Is it possible to go DDW faster than windspeed?

[Guest]

http://blog.makezine.com/archive/2010/1 ... black.html

[Absolution 7]

Maybe they have invented perpetual motion too!

[wheels 543]

No it's rubbish.

You can not make something from nothing. Even if you had a completely efficient transfer of energy from Propellor/gearbox to wheel drive and then frictionless tires, even though they still grip, you will run out of any energy input once you reach windspeed. There is nothing to push the vehicle through that barrier into a forward windspeed. Then once you reach a forward windspeed, you need to change the gearbox direction so the propellor is still pulling the cart forward.

There is another way of proving this wrong.

If you could get the cart to do 2.8x the windspeed, then you have a headwind. So why would it stop at 2.8x. What would happen is the wind would continue to input energy and the speed would continue to increase and thus the wind would input even more energy and so on and you now have perpetual motion, till the point of where the Blade disintergrates or you reach warp speed.

In fact, you can not even make a cart go directly into the wind. Simply put, the energy you gain from the wind is not enough to overcome the energy the wind exerts on the craft.

And one other way to describe it, in a way we can all understand. It would be like trimming all sails to centreline and pointing directly into the wind. You simply go backwards.

[Variant 2]

It is all simple physics. It is easy to understand once you realize that the wheels are ALWAYS providing the torque to the blade (not the other way around). The blade is effectively opposing true wind even when the cart is going faster than the wind.

[SloopJohnB 322]

Hey there's 153 pages on SA

 

Happy read.

 

http://forums.sailinganarchy.com/index. ... opic=82175

[wheels 543]

It is all simple physics. It is easy to understand once you realise that the wheels are ALWAYS providing the torque to the blade (not the other way around). The blade is effectively opposing true wind even when the cart is going faster than the wind.

Come on Variant, think about it. Think through very carefully about what you just said.

That is if you are using the wheels to spin the prop. But then, how do you turn the wheels. You would have to push the cart to turn the wheels to spin the prop.

You simply can not produce more energy than what you put in. It is impossible and is a fundemental law of phyisics. The only time you can break that law, in theory, is cold fusion. But that is still to be proven also.

Even if you took all losses away, the cart would get about as fast as the wind speed and no faster. But the losses are huge. You have friction from gears and angle drives and bearings and biggest of all, the propellor is one of the most inefficient things that exist. It works solely because of friction. It takes as much energy to turn it through the air as it does to pull itself through the air. And all the combined losses are increasing as the energy from the wind decreases.

Basically, if this was true, then the worlds energy problem would be solved. We would have perpetual motion. You don;t need a back wind. All you would have to do is push the cart and you are off.

[Variant 2]

Come on Variant, think about it. Think through very carefully about what you just said.

That is if you are using the wheels to spin the prop. But then, how do you turn the wheels. You would have to push the cart to turn the wheels to spin the prop.

quote]

 

 

You are right about needing to push the cart to get the wheels to spin. The wind does this. The cart begins moving by the wind pushing on the whole structure (including the blade) then as speed builds the blade spins as to oppose the wind, therefore pushing the cart faster.

[Guest]

Those guys from NALSA are not going to ratify the record without being completely convinced that they are not being hoodwinked in some way.

NALSA have it on their homepage, so are aware of the claims being made.

They also list a new landsailing record at 126.2 MPH!

 

The flat earth proponents were strong enough in their conviction to burn people at the stake wern't they wheels.

I don't understand how it worked, but it clearly does.

[BelowPAR 0]

Haha i like this ! . No idea about this perpetual motion theory? Might do some reading... (after the last exam tomorrow, which incidentally is on the physics of fluid flows..)

 

On one hand it makes stuff all sense, but im intrigued now...

[wheels 543]

then as speed builds the blade spins as to oppose the wind, therefore pushing the cart faster.

That is completely backwards. Where is your power coming from?? The wind is going to try and spin the prop the opposing way. It will actually negate the force of the wind against the art.

So where did the wind come from in their Treadmill test??? And the cart on the treadmill only crept forward. Lifting the treadmill up to create an uphill affect still had the cart move forward. So would that not mean when the cart/treadmill was level, it should not have accelerated faster and faster till it shot off the belt and flew through the room??

