Blackbird Land Yacht "DDWFTTW" 2

[LuckyDuc]

Did anyone follow this back in 2010? Somehow I missed it until I read where the craft was for sale:

http://autos.yahoo.com/blogs/motoramic/wind-powered-blackbird-sale-rewriting-physics-134230233.html

Wikipedia page is here:

http://en.wikipedia.org/wiki/Blackbird_(land_yacht)

There was a great deal of "Its a hoax and you are an idiot if you think otherwise" internet banter going on until they proved it. I am a mechanical engineer, but have not had to commit calculus or draw a free body diagram in YEARS! I've always enjoyed mindbenders like this and wouldn't mind discussing a few "nuances" and details of the project if anyone is game???

LD

 

[JStephen]

A related question: Can you build a wind-powered boat (ie, windmill on top, prop underwater) that can sail directly into the wind? On land, it'd be easy, just a matter of gearing, speed, and efficiency. On water, it's not obvious to me if it's possible or not. I tend to think "yes".

 

[drawoh]

JStephen,

Read up on Flettner Rotors. Somebody built a ship powered by them. The data you desire may exist.

--
JHG

 

[KENAT]

I wondered some time ago about the possiblilities of a vertical axis wnd turbine powered ship - probably not Savonius but lift based or hybrid.

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[rb1957]

things i know ...
1) a sailboat can go faster than the wind. it's to do with apparent wind direction, the vector sum of the boat spped and the wind speed, as the boat speed increases the apparent wind clocks around alittle taking the wind to a most advantageous point. this is usually with small displacement boats (hobie cats) and on a reach (across the wind).

2) a (conventional) boat can't sail directly into the wind. it needs an angle to the wind to generate lift from the sails. thinking about it, i guess you could have a moving clew (the fwd attach point of the sail, conventionally fixed on the CL) and i guess a moving (rotating) keel (as the keel generates side force to balance the sail).

i don't know why this thing is called a "land yacht", "land plane" might be more appropriate for a propeller driven vehicle, but i guess they're leveraging the idea of a vehicle being moved by wind power. i think it could go faster than the wind, depending on how efficiently it can generate power (from the propeller). the whole thing depends on getting the propeller turning, in still air you could use a motor to drive the propeller, and the propeller could then produce thrust, and then the vehicle speed would add to the rotation velocity of the propeller creating more apparent wind over the blades allowing for you to turn off the motor.

but something i also know ... an airplane engine produces more power to drive the propeller than the propeller uses to drive the plane forward. i mean i doubt that a propeller in freestream airflow can generate enough power to drive itself forward (that sounds more than a little perpetual motion). it might be able to work downwind, using the power of the wind.

Quando Omni Flunkus Moritati

 

[TheTick]

There are still some hinky things about this whole effort. In some articles, it is stated that the vehicle can not reach faster-than-wind speeds without getting some sort of assisted start. That means energy is being put into the system that was not developed from wind power.

 

[IRstuff]

I don't think that particularly matters. Whatever initial energy is put it, friction must eventually dissipate that energy, so if the wind is abel to maintain the vehicle's continual motion, the initial energy doesn't necessarily play into the net effect. That's not any different than needing a conventional engine to get a scramjet vehicle going fast enough to use the scramjet at all.

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[LuckyDuc]

Thanks for joining in, folks!

This is how I've been looking at it

Going upwind is a no brainer. The wind is the source, earth is the sink. The propeller takes energy out of the wind, transfers it to the wheels and away you go. Want to go faster? Get a bigger prop (or more of them) to extract more energy from the stream.

Downwind is the tough one. I am imagining the moving ground as the source (has to be push started, right?) and the zero (relative) velocity air as the sink. Ground spins wheels, wheels turn prop, prop claws its way through the air. NOW THEN...that gets us idling along right AT the speed of the wind. Where does the energy to accelerate to 2.8X come from?

 

[Compositepro]

This topic has been discussed a couple times before.

http://www.eng-tips.com/viewthread.cfm?qid=274009

 

[LuckyDuc]

EUREKA! The prop is geared so that it is spinning very fast in this "still" air. Then off you go until thrust = drag.

...right?

 

[LuckyDuc]

"This topic has been discussed a couple times before. http:..."

