Interesting mail about rigid sails, snipped from Xfoil yahoo group
- This way of making a Bertrand-style rig woud be interesting -
the forward part being either NACA 0012, or the other profilr
tested by Tom, and the rear part the NACA.
The problem would be the flap, which I guess would have
to be rigid - but it should sure be an effective rig!
Re: airfoils for wingsail
Posted by: "Tom Speer" me@... tspeer007
Date: Tue Oct 30, 2007 8:46 pm ((PDT))
> Could sombody advise me which airfoil will fit as wingsail to theTypically, most rigid wing rigs use two (or more) symmetrical sections
> sailing boat???
in tandem, forming a slotted flap. Symmetrical sections are required so
the boat can sail equally well on both tacks. A flap is required to
increase the maximum lift and achieve adequate acceleration from a given
wing area. Two elements, both using NACA 0012 sections, with the
forward (wing) element being 60% of the total chord and the aft (flap)
element being 40% of the total chord has proven to be very successful in
The simplest arrangement has arms extending from the leading edge of the
flap that are pinned at approximately 90% of the wing chord forming a
hinge that just allows the flap leading edge to clear the trailing edge
of the wing. When deflected, it forms a slotted flap. By varying the
hinge location, one can adjust the size of the slot
(http://www.tspeer.com/temp/ShapeFlapPivot.gif). Some rigid wing rigs
have an additional plain flap on the wing trailing edge that allows the
slotted flap to pass by, allowing more overlap, and adding some camber
to the wing to start turning the flow ahead of the flap itself.
The flap section can be comparatively thin because the leading edge
suction peak on the flap can be suppressed by closing down the slot
(http://www.tspeer.com/temp/VelocitiesFlapPivot.gif). It should have a
front-loaded pressure distribution to accomplish much (most?) of the
pressure recovery for the whole wing. The forward element will have
significant aft loading even though the section is symmetrical, because
of the influence of the flap. It can have a fairly mild adverse
pressure gradient at low to negative angles of attack for significant
runs of laminar flow. Since any sail operates at low Reynolds numbers
in light winds, it should be designed such that the transition point
(which will be via laminar separation bubble) moves smoothly forward as
angle of attack is increased
(http://www.tspeer.com/RigidRigs/40flap/S902fa20.htm). This will ensure
the laminar separation bubble is short and the section will not be
subject to premature leading edge stall.
Typical flap deflections are around 20 degrees for a high lift/drag
ratio at moderate to high lift coefficients. At high speeds (say, 30 kt
and up) the flap deflection may be nearly zero. Beware of negative flap
deflections - they cause negative pitching moments that can allow the
wing to self-trim at an angle of attack, leading to loss of control
unless the wing is provided with a positive means of controlling the
rotation (as opposed to a conventional sheet, which only restrains the
sail in one direction).
A rigid wing should be mass-balanced about the pivot axis to avoid
flutter. Wings can be particularly susceptible to flutter when tacking
because they can flop from side to side when the steady aerodynamic load
is removed. A wing that flutters causes very high drag that can cause
the craft to pitchpole backwards.
Unfortunately, this kind of multi-element section cannot be analyzed
with XFOIL. It requires a program like MSES. Or you can analyze the
wing and flap in 3D using a panel code such as CMARC