It's complete rubbish. Come on guys, we have to live by the laws of physics unfortunately. It's bogus.

[Guest]

I'm with you on this below-par.

It doesn't seem right, but there is clearly something going on there.

[wheels 543]

There is certainly something going on. The comments made seem to be from some guy not actually related to the blcakbird project, at least from what I can gather so far. So what is the BB project actually trying to do? Be wind powered and set a speed record doing so, now that i can believe. But i can not find any info in that regards yet. What i find strange is all the very strange Blogs that this article seems to be linked too. Yet nothing of any note. Is that sort of normal???

[Guest]

I've not read any of the links on this page yet, going back now.

http://www.nalsa.org/DownWind.html

[Guest]

From one of the documents in the link posted earlier.

 

The following case study will demonstrate the principle in simple terms:

Consider a cart with an electric generator driven by the rear axle and a propeller driven by an electric motor. We’ll assume the following reasonable set of parameters:

Electric generator efficiency: 85%

Electric motor efficiency: 85%

Propeller efficiency: 85%

Coefficient of rolling resistance: 0.02

Vehicle gross weight: 650 lbs

Coefficient of aerodynamic drag: 0.3

Projected frontal area: 20 sq-ft

For the purpose of this analysis we won’t consider the issue of accelerating to speed, but rather the cart’s ability to maintain faster-than-the-wind speed and further accelerate from that point. Thus we’ll tow the vehicle up to a speed of 20 mph in a 15 mph tail-wind and then let it loose. In this situation the vehicle will experience a relative head-wind of 5 mph. We’ll adjust the generator output such that it produces 20 lbs of retarding force at the wheels. This tells us the wheels will be putting power into the generator at a rate of 20 mph x 20 lbs (400 mph-lbs). But the generator will only produce 340 mph-lbs due to its 85% efficiency.

We deliver that power to the electric motor. But we get only 289 mph-lbs at the motor's shaft due to the motor's 85% efficiency. This power is working to spin the propeller, but the propeller does only 245 mph-lbs of work on the air due to its 85% efficiency. Given the vehicle’s relative airspeed of 5 mph, we can see that the prop will be producing 49 lbs of thrust.

Given our vehicle gross weight of 650 lbs and coefficient of rolling resistance of 0.02, we can calculate that we’ll lose 13 lbs of thrust to rolling resistance. We lose another 20 lbs at the wheels due to the retarding force caused by the electric generator. This leaves us with an excess of 16 lbs (49-13-20).

Finally, we have to consider the aerodynamic drag we experience in this state:

Aero_drag = Drag_coeff * frontal_area * ½ * rho * Vel * Vel

In which rho is air density and vel is the relative air velocity experienced by the vehicle.

Aero_drag = 0.3 * 20 * ½ * 0.002329 * 7.333 * 7.333 = 0.376 lbs

Where 0.002329 is the air density in slugs/ft^3 and 7.333 is our velocity in ft/sec.

Subtracting our aero drag of 0.376 lbs from our excess thrust of 16 lbs, gives us a remaining excess thrust of 15.6 lbs. This shows that the vehicle will continue to accelerate from this state.

The simple explanation is that the vehicle acts as a lever between two media (the ground and the air). Like any lever we can trade a small force over a large distance for a larger force over a smaller distance. This is how we get more thrust from the prop than we create drag at the wheels (since the wheels are moving over the ground faster than the prop is moving through the air –due to the tailwind)

Something to chew on.

[Guest]

That's a classique question! Yes it's possible to go DDW faster than windspeed. Why? It's only a story between true wind and apparent wind. In fact you don't really go down wind, you should keep at least a small angle between the boat direction and the wind direction. That's the key.

 

How to do it? Take a fast boat like a catamaran (Alinghi for example) or better a mix between skate board and fun board. It's also possible with a monohull but its' more difficult.