Had they used the term 'blackbird' I would have found it! lol

 

[rb1957]

i think that's a little simplistic.

going up-wind ... you're right, for a given propeller disc, there's a maximum vehicle speed. initially the blades would be set at an angle of incidence to the wind to maximise torque (like a wind turbine). as the blades start turning they create an in-plane velocity which combines with the wind to create an apparent wind vector, and the blades will now rotate to an incidence that maximises thrust (to get the vehicle moving). now the apparent wind is the vector sum of the vehicle's velocity, the blade's turning velocity, and the wing; and speed will be limited by the thrust produced by the propeller and the aerodynamic drag and othe losses. presumably the vehicle would start against brakes (the the wind would be pushing the vehicle backwards), and the vehicle will only move forward once the thrust produced exceeds the aerodynamic drag.

going down-wind ... the same procedure would happen ... initially the blades produce torque to turn, to rotate the apparent wind; and then produce thrust to move the vehicle. here though, the windage is helping the vehicle, so "drag" is initally +ve (a thrust driving the vehicle), then 0, and then -ve (retarding the vehicle like drag normally does).

thinking about the blades ... either the profile is dependent on the vehicle direction (up- or down-wind) which sounds silly (you'd have to change propeller depending on which way you were going) or the disc rotation would change (CW for up-wind, CCW for down-wind); i think this'd allow the same blade to maximise torque (initially) wih the same settings. mind you the propeller incidance control would be moving (in pitch) the blades in different directions but thats quite easily done. So the transmission would need to be able to take either rotation and produce fwd speed ... again QED.

Quando Omni Flunkus Moritati

 

[Compositepro]

"As they say, the wind does not turn the prop. The wind pushes against the prop. disc, which is the area swept by the rotating propeller (like a sail). This forces the vehicle forward. The wheels drive the prop. It is just like sailing. The forces that have to balance are the push of the wind against the propwash versus the drag of the wheels (which are driving the prop) and the aerodynamic drag on the body.

The part that is hard to visualize is how wind can push on an object that is traveling faster than the wind and in the same direction. The prop is pushing air back into the wind so that after the vehicle has passed, the wind speed has slowed-down even though the vehicle is going faster than the wind. This means that energy has been extracted from the wind."

"Replace the propeller with a sail of the same diameter. It is plain to see that the wind will drive the vehicle and energy can be extracted from the wheels. This energy comes from slowing the speed of the wind. With a sail as the vehicle speeds-up it, the amount of energy it can extract from the wind approaches zero.

Now with the propeller you can generate thrust with this extracted energy to make the vehicle go faster than with a sail. When the vehicle is at wind speed there is no air resistance so it doesn't take much energy to make it go. The propeller is providing thrust. The wind speed of the wind that has vehicle has passed through has been reduced so that means that energy has been extracted.

There are no violations of physics here."

 

[TheTick]

What's missing from any of this is a simple energy audit. How much energy was in the system ("yacht") when it was released and how much was there at the end of the run?

Net speed is not the same as energy. Energy can be stored mechanically and/or electrically and then transmitted to the wheels. They are towing this thing up to start speed with a truck. That does not tell us how much energy is being imparted to the system. Are they using the tow to get the prop up to speed? How fast is the prop going when the yacht is released, and also at the end of the run?

There are toy cars that do the same thing. The car is pushed and a gear set puts the push work into a mechanical system. When the pushing stops, the system shifts and the car keeps going and even accelerate as the stored mechanical energy is released.

Obviously, something happened to accelerate the yacht. But, this wouldn't be the first time someone was wrong about how their invention appears to work. Happens all the time.

 

[Compositepro]

It is not towed to speed and there is no energy storage involved. The following video is easier shows things more clearly.

http://www.youtube.com/watch?v=5CcgmpBGSCI&feature=player_embedded#!

 

[TheTick]

I misread my information. The "tow" in this report was on a hypothetical example. Gotta back off on that point, I guess.

http://blueplanettimes.com/?p=3922

 

[rb1957]

can you simplify the concept by adding a "barn door" upwind of the propeller. The wind pushes against the barn door, starts the vehicle moving, and the wheels then drive the propeller. it would seem that as the vehicle's (down-wind) speed increases then the pressure applied to the barndoor (by the wind) reduces.

if wind pressure against the propeller disc is the driving force, it is hard to see how this works (down-wind) when the vehicle speed exceeds the wind speed.

I think something else should be considered ... if the propeller is doing work against the airflow, then it is taking this energy from the wheels ... torque has to be supplied by the wheels, in order for the propeller to do work against the air. that means you've increased the drag load onto the ground, no? and so increased the force requird to drive thevehicle.

Quando Omni Flunkus Moritati

 

[TheTick]

I may be coming around on this.

Regardless of the wind velocity (both absolute and relative), if there's any wind at all, then there is a stream of matter with kinetic energy waiting to be extracted.

Beyond that, I'm losing my grip!

So many times in my career I've had to battle against laymen's erroneous perceptions. I may be an educated engineer, but I see now I'm at my limits of understanding. My understanding may not be thorough enough. What seems sensible to me is probably not enough of the picture.

 

[MintJulep]

Isn't it basically:

 

[GregLocock]

I think intuitively the moving directly upwind example is much easier to grasp - take a wind turbine, mount it on a little electric powered crawler, and obviously for some ratio of Vw to V there must be enough F to push the thing forward.

Incidentally the wiki links iclude lots of maths, I'm sure the maths is correct, but the assumptions may not be. The 1967(?) paper includes a whole heap of performance predictions and so on.

Cheers

Greg Locock


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