AB C
D
E
F
Right: the internal sliding ‘gusset’ added (top) to the Thinnair elevator where it meets the daggerboard to reduce local pressure loss. Above:A –generic Mach2 ‘high-lift’ foil designed for early take-off; B – Hollom DHDR2. Our first design where 5º of flap up or down reduces section drag due to the way that the back of the foil is designed through the flap hinge. It’s a good foil but the flap is narrow and thin. Thinness is the bigger issue as it’s impossible using standard-modulus carbon to get the flap stiff enough which makes for an interesting downwind ride; C – This is our current long-span design. Wider flap, which means it’s further into the body of the foil and close to its thickest section thus making the flap much easier to make stiff. From the profile shape you can see this foil and E (our small span design) both feature elliptical profiles. The chord thickness is a little less than is used on the other foils but camber is similar. Our long-span foil has a very different shape from the Thinnair short span as they operate at different Re numbers. The long span, to summarise, has a fatter nose compared to our short span but it’s no fatter overall; D – The Ellway-Maguire Exocet long span foil has been the class benchmark for a number of years, winning the last three worlds, although the design has been around since 2011 which shows how long it can take to get the best out of these complex machines. Thickness/chord ratio is between 12.2% and 13% depending on paint thickness (sic). Section shape appears similar to NACA 63-412 but differs slightly through the flap after the hinge; E – the short-span Thinnair foil. Our data says this is quicker than the long span after around 17kt of boat speed as the induced drag starts to drop away, while on the long-span foil it is starting to climb the parasitic side of the drag bucket; F – the Exocet small-span foil is similar to the Exocet long-span foil (D) apart from being shorter! So, having said all that, David [Hollom] tells me that if we have found a 20% reduction in section drag then, all other things being equal, we will get a speed gain of less than 0.4kt and that advantage reduces with speed… underlining that Moth speed is not‘all about the foils’, as many claim – Mike Lennon
neutral flap but pleasingly exhibited a drop of around 20 per cent once the flap was deployed. It had to be a winner? Would that life was that simple! They say, in my neck of the woods, ‘You don’t get owt for nowt’ and it’s very very true. Problem is that such a section results in a very thin flap and because, for a solid object (and the lay-up is solid), stiffness, in this case torsional stiffness, varies as the cube of size, the flap is not as torsionally stiff as we would like. As the actuating mechanism is on the centreline and, because of this deficit of torsional stiffness, the tips of the flap are not moving as much as they should which leads to less control in waves than we would like. No doubt the use of an ultra-high modulus carbon would (and in due course will) help but the cost of obtaining it in small enough quanti- ties for the production of a limited number of small items is currently prohibitive. The redeeming factor was that from the outset the foil was very quick against other known foil designs, although with the foils flexing control was far from ideal. After much experimenting it was decided that speed without sufficient control was not a winning combination so the decision was made to design a new set of horizontal foils of more conventional
52 SEAHORSE
design, having flaps large enough to give the stiffness required for control in waves. Different Res require different-shaped foils to minimise drag. Additionally, differ- ing operating Cls will require different amounts of camber. Using advanced design methods it is possible to design a foil sec- tion that has minimum drag at the required Re and a wide enough low-drag bucket (a range of Cls within which drag is low and on a lift-drag polar looking like a bucket) to accommodate the range of Cls over which the foil is expected to perform, again at minimum drag. It is, of course, possible to extend the range of Res over which the section is efficient but it will be less efficient than one that is more closely targeted. Bearing the above in mind and as it is necessary to have two horizontal foils – a large-span foil for light to medium condi- tions where speeds and thus Res are lower and a smaller-span, smaller-area foil, for heavier air where speeds and Res are higher – it was decided to design two new sections, targeting each to the operating Res of each foil.
We were happy with the theoretical drag figures for the original foil as they were considerably better than what the competi- tion were apparently using; so we were over the moon to be able to improve
considerably on that first foil and with a flap that was now large enough to give us the stiffness required for control in waves. Planforms of all our new horizontal foils are elliptical, or nearly so. The elliptical planform has a number of advantages. For a given span it gives minimum induced or vortex drag (drag due to lift). It is, however, a popular misconception that it gives the lowest induced drag. It only does that for a given span, but more important for most aircraft and birds is minimum induced drag for a given wing-bending moment. The aircraft designer is interested in weight and induced drag. The lower the wing-bending moment the lower his struc- tural weight and the lower his structural weight the greater will be his payload, and both weight (or lack of it) and low induced drag extend the aircraft’s range. The bird requires muscle and energy to resist wing bending. In both cases a lower wing-bending moment and a lower induced drag are of great benefit. For the aircraft it can carry more over a greater distance and for the bird it can fly further with less effort.
As a matter of interest the planform that best achieves this is triangular with a pointed tip. Most birds that fly great distances have an approximation of this
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