 

For my example we take a 20kts wind

 

1st action:

boat speed 0kts boat position Beam Reach -->If you have a good boat you should reach 20kts

 

2nd action:

You have know an apparent wind coming at 45° at 28,2kts. You have two choice to go faster, trim your sail, or bear away. We decide to bear away to be Beam Reach. We going know at 20kts at 135° from true wind and accelerating. If your are lucky you will maybe reach 28,2kts. Now you go DDW at the same speed than the wind speed (Your VMG is -20kts). If you have only few friction you can do that many time because Beam Reach is faster than Close Hauled. Every time you do it, you should accelerate because you bear away.

 

It's done, you go faster than the wind speed (your VMG is bigger than the true wind)

 

If we can't bear away and accelerate indefinitely, it's because at one moment the friction force are bigger than the wind force in you sail. In practice the smallest friction is found with a sand-yatch and they reach 3 or 5 time the speed wind. It's means they going down wind almost 2 or 4 time faster than the wind. If it's only almost it's because you should keep at least a small angle. In this case on a sand yacht the apparent wind is Close Hauled and the true wind is more than Broad Reach

 

Myth-buster???

 

Enjoy it and try a sand yacht to live this experience!

 

My english is not perfect but you have an answer without "Coefficient of aerodynamic drag" and complicate stuff!!

[smithy09 50]

I think I get it, and I believe it's true. The best explanation I found is:

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.

 

There is a full explanation with equations and diagrams here:

https://docs.google.com/View?docID=0AdRsKX7aaZTPZGRnbjhkajdfMTY0aGRzNWtnaGM&revision=_latest&hgd=1

 

Ingenious really!! Makes you wonder how many other simple things are out there to be discovered...

[Fogg 427]

Of course you can't create energy out of nothing or extract more energy than is available from the sources of input. But the mistake I think many people are falling for here is to simply equate the speed of the wind with the speed of the cart and saying that because the cart exceeds wind speed that is is somehow miraculously finding 'new' energy from somewhere. The energy balance equation is way more complex than trying to simply balance speed of wind with speed of cart.

 

So yes the cart can exceed wind speed but no it is not perpetual motion or new energy. It's just an ingenious way of demonstrating energy balance/conversion in a way that blows your mind!

[wheels 543]

Slacko, All the best with that exam today mate. I hope you remember everything you need and fluke everything you have forgotten. Take a deep breath and relax. double check all your answers. If you get time, come at your answer from another point of view.

There is no doubt that the vehicle in the pic is very energy efficient. Although that also works against it because the wind will have trouble pushing it. If the wind was pushing it, then why is the propellor at the front??

Anyway, forget that part, sooooothe vehicle is efficient and has certainly been desinged to have the least wind resistance as possible.

 

Just to be clear, i am coming at this from a DDW speed only. No side winds, no apparent winds no anything that can use angles of wind, it is a direct downwind that then once fast enough will be a direct head wind, with no angles to wind.

 

Slacko, you threw in some numbers. But if you want to throw the numbers in, have a look at this. This is JUST for propellor efficiency.

 

(1) η = T*V / Pavail

 

where η is efficiency, T is Thrust, V is Velocity, and Pavail is Power Available, or power going into the propeller. Basically, power out divided by power in. This equation is very useful for many cases, but you should see a problem in that as your velocity goes to zero, no matter how much thrust you're producing, your efficiency goes to zero. So how do you know how good your prop is doing at low speeds or statically? Well, there's another term that can be used, Figure of Merit, which is Induced Power divided by Power Available, or how much of your power is going into accelerating the air. A Figure of Merit of 1 is the best you're ever going to do. You can't accelerate the air any more than that. So, Figure of Merit gives a pretty good indication of how well the prop is doing at any airspeed. Figure of Merit is calculated as:

 

(2) F.O.M. = T*(VA + VI) / Pavail

 

where F.O.M. is Figure of Merit, T is Thrust, VA is craft velocity, VI is induced velocity at the propeller, and Pavail is Power Available.

