Left: Ineos persisted with their complex W-foil arrangement in New Zealand in 2021 then returned to the theme – but this time only very briefly – on their test boat this year (above). Strange decision… in practice by the time the foils went on their 2021 raceboat the team knew they had a bit of a hound so that any appendage data generated would be unreliable. Nice flow solution but a mechanical nightmare
A triangular wing planform moves the
aerodynamic or hydrodynamic centre further inboard thus reducing the wing- bending moment for any given load. Thus, for a given wing-bending moment, if span is unrestricted, the span can be greater than for an ellipse and span matters more than span-wise wing loading in reducing induced drag. That is why modern airliners have a
very triangular wing planform but without the pointed tip. The problem with pointed tips is that the Re becomes very low at the tip and this promotes early tip stall, which is very undesirable in an aircraft. This is because, as it is unlikely that the stall will occur at the same time on both wings, one wing will probably stall before the other; and if the stall starts at the tip the conse- quent rolling moment will be large, as will the resultant wing drop which in an air- craft is very dangerous. The Spitfire’s elliptical wing actually
had about 2° of washout at the tip for this very reason; so the much vaunted elliptical planform didn’t actually create an elliptical spanwise lift distribution. In the case of the airliner, reducing the
wing-bending moment enables a lighter wing structure, which means a bigger pay- load or a reduction in fuel burn. In the case of an America’s Cup yacht, it enables the designers to design a thinner wing section and thus raise the speed at which cavita- tion becomes a problem. This is one solution to the problem of
reducing wing area to reduce profile drag, while at the same time minimising wing T/C ratio to raise the speed at which cavi- tation becomes a problem. But as always there is more than one way to skin a cat, or a monohull, for that matter. Sweep works by reducing the effective
velocity of the onset flow and thus the local velocity around the foil and, as it is the speed of the local flow that causes the low pressure that in turn causes cavitation, it will enable a higher boat speed to be achieved before cavitation is felt. You may well ask how sweep reduces
the velocity of the onset flow. The easiest way to visualise its effect is to imagine that you have a foil that can be horizontally rotated inside a wind tunnel but without (this is a visualisation…) breaking the seal of the wing with the tunnel sides. If the wind speed in the tunnel is, say,
20kt and the foil was rotated horizontally, then the only velocity that the wing experi- ences is that velocity that is normal to its leading edge – which is reduced by the cosine of the rotation (sweep) angle. For example, if the speed at which cavitation became a limiting factor was 50kt then with 45° of sweep that limiting speed would be raised to about 70kt. It’s not all gain, however, because, as lift
varies as velocity squared, and as the only flow velocity the wing feels has been reduced by the cosine of the sweep angle, the lift goes down by the square of the cosine of the sweep angle, as does the profile drag; the consequence is that the wing now has to have a greater area to produce the required lift. It’s not completely negative, however…
remember the engineering. Because sweep demands that the wing is bigger in area, assuming the span stays constant, the chord must get longer and for the same strength, and therefore it can be substan- tially thinner. Thus not only is the sweep helping with
the cavitation problem, the thinner foil also helps and, because the profile drag has reduced, the extra area comes at no profile drag penalty… As a bonus, sweep also reduces induced drag, though, for mini- mum induced drag for a given span the planform is somewhat different from an ellipse. Finally, sweep also has a dihedral effect, which gives the craft greater lateral stability in roll, making it easier to sail. What more could you want? Finally, we have the double cranked
W-shaped foil which Ineos tried in 2021 and again, rather briefly, in 2023. I guess the idea is to make a foil with dihedral work without some of the drawbacks of dihedral. Dihedral, because it shortens the
amount of foil arm in the water, reduces the combined foil and foil arm wetted surface area – so reducing the profile drag of the foil system. The trouble is that the dihedral angle, to stay within the foil wing box and to maximise the shortening of the foil arm, needs to be about 20° from the horizontal. This is very near the foil assembly angle required to provide side force and lift in the right proportions, so that the outboard wing ends up near- parallel to the water surface which, if it gets too close to that surface, causes extra drag as well as other problems. By having a far steeper dihedral angle the foils come to the edge of the box before they get to full span, so that a horizontal section at either tip is then required to extend the foil to full span. This has the effect of keeping the dihedral angled part of the foil away from the surface and gives plenty of warn- ing if it gets too close to that water surface. There would appear, however, to be
several problems with this arrangement. Firstly, changes in angle of attack are not the same on the dihedral part of the foil as they are on the horizontal part. If the dihedral angle is 30° a change of just 1° on the horizontal part of the foil will translate to 1*Cos 30, which is 0.87° on the dihedral section, so that different parts of the foil system will be working at different lift coefficients depending on the change of angle of attack; generally the tips will now be working harder than the inboard dihedral sections. It also means that, because changes in angle of attack will only have 87% of the effect over a large part of the foil system, now controlling ride height will not be as positive as it should be. Also… because there are more junctions
and the flap mechanism has to be outside the foil in the full flow, the profile and junction drag will be considerably higher. Just think about it. With the fairings around the flap mechanism there are now 14 junctions instead of the two or possibly four in a normal layout. All in all, I think it’s a complicated way q
of wasting money. SEAHORSE 51
GILLES MARTIN-RAGET
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