## Car Goes Downwind 2.5 Times Faster Than the Wind

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Papa   1 kW

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### Car Goes Downwind 2.5 Times Faster Than the Wind

Try an wrap yur noggin around this: http://www.fasterthanthewind.org/

"The downwind car is designed with a propeller that pushes it along like an aircraft's propeller does, it is not a windmill or a turbine. Instead of an engine to turn the propeller, the propeller is geared to the wheels of the car, so that as the car moves forward, the propeller spins in such a way that the car will speed up if there is energy available from a tailwind. The following explanation will show how this works by analyzing the forces acting on the vehicle and the energy flow through the system.

In order for the car to travel directly downwind faster than the wind using only the wind for power, the total resistive force on the vehicle must be less than or equal to the thrust provided by the propeller (force balance) and the power provided at the wheels must be greater than or equal to the power consumed by the propeller (power balance). (Note: In this explanation we are describing the power balance as seen by an observer riding with the car, so the power for the propeller appears to come from the wheels. A similar analysis can be done from the ground-based reference frame, but it is more complicated.) The force equation tells us that the sum of the wheel resistance equals the propeller thrust when traveling at maximum speed (we are ignoring vehicle drag for simplicity in this explanation). The power balance tells us that the wheel force times the car speed (power input) must be equal to or greater than the propeller thrust times the relative velocity of the air (Vcar-Vwind) divided by the propeller efficiency (power output). Note: we are also neglecting drive train losses in this simplified analysis.

Figure 1: Force and power balance for a downwind car.

Understanding the power balance assumes that the reader knows that power is equal to speed times force. For the propeller, the speed we must use is the airspeed relative to the propeller, not the speed of the car, because the propeller is working against the air instead of the road surface. The power available from the wheels is proportional to the speed of the car ( Pinput = R*Vcar) where R is the wheel resistance force and Vcar is the car speed. The propeller power consumed is (Poutput = T*(Vcar-Vwind)/etap) where T is the propeller thrust, Vwind is the wind speed, and etap is the propeller efficiency which is related to the "slip" and viscous drag of the propeller in the air. If we combine the force and power equations and simplify the expression, the result is a simple equation that states that if the propeller exceeds a minimum level of efficiency, the car can travel faster than the wind downwind (figure 2).

Figure 2: Minimum required propeller efficiency for the car to work.

This equation defines the minimum propeller efficiency for the vehicle to work at a given ratio of wind speed. In reality, the required propeller efficiency will be higher since we neglected vehicle drag and drive train losses in this analysis. Figure 3 shows that even at very low propeller efficiency the car can run faster than the wind downwind and as the efficiency approaches 1.0 the speed ratio goes to infinity (assuming no other losses).

Figure 3: Variation of carÃ¢â‚¬â„¢s speed ratio as a function of propeller efficiency.

The reason the car works is that the propeller is pushing on air that is moving slower than the speed of the car because there is a tailwind. This allows energy to flow into the system for propulsion, something that would be impossible if there were no wind. Another way of looking at this is the tailwind allows the propeller to make more thrust for the same power than if there were no tailwind. The propeller must be geared to the wheels for this to work or else there is no way to keep it turning and accepting power from the wind."
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dogman dan   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Makes total sense to me. A sail is a wing, and so is a propellor. If it generates lift, it generates thrust.

dak664   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

The wheels are geared to the propeller so either the wheels are driving the propeller or the propeller is driving the wheels. If the wheels are driving the propeller the car would go faster if it was disconnected, so the propeller must be driving the wheels. If the car is going faster than the tailwind the propeller must actually be driven by a resulting *headwind*.

Not hard to see this can be the case if the car is initially at rest with a headwind. The propeller spins with the headwind and the gearing drives the car forward as long as the tires don't lose traction. If the headwind has low velocity the car can indeed go many times faster than the headwind. Now make it a tailwind with the car at rest, with the gearing reversed for the car to start forward. When the car speed reaches the wind speed there is nothing driving the propeller anymore. It is a steady state equivalent to the car at rest with no wind at all! At this point, if the gearing is reversed and there is a headwind gust (or equivalently a sudden drop in tailwind), the car could experience an acceleration surge from the stored momentum in the propeller acting against the wind gust, and in the short term it could go 2.5 faster (especially if the tailwind has dropped to a low value which would increase that ratio).

