Directly downwind faster than the wind.

[Wizofoz]

I've been involved in a number of science forum threads dealing with the physics behind a craft powered only by the wind being able to go directly downwind faster than the wind.

The level of emotion and animosity these discussions can bring is truly awe inspiring.

A couple of the chief protagonists have recently set (waiting to be ratified) the inaugural World Record, administered by National American Land Sailing Association.

There blog is here:-

Ride Like the Wind (only faster) (http://www.fasterthanthewind.org/)

They travel DIRECTLY downwind (not tacking or gibing, but in a straight line) at a sustained speed 3.2X the speed of the wind.

It is a counter intuitive concept, but interesting from an aerodynamic point of view.

[KiloB]

I fear this is just the latest iteration of the 'Perpetual Motion Machine' Gig.

Let's say the Machine moves off downwind and accelerates to the wind speed. At that stage the RELATIVE WIND is zero. To accelerate further the wheels would have to provide additional energy to the Prop, so they would slow down. So the Machine would slow down. So, you can never go faster than the wind speed directly downwind.

Pity though; If it worked us flying types could replace the wheels with a 'drive' Prop instead of the wheels and go flying for free!!;)

[ThinAirDesigns]

KiloB:

... you can never go faster than the wind speed directly downwind


You might want to rethink that position as the North American Land Sailing Association (NALSA.org) just oversaw testing of the vehicle this last weekend and collected data from 18 recording sensors showing it traveling more than 3.5x the speed of the wind while moving dead downwind. Expect a NALSA ratified record to be issued very soon for a speed above 3x the speed of the wind.

Basics:
-- the spinning rotor is a propeller, not a turbine
-- the wheels provide the torque to turn the rotor (always).
-- the rotor does not provide the torque to turn the wheels (ever).
-- it will take off from a standing start on it's own.
-- it is sometimes pushed by hand up to a few mph to save time during testing (it starts very slow otherwise)
-- there's no "null point' at windspeed.
-- It will maintain it's speed well above wind speed indefinitely
-- By design, this particular one works best when aimed directly downwind.
-- One can be built to go faster than the wind in any direction
-- A simple gearing change will cause it to go upwind rather than down

It's very counterintuitive, but it's real.

[clunckdriver]

Also farting helps!

[Mad (Flt) Scientist]

-- it will take off from a standing start on it's own.


That's the bit I'm having the biggest difficulty grasping. If the wheels are driving the prop, and the vehicle is stationary, then how can it start doing anything. If it were the other way round, driving the wheels from energy extracted from the air, then the direction of energy/power flow would seem to be more reasonable.

[ThinAirDesigns]

If the wheels are driving the prop, and the vehicle is stationary, then how can it start doing anything.


The bluff drag of the device in the wind is enough to start it rolling and then the process starts from there.

Here's a video of an early run where you can see no one touches the device as it starts.

YouTube - BUFC very first run (http://www.youtube.com/watch?v=7SYvg40NHtc)

I just submitted a post with a link to a video showing the vehicle self starting as described, but apparently the link caused the post to go for moderation. Hopefully it will show up eventually.

For info, new posters (I think until 5 posts or thereabouts) are moderated regardless of content across the site. Creates a problem with an interesting thread where posts accumulate quickly .. due to the need for a mod to be able to review and OK the post. Fortunately, for OK posters, the nuisance is short lived and goes away fairly quickly. Posts now tidied up and the video link visible.

Needless to say we're all fascinated by the thread and, as another sailor has indicated, the basics of getting something for not very much from the wind is well established .. now, if I can just get my trailer sailer to go faster I might end up becoming competitive ... - John

[janrein]

Very interesting!

To KiloB & M(Flt)Sc

If the device would be able to keep itself moving without any wind blowing over the ground, using this combination of wheeling, gearing and aerodynamics, then it would be a true perpetuum mobile. This one isn’t and as I understand is not meant to be a perpetuum mobile either.

In wind however, as long as a relative wind is maintained between the vehicle’s aerodynamic components by also mechanically connecting the vehicle to the ground, to provide an “anchor” for the lift to oppose itself against, energy can actually be extracted from the wind.

Sailing vehicles are known to be able to move faster than the absolute wind, when they move so as to increase the relative wind. Typically the relative wind is larger than the vehicle’s motion, which is faster than the absolute wind. Such a vehicle accelerates by the lift force in the sail, thereby accelerating the relative wind, enabling the vehicle to extract more energy from the larger air mass flow over the sail, thus creating more lift accelerating the vehicle further until some equilibrium is reached, with the lowest friction vehicles (ice-skaters ranking top of the list) the highest speeds, faster than the wind, can be reached. Only NOT directly downwind, and NOT directly into wind. Some angle to the wind is required to have an efficient lift producing configuration.

Now when the direct and simple mechanical coupling between the vehicle’s motion and the motion of it’s sails (sails mounted statically to the vehicle’s hull) is substituted by a more complex coupling through wheels “anchoring” the lift forces and using a gearing connecting the vehicle’s linear motion over ground to a rotary motion of its sails (propeller blades now), then actually the oblique-to-the-wind motion of the sails is maintained in a coiled-up movement while the vehicle itself can move directly down-wind. The vehicle being initially propelled by poor aerodynamics of its stalled propeller blades will slowly accelerate, thus accelerating the blades until proper lift is produced, ever more efficient with ever more air passing over the blades, the blades thus being able to extract more energy from the wind until some equilibrium is reached. That may be well above the speed of the absolute wind, thus becoming faster than the wind. Disconnect the gearing and the vehicle will again slow down until (much) slower than the wind.

To ThinAirDesigns,

Any plans for a DUWFTTW device? Any numbers on that? Just curious.

jr

[ThinAirDesigns]

Hi Janrein.

As you likely know there is a sponsored wind powered vehicle race that does race upwind. It's Aeolus race and his held in the Netherlands on a (somewhat) yearly basis.

The current record for directly upwind is ~0.64 windspeed and we're pretty sure that with the propeller switched out for a set of turbine blades would be able to exceed the speed of the wind upwind as well. The student vehicles in the Aeolus race are governed by design requirements that we are not bound by.

From an aerodynamic drag standpoint, 3x downwind and 1x upwind are the same and since the Blackbird has demonstrated greater than 3x, our confidence is high that the Blackbird could own both records.

We're hoping to perhaps get a new set of students at our University interested in the upwind record and do that next year.

You nailed the technicals of the device by the way -- kudos.

JB

[janrein]

To ThinAirDesigns

Aeolus race in the NL, that does ring a bell indeed, in the distance. I should have known that.

You indeed answered to my curiosity about the possible numbers on the DUWFTTW device.

Good luck with your project, downwind first I understand.

Wish you bright and enthusiatic students!

jr