CB model designs Yard Stick, CBMD-002 Construction Manual

Construction Manual

CB MODEL DESIGNS

Kit No. CBMD-002

www.cbmodeldesigns.com

Copyright CB Model Designs 2008

Thank you for purchasing a Yard Stick kit. We hope you will find building and flying the Yard
Stick one of the pleasurable things in life and achieve many hours of enjoyment from it.

Wing Construction

Cover the wing plan section with wax or parchment paper prior to construction to prevent parts
sticking to the drawing.

Begin wing construction by laminating the left and right hand leading edge bows from three
layers of 1/32 x 3/16 balsa. These parts are easily formed directly over the plan by pinning scrap
balsa blocks to the board and forming the strips against them to maintain the profile shown on
the drawing. Adjust blocks to control the profile to the outside edge on the drawing, and to the
mating joint for T-4. Achieve this using the stock with no glue applied. Once this setup is
established, use a thin coat of white glue between the three layers of balsa and reposition
against the blocks on the plan and against the building board surface. Make sure there is excess
material inboard of the polyhedral break-this will be trimmed back at the joint location once the
bow laminations have dried.

While the bow adhesive is drying, assemble the wing tip segments T-1, T-2, T-3 and T-4 directly
over the plan. Use white glue at the joints and position each segment to match the drawing. Use
pins placed against the sides of the parts to maintain position, as pin placed through the parts
may cause splitting or otherwise weaken the final product, which will be sanded to a tapered
section to match the airfoil contour.

While the tip trailing edge assembly is drying, position the laser cut trailing edge segments to the
plan. It is suggested to use a balsa strip pinned to the building board along the edge of the
trailing edge line on the plan to support the location of the trailing edge segments during
assembly. The same suggestion applies to the leading edge segment installation as well, as
both boundaries of the wing will be well controlled dimensionally if this is done.

When the tip bows are dried, remove any blocking near the polyhedral joint to allow access for
trim of the bow. Use the laser cut leading edge segment end as an angle gauge to mark the cut
line for the polyhedral joint onto the tip bow. Use a new, sharp razor to cleanly cut the tip bow
along this line. Locate the leading edge segments in position over the plan based on the
dihedral and polyhedral joint locations. All segments should touch at the bottom edge. Block the
leading edge segments against the strip stock used to control the boundary. Again, avoid pins
through both the leading and trailing edge segments, as these items get much thinner during the
finishing steps of the wing construction and will be weakened by pin holes.

When the tip trailing edge assembly is dry, remove from the plan. This section will be installed
off the plan based on the rib geometry and offsets for undercamber. Position the inboard and
outboard main spars over the plan using scrap balsa blocking pinned to the plan to maintain
vertical position as well as forward and aft position. Note-the lower side of the spar has standoff
tabs for proper offset for the rib undercamber. The wing leading edge bows should be left in
position on the drawing as they were during lamination and trim for the polyhedral joint. Fit the
tip trailing edge assembly in place over the top of the main spar, against the trail ing edge
segment at the polyhedral joint, and against the backside of the leading edge laminated bow.
Use a few scrap balsa blocks pinned at the edge of the tip outline on the drawing to control the
boundary of the trailing edge segment as you best fit all the joints. Do not glue anything yet.
Position wing ribs W-2, W-3 and W-4 over the main spar according to notches in the spar. Best

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fit wing ribs to the drawing by moving inboard and outboard slightly with the spar. Engage the tip
trailing edge assembly notch to the end of the spar. The main spar dihedral and polyhedral joint
ends will not touch un til the dihedral is rigged at assembly.

The tip trailing edge assembly should be off the building board from the polyhedral break
outboard. It is supported at the spar end by the removable tab on the spar lower edge. Ribs
W-3 and W-4 also have standoff tabs to ma intain the proper assembly rig, and the trailing edge
assembly should be set to assemble flush with the top edge of these ribs. This also applies in
general for all wing ribs-set the top edge to be flush at the upper surface of the trailing edges.
The reason for this will become apparent during the final sanding and shaping step. If
necessary, add scrap balsa shims under the tip trailing edge assembly to support during the
bonding assembly. Position the forward edge of T-4 to be centered on the aft surface of the
leading edge bow. This will create a slight undercamber and washout effect in the tip, which is
highly desirable.

