I had a great email discussion recently with a windfoil friend who has prior experience with hang gliding. He shared many important things about the physics of foils with me. I thought it might be helpful for everyone if I put those thoughts into a blog post so we can all refer back to it. Here it is:
Friend: After mulling this over I think what is happening is a combination of Adverse Yaw and sail/mast directional force on the board causing it to exacerbate the wing adverse yaw induced slipping. Below is a long description of what I believe is happening and why. I may not be correct and I do believe that there are a lot of different and interacting forces on the wing coming into play. I am sure the hydrodynamic engineering mathematical analysis of this would be very complicated. So, I hope this may help us both to better understand what is going on and I am anxious to hear what you think about my analysis.
Finally, I don't know if any of it really answers how to better control the yaw/slipping/sliding tendency of the RS-1300 and similar wings but perhaps better knowing why its happening and anticipating it, we can better adjust or weight, foot pressure, sail positions etc. to counter some of it.
Ideally in an aircraft or foil wing you want a smooth coordinated turn where the wing rolls into the turn to the desired degree of bank. It then continues turning while maintaining that degree of bank. Additionally a positive angle of attack of the wing is maintained by back stick providing up tail elevator action, or back foot pressure when foiling.
If you don't coordinate your turn the bank angle will keep increasing due to ever increasing speed and subsequent increasing lift of the outer wing. Simultaneously the angle of attack decreases resulting in steeper and faster turning. In a plane this ends up to be a very fast deep diving spiral turn which can quickly over-stress the airframe due to excessive G forces, be difficult to pull out of and possible structural failure.
However, lucky for us foilers, it will just quickly roll our board steeply to the inside, breach the outer side of the wing which will then stall and we wipe out.
So, ideally we desire a nice coordinated turn when we jibe. However, the higher the aspect ratio foil wings with their longer and more narrow designs; when you initiate a turn, the increasing speed of the outer wing also starts producing more drag resulting in adverse yaw. This in turn causes the turn to be uncoordinated and the whole wing, foil and board begin slipping to the inside of the turn. Not having a moveable rudder it is difficult to counter the adverse yaw and slippage. We may try to counter this by pivoting the board to the inside by torquing both feet towards inside of the turn. This would yaw the board back into a more coordinated turn but that would only help momentarily as once we stop pivoting the wing will continue to yaw back to the outside.
Now when we introduce the forces that the sail rig and mast act on the board and foil, a whole new set of factors comes into play. When you initiate a jibe initially the mast is tilted slightly to the inside of the turn. At this point the forces on the mast are projected down into the board through the mast foot as well as pulling it forward as long as the sail is powered up. These forces can be affect as well from the weight of the sail/rig and the rider pulling or pushing down on the boom with their hands to some degree. Then when the mast is initially tilted to the inside, the pressure on the board from the mast will want to push the board both down and towards the outside of the turn. Consequently the board is pushed outwards and downwards simultaneously both trying to flatten out the board's bank angle and serving to have some counter effect on the board/foil slipping towards the inside due to adverse yaw.
Then as you get to deep broad reach and downwind you are tilting the mast back more upright (perpendicular) to the board. This just generates a downward force on the board from the sail rig & mast but probably has less effect on the yaw / slipping. However, as the mast is then tilted to the outside of the turn as the board goes from downwind to broad & beam reach on the new tack, the force from the mast is now also pushing the board towards the inside of the turn. I believe this then exacerbates the slipping to the inside and the bank angle of the board to the inside. This is likely why we have to pay attention to both heel and toe pressure during the jibe to help level out the board and prevent falling off on the inside.
Another factor that likely adds to yawing and slipping of these higher aspect wings such as your RS 1300 is that they are much flatter with minimal dihedral or anhedral. Maliko 200 for example has a lot of anhedral. A lot of aircraft have dihedral. In a slip to sideways both act to push the inside wind up or down. Think of the M200 with its curved downward tips. If you try to push that wing sideways through the water the downward curvature will push against the water and as the tip lowers the wing wants to keep rolling downward because of the curvature of the design. In an airplane with dihedral ( wings sloping upwards from the root cord (center of wing), when pushed sideways like in a slip, the inside wing will want to go up helping return the craft to level flight.
Now taking a flatter wing such as the RS-1300, when you push it sideways in the water it will likely just want keep going sideways to a much greater degree than the Maliko will. This in turn makes the wing more prone to slipping / sliding in a turn and without anhedral or dihedral, it will be less inclined to want to roll into a turn like a Maliko.
