DDWFTTW

Submitted by Chilidog on Tue, 08/17/2010 - 21:31
Ok, this is slightly off the topic of Flying Scots, but it is definitely sailing related. DDWFTTW = Directly downwind, faster than the wind Here is the question: Is it possible to go directly down wind, powered by wind power alone, faster than the wind? before you snort and dismiss this question, check this out http://www.fasterthanthewind.org/ this is way cool.
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Comments

Not a chance of this being real.

Submitted by lacoleman on Sat, 10/30/2010 - 11:20.
Not a chance of this being real...unless there is a strong wind gradient between the wind meter and the sail/prop. As any pilot can tell you, there can be a big wind change between ground level and 20 feet in the air.

This has been on the Forum some time ago.

Submitted by karafiath on Sun, 10/31/2010 - 18:38.
This has been on the Forum some time ago. Look at the video carefully. The mast wind feather shows the wind not from directly aft but from the side and aft. A completely steady state analysis shows that it is not possible to sail faster than the wind in the directly downwind direction because the craft will have zero relative wind and the propeller will stop turning. If the craft accelerates through the zero relative wind spot either because of a wind gust or because of it's momentum then it would be encountering a head wind and the propeller pitch would have to reverse to generate some momentary thrust. However the slightest air drag will cause it to slow thus slowing the encountered wind because the craft is generating it's own wind. Propeller thrust will be lost and the craft will slow down eventually to the point where it has no more self generated headwind. Note that this is different than sailing into the wind where the wind speed is constant and as the craft slows the relative wind increases thus generating more thrust.

quote:[i]Originally posted by lacoleman[/i] [br]Not a chance of

Submitted by ThinAirDesigns on Fri, 11/05/2010 - 13:00.
quote:
[i]Originally posted by lacoleman[/i] [br]Not a chance of this being real...unless there is a strong wind gradient between the wind meter and the sail/prop.
Your comment regarding gradients is right on lacoleman -- there *IS* a lot of difference in wind velocity between ground level and 20ft in the air. In this particular case however, reviewing the rules for the NALSA ratified world record shows that the rule-makers were also aware of this and wrote the rule as such: "All primary wind speed measurements will be made between propeller hub height and 5 feet above hub height" This is why in all the picture from the world record run day you will see that all the wind recording devices (there were many) were placed up on stalks to above the height of the propeller hub. http://picasaweb.google.com/davidhglover/DDWFTTWJuly3RecordDay?feat=flas... dwfttw is easily possible without taking advantage of any gradients. NALSA.org independently confirmed this by placing 18 recording sensors on the vehicle and along the course and ratifying a world record of 2.8x the speed of the wind. http://www.nalsa.org/BlackBirdDDWSR/Observers%20ReportNALSA%20C4Blackbir... Kimball Livingston -- noted sailing race official, sailing author and editor of Sail Magazine was on hand as a witness for the runs and posted the following writeup. http://kimballlivingston.com/?page_id=4116