EVALUATION OF MULTI-ELEMENT WING SAIL

5 th High Performance Yacht Design Conference
Auckland, 10-12 March, 2015
EVALUATION OF MULTI-ELEMENT WING SAIL AERODYNAMICS FROM
TWO-DIMENSIONAL WIND TUNNEL INVESTIGATIONS
Alexander W. Blakeley 1 , abla089@aucklanduni.ac.nz
Richard G.J. Flay 2 , r.flay@auckland.ac.nz
Hiroyuki Furukawa 3 , furukawa@meijo-u.ac.jp
Peter J. Richards 4 , pj.richards@auckland.ac.nz
Abstract. Following the 33rd America's Cup which featured a trimaran versus a catamaran, and the recent 34th America's Cup in
2013 featuring AC72 catamarans with multi-element wing sail yachts sailing at unprecedented speeds, interest in wing sail
technology has increased substantially. Unfortunately there is currently very little open peer-reviewed literature available with a
focus on multi-element wing design for yachts. The limited available literature focuses primarily on the structures of wings and their
control, rather than on the aerodynamic design. While there is substantial available literature on the aerodynamic properties of
aircraft wings, the differences in the flow domains between aeroplanes and yachts is significant. A yacht sail will operate in a
Reynolds number range of 0.2 to 8 million while aircraft operate regularly in excess of 10 million. Furthermore, yachts operate in the
turbulent atmospheric boundary layer and require high maximum lift coefficients at many apparent wind angles, and minimising drag
is not so critical. This paper reviews the literature on wing sail design for high performance yachts and discusses the results of wind
tunnel testing at the Yacht Research Unit at the University of Auckland. Two wings with different symmetrical profiles have been
tested at low Reynolds numbers with surface pressure measurements to measure the effect of gap geometry, angle of attack and
camber on a wing sail’s performance characteristic. It has been found that for the two element wing studied, the gap size and pivot
point of the rear element have only a weak influence on the lift and drag coefficients. Reynolds number has a strong effect on
separation for highly cambered foils.
NOMENCLATURE
AWA
c
C l
C d
D
g
L
R
S
TWA
V A
V mg
V S
V T
A
H
Apparent wind angle
Chord
Lift coefficient
Drag coefficient
Drag on wing and any sails
Gap
Lift on wing and any sails
Resistance of hull and appendages
Side force on hull and appendages
True wind angle
Apparent wind velocity
Velocity made good
Yacht velocity
True wind velocity
Angle of attack of front element
Angle between rear and front elements
Apparent wind angle
True wind angle
Aerodynamic drag angle
Hydrodynamic drag angle
Leeway angle
1. INTRODUCTION
Following the 33rd America's Cup in 2010 which
featured a trimaran versus a catamaran, and the recent
34th America's Cup in 2013 featuring AC72 catamarans
with multi-element wing sail yachts sailing at
unprecedented speeds, interest in wing sail technology
has increased substantially. Unfortunately there is
currently very little open peer-reviewed literature
available with a focus on multi-element wing design for
yachts. Hence a wind tunnel study of multi-element wing
sails was carried out by the University of Auckland.
Some preliminary results of this research are available in
[1].
Solid wing sails offer several advantages over flexible
sails for high speed sailing. One advantage is that the
wing has an internal structure which is used to give the
wing its shape. The shape is not dependent on the tension
in the lines at the sail corners. The shape of a flexible sail
cloth mainsail is highly dependent on the tension in the
mainsheet, particular the vertical component which is
required to keep the leech from twisting excessively. In a
wing sail, this vertical component is eliminated
completely from the mainsheet, which is used simply to
alter the angle of attack using a horizontal force. The
twist in the solid sail is controlled by internal control
lines which are under considerably less tension than a
conventional mainsheet. Thus the angle of attack can be
changed relatively easily and quickly, and the power of
the sail can be controlled much more easily than in a
conventional soft sail. This makes sailing a high
performance catamaran much easier with a wing sail than
a soft flexible sail.