 

Now, we'd like to turn this equation around to solve for thrust for a given Figure of Merit. The only problem is that induced velocity is a function of thrust. So, let's figure out a way to solve for induced velocity. The fluid flow analogy to F=ma is

 

T = ρAV ΔV

 

where ρ is density, A is the propeller swept area, V is the velocity at the propeller, and ΔV is the amount that the air is accelerated by the propeller, the difference between the freestream and some point downstream of the rotor where no more acceleration takes place. It's pretty well known that at the propeller, the air has accelerated one half of what it will do downstream (VI = 1/2 ΔV). So, at the propeller, the velocity is VA + 1/2 ΔV. Substituting this into the equation above yields:

 

T = ρA (VA + 1/2 ΔV) ΔV

 

Rearranging this equation into something that can be solved using the quadratic equation yields:

 

1/2 ΔV^2 + VA ΔV - T/ρA = 0

 

Solving with the quadratic equation gives:

 

ΔV = [-VA ± sqrt(VA^2 + 4*1/2*T/ρA)] / 1

 

And remembering that VI = 1/2 ΔV:

 

(3) VI = [-VA ± sqrt(VA^2 + 4*1/2*T/ρA)] / 2

 

where the addition solution, not subtraction, gives the correct answer for most conditions at which a propeller will be operating.

 

Now, let's go back to equation (1), and solve for thrust

 

(4) T = η*Pavail / VA

 

Substitute that back into equation (3):

 

(5) VI = [-VA + sqrt(VA^2 + 4*1/2*1/ρA*η*Pavail / VA)] / 2

 

Now we've got the three equations that we'll need to solve for thrust for a given figure of merit. Taking the equation for Figure of Merit (2), and solving for Power Available yields:

 

Pavail = T * (VA + VI) / F.O.M.

 

Substitute this into the equation for efficiency (1), and simplify to get:

 

η = VA * F.O.M. / (VA + VI)

 

Subsitute equation (5) into this to get:

 

η = VA * F.O.M. / { VA + [-VA + sqrt(VA^2 + 4*1/2*1/ρA*η*Pavail / VA)] / 2 }

 

Static Thrust is a special case, since both airspeed and efficiency will be zero. This makes the final equation above that we had derived become invalid, since the denominator will be zero. But, it is extremely easy to solve for induced velocity, and then thrust from there using the F.O.M. equation.

 

T = [F.O.M. * Pavail / sqrt(1/2 * 1/ρA)]^(2/3)

 

So what does that all mean???

Well it takes about 550ftlb of torq to get 1Hp of thrust from a propellor at maximum efficiency. Maximum efficiency is achieved when the airspeed is approaching something around 200Kts. Even then the maximum efficiency in theory is 90%.

At 20kts, the prop has an efficieny more like 18%. 50kts = 60%

 

Now all that is Just for the prop.

What everyone is failing tounderstand is that as the vehicle apporaches windspeed, where is the energy coming from? You have to have energy to spin the prop or turn the whees faster than the wind and thus producing more ergy than is being applied.

Either the prop is turning the wheels, or the wheels are turnign the prop. Either way, either has to providing more energy than is being put into them.

[wheels 543]

Actually, I just though of the simplest way of explaining this. Lets say you take a Sail and this sailboat is the best ever made and it has absolutely no friction in the water at all. And lets say it has no mass to have to get moving. We put up a huge spinniker. Then we put at the back, a huge fan that is blowing into the Spinniker. Are we going to propell our boat Forward???

OK, so lets just say somehow we do propell our boat forward. That means that somehow we have more energy pucing us forward than the fan has produced, but then we reach wind speed the Spinniker will not fill. OK so lets say somehow we keep accelerating forward. Well eventually our spinniker is going to revers and impead forward speed. None off all of above is ever going to happen. Even if once you get to wind speed, you turn the fan around and use it to push, you have to overcome your spiniker opposing you and that is exactly what is being asked of this machine.

 

Please note, the author of that article does NOT seem to be realted to the Blackbird Wind cart project and I would suggest the Blackbird is doing something very different.

[wheels 543]

Actually if you look very closly at the Vid, I reckon the wind arrow is not showing wind directly behind. It looks more like the wind is coming from an angle biased to the left of driectly behind.

Even when they reached theoretcal windspeed and thus should have no wind at all, why did the arrow swing all the way clockwise. It should have just stopped moving and then flipped around to forward. Instead it wavered all the way around.