DDWFTTW - Directly Downwind Faster Than The Wind

[spork]

That torpedo had been mentioned on other forums. I agree completely of course. I assure you it goes nowhere with the deniers.

If you're interested in seeing the truly hard of thinking, we have two such people that are astonishingly so here:

http://talkrational.org/showthread.php?p=610758#post610758

You should try the torpedo argument on them - but be prepared for extreme frustration.

 

[ThinAirDesigns]

San Jose State University Aero Professor and Stanford Phd Dr. Nikos Mourtos along with a team of students and advisors have taken on a project to construct and document DDWFTTW in a more thorough way than ever before. Their goal is to achieve a documented 2x windspeed DDW.

Follow their blog at www.fasterthanthewind.org

Remember that in the blog format the latest posts show up on top. To view the entries from the start, click on the "2009" link on the right (below the list of followers) and scroll to the bottom.

Enjoy.

JB

 

[spork]

We're starting to make some progress on the prop. The first pic shows 5 of the 8 segments stacked on the carbon spar (windsurfing mast). The finished prop will be 16' dia.

 

[spork]

Moved the prop and prop-table into my garage today for finishing. The first five (of eight) sections are now bonded to the spar and curing.

 

[Guest625101138]

When are you aiming to do the first test run?

Rick W

 

[powerabout]

Do they have to be push started?
Coz the wind will want to wind the prop and reverse the cart upwind, and thats not going to happen or is it?

 

[ThinAirDesigns]

Rick: We hope to have prop stand and dyno testing done by roughly an April timeframe. We get good winds here in the spring.

Powerabout: self-starting depends on the conditions. All of the ones we have built have been able to self start if the wind is strong enough. We are not sure how this large one will self start in the design conditions of 20mph wind.

We can add flaps across the back that create drag and then fall down flat as we near windspeed if that is needed. Variable pitch on the prop also can help and it's not outside the realm of possibility that we might do that at some point. We're starting out with the simplest version and we'll go from there.

JB

 

[spork]

Making reasonable progress on the propeller. The chassis has taken shape, and now the prop. There's still a good bit of work left on both, but then we get to the spinny bits.

 

[Guest625101138]

That prop looks substantially bigger than the 3m you originally suggested.

I did some testing of a small turbine to windward. The water propeller was not optimised but rather I used bits I already had. You can gauge the performance here:
http://www.youtube.com/watch?v=CCN2MlbVJG0

I hope your downwind performance is a bit more dramatic.

Even with the 2.2m diameter turbine it was a lot of power in strong wind. You should not suffer this problem as you have to make your own wind.

Rick W

 

[spork]

That prop looks substantially bigger than the 3m you originally suggested.

Hmmmm.... I'm an old man with a failing memory, but I don't recall suggesting a 3M prop. As far back as I can recall, we've been talking 16' to 20'. This one is 16'.

I did some testing of a small turbine to windward. The water propeller was not optimised but rather I used bits I already had. You can gauge the performance here:
http://www.youtube.com/watch?v=CCN2MlbVJG0

I hope your downwind performance is a bit more dramatic.

That's spectacular. A wind powered boat progressing directly into the wind is plenty dramatic. And I'm loving your smug look as you head toward the finish line!

 

[Guest625101138]

You are correct. ThinAirDesigns gave 15' here:
http://www.boatdesign.net/forums/boat-design/wind-powered-sail-less-boat-24669-17.html

I determined this to be on the small size but then you are aiming for higher winds than I was working on.

Rick W

 

[powerabout]

Can someone show me a diagram/drawing of the air flow across the blades from stopped to running to wind speed to more than wind speed.
Like many I am having trouble grasping this.
Thanks

 

[spork]

Can someone show me a diagram/drawing of the air flow across the blades from stopped to running to wind speed to more than wind speed.
Like many I am having trouble grasping this.
Thanks

First of all Bob, thanks for being in the rare minority by approaching this with an open mind. Yes, it can definitely be tricky. Bear with me here...