1 PhD Student, Yacht Research Unit, Department of Mechanical Engineering, University of Auckland, NZ
2 Professor, Director Yacht Research Unit, Department of Mechanical Engineering, University of Auckland, NZ
3 Associate Professor, Department of Mechanical Engineering, Meijo University, Japan
4 Associate Professor, Department of Mechanical Engineering, University of Auckland, NZ
37

Multi-hull yachts have considerably less hydrodynamic
drag than mono-hulls since they have much lower
displacements than monohulls of similar length. This is
because they derive their righting moment from their
wide beam, and not from having a heavy keel.
Furthermore they heel only a small amount. A dangerous
feature of multi-hulls is that once the overturning
moment exceeds their maximum righting moment, they
will overturn unless the overturning moment is reduced
quickly. When coupled with a high performing solid sail
or wing, this means that they are able to sail with small
apparent wind angles both upwind and downwind as
illustrated in Error! Reference source not found.. It
was shown by Lanchester [2] that the minimum angle
( A ) at which a boat can shape its course relative to the
wind is the sum of the under and above water drag
angles, namely H and A . A wing sail operating at low
angles of attack has a low aerodynamic drag angle, and
so the result of this is that high performance multi-hulls
with wing sails operate at low apparent wind angles in
both upwind and downwind sailing. These angles are
illustrated in Error! Reference source not found.. This
means that it is only necessary to have aerodynamic data
for wing sails for low angles of attack.
V A
A
V S
Upwind
V T
T
V mg
V mg
Figure 1 Velocity triangles for up and downwind sailing
showing the low apparent wind angle A which occurs for
fast multi-hulls with wing sails.
Understanding the performance of multi-hulls can be
enhanced by analysing a free-body diagram of the yacht
and considering the forces in the horizontal plane. This is
close to reality, since they do not heel much. The
aerodynamic and hydrodynamic forces on a catamaran
are shown in Error! Reference source not found.. Sail
forces are lift (L) and (D) being normal and parallel to
the apparent wind direction, and hull/appendage forces
are sideforce (S) and resistance (R) which are normal and
parallel to the yacht’s course through the water. There
may be a small leeway angle (). Note that for
equilibrium, the hydrodynamic and aerodynamic forces
in the plane of the water must be equal and opposite, so
. Furthermore, as noted above,
. Reducing D and R lead to reductions in
and and thus to a reduction in AWA, which will lead
V S
V A
A
Downwind
V T
T
to increased velocity made good (V mg ) to the top or
bottom mark.
S
H
Error! Reference source not found.Figure 2 Sketch of
horizontal forces acting on a catamaran showing the
important directions.
Ignoring the structural aspects and considering primarily
the aerodynamic aspects, high speed multi-hull yachts
with wing sails need wings that are symmetrical so that
they can tack and gybe at will. They need to be able to
develop high lift and have low drag. They need to be able
to twist so that the height of the thrust can be lowered
when required to reduce the overturning moment. From
observations of existing vessels, it is apparent that severe
twist is important so that the direction of the lift force can
be reversed at the top of the wing to provide a righting
moment. Generally it is apparent from the 2013
America’s Cup yachts that for a two-element wing, the
chords of the main wing and flap are usually of similar
length, and the gap between them is relatively small. The
first element is thick so that it can take the required
structural loads, while the flap is thin. Typical thickness
ratios are 25% and 9% for the first and second elements
respectively.
2. SOME THEORETICAL CONSIDERATIONS
By applying the sine rule to the velocity triangles in
Figure 1 it is evident that
The velocity made good is given by
R
, so (1)
(2)
(3)
Thus to improve performance (i.e. increase the ratios
and ) at a fixed heading (i.e. fixed ) it is necessary to
reduce . Figure 3 shows curves of the ratio for
constant and for various , which are evidently
circular arcs. Figure 3 shows clearly how smaller
L
D
A
38

gives more speed. Maximum yacht speed occurs when
and it is the diameter of the circule. The
maximum velocity made good is determined from the
horizontal tangents to the circle, and occurs for the
angles and for upwind and
downwind respectively.
