No, because the propeller efficiency(as used here) becomes negative, if the propeller is working in reversed flow. The thrust force is still a positive finite number. You explain yourself why this limiting case(airspeed = 0) is difficult to handle with energy formulas:Then, as the vehicle slows just below wind speed, we are approaching infinite retarding force because of the sign change).Force by itself does not equate into energy usage. One must take velocity into account too... 10 lbs of force at zero miles per hour is ZERO ENERGY. 10 lbs of force at 100 MPH is a lot more zero energy.
It is the usual definition of propeller efficiency. And efficient propellers achieve 90% here, but not at static condition (airspeed = 0). When dealing with airplane propellers you are usually not mainly interested in the static condition or reverse flow. And here you also don't need this cases to show that steady state above wind-speed is possible.I believe the error in equation (2)--The author talks about the efficiency of a propeller and use it as an equation to generate thrust.
Turbines and propellers serve different purposes, and so the common definitions of "efficiency" for them are different. But it doesn't matter how you call that term. Fact is: you can design a propeller that will achieve:And the problem is that is not the efficiency of a turbine/propeller,
thrust_force * free_stream_airspeed / prop_shaft_power = 0.9
Again: the propeller is not extracting the energy from the flow. It is delivering energy to the flow (in the propellers ref. frame). Above wind-speed it accelerates the apparent headwind, thus slows down the true tailwind.Basically, Formula (2) states that if you use a mechanical device to extract the energy from the flow--
As I said: At design airspeed & RPM such propellers achieve 80-90% efficiency. Combine it with 80-90% transmission efficiency (wheel -> shaft) and you have 64%-80% total efficiency. That allows you to achieve 2-3 times wind speed if you account for drag.I agree with the first statement--in reality, they would probably be lucky to get 30-50% energy transfer from the wheels to the propeller (as thrust).
In simplified terms: The thrust force of the propeller is the negative change of airs momentum per time.There is nothing in the equations that account for the change in mass flow of the wind...
The forward thrust is not just a result of the propeller rotation. The true tailwind also contributes here, by reducing the apparent headwind, and thus allowing the propeller to produce more thrust. Without the true tailwind the propeller would never develop more thrust, than the wheels need to brake in order to turn the propeller.For example, if the entire forward motive force of vehicle is provided by air propeller and the entire energy to drive the propeller comes from the wheels.
You could even measure it well above hub-height, they would still be faster. Here is an early prototype. Note the streamer above the propeller:According to the rules, the wind speed is supposed to be measured at hub height--so that should not introduce a huge error...
http://www.youtube.com/watch?v=EEuAqq8FINw#t=2m30s -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
You will find a lot of discussions on this. And some people made explanatory animations with simple mechanical analogies:
These are funny too:
http://www.youtube.com/watch?v=E7vcQcIaWSQ
http://www.youtube.com/watch?v=k-trDF8Yldc -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Tacking is done to go in the opposite direction from the wind, another thing that seems counterintuitive on the face of it. You can't, of course, go in exactly the opposite direction, but you can go less than 90 degrees from directly upwind, and by switching back and forth between port and starboard tacks you can get directly upwind from where you were.That's called Tacking, when done in a sailboat. And as I understand it, it is at an angle and against the wind.... back and forth, sideways, Understood using vectors and trigonometry. -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
The same dream, different design.
http://www.popsci.com/archive-viewer?id=GgEAAAAAMBAJ&pg=102&query=wind-powered+car -
Re: Wind-Powered Car Actually Moves Faster Than Wind SpeedThe same dream, different design.
http://www.popsci.com/archive-viewer?id=GgEAAAAAMBAJ&pg=102&query=wind-powered+car
That's a nice compact design, but it won't go directly downwind faster than wind or directly upwind (on wind alone). Similar one but too big for roads achieved 126mph in 2009:
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Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Tacking is done in both directions. Tacking downwind is even more counterintuitive than upwind, if you outrun the airmass (achieve a downwind-velocity-made-good greater than true windspeed):Tacking is done to go in the opposite direction from the wind, another thing that seems counterintuitive on the face of it.
Here analogously the vectors at the propeller blade airfoil of the DDWFTTW vehicle at 1.5 directly downwind:
The transition from tacking downwind to the propeller is shown here:
http://www.youtube.com/watch?v=UGRFb8yNtBoYou can't, of course, go in exactly the opposite direction
With a conventional sail craft, no. With rotating airfoils you can do both:
- directly upwind
- directly downwind faster than wind -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
They can. The vectors are not exactly the same but similar. Going dead upwind has been done many times:I still don't see how it was done... Otherwise, they should be able to go dead upwind too (same vectors).
http://www.youtube.com/watch?v=KQu4hUKnoVE&feature=player_detailpage#t=130s
http://www.youtube.com/watch?v=A-aX6z_qlBg
http://www.youtube.com/watch?v=oKqC5JsurOk
But the BlackBird might be soon the first one to go directy upwind faster than true wind.
As I already said: The propeller blades are also moving at an angle to the true wind, even when the vehicle moves parallel to the true wind. See vector diagram above.When I followed the links, I saw mostly sail boats (yes, they can get within 30 degrees of up wind and can go faster than the wind--again when the wind is coming from the side, not front or back. -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Certainly, I would agree that moving upwind with a propeller and a gear box is possible at slow ground speeds (as the youtube video).
