Help trying to come to terms with the Blackbird Land Yacht 1

[TMcRally]

I'm not an engineer so be nice

Sorry to drag this topic up again, I have read the info here and elsewhere and still struggle with it. Can it be explained or demonstrated with vector drawings with a large amount of layman terms.

I'm in the camp that says no such thing as a free lunch. If it takes (I'm just making up the numbers incert your own) 2kw to drive that cart at 50kn with a 20kn tail wind then the wheels must be able to generate 2kw net positive power against a 30kn headwind. I just can't make that work for me.

How I understand the demonstration.
The wheels drive the propeller and it starts stationary and the cart moves off blown DDW simply pushed by 20kn of breeze. As it moves, the wheels rotate and drive the propeller which I assume is geared to move enough air to provide positive thrust to the cart in the direction of travel. Initially the propeller is only capable of reducing drag which reduces the force on the cart. If geared at 2:1, at 10kn cart speed over ground the propeller is not producing any effort +/-. The cart has significantly less than 1/4 of the drag/parachute effect as it did when it was stationary and significant increases in friction through the drive with no benificial effect. How can it possibly continue to accelerate through this and then produce more power than it uses,

It must be a scam..... No ?

 

[Sparweb]

Didn't you watch this year's America's Cup races?

Don't worry, it's all on Youtube:

https://youtu.be/Tyn-Vyuzrbc?t=800

Sailing at 40 knots in a 12 knot wind. How do they do it?

Outsailing the wind isn't new.
2010 Americas Cup, Race 1 won with a 20 knot average speed in a 5-10 knot wind.

https://en.wikipedia.org/wiki/2010_America's_Cup

So my answer to your question about that lame land yacht is "yawn". Figured out a long time ago.
Now excuse me while I go back to watching those awesome winged boats just tearing across the water like mad.

Please remember: we're not all rednecks!

http://www.sparweb.ca

 

[3DDave]

It doesn't use more power than it produces, it does harvest power from the differential between the wind and the ground.

https://lockhaven.edu/~dsimanek/museum/ddwfttw.htm

and

http://dwfttw.blogspot.com/#2

A more extreme version of this are the wind-shadow gliders, aka dynamic soaring, which has gotten to 835kph for a radio control unpowered airplane by a difference in the speed of wind vs air stagnated behind an obstuction.

 

[TMcRally]

SparWeb, A different situation. The AC75's can't maintain sailing above or equal to wind speed Direct Down Wind (DDW), they gybe at around 18° apparent to make best Velocity Made Good (VMG) when sailing downwind.

I am happy with understanding sailing above wind speed in other situations.

These Blackbird Land Yachts claim 2.8x windspeed, starting from, and always traveling DDW.

 

[TMcRally]

3DDave, fascinating to watch, thanks for the leads but it's not the same.... Or is it ?

In the concept of shadow gliding they are using the differential between the air flow speeds to increase speed. I'm sure you are going to say the cart uses the differential of the air speed and ground speed but I can't make that leap in my head.

 

[rb1957]

AC75s sail faster than the true wind because of apparent wind, yes, it is a little "perpetual motion"-ish or "free lunch"-ish but it is !

and no it's not through gybing, they do the same up wing, even their VMG exceeds the true wind.

the previous edition foiling cats did the same.

now, can you do this on land ... I guess if you've got a salt flat. you can't do this in "any" direction, your direction is still relative to the true wind direction.

another day in paradise, or is paradise one day closer ?

 

[rb1957]

this thing is the same. the initial thrust exceeds the drag so the vehicle accelerates. This rotates the apparent wing direction (for the propeller) in this case. Adjust pitch to the new direction, and get more thrust, more speed etc (including more drag). The AC75s do their magic by a sudden dramatic drop in drag when they get up on their foils.

Now I have to think about what if at speed they swap the drive on the prop to charge a battery. Even if the vehicle starts to coast and slow you'll have some charge in the battery. I'm thinking that it is the wind power being harvested, just as in a windmill.

Now this thing I think says, in a no wind condition, start turning the prop, get thrust and forward motion, adjust pitch on the blade, get more thrust, go faster, etc. The initial work is done by the drive train and battery. This initial drive stops (yes?, TLDR, well I read a long time ago and forgot the details) and the vehicle has forward motion, even if just coasting ... more than the original work done ? Has this thing gained work/energy from it's own motion ?

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

The blackbird works exactly like a sailboat.

A conventional boat on an upwind tack is subject to apparent wind, and the crosswind component is balanced by force on the boat's keel. The positive work applied to the boat by the energy from the crosswind is balanced because as the boat heels leeward, the boat's CG is raised from the position it would be in with the sails down (or no apparent wind).