Thus the importance of being able to determine and
minimise the aerodynamic drag angle has been
demonstrated. Wing performance between candidate
options can therefore be compared on the basis of the
aerodynamic drag angle as it influences directly.
1
0.5
0
‐0.5
V T
‐1
‐1.5
‐2
T
V S
A
V A
bA = 25 deg
bA = 45 deg
bA = 90 deg
bA = 150 deg
0 0.5 1 1.5 2 2.5
Figure 3 Velocity triangles (speed polars) for fixed values of
A for a range of T .
3. LITERATURE REVIEW
The limited available literature on wing sails focuses
primarily on their structure and control, rather than on
their aerodynamic design. While there is substantial
available literature on the aerodynamic properties of
aircraft wings, the differences in the flow domains
between aeroplanes and yachts is significant. A yacht sail
will operate in a Reynolds number range of 0.2 to 8
million while aircraft operate regularly in excess of 10
million. Furthermore, yachts operate in the turbulent
atmospheric boundary layer and require high maximum
lift coefficients at many apparent wind angles. A brief
review of some of the literature on wing sails is
presented below.
Elkaim [3] at the University of California undertook a
large amount of work on wing sails while involved in the
Atlantis Project, which focused on designing an
Autonomous Marine Surface Vehicle for a wide range of
applications. A rigid wing sail was used on this vehicle
because it was regarded as more aerodynamically
efficient than a soft sail, by pivoting it near the centre of
pressure it required less force to actuate than a regular
soft sail, and it could be designed to be self-trimming.
Marchaj [4] reviewed some research on wing sails,
pointing out their advantages in some situations. In
discussing the 1976 C-Class race between Aquarius V
with a soft sail, and Miss Nylex with an advanced wing
sail, he pointed out the advantage of the soft sail with the
lighter rig in light conditions, compared to the more
aerodynamically efficient but heavier wing sail rig.
Some wing sails have been made of double-surface soft
sails, and are a good idea for cruising as they can be
reefed and dropped when wind conditions are
unfavourable, e.g. the Wally and Omer wing sail design
[5]. They do not have all the benefits of rigid wing sails,
e.g. the leech tension needs to be maintained, requiring a
sophisticated boom and powerful mainsheet.
C-Class catamaran racing has often produced very
innovative ideas regarding wing sails. Examples are the
slot gap on the yacht Cogito, described by MacLane [6],
which could be changed onshore, but was fixed once
racing. Killing and Clarke [7] describe further
enhancements of C-Class wings for the yachts Alpha and
Rocker, which had more advanced cross-sectional shapes
than Cogito which were developed using software
developed by Drela at MIT [8,9].
There are maximum heeling and pitching moments that
can be applied to catamarans before they tip over. Thus
in strong wind conditions it is advantageous to lower the
height of the line of action of the side and thrust forces.
Thus it can be shown that the optimum lift distribution
for a catamaran is such that it changes sign at the top of
the wing, requiring large amounts of wing twist [10].
Wing sails can be designed with mechanisms for
producing such large amounts of twist, which is
impossible for soft sails, and so this is another significant
difference between wing and soft sails.
One of the commonest areas of wing sail use is for highspeed
land yacht sailing, more specifically ice yachts.
These yachts skate on ice and are sometimes powered by
large wing sails. They are capable of very high speeds.
The Greenbird project is arguably one of the most
successful versions of wind powered land/ice yachts. On
land, the Greenbird holds the land speed record of 126.2
mph, while on ice, they hope to beat the current record of
84 mph by setting a speed equal or higher than the
126.2mph that they set on land [11].
Recently relatively small yachts have been
experimenting with wing sails. The X-Wing sail project
[12] is a 2-element wing designed for small sailing
dinghies, specifically the “Sunfish” class. The designer
was inspired by the 2010 America’s Cup and has
designed a low-cost wing for those wanting to
experiment with the technology.
