Why Sails Twist: Difference between revisions
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| width="100%" align="CENTER" | '''<font size="2" color="#000000">Reason III - Circulation-Induced Tip Stall<br /><br /></font>'''<font color="#000000">The discontinuity at the head (sail and then nothing) causes 3-dimensional circulation flow at the tip. A component of this flow is in a direction which serves to increase the angle of attack. At high (near critical) angles of attack, the circulation flow is increased resulting in tip stall. In aircraft, this phenomena is reduced by 'washout' (twist), a more stall resistant foil section, a winglet, or a combination of all three. In a sail, twist serves to decrease the angle of attack at the head thereby reducing tip stall.and improving efficiency and performance.</font> | | width="100%" align="CENTER" | '''<font size="2" color="#000000">Reason III - Circulation-Induced Tip Stall<br /><br /></font>'''<font color="#000000">The discontinuity at the head (sail and then nothing) causes 3-dimensional circulation flow at the tip. A component of this flow is in a direction which serves to increase the angle of attack. At high (near critical) angles of attack, the circulation flow is increased resulting in tip stall. In aircraft, this phenomena is reduced by 'washout' (twist), a more stall resistant foil section, a winglet, or a combination of all three. In a sail, twist serves to decrease the angle of attack at the head thereby reducing tip stall.and improving efficiency and performance.</font> | ||
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[[Category: Tips | [[Category: Instruction, Tutorials and Tips]] |
Latest revision as of 15:04, 25 June 2021
This article originally appeared on windwing.com and is now available via the Internet Archive
Since much material is often subsequently lost to public view, we are placing a copy here for future use.
Why Sails Need Twist | ||
There are three reasons for twist in a sail. In descending order of magnitude and importance they are: I - Increase the wind range | ||
Reason I - Increased Wind Range | ||
The total aerodynamic force may be split into two components - a lift component which is perpendicular to the flow and a drag component which is in the same direction of the flow. These components are both proportional to the 'sheeting' angle. As the sheeting angle increases, lift increases to a maximum which is reached at the critical 'stall' angle. Above the stall angle, a rapid and significant loss of lift results. The following plot illustrates the effect of 'sheeting' angle on the lift coefficient for a camberless sail. (A 'cambered' sail will have a non-zero positive coefficient of lift at zero sheeting angle due to its 'pre-inflated' mechanically induced shape.) | ||
The relationship between the lift and drag components is a measure of sail efficiency. At zero sheeting angle, the sail still has a drag component due to the frontal area (shape) and 'wetted' surface (overall area.) As the sheeting angle and lift increase, the drag also increases. This additional drag is known as 'induced' drag. The following plot illustrates a typical L / D relationship: | ||
Reason II - Wind Gradient or 'Shear' |
Windspeed vs. Height plots compliments of W. L. Kleb |
Reason III - Circulation-Induced Tip Stall The discontinuity at the head (sail and then nothing) causes 3-dimensional circulation flow at the tip. A component of this flow is in a direction which serves to increase the angle of attack. At high (near critical) angles of attack, the circulation flow is increased resulting in tip stall. In aircraft, this phenomena is reduced by 'washout' (twist), a more stall resistant foil section, a winglet, or a combination of all three. In a sail, twist serves to decrease the angle of attack at the head thereby reducing tip stall.and improving efficiency and performance. |