myiW Current Conditions and Forecasts Community Forums Buy and Sell Services
 
Hi guest · myAccount · Log in
 SearchSearch   ProfileProfile   Log in to check your private messagesLog in to check your private messages   RegisterRegister 
What is tailwalking and what is wheelies?
Goto page Previous  1, 2, 3, 4  Next
 
Post new topic   Reply to topic    iWindsurf Community Forum Index -> Windsurfing Discussion
View previous topic :: View next topic  
Author Message
techno900



Joined: 28 Mar 2001
Posts: 3914

PostPosted: Fri Jul 29, 2011 9:52 am    Post subject: Reply with quote

You nailed it dOuglass!

Wheelie's can be intentional or accidental. The intentional I explained above and the accidental is a result of tail walking to the extreme.

I have only done it on my formula board, but on at least three occasions in overpowering gusts and heading up to control speed, I inadvertently let up on the downward boom pressure (sitting in the harness) resulting in the nose of the board going vertical with the entire board out of the water, and me flopping into the water on my back while hooked in. No injuries or damage from doing this, but it is quite exciting.
Back to top
View user's profile Send private message
rigitrite



Joined: 19 Sep 2007
Posts: 514
Location: Kansas City

PostPosted: Fri Jul 29, 2011 10:04 am    Post subject: Reply with quote

What you're describing is deflection, not lift. It may seem like lift, but it's not. You wouldn't say that a canoe paddle or a boat rudder creates lift when you use them to turn the vessel. The effect you're describing is real, but it's the actual molecules of water hitting the fin and imparting a force to cause motion in another direction. Same thing happens with the sail: actual molecules of air hitting the material and imparting force to the board, but that's only like 20% of the power in a sail on a beam reach. LIFT is what powers the sail, and were your sail symetric, like a fin, then you'd barely be able to move.

The question you really wanna ask is: If the fin creates no lift (it doesn't) then how come a huge fin can make you tail walk even when you can barely plane? (we all know this is true.) The answer is counter-intuitive, but makes sense.

Dane, I've been to your house before. : )
Hope to someday sail Smithville when you're there.

_________________
Kansas City
Back to top
View user's profile Send private message
bred2shred



Joined: 02 May 2000
Posts: 981
Location: Jersey Shore

PostPosted: Fri Jul 29, 2011 11:38 am    Post subject: Reply with quote

rigitrite wrote:
sorry guys, fins don't create lift: they're completely symetric. Please review Bernoulli's principle for an explanation of lift.


Sorry dude, but you're completely wrong. Please review fluid dynamics 101 because you clearly have no clue what you're talking about. By definition, lift is the component of force generated by a fluid flow over a surface which is perpendicular to the fluid's direction of flow. Whereas drag is the component of force generated by a fluid flow over a surface which is parallel to the direction of fluid flow. Therefore, the horizontal force generated by a fin which is perpendicular to the board's centerline is, by definition, LIFT. The same goes for a sail- the force that pulls you sideways/forward is lift. The force that limits your top speed is drag.

There is no question that symmetrical foils generate lift (and drag). A sheet of plywood can generate lift. The key is that the foil has an incident angle to the flow (i.e., the foil must be angled to the flow in order to generate lift regardless of whether it is symmetrical or assymetrical). Assymetric foils are typically more efficient at generating lift, but a foil does NOT need to be assymetrical to generate lift.
sm
Back to top
View user's profile Send private message
rigitrite



Joined: 19 Sep 2007
Posts: 514
Location: Kansas City

PostPosted: Sat Jul 30, 2011 5:01 pm    Post subject: Reply with quote

Quote:
Sorry dude, but you're completely wrong. Please review fluid dynamics 101 because you clearly have no clue what you're talking about.


Oh goodie, a pissing contest. Well, since you're gonna be a prick about it....
From "Engineering Fluid Mechanics" Roberson/Crowe 4th ed. pg 470
"The sum of the forces (pressure, viscous, or both) that acts normal to the free-stream direction is the lift, and the sum that acts parallel to the free-stream direction is the drag."

Since you clearly can't add, the "lift" on either side of a symetric foil is the same, therefore, X - X = 0. So lift = 0, get it...no lift? That's what I'm talking about. Symetric fin does not create lift, it just opposes side slip (force of the sail pulling normal to the liquid free-stream.) Since there is no pressure differential, there is no "lift". QED
I think it is YOU who need to review your fluids.

_________________
Kansas City
Back to top
View user's profile Send private message
rigitrite



Joined: 19 Sep 2007
Posts: 514
Location: Kansas City

PostPosted: Sat Jul 30, 2011 5:15 pm    Post subject: Reply with quote

BTW, when does your fin ever have an incident angle??? Never.
As far as incident angle = deflection angle, that's just semantics. I wouldn't call that lift, but that's not to say that it wouldn't fall within a technical definition. It's immaterial to the discussion at hand though, since your fins incident angle is always = 0, and the sum of the forces normal to the fin is still ZERO. Thus, no lift.....
Back to top
View user's profile Send private message
isobars



Joined: 12 Dec 1999
Posts: 20132

PostPosted: Sat Jul 30, 2011 6:13 pm    Post subject: Reply with quote

rigitrite wrote:
when does your fin ever have an incident angle???