39

The literature cited above shows that work on wing sail
development has taken place for a variety of reasons, but
mainly to obtain high speeds or to improve control, and
that developments have occurred in both small as well as
larger craft. However, it is evident that there is a paucity
of multi-element wing aerofoil data obtained specifically
for yacht sail design. This paper discusses the results of
wind tunnel testing at the Yacht Research Unit at the
University of Auckland to help provide this information.
4. CFD OPTIONS
To analyse the flow around upwind sails, the potential
flow assumption and the panel method are basic
methods, but they cannot model viscous effects which
are dominant in locations of flow separation. The Navier-
Stokes equations can be solved directly (DNS) in cases
with flow separation phenomena. When the Reynolds
number is high and the flow becomes turbulent, it is
difficult to use DNS because it needs an enormous
number of grid points. The turbulence must be modelled
with turbulence or sub-grid models. Reynolds Average
Navier Stokes (RANS) turbulence models obtain a
stationary solution that is representative of the mean
flow. They are robust and easy to use, but cannot be
adopted to unsteady flow. Large-eddy simulation (LES)
is used to model the larger, three-dimensional unsteady
turbulent motions. In LES, the smaller scale motions are
filtered and subsequently modelled. LES can be expected
to be more accurate and reliable than RANS models for
flows where large-scale unsteadiness is significant.
The program MSES [8] is a coupled viscid/inviscid Euler
method. It solves the Euler equations on a discrete two
dimensional grid, coupled with an integral boundary
layer formation. MSES was created to analyse the flow
around an aerofoil more accurately than inviscid codes
by incorporating viscous effects and a boundary layer
formation into its solutions. Therefore it theoretically
offers more realistic results than a vortex lattice method
(VLM), although it possibly requires more computational
power. A derivation of MSES is the MISES [9] code,
which has been developed to determine the flow around
multi-element aerofoils.
While CFD analysis of wing sails is highly advanced and
capable of accurate predictions, it is not easy to predict
sail performance if there is flow separation. As
mentioned in the present paper, it is important for
designers of wing sails to know where flow separation
occurs, and physical testing such as wind tunnel testing
can help in this regard. Wind tunnel testing can also give
good information for checking the output from CFD
codes, although they may be unable to give results for the
very high Reynolds numbers appropriate for large yachts
sailing very fast, e.g. America’s Cup AC72 yachts.
However, the Reynolds numbers from wind tunnels
would be commensurate to those for smaller C-class
yacht wing sails.
CFD analysis of wing sails is not considered further in
the present paper.
5. EXPERIMENTAL SETUP
5.1 Wind Tunnel
The experiments were performed in the University of
Auckland open-return wind tunnel. The wind tunnel has
been specifically designed for testing yacht sails and has
a standard operating cross section of 7 m wide by 3.5 m
high. The flow is produced by two 3-m diameter 4-
bladed fans and then driven through a 1 m thick
honeycomb screen and two tight mesh screens to remove
swirl and to give a uniform velocity profile and relatively
low turbulence flow. Permanent pitot-static probes set up
well upstream of the wing recorded the dynamic and
static pressures whilst another probe measured the
atmospheric pressure outside the working section.
The wind tunnel walls were brought inwards to give a
2.5m wide by 3.5m high test section and the wing models
were located near the outlet of the nozzle as shown in
Figure 4.
Figure 4 Photograph of wing model in wind tunnel showing
base of support struts and endplate marking for angle
fixing.
5.2 Multi-element Wing Construction
The multi-element wing used in the wind tunnel
comprised a NACA 0025 main element and a NACA
0009 aft element with a 50:50 chord ratio. The wing
spans 2.5 m across the width of the wind tunnel and has a
chord length of 1 m (with zero aft element deflection and
no gap). 20 mm thick MDF boards at each end of the
model contained the placement locations for the elements
and tested configurations. Once attached to the wind
tunnel walls the model can be rotated and then securely
fixed to achieve various angles of attack. By constructing
the model in this fashion the trailing vortices as
experienced by a finite spanned wing are virtually
eliminated.
