Opposite: and then of course there is the case when both foils are out of the water. Above/top: Figure 1, the generic Drag vs Speed bucket with minimum drag at the point of intersection; and Figure 2 (above) with drag being reduced in increments of 10% while the speeds remains as before. Top left: the Mark Mills/KND-designed coastal racer Flying Nikka mimics the AC75s in philosophy but less so in the detail as she must sail in a wider range of conditions. The AC75s carry ballast just in the foils, with no keel, while Nikka has a fin and bulb to be eligible for existing regattas (and for safety); then where the AC75s employ trim tabs on the foils, to change foil angle of attack on Nikka the foil arm arrangement itself is rotated. Left: illustration of how foil sweep drives the proportion of the aircraft’s velocity that is the airflow normal to the chord
to speed up and pulling back to slow down. That is why, on the approach to landing, you can very often hear the engines spooling up and down as the pilot adjusts the power (not elevator) to main- tain his approach path. This point of minimum drag will occur at about 23kt under the new AC75 rule when, as previ- ously mentioned, both induced and profile drag will be about equal. Bearing in mind that design features that
reduce induced drag generally increase profile drag, and vice versa, it might be interesting to see the effect of reducing each of these drags separately. Fig 2 shows the effect of reducing each drag by 10% while holding the other constant. As can be seen, minimum drag is the
same in both cases but occurs at a higher speed when profile drag is reduced – and at all speeds above minimum drag the total drag is then lower than before. The aircraft or foil-borne craft with reduced profile drag thus flies faster at all lift/drag ratios even at speeds below minimum drag. This might give the impression that it is
better at all speeds. However, in the case of the aircraft, at minimum sink (best ratio of Cl1.5/Cd), although the lift/drag ratios and thus the glide angles will be the same,
the aircraft with less induced drag will be flying more slowly and will thus proceed down that glide path more slowly. It will therefore sink more slowly in still or sink- ing air, or gain height more rapidly in a thermal. For a competition glider, for example,
this is a great advantage and is one of the reasons for their characteristically high aspect ratio wings. But for an AC75 it would seem the emphasis might better be on reducing profile drag rather than induced drag, even though lift-off may then occur at a higher true windspeed. To state the obvious, reducing foil
wetted area will normally reduce profile drag. Foil wetted area can be reduced either by reducing the span or by reducing the chord or by a combination of both. Reducing the span while keeping the
chord constant will normally reduce pro- file drag more than by reducing the chord while holding the span constant. There are two reasons for this. Firstly, reducing the chord reduces the Reynolds number (Re) at which the foil runs and profile drag
‘A bad workman always blames his tools’ Anon
coefficient (Cd) normally rises as Re falls; thus, although the area has gone down, because the Cd has risen, the profile drag will have gone down by somewhat less than the reduction in area. On the other hand, reducing the span while leaving the chord constant, because the Re remains the same, results in a profile drag reduc- tion equivalent to the area reduction. Secondly, there is the engineering.
Reducing the span normally reduces the foil bending moment allowing a thinner wing, which again, normally, reduces profile Cd, so that by reducing the span the profile drag will reduce at a greater rate than the reduction in area for the same strength. In contrast, reducing the chord while holding span constant, normally keeps the foil bending moment constant so that, allowing for the loss of material due to the reduction in chord, to maintain the same strength the foil has to be physically thicker. This, together with the reduction in chord, produces a larger thickness/chord ratio (T/C) and profile Cd rises both due to the lower Re and the bigger T/C ratio. This rise in T/C ratio is not insignificant
and has ramifications further down the line. As a simple example, halving the chord on a solid foil while maintaining the
SEAHORSE 49
FABIO TACCOLA
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