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15th December 2007 02:16 AM
Unregistered With all repect expander thats been crux of thread for a while.
We know some have said high aspect is more efficient; some have said more efficient in certain situations and a couple have said Aspect ratio does not affect overall lift.
If Floyd`s quotes (from some Scientuific papers; listed in post) are taken to have any substance then overrall lift for a planing body is simply a matter of area.
Your post implies a longer leading edge gives more lift.
If planing is a result of momentum exchange over entire area there is no reason to just assume high aspect is any more efficient.(It would simply be matter of max area for minimum leading edge/trailng edge)
On the other hand if planing is a result of lift generated by leading edge then it stands to reason higher aspect would generate more lift.
Nobody has actually said its one or the other (for a fact) with evidence.(Infact evidence provided is contadictory)
Personally reckon leading edge argument is flawed for reasons mentioned in my last post.(ie boards do not have flow over top surface so wing anology is very weak)
14th December 2007 03:55 AM
Expander
Quote:
Originally Posted by Unregistered View Post


...a planing surface (ie a board) will not create any lift at zero angle of attack because at zero AoA there iis no momentum exchange.Board /hull or any planing surface needs to be at a slight angle to direction of travel...
--

As I've written in previous post, "...in these context, in air or in water, an HIGHER Aspect Ratio has pratical effect of increasing LIFT (above all when angle of attack increases)..."

And when a windsurfing board runs on water it always (and inevitably) assumes a certain Angle of Attack for board design (scoop line).
14th December 2007 03:14 AM
Unregistered Great explanation from expander;;;
but it does assume there is a very close anology between lift annd drag relatiopn ships for wings/fins nd to that of planing surfaces.(Which has been in doubt in a few posts)

Not sure its anything to do with it but;

An aircraft wing will give lift at a zero angle of attack.(Bernouli /venturi principle;ie air over top surface travels much further than that underneath; creates pressure difference between upper and lower surfaces and cause luift.

A planing surface (ie a board) will not create any lift at zero angle of attack because at zero AoA there iis no momentum exchange.Board /hull or any planing surface needs to be at a slight angle to direction of travel. (Ie even flat slalom boards are sat deeper at tail than front edge)

Taking all this into account dont really think we can assume wing anolgy is a good one. (Especially when we are making conclusions based on them)
14th December 2007 02:07 AM
Unregistered expander-
thanks for the great graphics
14th December 2007 01:46 AM
Expander --

A small contribuition about ASPECT RATIO issue and relative planning abilities of a windsurfing board.


About Aspect Ratio (AR).

Aspect ratio (AR) is an indicator of the general performance of an (hydro) aerodynamic surface.
In aerodynamics, the Aspect Ratio of a wing is defined as the square of the span divided by the wing area.
It is a measure of how long and slender a wing is from tip to tip.
"HIGH" Aspect Ratio aircraft wing indicates long, narrow wings, whereas a "LOW" Aspect Ratio wing is short and stubby.




In windsurfing board design, "HIGH" Aspect Ratio term indicates wide Formula boards while "LOW" Aspect Ratio term indicates Slalom narrow boards.




In these context, in air or in water, an HIGHER Aspect Ratio has pratical effect of increasing LIFT (above all when angle of attack increases).
An analytic confirmation could be verify just considering LIFT (L) and DRAG (D) of a hydro/aerodynamic surface, and plotting a graph to see how COEFFICIENT OF LIFT (CL), given from ratio of Lift over Drag (L/D), works for surfaces with a HIGH and LOW Aspect Ratio (AR):




It is clear that, for a surface with HIGH value of AR (graph on left), Coefficient of Lift (red curves) is much higher than a surface with LOW value of AR (graph on right).
At same Coefficient of Lift, higher is L/D ratio, MORE efficient is a surface.
Therefore, the HIGH Aspect Ratio surface (Glider wing or Formula windsurfing board) will glide much further and will be characterized by better skimming abilities than a LOW AR surface, above all when wind is in the low-end and board is crossing from full displacement to skimming condition.

Schematizing,

IN AERODYNAMICS:

HIGH Aspect Ratio wing --> Higher Lift Coefficient --> lower stalling angle of attack (Gliders);
LOW Aspect Ratio wing --> Lower Lift Coefficient --> high stalling angle of attack (Fighter Jets)

or, IN WINDSURFING BOARD DESIGN:

HIGH Aspect Ratio board --> Higher Lift Coefficient --> better low-end planning abilities (Formula);
LOW Aspect Ratio board --> Lower Lift Coefficient --> better speed performances (Slalom).

A really empirical confirmation of better efficiency of a HIGH Aspect Ratio surface on a LOWER one is given by different kind of FLAGS: yes, I'm speaking about very common flags like ones visible in every beach in summer season; well, like visible in following drawing, a HIGH AR flag is more efficient of LOW AR flag also when wind is weak or tenuos for other one (it goes on flapping while other flag is become flabby).



I hope this contribuition could be useful to clarify entire matter.


- EXPANDER.
12th December 2007 09:50 PM
Ken In conclusion - for the regular folks:

Wide = quick to plane, but hits the wall at some point in high winds (39 knots - see GPSwindsurfing.com). Wow!!!!!

Skinny = slow to plane, but keeps going faster in the high winds (50 knots).
12th December 2007 08:39 AM
steveC I'm not so capable that I understand the real scientific/technical stuff, but my "off the top" take is that high or low aspect leverage is kind of relative to the playing field. I guess it depends on your focus. Needless to say, if it's really cranking, the narrow emphasis has obvious advantages relative to control. In light stuff, the opposite is true.

I think that it's fair to say that both wide and narrow width boards have a distinct place in the picture. A skewed perspective one way or another will affect the performance envelope. Of course, a steely nature will often figure too in the outcome.
12th December 2007 06:06 AM
C Guy Also, I'm not an aerodynamicist's underpants and so therefore I know nothing; it's just that sometimes I get to ask those who do know their stuff.
12th December 2007 06:00 AM
C Guy It wasn't a competition, so no one won or lost.

As far as I can see, Floyd is right and we can't just say that high aspect is more efficient. It's just more efficient for some purposes. The high aspect planing surface does work and for much the same sort of reason as skinny wings work (ie reduction in tip loss etc).

Savitsky's 1964 paper has a good way of looking at it - if you had an extremely wide span planing surface, the tip loss is minute and almost all of the effect of the water being shoved down is acting to lift the board.

In contrast, if you had a very long skinny surface, basically all the water slips out to the side so there's nothing underneath to lift the board up. Sure, there IS actually water underneath but its effect is diffused by distance - just like the fact that we don't get squashed by air being shoved down underneath a jumbo passing overhead

But where Floyd is right to say that we may be being oversold the high aspect effect is that the marketing spin ignores the downsides - ie while induced drag when planing DROPS by the square of the width, form drag when not planing INCREASES by about the square of the width and wavemaking drag is increased by a short fat surface. And to get to plan, we have to overcome form drag, wavemaking drag etc.

So skinny works in some conditions, fat in others, and to say that fat = good is surely wrong. And for my two cents worth, we need to get away from emphasising top end speed in planing conditions so the value of long and skinny can't be overlooked.

PS Floyd, it is fascinating looking at the way people look at lift - momentum transfer seems cool, but circulation and Euler equations do my head in! :-)
11th December 2007 06:16 PM
Floyd Nobody won !
No; we`re all confused.
AR- Aspect ratio. Of sail ; luff squared divided by area.Of board; planing width divided by planing length.
Yes
Dont know .
Its cold.
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