40

Each element has 64 independent pressure taps which
measure pressure differentials through 1 mm diameter
holes placed around the centreline of the windward and
leeward surface of each wing, sufficiently far away from
the wind tunnel side-wall boundary layers. They are
dispersed from the leading to the trailing edge with a
higher concentration around the leading edges of the
elements. The pressure taps can be seen in Figure 5.
They are slightly off-set from the stream-wise direction
to avoid interference from wakes of upstream holes
interfering with downstream taps. Tubes connecting to
each individual pressure tap feed out an end of each wing
to a box containing pressure transducers. The pressures
were logged at a rate of 200 Hz over a 30-s period. The
pressure transducer box was located downstream of the
wing out of the free-stream.
5.3 Experimentation Variables
The variables altered during the wind tunnel testing were
as follows:
• Front wing model angle of attack, relative to
main element chord
• Aft element chord-line deflection, , relative to
main element chord
• Gap between main and aft element, g, defined at
zero aft deflection angle , as a % of the main
wing chord
• Aft element pivot point position, as a % of the
main wing chord.
• Free stream flow velocity, V.
Figure 6 shows these variables with respect to the model.
It also defines the pivot location for the aft element and
defines the gap between the foils.
Figure 5 Close up photograph of leading edge of a wing
model showing the 1 mm diameter pressure tap holes.
The pressure measurements, along with the locations of
the pressure taps can be used to obtain a pressure
distribution across the surface of each element.
Aerodynamic forces result from these pressure
distributions acting over the surface of the wings. By
integrating the pressure distributions in the vertical and
horizontal directions, the vertical and horizontal forces
can be computed, respectively. Since pressure acts
normal to a surface, the surface curvature of the aerofoil
between two points should not be neglected. However,
the pressure taps were distributed closely together around
regions of high curvature on the wing elements and
therefore a straight line approximation of the surface
between any two pressure taps was regarded as
sufficiently accurate for the present work. The integrated
x and y forces were firstly resolved into directions
relative to the chord of the aerofoil. Therefore their
resultants were then decomposed appropriately to find
the lift and drag relative to the free-stream velocity.
Finally non-dimensional representations of the
coefficients of pressure (Cp), lift (Cl), and drag (Cd)
were computed.
Figure 6 Wing variables altered in experiment.
Reynolds number Re is defined by the reference length
2c (=1.0 m), the reference velocity V and the kinematic
viscosity for air.
41

6. RESULTS AND DISCUSSION
6.2 The influence of angle of attack
6.1 Pressure coefficients
Figure 7 Example of pressure coefficient distribution. =
10°, g = 1%, = 2°, Re = 800,000.
The shapes of the main and aft elements are NACA0025
and NACA0009, respectively [13]. The chord of the
main element c1 is 500 mm, and it is same as the chord
of the aft element c2. The total wing chord c = c1 + c2 =
1 m.
Figure 7 shows an example of the pressure coefficient
variation over the foils. In Figure 7 the deflection angle
of the aft element is 10°, the gap between main and aft
elements is 1% referenced to the main element chord c1,
the angle of attack, is 2° and Reynolds number based
on the total wing chord c as the reference length is
800,000.
Figure 8 Example of stall. = 10°, g = 1%, Re = 700,000.
Figure 8 shows the distributions of the lift coefficient C l
and the drag coefficient C d versus angles of attack C l
increases almost linearly with increase in up to 10° and
C d also increases a little. C l reaches a maximum value at
around = 10°. At larger angles of attack the wing stalls
and C l decreases sharply and C d increases rapidly, as
expected.