It's best to first consider an ice-boat on a downwind tack (say 45 degrees off of directly downwind). We'll consider the wind over its sail, and the lift and drag vectors that make it capable of achieving a direct downwind VMG much greater than wind speed.

With that in mind we can imagine the ice-boat on that same course, but on an ice-covered planet. But this ice-covered planet is a cylinder rather than a sphere. So the ice-boat makes one continuous downwind spiral around this spherical planet, outpacing the wind that blows everywhere in the same direction as the planets axis.

From there we can imagine a second identical ice-boat directly across the planet's axis from the first - on a similar spiral downwind course. Now we realize that if we simply change the scale vastly, we can think of those two sails as the ends of our prop blades.

By constraining the ends of our prop blades to follow that same downwind spiraling path, we make them identical to the ice-boat sails. To do this we attach the prop-shaft to our axle through a 90-degree gear drive. In this way we can assure that if the vehicle goes 1 foot downwind, the prop tips must move 1 foot circumferentially - just as the blades of the ice-boat do for the ice-boat sail.

I'll wrap this post up, and post the diagram in the next. The diagram describes the flow over the ice-boat sail, but it is identical to the flow over our prop blades. Let me know if this makes sense.

 

[spork]

Here is the vector diagram for an ice-boat that’s maintaining a course such that its downwind velocity component is faster than the wind. I've assumed a boat going 45 degrees downwind at a speed of twice the wind speed. The ticket is to then compute the necessary L/D of the sail and keel to make this possible (this would have the boat making a downwind velocity component of 1.414 times the wind-speed).

Given the wind velocity and the boat velocity, we can easily compute the apparent wind over the sail. From this we see the direction of lift and drag on the sail. What we need to do to make sure this sailing configuration is possible is to insure that "alpha" is small enough so that the resultant force has a forward pointing component (relative to the boat). In this case alpha would have to be 45 - 16.3 degrees or 28.7 degrees (or less). That relates to a sail whose L/D is 1.83. Obviously this is easily achievable. However, I've assumed a keel with infinite L/D in this case. The drawing gets a little more cluttered if we include the keel's L/D. So I'll try to describe it in words. I'm going to assume an L/D of 10:1 for the keel (easily done with an ice-boat). This will trim 5.7 degrees off of my 28.7 degree budget. That leaves me with an allowable 23 degrees(L/D = 2.35:1) to achieve this configuration. So with a keel that has a 10:1 L/D and a sail/boat that has an overall 2.35:1 or better L/D this configuration can be achieved (we can achieve a downwind component about 40 percent faster than the wind speed).

In each case the vehicle pushes the fluid backward relative to the vehicle itself, but not backward as quickly as the vehicle is moving forward. It’s all about exploiting the energy available at the wind/ground interface. This is why when we compute the energy produced by the prop it is less than the energy harvested by the wheels (on our prop-cart). In the real world this will always be true.

 

[spork]

And just one last piece...

When the cart is stationary facing downwind, the wind will want to push it as a bluff body. It will also want to turn the prop as a turbine. This is the opposite direction from the actual operation of the prop. The direction the prop ultimately starts turning is dependent on the gear ratio between prop and wheels, and the pitch of the prop. For a downwind vehicle the prop must be designed to advance more slowly through the air than the wheels do over the ground. If the opposite were true the vehicle would advance directly into the wind as Rick's boat does in his video.

So as the cart begins moving downwind, the prop is forced to turn by the wheels. As the speed increases, the angle at which the relative wind hits the prop blades begins to look more like our vector diagram. Well below wind speed, the tips of the prop cease acting as bluff bodies, and become lifting surfaces. As the speed continues to increase, more of the prop becomes unstalled. When we reach our steady-state velocity above wind speed, the vector diagram above describes the flow over our prop blades.