John in CR   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Dogman, if it was only that easy. The prop is actually turning against the direction that the wind would force it.

Yeah, it's a real noggin wrapper. I saw video of a wood rig a few years ago, but I didn't realize anyone built a large one. I wonder what gearing ratio they use. Hey Papa, do you think the optimal gearing ratio varies with wind speed or does only the prop determine it?

John

hydro-one   10 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

impossible.
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liveforphysics   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

hydro-one wrote:impossible.

This same principal has been applied to sailing and other sail powered things for a couple thousand years. Definitely not impossible. Rather than a tacking-angle to convert a low speed, high force vector into a lower force, higher speed resultant vector, they just did it with a prop and gears.

It's tacking (the way boats sail much faster than the wind speed), it's just done with a prop and wheels rather than an angled sail.

If I'm not mistaken, the little desert/ice sail-powered rigs do better than 2.5times wind speed?

This might help folks with the energy conservation concept and exceeding wind speed velocity:

http://www.animations.physics.unsw.edu. ... iling.html
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swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

It seems simple to me. If you used the largest lightest propeller you could possibly find, you could capture lots and lots of energy to propel something that's just your mass. By the equation of kinetic energy, you're then going to go really fast.

I don't understand if they were able to sustain 2.5 times faster than the down-wind. I could understand hitting 2.5 times for a moment because of stored energy, but it seems to me that the wind wouldn't be capable of beneficially rotating the propeller past that speed to continually convert wind energy to kinetic energy. Unless the wind happened to be perpendicular or at an angle to the craft or hitting the craft (I'm now thinking that it is - but the title says "Downwind".)

EDIT: It appears I misunderstood it. The wind is heading against the craft's direction. Okay, that makes perfect sense. I wonder if you could find a light/large enough propeller, you could have a wind powered aircraft?

EDIT 2: Or, wait, no... that picture shows the wind heading in the same direction as the craft.
Last edited by swbluto on Jul 11 2010 11:50am, edited 3 times in total.

TylerDurden   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

swbluto wrote:I don't understand if they were able to sustain 2.5 times faster than the down-wind. I could understand hitting 2.5 times for a moment because of stored energy, but it seems to me that the wind wouldn't be capable of beneficially rotating the propeller past that speed to continually convert wind energy to kinetic energy. Unless the wind happened to be perpendicular or at an angle to the craft (I'm now thinking that it is - but the title says "Downwind".)
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Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

John in CR wrote:I wonder what gearing ratio they use. Hey Papa, do you think the optimal gearing ratio varies with wind speed or does only the prop determine it?
It appears that they incorporated both variable pitch prop hub and a conventional bike cassette (sans derailleur) gearing. The following pics were posted mid-March so I don't know if changes were made later.

Here's an interesting quote, "As we approach and perhaps even exceed 3 times the speed of the wind, we're getting rather twisty on our prop shaft. We're now in the 500-700 ft lb range torque wise"

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John in CR   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

liveforphysics wrote:
hydro-one wrote:impossible.

This same principal has been applied to sailing and other sail powered things for a couple thousand years. Definitely not impossible. Rather than a tacking-angle to convert a low speed, high force vector into a lower force, higher speed resultant vector, they just did it with a prop and gears.

It's tacking (the way boats sail much faster than the wind speed), it's just done with a prop and wheels rather than an angled sail.

If I'm not mistaken, the little desert/ice sail-powered rigs do better than 2.5times wind speed?

This might help folks with the energy conservation concept and exceeding wind speed velocity:

http://www.animations.physics.unsw.edu. ... iling.html
It is possible, though Luke hasn't really grappled with it yet. The wind is not directly making the blades turn like it does on a wind turbine, or like on a sailboat sail either, since neither of those can move with the wind faster than the wind is blowing. It more like, but just the opposite of, the boats with a wind turbine above deck that turns a shaft for a propeller in water that are able to sail even directly into the wind. In this case, the wheel on the ground is what turns the blades.