I can envision why the wing foiler tries to yaw his high aspect ratio wings first when starting to jibe. Since these wings are more resistance to just rolling into a turn because of their flatter design, initially yawing towards the outside of the turn the helps the wing start to slip towards the inside allowing the board to begin turning towards the inside. Make sense?
Finally, I don't know if any of it really answers how to better control the yaw/slipping/sliding tendency of the RS-1300 and similar wings but perhaps better knowing why its happening and anticipating it, we can better adjust or weight, foot pressure, sail positions etc. to counter some of it.
Ideally in an aircraft or foil wing you want a smooth coordinated turn where the wing rolls into the turn to the desired degree of bank. It then continues turning while maintaining that degree of bank. Additionally a positive angle of attack of the wing is maintained by back stick providing up tail elevator action, or back foot pressure when foiling.
If you don't coordinate your turn the bank angle will keep increasing due to ever increasing speed and subsequent increasing lift of the outer wing. Simultaneously the angle of attack decreases resulting in steeper and faster turning. In a plane this ends up to be a very fast deep diving spiral turn which can quickly over-stress the airframe due to excessive G forces, be difficult to pull out of and possible structural failure.
However, lucky for us foilers, it will just quickly roll our board steeply to the inside, breach the outer side of the wing which will then stall and we wipe out.
So, ideally we desire a nice coordinated turn when we jibe. However, the higher the aspect ratio foil wings with their longer and more narrow designs; when you initiate a turn, the increasing speed of the outer wing also starts producing more drag resulting in adverse yaw. This in turn causes the turn to be uncoordinated and the whole wing, foil and board begin slipping to the inside of the turn. Not having a moveable rudder it is difficult to counter the adverse yaw and slippage. We may try to counter this by pivoting the board to the inside by torquing both feet towards inside of the turn. This would yaw the board back into a more coordinated turn but that would only help momentarily as once we stop pivoting the wing will continue to yaw back to the outside.
Now when we introduce the forces that the sail rig and mast act on the board and foil, a whole new set of factors comes into play. When you initiate a jibe initially the mast is tilted slightly to the inside of the turn. At this point the forces on the mast are projected down into the board through the mast foot as well as pulling it forward as long as the sail is powered up. These forces can be affect as well from the weight of the sail/rig and the rider pulling or pushing down on the boom with their hands to some degree. Then when the mast is initially tilted to the inside, the pressure on the board from the mast will want to push the board both down and towards the outside of the turn. Consequently the board is pushed outwards and downwards simultaneously both trying to flatten out the board's bank angle and serving to have some counter effect on the board/foil slipping towards the inside due to adverse yaw.
Then as you get to deep broad reach and downwind you are tilting the mast back more upright (perpendicular) to the board. This just generates a downward force on the board from the sail rig & mast but probably has less effect on the yaw / slipping. However, as the mast is then tilted to the outside of the turn as the board goes from downwind to broad & beam reach on the new tack, the force from the mast is now also pushing the board towards the inside of the turn. I believe this then exacerbates the slipping to the inside and the bank angle of the board to the inside. This is likely why we have to pay attention to both heel and toe pressure during the jibe to help level out the board and prevent falling off on the inside.
Another factor that likely adds to yawing and slipping of these higher aspect wings such as your RS 1300 is that they are much flatter with minimal dihedral or anhedral. Maliko 200 for example has a lot of anhedral. A lot of aircraft have dihedral. In a slip to sideways both act to push the inside wind up or down. Think of the M200 with its curved downward tips. If you try to push that wing sideways through the water the downward curvature will push against the water and as the tip lowers the wing wants to keep rolling downward because of the curvature of the design. In an airplane with dihedral ( wings sloping upwards from the root cord (center of wing), when pushed sideways like in a slip, the inside wing will want to go up helping return the craft to level flight.
Now taking a flatter wing such as the RS-1300, when you push it sideways in the water it will likely just want keep going sideways to a much greater degree than the Maliko will. This in turn makes the wing more prone to slipping / sliding in a turn and without anhedral or dihedral, it will be less inclined to want to roll into a turn like a Maliko.
I can envision why the wing foiler tries to yaw his high aspect ratio wings first when starting to jibe. Since these wings are more resistance to just rolling into a turn because of their flatter design, initially yawing towards the outside of the turn the helps the wing start to slip towards the inside allowing the board to begin turning towards the inside. Make sense?
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