Thread: Apollo fool,,,,
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Old 14th December 2007, 01:46 AM   #59
Expander
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Join Date: Sep 2006
Location: Santa Croce lake, Italy
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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.

Last edited by Expander; 14th December 2007 at 04:19 AM.
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