Posted by: Ken Smith on November 04, 2005 at 00:01:33:
In Reply to: Re: Chicago area DN Clinics posted by Paul Goodwin on November 03, 2005 at 08:57:32:
Peter:
Way down at the bottom of the open forum board are the following two relevant posts. I believe there is no more reliable information, and Paul's advice is good. I am told one of the land yacht guys did an optimization study using an airfoil plank, the idea was to generate down lift in a hike. Thre were some flapped planks built for a while, but the flap and the downward lift generated too much drag. Low drag can be achieved with fairly symetric shapes, as Paul described.
R/
Ken
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Posted by: R Cummins US3433 on January 18, 2000 at 12:58:06:
In Reply to: Wind Tunnel Tests of Runner Planks posted by Brack on November 18, 1999 at 22:55:46:
Once upon a time, about twenty years ago, we did some work on this sort of thing. Low speed wind tunnels are interesting, and you can come up with some drag coefficients and other numbers that may or may not tell you much. I have also had the opportunity to listen to the airplane folks at EAA talk about this sort of thing at length.
Aerodynamic shapes are better, and you need to keep that in mind when you build things. Rounded edges are good, airfoils are better, but how much is hard to say. If you want to go nuts, you can (or could) get airfoil shaped cable from the airplane folks.
The real problem is surface friction. Your body presents more irregular surface area than anything else in the boat. This is the reason that many of the top racers wear skin tight body suits when they are sailing.
In the end, I would spend my time making sure the plank doesn't twist the runners out of alignment when it flexes. A plank that twists the runners out of alignment is likely have a greater adverse effect on speed than a plank with square edges.
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Posted by: Bob Dill on January 19, 2000 at 09:30:39:
In Reply to: Re: Wind Tunnel Tests of Runner Planks posted by R Cummins US3433 on January 18, 2000 at 12:58:06:
Aerodynamics of runner planks:
The DN experiences a maximum apparent wind of about 50 mph (in 20+ true wind) at more moderate true wind speeds 40 mph apparent more typical. A 1.25" thick runner plank has about 0.6 square feet of frontal area. The drag coefficient of a long narrow rectangle is about 1 giving about 2.5 lb of drag at 40 mph apparent. Simply rounding the leading edge will reduce the drag coefficient to about 0.5 and making an elliptical plank section will get you down to about 0.1 or 0.2. I strongly recommend rounding the leading edge. Elliptical sections are pretty easy to do. Full airfoil sections are not worth the bother. Overall the plank is one of the bigger aerodynamic factors that you can do something about. Bob Cummins suggestions about clothing are right on the money as well.
Bob suggested airfoil stays (if you want to go nuts). They work very well on biplanes as between the wing tension members. They do not work well on boats for several reasons. The angle of attack of the stays changes too much on opposite tacks. The loose stay will tend to get sideways creating even more drag than a round wire. The tensioned stays may tend to oscillate as they are too long to have enough torsional rigidity to prevent this. If that is not enough, they will run afoul of the class rules.
Bob