What makes it work is that it captures the force of the wind relative to the ground, not relative to the vehicle. Imagine a fixie bike with a propeller being driven too, and at first no wind at all. The gearing on the bike wheel is so low that the propeller is actually providing some thrust, so it's screwing through the air faster than your ground speed. Then suddenly the wind comes from behind and pushes against your fan bike's thrust, resulting in more effective thrust and making you go faster, but going faster makes the propeller turn faster because it's a fixie and going faster means more rpms for everything, creating even more thrust against the wind that speeds you up and speeds the prop even more.

swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

John in CR wrote:
liveforphysics wrote:
hydro-one wrote:impossible.

This same principal has been applied to sailing and other sail powered things for a couple thousand years. Definitely not impossible. Rather than a tacking-angle to convert a low speed, high force vector into a lower force, higher speed resultant vector, they just did it with a prop and gears.

It's tacking (the way boats sail much faster than the wind speed), it's just done with a prop and wheels rather than an angled sail.

If I'm not mistaken, the little desert/ice sail-powered rigs do better than 2.5times wind speed?

This might help folks with the energy conservation concept and exceeding wind speed velocity:

http://www.animations.physics.unsw.edu. ... iling.html
In this case, the wheel on the ground is what turns the blades.
I was thinking about that, but it didn't seem immediately obvious because you'd then just be converting kinetic energy from your speed into wind energy which means you should slow down to the at least the wind speed. But... if the propeller itself is moving faster than the wind speed because it's tied to your vehicle's speed (Through the chosen gear ratio), then it seems possible for the propeller to continue to generate thrust because propeller "generated air" speed is greater than the wind speed. If they were the same, then obviously the propeller wouldn't be pushing which is what I originally was puzzled over.

However, this doesn't seem like a steady state condition ... or maybe it could be? It seems mind-bending if it is. I'd think that as you convert kinetic energy into thrust, you'd lose kinetic energy... but thrust itself continues to add to kinetic energy. So, as long as the amount of kinetic energy added is greater than that amount robbed, you'll have a net gain in kinetic energy and thus speed past the wind speed.

Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

OK,... something's screwy here...

Here's a screen grab from the first vid posted at, http://www.fasterthanthewind.org/

The car is just being launched and the prop is turning (about 14 sec into the video). But note that the prop is turning clock-wise (as viewed from the rear of the vehicle) - and also note the pitch angle of the blades.

Bare-in-mind, that there is one-way ratchets in both drive wheels, so the wheels CANNOT be driving the prop. There appears to be a strong tailwind... so why is the prop rotating clock-wise? With the pitch angle of the blades (see attached pic), shouldn't it be rotating counter-clockwise?
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wind-car.jpg (42.48 KiB) Viewed 3724 times
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swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Oh, wait... they ARE heading into the wind. It's a headwind, not a tail wind, if you look at the anemometer in the video. Given that, then it makes perfect sense they could go faster than the wind. As long as the drag from the vehicle doesn't exceed the thrust generated from the propeller, they should be able to easily go faster than the wind. How much they reduce drag will be telling of how much faster they can go...

The drawing in the first post seemed ... not perfectly clear.

Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

swbluto wrote:Oh, wait... they ARE heading into the wind. It's a headwind, not a tail wind,
In the video, they don't see a headwind until 1:35 into the run. So why did the prop rotate clock-wise prior to experiencing a headwind?
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swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Papa wrote:
swbluto wrote:Oh, wait... they ARE heading into the wind. It's a headwind, not a tail wind,
In the video, they don't see a headwind until 1:35 into the run. So why did the prop rotate clock-wise prior to experiencing a headwind?
If you look at the anemometer at 1:14, you'll see that they're heading into a headwind prior to 1:35. At 1:35, the weather vane switches directions because the ...

Oh wait, I'm just now reviewing what the weather vane is indicating. After thinking about the wind's interaction with the "flapper" on the back, the arrow points from where the wind is coming from, not where it is going. So you're right, it must've been a tail-wind. At 1:35, it must have converted into a "head-wind" because the vehicle was moving fast enough to generate its own "head-wind".

The only explanation would seem to be the wheels drive the propeller, and the drag from the tail-wind exceeded the thrust of the propeller at that propeller speed, and so they moved in the direction of the tail wind. So, the propeller was acting more like a "parachute" than a propeller at that low speed, and the direction it turned was forced by mechanical coupling.