6.3 The influence of aft element deflection
The horizontal axis of the graph indicates the
dimensionless distance along the chord direction. The
vertical axis shows the pressure coefficient. The position
x/c = 0 corresponds to the leading edge of the main
element, and x/c = 0.5 indicates the trailing edge of the
main element and the leading edge of the aft element.
The vertical axis has been inverted with negative values
at the top. Thus the red line indicates the pressure
coefficient at the top of the elements, and those along the
bottom are shown by the blue line. Like ordinary wings,
the pressure at the top is negative, and lift occurs due to
the pressure difference between the top and bottom of
elements. The distance between the lines is indicative of
the normal force acting on each element.
= 0
= 0
= 15
= 15
= 30
= 30
Figure 9 C l and C d versus for varying aft element
deflection. g = 2%, Re = 700,000.
42

= 0
= 15
= 30
= 25
Figure 10 Pressure coefficient distribution for varying .
g = 2%, Re = 700,000, = -1°.
Figure 9 shows C l and C d versus for varying aft
element deflections. Deflecting the aft element
downwards increases C l substantially, although the
values of C l for = 15° and 30° are almost the same. The
gradients for each are nearly identical. The stall angle
for at high aft element deflections (15°) is lower
than that at = 0°. The drag coefficients are substantially
higher when the aft element is deflected at angles of 15°
and 30° compared to = 0°. It can be seen that
increasing the flap angle from = 15° to = 30°
produces almost no increase in lift and a significant
increase in the drag.
To show why the lift is so low for = 30°, Figure 10
compares the pressure distributions for three different
flap deflection angles for = -1°. At = 30°, the
pressure coefficients are smaller than for = 0° and 15°,
and the difference between the values at the upper and
lower surface is small. In particular, it can be seen that
the large aft element deflection of = 30° produces
virtually no pressure difference across it, as well as
virtually no pressure difference across the front element
as well.
Figure 11 C l and C d versus for varying aft element
deflection. g = 2%, Re = 300,000, pivot = 80%.
Figure 11 shows another example when the cambers are
varied. C l at ° is larger than that at ° over
whole rangebefore the flow stalls. C l at ° is
larger than that at ° when is small, but the slope
of C l -a curve for ° is lower than the other curves,
and C l at ° becomes larger beyond °This
means C l has a local maximum at a certain camber.
6.4 The influence of gap size
Figure 12 shows a comparison of wing performance for
different gap sizes. As can be seen in the figure, the
distributions of C l and C d for each gap are almost the
same and the only small difference is that g = 1% gives a
slightly higher C l at = 10°. In other tests in this study it
has been observed that the influence of gap size is very
small.
43

Figure 14 C l and C d versus for varying gap size for
negative gaps, = 15°, pivot = 90%, Re = 700,000.
Figure 12 C l and C d versus for varying gap size for = 10°
and Re = 700,000.
Figure 14 shows lift and drag coefficient result when the
gap is negative, i.e. the main and aft elements are
partially overlapping. Note that this is only physically
possible for non-zero values of . While C d at g = 0 and
at g = -2% are amost identical, C l at g = 0 is larger than
that for g = -2% near the angle where the flow stalls.
Thus negative gaps do not seem to be beneficial.
6.5 The influence of aft element pivot point position
Figure 13 C l and C d versus for varying gap size for =
10°, Re = 300,000.
Figure 13 shows the effect of varying gap size at a
smaller Reynolds number. While the flow stalls at
° in Figure 12, C l begins to decrease at a smaller
in Figure 13. Thus a smaller Reynolds number causes
stall to occur at a smaller
Figure 15 C l and C d versus for varying pivot points for =
10°, g = 0%, Re = 700,000.
The pivot point is the location about which the aft
element rotates as shown in Figure 6. It is defined by the
distance aft of the leading edge of the main wing in terms
of % main element chord. Thus for example, when the
pivot point is 0%, it means that the pivot point is at the
leading edge of the main element, and 100% means that
44

pivot point is at the trailing edge of the main element.
