Dihedral on the M. 20 is set by using the dihedral gauge from
the plans. Tabs under the center section ribs set the dihedral
break in the middle of the wing.
Washout is added to the wing as part of the sheeting process. Note
the shim under the back of the wingtip.
Masking tape protects the
sheeting while the soft balsa
LE is sanded to shape.
the wing is to
ribs, where the
need to be
solution is to use
a dihedral gauge.
This might be
printed on the
plans. If so, cut it out and prop it against the center rib. After
the rib is aligned to the angle of the gauge, glue the rib in
Plans usually provide details for how to check the assembled
wing’s dihedral. This normally means instructions about how
to measure the space under the wingtip when the wing is on a
flat surface. In addition to improving flight stability, getting the
correct dihedral angle can affect how the wing fits the rest of
The M. 20’s wing has a dihedral break slightly inboard of
the landing gear. Designers who are facing this issue will often
frame each panel separately and join them later. To enable the
M. 20 wing to be built in just two panels, the ribs from the
break to the root have tabs to lift them to the correct height
from the board. This ensures that the angle of the dihedral
break will be on target. A paper gauge from the plans takes
care of the dihedral between the two completed wing panels
by setting the angles of the center ribs.
Builders also need to know about washout. Many aircraft
have an intentional twist designed into the wing. The direction
of the twist is important. Washout means that the wingtip is
twisted counterclockwise when the aircraft is viewed in profile
and heading to the left.
The purpose of washout is to control how the aircraft stalls.
Washout decreases the angle of attack of the wingtip, so that
it will stall later than the root. This helps prevent the dreaded
tip stall—a condition where the tip of one wing suddenly stops
lifting and the aircraft unexpectedly snap rolls.
Washout is added to open structures without
sheeting by lightly twisting the wing while shrinking
the covering. Normally, only a couple of degrees of
twisting are enough. The open structure is flexible
and easily adjusted.
If the wing will be sheeted, the builder needs
to exercise more care. When the wing is partially
sheeted, it will become more rigid. The washout
needs to be in place when the sheeting goes on.
The M. 20 is sheeted from the leading edge (LE)
to the main spar on top and bottom with 1/16-inch
balsa. To get the washout right, pin the finished wing
panel down flat to the board. Shim the wingtip with a bit of
1/8-inch balsa under the trailing edge.
Fit the upper sheeting panels then dampen their outer
surfaces with water so that they curl into place. Carpenter’s
glue allows their placement to be adjusted. After the upper
panels fully cure, the wing is much less flexible. The bottom
side can now be sheeted free from the board, setting the
washout in stone.
Many designs use a strip of soft balsa glued to the front of
the wing panel to form the rounded LE of the airfoil. The
M. 20 is designed this way. Now that all of the sheeting is in
place, glue a strip of 1/4-inch soft balsa to the slotted balsa LE
that holds the ribs in position.
Quite a bit of the soft balsa will be removed during the
shaping of the LE. An X-Acto knife or a razor plane works
well to knock off the big stuff. Move on to the sanding bar
with 60-grit sandpaper. It’s a good idea to protect the sheeting
with masking tape so that it doesn’t get sanded away. Sand the
soft balsa until it blends smoothly with the sheeting on the top
and bottom of the wing.
The soft balsa now has three surfaces: one that is tangent
to the top sheeting, a second that is tangent to the bottom
sheeting, and the third is the flat front face of the LE. The last
step is to carefully blend these three surfaces into a smooth
radius. This step is important—a nice, round LE improves the
30 Model Aviation SEP TEMBER 2015 www.ModelAviation.com