There's still something quite weird about accelerating into a head-wind, despite how much it seems understandable in physics concepts. If that's understood, then allowing a vehicle to accelerate due to a tail wind until going fast enough to generate a "head-wind" should be equally understood. It still seems weird. It's like there HAS to be some fatal flaw I'm not seeing.

Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

swbluto wrote:The only explanation would seem to be the wheels drive the propeller, and the drag from the tail-wind exceeded the thrust of the propeller at that propeller speed, and so they moved in the direction of the tail wind. So, the propeller was acting more like a "parachute" than a propeller at that low speed, and the direction it turned was forced by mechanical coupling.
The site clearly states that the rear drive wheels have one-way ratchets installed to permit towing the vehicle without spinning the prop. That said, the prop should be able to freely spin backwards (counter clock-wise) in the presents of a tailwind. So what am I missing here?

Now 'if' the prop shaft were also an armature... and you had a battery...
Last edited by Papa on Jul 11 2010 3:32pm, edited 1 time in total.
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swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Papa wrote:The site clearly states that the rear drive wheels have one-way ratchets installed to permit towing without spinning the prop.

"permit" doesn't mean the same thing as "always only". I permit you to reply without blinking, but that doesn't mean I *don't* permit you to reply while blinking.

But, if you're suggesting it's a fraud... it might be. If they could bicycle until they got to the point of a "headwind", then it seems like it could take off after that. Maybe. I'd need to better review the physics. It seems like there emphasis was more that it could sustain a speed faster than the wind on wind power alone, not necessarily get to that speed on wind power alone.

Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

swbluto wrote:But, if you're suggesting it's a fraud...
I wasn't trying to suggest anything. Just an observation.
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swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

I wonder if you put a small prop on a straw and put a "one way rachet" on it such that going backwards on the straw was impossible for the prop, but frontwards on the straw was possible, if you blew wind at it, if the prop would advance forward. It seems that the thrust wouldn't exceed air friction but, then again... the drag of the propeller spinning at wind speed is minimal isn't it? But, then again, the thrust should be minimal since the pressure difference is roughly equal since the "wind speed" is the same infront of the propeller and behind it. Now if you stopped breathing for a split second, then the propeller would have momentum that would allow it to move forward due to the propeller's thrust until the blowing resumed.

Now, I have no idea if this is applicable to this, but it seems like it'd be interesting to try.

swbluto   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Papa wrote:I wasn't trying to suggest anything. Just an observation.
No hostility intended.

If it can't be explained by non-fraud mechanisms, then that would suggest fraudulent activity. So far, it doesn't seem like you've found/accepted a non-fraudulent explanatory mechanism.

johnrobholmes   1.21 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

The prop is acting more as a sail at slow speed as it is turning the wrong way for the wind, and as they accelerate to zero apparent wind speed the prop resistance to turning is lowered further and further. As it hits zero apparent speed and gets any sort of frontal gusts, the prop goes into action and will increase speed if the pitch is right. So with the back wind, the vehicle is just pushed to speed by the wind. Once it gets to frontal wind the prop is efficient enough to drive the wheels.

Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

johnrobholmes wrote:The prop is acting more as a sail at slow speed as it is turning the wrong way for the wind,...
What do you estimate the 'tailwind' speed in the video I referred, when the vehicle is at rest? 20.. maybe 30 MPH? This is what a typical wind generator looks like in 20-30 MPH wind:

Note which way the blades rotate in relation to blade pitch on the wind generator.
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John in CR   100 GW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Papa,

The wheels turn the prop. The prop is not a turbine. The prop is spinning in the proper direction which is against the wind, because the wheels cause it to. To start the pitch of the blades was near neutral for the wind to give a push and get it started. Then the prop starts turning and providing forward thrust against the wind. Later, what looks like a headwind is actually demonstration that it is in fact traveling faster than the wind, the wind never changes direction, though the apparent wind does.

John

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Hard to estimate just on prop speed. Since it is variable pitch the "airspeed" and rpm of the prop change.

Papa   1 kW

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### Re: Car Goes Downwind 2.5 Times Faster Than the Wind

Ok... I've got bassackwards...
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