Nicely trimmed close to the water, when everything is right the sealing effect of Ineos’s long and dead-straight skeg looks obvious. The trick is to maintain a small gap with the correct trim when the seas begin to pick up. There were moments during the 2024 Cup Match – mainly off the wind – when Ineos was more than a match for Team New Zealand, the problem was doing it for long enough to make more of a fight of it. As Ineos technical head Geoff Willis pointed out (Issue 540) the British team really needed another six months to get everything working to its full potential; but give Team NZ another six months too and it may just have gone full circle
spanwise twist (wash in or wash out) on all the planforms, but he varied how the ellipse was constructed. His first wing had the ellipse built off a
straight leading edge so that all the curva- ture was on the trailing edge (à la Team New Zealand 2024). His second was constructed around a straight quarter chord line that gave a wing planform reminiscent of a Spitfire or P47 Thunderbolt. His third wing had a straight trailing edge with all the curvature, to form an ellipse, on the leading edge. His last wing had a trailing edge that was curved to form an elliptical trailing edge, the rest of the ellipse being built off this trailing edge so the result was a swallow-like wing. Naturally they were all of the same area and span. When he ran them through his program
the wing with the least drag was the last wing with the trailing edge behind the back of the root chord à la swallow or swift. The wing with the greatest drag was the first, with the straight leading edge and thus a tip right at the leading edge of the wing. When he published his paper it caused
something of a spat between scientists. Another very distinguished aerodynamicist, Richard Eppler, disagreed and went as far as building small model gliders with each of the wing planforms and flying them in a gymnasium so there was no or little wind effect. He launched them all from the same height using a constant energy launch sys- tem so they all started with the same kinetic and potential energy… and guess what? Yes, Van Dam was right and they all
flew a distance equivalent to their different theoretical efficiencies. And Eppler was gracious enough to apologise. The lesson
50 SEAHORSE
seems to be that the further back the tip vortex forms the lower the induced drag. But why should that be? Well, to understand that we need to look
at the origins of induced drag. We must first consider the case of a wing of infinite aspect ratio (a wing with no tips), which although not possible in a flying machine is possible in a wind tunnel by having a wing that extends from wall to wall with no gaps. In this case the flow, which is along the length of the tunnel, at an infinite distance in front of the wing develops an upwash as it approaches the wing. This is because the attachment point
(stagnation point) will be at some point on the leading edge of the wing somewhat below the nose depending on the lift coeffi- cient (Cl) that the wing is working at. And because of this the flow must develop an upwash to navigate over the top surface of the wing. This upwash is then matched by an
equal downwash behind the wing so that there is no net up or downwash and, as it is the net downwash that is the root of induced drag, and as there are no tip vortices to increase the downwash or absorb energy, there is no induced drag. Compare that with a real wing with
tips. The stagnation point will still be on the lower surface of the nose so there will still be an upwash in front of the wing. However, due to the difference in pressure between the upper and lower surfaces the high pressure on the underside of the wing will want to migrate towards the lower pressure on top of the wing and, when this occurs at the tip where the two surfaces join, the resulting flow of air around the
tip then develops into a vortex, which we call the tip vortex. Looking from behind, the resulting
vortex flow is obviously circular but the flow outboard of the tip will be upwards and the flow inboard of the tip down- wards (Fig1a). The effect is a downwash between the two vortices, which affects the whole wing (Fig1b). This downwash is in addition to the downwash experienced by a wing of infinite aspect ratio so that the net downwash is now greater than the upwash, which makes the wing work in a sloped airflow. As that slope is backwards it is as if the
wing was flying uphill. Local lift, in this case, is always assumed to be working at right angles or normal to the local airflow so the lift vector will be leaning backwards and thus have a component of drag (Fig2). That drag is induced drag (in other words, the energy contained in the tip vortices). The path of the flow between upwash in
front of the wing to downwash behind it is a curve, and the further back any point on that curve is the steeper the flow, and the further forward it is the less steep (or even positive) will be the gradient. Thus the posi- tion of the wing within that body of curved air will determine the lean angle of the local lift vector, and thus the level of induced drag. Further forward the lower the induced drag and further back, up to the point the downwash diminishes, well behind the wing, the greater the induced drag. It is not possible to move the wing
within that body of air, but it is possible to move the body of air and its direction with reference to the wing. By moving the tip vortex back, as in the crescent-shaped
ALAMY
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150
