# Downwind faster than the wind -- theoretical limits?

Some clever makers have built a wind-powered cart that can travel downwind faster than the wind is blowing. Here’s a video that shows it in action. Watch for the point where the red flags stop blowing forward and start blowing backwards – that’s when they pass wind speed.

Having demonstrated the basic concept, apparently now they’re working on making a model that will travel downwind 3x faster than the wind. My question is: Is there a *theoretical *upper bound on this multiplier, or is the top speed of such a craft determined entirely by practical considerations like the friction in the mechanical linkage between the prop and wheels.

"any boat which runs ‘square’ must necessarily sail downwind at some speed less that the wind’s speed whereas any boat which tacks downwind has no theoretical limit to its speed.

A fast keelboat such as a Soling can sail at 30 degrees off the apparent wind, an 18ft Skiff at 20 degrees, and an iceboat at 7 degrees, equating to around 6x the wind

“On March 26, 2009, the Greenbird, driven by Richard Jenkins at the dry Ivanpah Lake in Nevada, broke the record for the fastest speed attained by wind-powered vehicle, having reached a maximum velocity of 126.1 miles per hour (202.9 kilometres per hour), with wind speeds at about 30 mph”

penultima thule: Your links deal with sailing faster than the wind, but not directly downwind as is the case in the OP’s link.

I think the key here is that the propeller blades - or at least their outer sections - are not traveling in a straight line downwind. They are doing the same thing as a sail on a fast boat (say, an iceboat): traveling at an angle to the wind. This produces a rotary motion that is arranged to drive the wheels.

You’ve got it backwards - the wheels drive the prop. MPSIMS thread here.

In answer to the OP, I believe there are no theoretical upper bounds on how fast this mechanism can work, but as a practical matter losses to air resistance are going to kill you once you get going 3-4x windspeed.

Okay - I’m starting to get my head around it.

We have exceeded 3.5x several times now, though the official record will remain at 2.8x for the foreseeable future as we aren’t motivated currently to go though the expense of another official record setting effort.

There is not. There are many practical limitations however.

JB

I’m not sure why it’s conceptually difficult that something can move faster than the speed of something that’s powering it. Sure, it doesn’t happen for any old object, but normally if you put a motorized vehicle doesn’t normally fly - you have to shape the body correctly (aka give it air-foiled wings) to generate that from moving through the air.

The key is that the ground is moving at a different speed than the air. Connect the two and you can extract energy regardless of one’s speed by having a generator extract energy from the fast movement across one material and a motor connected to the generator to provide force to propel the object through the other material that isn’t moving as fast. The genius thing is that they can do so with small enough transfer losses that the energy extracted is greater than the rolling resistance.

My only question is how easy is it to get started? What kind of wind/wind gust do you need?

Hmm, I thought I was posting that in the other thread…appears slightly out of place in this one, and there’s also a horrendous grammar error in the second sentence due to changing what I was going to say halfway through.

Oddly, it got a lot easier, conceptually, for me to understand the concept when I stopped thinking of it as moving relative to the ground in the same direction as the air, and started instead thinking of it as moving relative to the air in the opposite direction as the ground.

And ThinAirDesigns, you guys really need a better acronym. “DDWFTTW” (I presume that’s “Driving DownWind Faster Than The Wind”?) doesn’t exactly trip off the tongue.

The way I got it was to imagine a low-flying zeppelin powering its propeller by lowering a wheel to touch the ground below. It made more sense once I stopped thinking of the wheels as anything other than a power source for the prop.

Blackbird will function fine (meaning go faster than the wind) in winds as low as 6-7mph. It will not self-start at those speeds however and requires a push to get it moving. Once the winds get up to 9-10mph, one merely need release the brake.

If we needed to get it rolling untouched by human hands at the lower windspeeds, there’s an easy way to do it – more drag. Retractable drag flaps or retractable drogue chute would do the job and wouldn’t impact top speed a whit.

JB

Directly DownWind Faster Than The Wind.

How about having the driver (pilot?) stand up - and spread his arms - to create some air resistance? (He can sit down once the thing is moving.)

While a perfectly valid source of drag, the pilot of the Blackbird is tucked in under removable fairings (place over him once inside). I wouldn’t be practical for him to attempt to get in and out during the process.

JB

Having thought about it more, it would certainly be possible to design the vehicle to have a large flat area in which to intercept the wind trying to push it, while have it narrow down to a more aerodynamic structure in the direction it will be traveling. Sure, you could have retractable flaps to introduce more cross-sectional area to get started, but it sorta feels like this entire exercise is about having an object that the wind pushes that, without further modifications, will go faster than the wind. If you’re allowed to make modifications to the craft as you go, the ability to make those modifications requires some sort of energy source. Unless the wind-powered movement powers the retraction of the flaps, you just added an energy source, at which point you might as well put a motor on it.

It is well known that by tacking downwind, a sailboat can make net downwind progress faster than the wind speed, and much better than any traditional boat sailing directly down wind.

If we replace the zig-zagging tacks of the boat’s sail with the helical path of the propeller blades, then it becomes obvious that this is another manifestation of the same phenomenon, and needn’t violate any physical principals.

As for limits, I can’t quite work it out, but suspect it is strongly related to the L/D of the propeller blades. In that case, the limit is only constrained by the prop diameter. In addition to just being unwieldy, a large prop will soon run against practical limits related to material properties…it has to be stiff enough not to flutter, while being thin enough to work for example.

There are also practical limits as to how fair you can make an airfoil. Eventually surface contamination from bugs or even dust becomes a problem. (Bugs significantly degrade performance of state-of-the-art sailplanes.)

Definitely has to be.

I’d accept the notion that aspect ratio puts a constraint on L/D. But you seem to imply that unlimited aspect ratio implies unlimited L/D, which I question.

Quite right, although modern airfoils have significantly reduced the seriousness of this problem.

I’m not sure I buy that explanation. Here’s why.

Let’s consider a wide river with parallel sides, where the wind is blowing exactly sideways to the river. Let’s say that the wind is blowing at a speed such that any little parcel of air takes two minutes to get from one side of the river to the other. Now you have a sailboat, and want to race it from one bank to the other.

If you sail it directly downwind, which would be directly across the river, it will take you more than two minutes to cross. Instead, let’s say you tack across. It still will take more than two minutes - your ground speed can be faster than the wind is blowing, but you’ll have a longer distance to travel. I don’t think there’s any way to cross the river in under two minutes whether you tack or not.

Now applying this to the Blackbird, the propeller really is going faster than the wind. It could cross the same distance in under two minutes. So I don’t see how the tacking explanation helps here.

Nope, look into “speed made good” on sailing sites. There is so much speed to be gained on a loose reach that just the downwind component of velocity IS greater than the wind speed. Apparently near double for state-of-the-art Americas Cup boats, but worthwhile even for more mundane craft.

It’s a lot simpler than tacking. The wind propels the vehicle, the vehicle uses the forward motion to push back against the wind. This sounds like it ought to be impossible, but it isn’t.

The same thing can be done with gears (as I did last night, for the other thread). Thismodel uses gears to convert its own motion into a ‘push’ against the toothed rack used to push it. The result is that when you move the toothed rack forward, the vehicle moves forward faster than it.