Remember the debate about downwind vehicles

spork

We've had a couple of heated debates here about whether one could build a wind powered vehicle that would go directly downwind faster than the wind. So we built one...

First run: http://www.vimeo.com/10476453
Second run: http://www.vimeo.com/10477373
Last run: http://www.vimeo.com/10476216

mender

Hopefully, large scale physical evidence of DDWFTTW travel will help moderate some of the extreme anti invective and allow a calmer discussion of this fascinating phenomena.

spork

Actually, the resulting discussion goes beyond calm - all way to non-existent.

russ_watters

....which then begs the question: where does the discussion go next?

spork

Ratification of an official DDWFTTW speed record (stay tuned), and then perhaps the overall land speed sailing record(?)

rcgldr

DDWFTTW vehicles require a ground force that is backwards relative to the direction of travel (wrt ground) in order to provide the power to drive the prop. However the ground force on a sailcraft is perpendicular it's direction of travel (wrt ground).

Perhaps you mean the component of speed directly downwind? DDWFTTW vehicles require all of the ground force to be upwind, while sailcraft only require a fraction of the ground force to be upwind, depending on the sailcraft's heading. I'm pretty sure that ice boats will have a faster component of speed directly downwind, but land (sand) craft have more drag and I'm not sure.

spork

What you're saying is correct of course, but the numbers suggest that a DDWFTTW vehicle can take the existing overall land sailing speed record (not just the downwind component). Keep in mind the DDWFTTW vehicle gets a better break on aero drag too.

Greenbird is going after the ice record now. So I don't know what numbers they'll achieve. I'm pretty sure we're not interested in the ice record any time in the near future.

rcgldr

Aren't they already in the 5x to 7x wind speed range? I recall something about 10 mph winds being ideal because the ice boats are designed with that in mind, achieving speeds of 50 mph to 70 mph? Beyond that and drag becomes an issue, so achieving the same ratio with a 15mph wind requires a different design (larger?), and the locations where they can use the ice boats are limited in size.

spork

I haven't followed the Greenbird really. I believe they set the land record at 126 mph in winds that peaked at 47 mph. I believe they tried to outfit it for the ice, and found out they needed to basically start over from the ground (ice) up. But I haven't looked at the wind or grounds speeds for ice records.

rcgldr

I found an article posted at several websites. Apparently a Skeeter can achive 6x wind speed in light wind, less than the 10 mph I thought it was. In a chart of a 2 lap race, top speed was about 65 mph with 15 mph wind, but the goal there was to run a 2 lap course as opposed to a speed run.

The fastest wind powered devices are probably radio control gliders while dynamic soaring. At youtube there's a video showing a run of 375 mph, and later that day they recorded a speed of 392 mph, but didn't video the run, just the max speed on the radar gun.

http://www.youtube.com/watch?v=WaQB16ZaNI4&fmt=18

update - make that 399 mph as of 2009-12-22, same model, same location:

http://vimeo.com/8356047

spork

Is that the NALSA article? I always thought it was more like 4x-5x

rcgldr

Yes.

Skeeter ... It can sail about 6 times the wind speed in light winds and tops out a little over 60 mph.

nalsa_article.pdf

I think that the DDWFTTW carts are a bit more surprising and impressive that they could work at all; outrunning the wind that propels them.

spork

Interesting. I bet you found that in places that JB or I posted references to it. I never noticed that line. I've been saying 4X - 5X for a long time.

Interestingly, that article was written by Bob Dill. He's on the NALSA B.O.D. and was our contact and host at the Ivanpah event. We're working with him now to ratify a category to set a downwind record in.

Subductionzon

So where are all of the deniers to this topic? Now that spork has extremely strong video evidence of the cart going faster than the wind have they all run away? Check out the last run where the cart drives through a cloud of dust that is being blown by the wind. I think schroeder especially owes spork at least an "I'm sorry".

DJM111188

Spork, at your request, I have posted my questions below. Anyone with insight please feel free to chime in. Also I have now a better understanding of what the term perpetual motion means. At the time when I contacted you I hadn't taken in to account that A)the wind is energy. But now I have revised the original question.

Hello I am not a frequenter of online forums but I recently was on the site and came across the "DWFTTW" machine and I felt the need to contact you. I have watched some of your videos on youtube. One of those was where you raced the turbine shaft vs prop shaft carts. I was wondering if you could clear some things up for me.

1) Are both carts using a 1:1 drive?
2) In the video you said the turbine shaft cart accelerated much quicker then the prop shaft cart correct? If this is true then would changing the gear ratio allow the turbine cart to have a much faster speed?
3) If your answer to number 2 is "Yes", then would the turbine cart be geared high enough to where it could keep up with the prop car? Or is the only reason a vehicle is able to travel faster down wind then the wind itself because of the wheels powering the prop?


I am just trying to learn so please take no offense to my inquiries. I would love to know how exactly this works. It seems to me that if 20mph of wind is applied to the turbine cart then it will never ever be able to reach a faster speed, even taking into account a gear ratio change, because when it would get to the 20mph threshold there would be no more force to turn the wheels. Am I correct in this assumption?

Also can you explain to me the end result in my hypothetical test? Say you had one of the carts that had a steering servo hooked up to the front wheels and you were able to find a stretch of 5 miles with absolutely flat and even surface. Lets say wind-speed could be controlled at exactly 20 mph. If you were to put the cart down and get it to travel the whole 5 miles directly downwind in a striaght line, is there any way to determine how fast it could actually go? More specifically would it have some magic number as a threshold top speed or would it continue to gain speed indefinitely as long as it had the room? If this description is too vague please notify and I will try to clear it up as best as possible.

Thanks for your time.

spork

Thanks. I'll do my best to answer your questions.

1) Those particular carts don't use a 1:1 drive. I don't recall the gear ratio off-hand, but I think it was 13:16 in both cases. Of course the gear ratio is only part of the story. To get the "vehicle speed ratio" (speed through air divided by speed over ground) you also have to know the wheel diameter and prop pitch. The difference between the two carts isn't the gear ratio per-se, but the sign of the gear ratio. The gear ratio for the turbine cart is -13:16. So the turbine turns the opposite direction from the prop of the prop cart.

2) No, the turbine cart can never reach wind speed. It uses the tail wind to turn the turbine, which in turn turns the wheels. If it reached wind speed it would no longer feel any relative wind, and would thus have no torque at all on the turbine.

3) There are several approaches to traveling directly downwind faster than the wind. But all require that the vehicle exploit the energy of the wind relative to the surface - not relative to the vehicle. The BUFC does this by gearing the prop to the wheels such that the wheels do the work to turn the prop. A turbine cart can never do this.

On the contrary. This is what I'm here for. I enjoy these kind of questions.

You're exactly right.

Not too vague at all. In fact it's quite well posed.

For a given VSR (vehicle speed ratio) the cart will have a theoretical top speed as a function of wind speed. For example, if the cart is trying to advance through the air at 1/2 the speed that it advances over the ground, it will never go faster than twice wind speed. As you change the prop-pitch and/or gear ratio such that the geometric VSR becomes closer and closer to 1.0, you'll get a higher theoretical multiple of wind speed - but at a price. To achieve more than about 4 times wind speed requires a cart with ludicrously efficient transmission, prop, etc. That being said, there is no theoretical bound to the vehicle's speed as a multiple of wind speed.

ThinAirDesign

Then you will be *really* surprised when we take Greenbirds record of 126mph and smash it in a DDWFTTW vehicle.

JB