Always, in a beam or close reach, and usually, in a broad reach. Our pointing angles and direction of travel differ unless we're running dead downwind. IOW, our boards crab. Thus ... incident angle (angle of attack).
Back to top
View user's profile Send private message
whitevan01



Joined: 29 Jun 2007
Posts: 607

PostPosted: Sat Jul 30, 2011 6:41 pm    Post subject: Reply with quote

isobars is so right on this. there are aerobatic airplanes that have symmetrical airfoils and yet they produce lift and fly. Why? as Isobars stated, angle of attack.
If you really look at an airplane as it flies in level flight, the nose is always pitched up a bit. Like an airplane, our boards have a slip angle which is the angle of attack.

Read "The Aero-hydrodynamics of Sailing" by C.A. Marchaj and any other fluid dynamics texts and you will see what I mean.

not that the book that was referenced by rigitright is wrong, it is correct. It is the interpretation that was incorrect.
Back to top
View user's profile Send private message
cgoudie1



Joined: 10 Apr 2006
Posts: 2385
Location: Killer Sturgeon Cove

PostPosted: Sun Jul 31, 2011 1:48 am    Post subject: Reply with quote

Oh Dude,

Bernoulli is rolling over in his grave.

Perhaps this is just a misunderstanding. The lift on a fin
is perpendicular to it's "flat" surface, and can be attained simply by
angle of incidence. It doesn't lift the board out of the water (per-say,
tough torque around the fin does have an effect)

I wonder how a stunt plane stays aloft while flying upside down? ;*)

-Craig

rigitrite wrote:

Oh goodie, a pissing contest. Well, since you're gonna be a prick about it....
From "Engineering Fluid Mechanics" Roberson/Crowe 4th ed. pg 470
"The sum of the forces (pressure, viscous, or both) that acts normal to the free-stream direction is the lift, and the sum that acts parallel to the free-stream direction is the drag."

Since you clearly can't add, the "lift" on either side of a symetric foil is the same, therefore, X - X = 0. So lift = 0, get it...no lift? That's what I'm talking about. Symetric fin does not create lift, it just opposes side slip (force of the sail pulling normal to the liquid free-stream.) Since there is no pressure differential, there is no "lift". QED
I think it is YOU who need to review your fluids.
Back to top
View user's profile Send private message Visit poster's website
jingebritsen



Joined: 21 Aug 2002
Posts: 3351

PostPosted: Sun Jul 31, 2011 7:57 am    Post subject: Reply with quote

what keeps us from going sideways when the sail lifts us in wind? the medium of air vs water shows in the difference in size of fin vs sail, correct? what are we pissing over? i need a lift from some better wind. maybe i should start a super PAC like Colbert, so i can voice the joy of my windsurfing in a sexier spot than where i live? i take donations, any size will do....
_________________
www.aerotechsails.com
www.exocet-original.com
www.iwindsurf.com
http://www.epicgearusa.com/
Back to top
View user's profile Send private message
maxsonic



Joined: 28 Jun 2000
Posts: 9

PostPosted: Sun Jul 31, 2011 1:34 pm    Post subject: Reply with quote

rigitrite wrote:
Quote:
Sorry dude, but you're completely wrong. Please review fluid dynamics 101 because you clearly have no clue what you're talking about.


Oh goodie, a pissing contest. Well, since you're gonna be a prick about it....
From "Engineering Fluid Mechanics" Roberson/Crowe 4th ed. pg 470
"The sum of the forces (pressure, viscous, or both) that acts normal to the free-stream direction is the lift, and the sum that acts parallel to the free-stream direction is the drag."
.


Unsure what you're trying to explain above, but in the same reference (2nd Edition, pg 443, Figure 11-20, attached) it is clearly evident that the NACA 0012 symmetric airfoil produces lift with ANY angle of attack (and positive free-stream velocity, of course). Compare its lift characteristics with the cambered NACA 2412 (data referenced from Theory of Wing Sections, Abbott and Van Doenhoff).

Here's another fluid dynamics argument that windsurfers like to tussle about...is spin out caused by "cavitation" or by increasing the fin's angle of attack to/past the symmetric airfoil stall point shown in Figure 11-20? I vote for the latter!

MAX



Capture.JPG
 Description:
 Filesize:  85.15 KB
 Viewed:  9316 Time(s)

Capture.JPG


Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    iWindsurf Community Forum Index -> Windsurfing Discussion All times are GMT - 5 Hours
Goto page Previous  1, 2, 3, 4  Next
Page 2 of 4

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You can attach files in this forum
You can download files in this forum

myiW | Weather | Community | Membership | Support | Log in
like us on facebook
© Copyright 1999-2007 WeatherFlow, Inc Contact Us Ad Marketplace

Powered by phpBB © 2001, 2005 phpBB Group