Figure 15 shows results for various pivot points. g and
Re are all constant. It can be seen that C l at pivot = 40%
is slightly larger than other results, but otherwise there is
little obvious effect, even for the very large changes of
position from 40% to 90%.
Figure 17 shows a comparison of the C p variation when
Re is varied. For Re = 800,000, the pressure coefficient
curve is smooth. On the other hand, when Re is 200,000,
the pressure coefficient has a step-like distribution at
x/c = 0.18. The authors speculate that this step-like
distribution at this low Reynolds number may be a
laminar separation bubble. Furthermore, there may also
be a laminar separation bubble on the pressure surface at
x/c = 0.4. When the Reynolds number is larger, flow
separation does not occur in these results and the curves
are smooth. The pressure coefficient distributions on the
aft elements are almost the same for both Reynolds
numbers.
Figure 16 C l and C d versus for varying pivot points for =
10°, g = 2%, Re = 300,000.
Figure 16 shows the results when Re is smaller
(300,000). It can be seen that C l and C d are almost
identical, but that the flow separates at smaller when
the pivot point is 40% compared to 90%.
Figure 18 C l and C d versus Re. = 10°, = 2°, g = 1%.
6.6 The influence of Reynolds number
Figure 19 C l and C d versus Re. = 10°, = 10°, g = 1%
Figure 17 Pressure coefficient variation over the elements
for two different Re, for = 10°, g = 1%, = 2°.
Figure 18 and Figure 19 show C l and C d versus Reynolds
number. In many cases in this study, C l and C d do not
change significantly when the Reynolds number is varied
as shown in Figure 18. On the other hand, when the aft
45

element deflection and the angle of attack are large, C l
and C d change with Re as indicated in Figure 19.
Generally, C d increases with increase in C l as shown in
Figure 11 and Figure 16.But, in Figure 19, C d becomes
small when C l becomes large for this value of
To analyse why this reduction in C d occurs, the
pressure coefficients corresponding to Re = 400,000,
500,000 and 600,000 in Figure 19 are shown in Figure
20.
has the same effect as roughening the surface of an
aerofoil, and causes early transition, and thus the
emulation of higher Reynolds number behaviour. Even
the turbulence from a small upstream rod [14] has been
found to be sufficient to substantially alter the separation
behaviour of bluff bodies.
For testing aerofoils like those in the present tests,
turbulence in the onset flow will affect the location of
transition, as will the presence of the pressure gradients
on the top and bottom surfaces. It is expected that the
tested wings will simulate the behaviour of C-class
catamarans, which operate at similar Reynolds Numbers.
It is not known with certainty in the present tests where
transition was occurring, and whether or not bypass
transition was present.
The effect of the transition location on the behaviour of
wings can be investigated by tripping the boundary layer
artificially using a wire or similar, and it is planned to
carry out such tests in the future.
Re = 0.4 x 10 6
Figure 20 Pressure coefficient variation over the elements
for three different values of Re. = 10°, = 10°, g = 1%.
The pressure coefficient over the top of the wing at Re =
400,000 is almost constant downstream of the leading
edge. This indicates that the flow has separated at the
leading edge of the main element for this small Reynolds
number. The wing is stalled over its entire surface, and
thus C l becomes small and C d is large. When the
Reynolds number is larger (Re = 600,000), no separation,
or only a very small amount of flow separation occurs,
and C d is again small. The pressure coefficient near the
leading edge of the main element is significantly large,
and C l is large. When the angle of attack and the aft
element deflection are large, it is necessary to operate the
wing at Reynolds numbers in excess of 600,000 in order
to avoid separation and poor performance. Note that the
pressure distribution at Re = 600,000 may show small
laminar separation bubbles on the suction surfaces of
both the fore and aft elements near their leading edges in
the strongly adverse pressure gradient regions.
The results presented in the paper were obtained with the
two-element, 1-m chord wing mounted between the walls
of the 2.5-m wide duct near the outlet of the open jet.