The blades on the blackbird's turbine are also subject to an apparent wind vector, because even though the vehicle is traveling dead downwind, the turbine blades are rotating. The only point in time where the apparent wind vector on the turbine blades is exactly the same as true wind is the instant where the brakes are released and the vehicle has not moved. As soon as the vehicle begins to move, the turbine blades are rotating and the apparent wind does not match true wind. In the case of the blackbird, the crosswind component of the apparent wind is balanced by torque on the turbine shaft - and that torque is used to provide additional motive power to the vehicle. At some point, as the vehicle speed approaches the exact wind velocity, the apparent wind on the vehicle chassis approaches zero BUT the apparent wind on the turbines does not. All that happens is that the apparent wind vector on the turbine blades changes angle until it is normal to the turbine shaft. During this phase that apparent wind is enough to keep the turbine rotating and continuing to provide motive power to the wheels. Once the vehicle reaches this phase, the apparent wind on the turbine blades continues changing angle, becoming more and more negative (towards the rear of the vehicle as it accelerates); at this point the apparent wind is driving the wheels and is no longer acting as a sail. As long as the power derived from turbine torque is larger than the total drag on the vehicle, it will continue to accelerate.

Think of the whole thing as a conventional sailboat on an upwind tack, except the energy harvested from the crosswind component of apparent wind is harvested as motive power instead of just balancing against dead weight.

The only condition necessary to make this work is whether or not you can design a turbine which makes positive power, in the same rotation direction, under apparent wind vectors on both sides of normal to the shaft. With efficient blade shapes and adjustable pitch, this isn't a difficult task for someone who knows what they are doing.

 

[rb1957]

yeah, forgot they start downwind (and not with a battery). That actually is a poor way to start, since the vehicle's velocity subtracts from the wind speed. I think they'd have more luck starting into the wind, the wind will create some thrust (sort of like a windmill, only they're designed to maximise torque rather than a propeller maximising thrust. If thrust exceeds drag it'll move fwd (once you release the brake), you'll probably have to adjust pitch to account for the rotational vector.

Unlike a boat (a la AC75) which has to reach (across the wind) to get the sail lift vector accelerating the boat.

I suspect they have the blades at minimum pitch to start with (maximum windage area) to et the thing moving. As it approaches the wind speed, zero apparent wind, they flip the blades to fine pitch, reducing the drag (and thrust). Maybe they have the blades initially locked, and releasing them they start to turn. I think this rotational speed is sufficient to create thrust (even in a zero apparent wind situation) and so the vehicle can accelerate downwind faster than the wind.

another day in paradise, or is paradise one day closer ?

 

[Compositepro]

This topic was discussed here a long time ago.
thread404-346203

 

[TMcRally]

Ok, I think I have it.

My stumbling block was when the carts propellers speed and pitch equalled the air speed I hadn't considered there would be any motive force generated by the propeller which is not right. The angle of attack needed so the cart can continue to acceleate is in the propeller blades. They are still rotating at 90° to the air flow (relative to the ground) and as such even though the pitch of the blades matches the air flow, the blades are still forcing the air to go around the assymetrical foil shape of the blade so still creating lift. Providing the air speed at which the foil shape is neutralized and the angle of attack is neutralized occur at different air speeds it will work.

Because it can always maintain an angle of attack the carts velocity is only limited by drag. No different to the AC75's or any other sailboat except the boats don't have a differential to allow an angle of attack when sailing at 0° or 180° to true.

 

[TMcRally]

Thanks all.

Happy to be corrected if I have it wrong.

 

[Compositepro]

"The blackbird works exactly like a sailboat."

That is not really true. There is one critically important difference. There are wheels that extract energy from the forward movement of the vehicle. This is the part that makes this problem unique and therefore difficult to grasp, because it resembles perpetual motion.

 

[Sparweb]

TMcRally,
But... but... where's the foot-stamping and vitriol? Has something changed on the internet, or did we just see someone learn something with an open mind from reasonable and well-humoured responses?
</sarcasm>
Thanks for the fun subject, T.

Please remember: we're not all rednecks!

http://www.sparweb.ca

 

[WKTaylor]

This was driving me crazy... Found-it... AIAA book!!!

Sailloons and Fliptackers: The Limits to High-Speed Sailing

https://www.abebooks.com/first-edit...-Bernard-Smith-AIAA-Washington/14032380792/bd

https://www.google.com/search?source=univ&tbm=isch&q=sailoons+and+fliptackers

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[SwinnyGG]

There are wheels that extract energy from the forward movement of the vehicle. This is the part that makes this problem unique and therefore difficult to grasp, because it resembles perpetual motion.

It's the opposite of that- the turbine is a turbine, not a propeller. The turbine does not generate positive thrust via air movement; it extracts energy from the airstream and provides positive torque to the wheels.

In the sense that it extracts power from the airstream and provides motive power, it's just like a sail.

Edit:

After some further reading, turns out I was wrong about this. In the downwind scenario the power to accelerate past wind speed IS provided by prop thrust; the turbine scenario is only correct when the Blackbird is traveling directly upwind.

It sounds like, from a few interviews, that in order to run upwind the vehicle configuration is actually changed (blades are changed and the asymmetrical axle is flipped). I was not aware of this and I think that is the source of my confusion.

So after that revelation I'd agree, the downwind concept of operation is not exactly the same as a tacking sail. I still like the analogy as a simple explanation, but CompositePro is right, it's not technically correct.