My analysis of Sailrocket
18 December 2011
I copied a diagram of VSR2 (wing doesn’t show well) and added in the major force arrows that apply. Be aware that these arrows are not correct in terms of scale (length) and some of their locations are guesses, however I believe I’m correct enough for us to learn something about what VSR2 has been experiencing. Note that as I stated in an earlier post, all sailboats work in the same way, the only difference being the location and shape of the parts, and how those parts are controlled.
Here is the key to the force arrows:
Wt is the true wind
Wa is the apparent wind
Vb is the boat velocity
Fa is the airfoil force (lift)
Fh is the hydrofoil force (lift)
Fr is the resultant force (Fa plus Fh)
Da is the aerodynamic drag
Dw is the hydrodynamic drag
Dt is the total drag (Da plus Dw)
Observe that Fr and Dt are equal, in line, and opposed. This is the force relationship when the craft is in a steady forward motion (Vb). The craft will have neutral helm. If we change the positions or magnitude of the forces on the craft, the crafts motion will change and we can learn something about what changed by observing the motion change. This is what I will discuss, based only on what has been posted on VSR’s website.
If we rotate the wing (airfoil) clockwise, we see that Fa also rotates clockwise, this causes the intersection between Fa and Fh to shift to the left and Fr also shifts left. Fr and Dt are no longer in alignment and the craft will yaw clockwise (round up). This is the problem that VSR 1&2 both have, although VSR2 has better control of this. This yaw problem is an issue while sheeting in (start-up) or sheeting out (stopping). The solution is to swing the arm forward (the equivalent of rotating the hull and foil counter-clockwise) which keeps Fr aligned with Dt. VSR2 is set-up to do this, but Paul explained that it’s too difficult to do on-the-fly; this has resulted in the start-up procedure Paul has been using.
At speed, with neutral helm, Fr and DT are in alignment, but any wind shift or drag change will change their relationship. This shows up as a force on the rudder which is part of the data the team collects and analyses. Since I don’t have access to that data, I can only “see” the major changes that Paul describes on his blog.
One of these changes that Paul mentioned was with relation to the conventional foil in which he stated that VSR2 did a big yaw away to leeward. He goes on to say that he thought something let go and aborted the run. He said that he thought the main foil ventilated causing the yaw. Let’s look at the diagram and see if we can learn anything about this.
If the conventional foil ventilates, one or both of these things will happen: the foil will lose lift and it will likely have a drag increase (the L/D ratio will get worse). Looking at the diagram, we see that if the main foil loses lift (Fh) it will get overpowered by the wing (Fa) and the foil will drift to leeward a bit. That drift will be countered by the forward positioned rudder and VSR2 should yaw to windward. Note that VSR1 had the rudder aft so under the same conditions it would have yawed to leeward. Rudder location in this case makes the difference. If the foil ventilated and drag increased, (Dw at the main foil) we see that Dt would have shifted to the right (the balance between Da and Dw shifted) Fr and Dt are now unbalanced and will cause the craft to yaw to windward.
I see no way VSR2 would yaw to leeward if the main foil ventilated (if anyone knows, feel free to correct me). Note that if the wind drag increased, the increase at Da would have caused the craft to yaw to leeward, but we have no data to suggest that happened. From all the above, I suspect that the drag on the main foil might have decreased (which would cause a yaw to leeward) and I wonder what would have happened if Paul had kept going.
“Hey Bob, the drag should go up with an increase in speed – you’re nuts!” Well Paul gave us a clue that could account for the drag decreasing. In the comments for Paul’s blog post of 9 Dec 11, Paul gave a reply to “Armchair Comments”. Paul: “The upper section of the foil is more responsible for generating vertical lift. If it generates too much then the back of the boat rises until we reach the transition (radius) of the foil where the vertical component lessens. If we go too far then only the bottom section is left in and that is angled so it is pulling down more than the rig is lifting i.e. we will have net negative lift and be pulled back down to the transition. The boat should seek balance around this transition.”
Paul’s quote above sounds like the main foil angle is set wrong. The wing and main foil should ideally be set parallel and positioned such that the forces through the centers of effort are directly aligned and in plane. That will cause those forces to be equal and opposite in that plane (viewed from fore or aft) and there will be no net up or down force (those forces cancel out). If the main foil is over angled (as the above quote suggests) and is producing negative lift, then VSR2 is producing needless drag as the upper section of the foil produces lift and the lower section produces “negative lift”. This drag will increase with the velocity and put VSR2 up against the wall.
Now the times when the conventional foil bore away (which I suggested might mean less drag) I can account for if there was flex in that foil with the increased load caused by the speed increase. If the foil flexed, it would have moved to a more parallel position relative to the wing and the “negative lift” would have decreased and the drag from the upper and lower sections of the foil fighting each other would have decreased.
Sailien yaws when it lifts onto its foils and the direction it yaws is dependent on which side is lifting up (reducing the drag on that side). Ride height should only be controlled by the upper section of VSR2’s main foil in my opinion.