And, I would guess that the limitations of pure upwind speed just depends on the efficiency of the mechanicals and the drag of the vehicle. Low drag and high efficiency would, it would seem, would not limit forward direct upwind speed to some "magic number" like a limit of wind speed as forward velocity.
Still trying to understand going downwind faster than the wind (direct tailwind).
Not sure how that differs from just pushing the car and getting relative wind from the initial shove in dead air...
But like a say, I do not claim that I understand it all either.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
this is all well and good, but i don't foresee them putting sails on cars to do this or even close. the application is too impractical to employ for cars on a street as there'd be nothing to the car and the sail would take up more vertical and horizontal room than can be legally utilized. this leaves it as an unrealistic approach to powering today's cars and is therefore a moot point even if it works so i'm curious as to what the relevant point is to this thread? -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed... this leaves it as an unrealistic approach to powering today's cars and is therefore a moot point even if it works so i'm curious as to what the relevant point is to this thread?
neil -- I can't comment on the point of the thread as I just arrived, but I can assure you (as the co-designer/builder of the device linked to in the OP) that our point had nothing whatsoever to do with any practical application at all. We didn't build the vehicle because we believed any more would ever be built and used.
In short, it was built to prove a point (and educate hopefully) -- the point being that something many consider impossible borders on trivial to accomplish. It's a mere engineering exercise and nothing more.
JB -
Re: Wind-Powered Car Actually Moves Faster Than Wind SpeedCertainly, I would agree that moving upwind with a propeller and a gear box is possible at slow ground speeds (as the youtube video).
Yes.And, I would guess that the limitations of pure upwind speed just depends on the efficiency of the mechanicals and the drag of the vehicle.
Yes.Still trying to understand going downwind faster than the wind (direct tailwind).
Not sure how that differs from just pushing the car and getting relative wind from the initial shove in dead air...
In the case you describe above, there is no relative motion between the air and the ground ('true' wind). Since it is a 'true' wind powered device, if there is none, it rolls to a stop.
Remember, the Blackbird *isn't* powered by the relative velocity of the air vs the device itself ('relative' wind), but rather from velocity of the air relative to the ground ('true' wind). On a still air day, no matter how fast you push the device, you still are dealing with air that is not moving relative to the ground and thus you have no power source.
JB -
Re: Wind-Powered Car Actually Moves Faster Than Wind SpeedBut Power = Force * Velocity...
BB, for those who understand the above equation it is often only a few very easy steps to understanding how the Blackbird works.
Since the Blackbird is traveling directly downwind, it travels across the ground with greater velocity than it travels through the air. Imagine if the device is traveling DDW at 20mph in a 10mph wind (2x windspeed). In such a steady state situation it's easy to see by plugging in the numbers into that equation that we are able to harvest exactly twice as much power from the wheels as we need to propel the vehicle via the propeller. Now that assumes a 100% transfer of power -- something that obviously isn't the case, but if at 100% efficiency, one is generating more power at the wheels than is needed at the propeller then it's only a matter of engineering the losses low enough so as to use no more than that extra that is shown as available.
Plugging the numbers into the above equation also makes it easy to see the difference between operating downwind and pushing the vehicle forwards when there is no wind (as you have mentioned several times). In that scenario, the numbers will show that the amount of power available at the wheels is identical to the power needed at the prop (no matter how fast you push). Of course with matching in and out power, there is nothing for losses and thus the vehicle slows.
Hope that helps.
JB -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Correct, and the same applies to the downwind direction. The maximum velocity in both directions is just a question of efficiency. See this paperCertainly, I would agree that moving upwind with a propeller and a gear box is possible at slow ground speeds (as the youtube video).
And, I would guess that the limitations of pure upwind speed just depends on the efficiency of the mechanicals and the drag of the vehicle. Low drag and high efficiency would, it would seem, would not limit forward direct upwind speed to some "magic number" like a limit of wind speed as forward velocity.
For upwind (negative velocity):
max_velocity / windspeed = 1 / ((1 - total_efficiency) - 1)
For downwind (positive velocity)
max_velocity / windspeed = 1 / (1 - total_efficiency)
Same as going upwind, you just swap the roles of ground and air. This shows the symetry in terms of a simple lever:Still trying to understand going downwind faster than the wind (direct tailwind).
http://www.youtube.com/watch?v=za_rPKSwiyc
This shows both directions for the rotor carts in a schematic way:
http://www.youtube.com/watch?v=FqJOVHHf6mQ
It differs by the velocity difference between ground and air. In the frame of the cart you have:Not sure how that differs from just pushing the car and getting relative wind from the initial shove in dead air...
F_air * V_air = total_efficiency * F_ground * V_ground
If V_air = V_ground (no true wind) then you can never achieve F_air > F_ground to accelerate. But with a true tailwind you have V_air < V_ground, so you can generate a propeller thrust greater than wheel drag. -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Certain sail boats,, and certainly ice boats will sail a lot faster than the wind!
Tony -
Re: Wind-Powered Car Actually Moves Faster Than Wind SpeedCertain sail boats,, and certainly ice boats will sail a lot faster than the wind!
Tony
That is correct, however none of those can travel directly upwind at any speed, and none of them can travel directly downwind as fast as the wind, let alone faster. The point of the vehicle in question is to demonstrate that those limitations are based solely on the design limitations of those craft and not some law of physics.
JB -
Re: Wind-Powered Car Actually Moves Faster Than Wind Speed
Just. Sayin,,, as they say!
T
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