The turbulence intensity of the flow was about 1%,
which is much lower than that in the atmosphere, but is
quite large compared to 0.1% in good quality
aeronautical wind tunnels. Turbulence in the onset flow
7. CONCLUSIONS
In this study, the influence of geometric factors such as
the aft element deflection, the gap and the aft element
pivot point have been investigated in a wind tunnel
study. From the study, the following conclusions can be
drawn.
C l and C d are significantly related to the aft element
deflection. As the aft element deflection is increased, C l
and C d also increase. It is the same aerodynamically as
adding camber to a single element wing. The addition of
aft element deflection increases C l , but too high a
deflection results in an increase in drag.
The influences of the gap size and the pivot point
location are found to be relatively small in this study.
The Reynolds number has a large influence on the
sensitivity of the wing to separation. As expected, a
smaller Reynolds number caused the flow to separate
more easily, especially with high aft element deflections
and high angles of attack.
Acknowledgements
We would like to recognise the following people who
have contributed to this project. We thank Joseph Nihotte
and James Turner for carrying out a large amount of the
wind tunnel testing, and for analysing some of the
results. We also thank David Le Pelley for his help and
advice in setting up and overseeing the wind tunnel
testing.
References
1. Blakeley, A.W., Flay, R.G.J. and Richards, P.J.,
Design and optimisation of multi-element wing sails
for multihull yachts, Proceedings of the 18th
Re = 0.6 x 10 6 46

Australasian Fluid Mechanics Conference 3rd - 7th
December 2012 Edited by P.A. Brandner and B.W.
Pearce. Published by the Australasian Fluid
Mechanics Society. ISBN: 978-0-646-58373-0.
2. Lanchester, F.W., Aerodynamics, Vol. 1, p 431, A.
Constable and Co., London, 1907.
3. Elkaim, G.H., “Autonomous Surface Vehicle Free-
Rotating Wingsail Section Design and Configuration
Analysis”. Journal of Aircraft, 2008. 45(6): p. 1835-
1852.
4. Marchaj, C.A., Sail Performance: Techniques to
Maximize Sail Power. 1996: McGraw-Hill.
5. Gonen, I. Omer Wing-Sail Ltd. Available from:
www.omerwingsail.com.
6. MacLane, D., “The Cogito Project- Design and
Develop of an International C-Class Catamaran and
Her Successful Challenge to Regain the Little
Americas Cup”. Marine Technology and SNAME
News, 2000. 37(4): p. 163-184.
7. Killing, S., “Alpha and Rocker - Two Design
Approaches that Led to the Successful Challenge for
the 2007 International C Class Catamaran
Championship”. Proceedings of the 19th
Chesapeake Sailing Yacht Symposium. 2009.
Annapolis, Maryland.
8. Drela, M. "Newton Solution of Coupled
Viscous/Inviscid Multielement Airfoil Flows,",
AIAA paper 90-1470, presented at the AIAA 21st
Fluid Dynamics, Plasma Dynamics and Lasers
Conference, Seattle Washington, June 1990.
9. Drela, M. and Youngren H., "A User's Guide to
MISES 2.53", MIT Computational Sciences
Laboratory, December 1998.
10. Peart, C.R.C., “Numerical Modeling of a wing-sail
Rigged Class in a Wind Gradient in Both Upwind
and Off the Wind Conditions”. MSc Thesis, School
of Engineering Science, University of Southampton,
UK, 2011.
11. Greenbird. Greenbird supported by ecotricity.
Available from: http://www.greenbird.co.uk/.
12. Sails, X.-W. X-Wing Sails - Wing Sails for Sunfish
and More. Available from:
http://www.solidwingsails.com/.
13. Abbott, I.H. and A.E. Von Doenhoff, Theory of
Wing Sections: Including a Summary of Airfoil Data.
1959: Dover Publications.
14. Gartshore, I.S. “Some effects of upstream turbulence
on the lift forces imposed on prismatic two
dimensional bodies”, J. Fluids Eng. 106(4), 418-
424, 1984.
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