Once the outer wing panel is prefitted and all parts positioned, bond in place with light application
of thin CA. Do not install the polyhedral joint rib W-1 yet. Dry assemble and best fit all the
inboard wing panel W-1 ribs to the drawing location. Do not install the center dihedral rib W-1
yet. When satisfied with the inboard wing panel dry prefit, bond the assembly with light
application of thin CA adhesive.

Install the 1/16 square turbulator spars in the outboard wing panels. Allow excess material for
scarf joint overlaps at the polyhedral joint location. Bond the spar ends to the wing tip and ribs.
Do not bond spars to rib W-2 yet-this will make it a little easier to assemble the polyhedral joint
and W-1 joint rib. Install the inboard panel turbulator spars, again allowing for material excess at
the dihedral and polyhedral scarf joint locations.

Carefully remove an outboard panel from the plan. Use the outboard joint rib gauge from the
laser cut sheet to install the joint rib W-1 against the end of the inboard wing panel still on the
board. Align carefully, but do not glue yet. Position the outboard wing panel onto the plan again,
making sure to pass the loose ends of the turbulator spars over the top of the joint rib W-1.
Carefully block the outboard panel tip off the building board to the 1-5/8" dimension shown on the
drawing. Position to obtain contact with the end of the main spars, trailing edge and leading
edge. Once the rig is established and all joint members are in contact, bond the trailing edge joint
first, then the main spar/rib joint, and finally the leading edge/rib joint, with the rib positioned at
the angle maintained by the gauge.

The turbulator spars should be overlapping at the spar notches in the joint rib W-1. Position one
on top of the other over the rib notch, and using a very sharp razor, cut both spars at the
approximate angle shown on the drawing, straddling the joint rib. The scarf joint should fall into
the notch, and the tapered edges in firm contact with each other. Apply thin CA adhesive to
solidify the joint to the rib and each spar. Repeat for the remaining turbulator spars and rib joints.
The same basic process applies to the dihedral rig of the wing center. Leave one side flat on the
building board and block the other side up to the dimension noted on the drawing, after
positioning the wing center joint rib W-1. This completes the basic wing assembly.

Sanding the Wing to final shape

Sand the lower surface of the trailing edge to blend into the lower contour of the wing as
determined from the wing ribs. If the ribs were installed flush to the upper surface of the trailing

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edge segments, you will note a small step at the end of each rib relative to the lower surface of the
trailing edges. This step needs to be sanded away to blend the lower surface of the trailing edges
to match the undercamber of the wing. Use a small sanding block and position the wing lightly into
your hand or against a working surface (wing upside down) and carefully work the lower surface of
the T.E. stock by removing the material uniformly across the forward edge, until it is evident that the
lower wing contour is smoothly blended to match the rib contours. At the leading edge, blend the
lower surface of the leading edge segments and tip bow to blend smoothly to the profile of the ribs.
Remove all the spar standoff tabs and very lightly sand any mismatches between the lower edge of
the spar and ribs to get a flush match at all joints.

Once the wing bottom is sanded, perform similar cleanup on the upper surface. Using a flat
sanding block, lightly sand the tops of the turbulator spars to be flush with the rib contours. Finish
shaping the upper surface of the leading edge segments and bows and blend into the bottom
contour with fairly sharp radius at the nose. Carry the leading edge radius into the wing tip and
blend as required to transition from leading edge to trailing edges. Finish sanding the upper
surfaces of the trailing edge segments and tip assembly to a tapered cross section, leaving a 1/32
thick edge across the entire trailing edge of the wing.

Perform final cleanup sanding with 220 & 400 grit paper, and apply two coats of nitrate dope to all
the lower wing surface edges of the ribs, spar, and boundary members of the wing frame, and also
the upper surface boundary and dihedral break rib edges.

Cover the wing with tissue provided starting with the bottom surface, making sure to bond to each
rib and spar, tissue grain in spanwise direction. Apply one coat of non-tautening nitrate dope
thinned 50% after shrinking tissue with alcohol or water mist.

Horizontal Stabilizer Construction

Begin assembly by placing scrap balsa strips along the leading and trailing edge lines of the
stabilizer drawing to control the boundary of the leading and trailing edge segments, as done for the
wing. Assemble the S-2 ribs with doubler ribs S-2a, making a left and right hand assembly.

Position the stabilizer spar over the drawing using scrap balsa blocking to maintain position, as
done for the wing. Position leading and trailing edge segments over the plan, using the balsa strips
to position against. Use no pins through the leading and trailing edge members. Position center rib
S-1 onto the spar and best fit the spar and rib location to the drawing. Position the S-2 rib
assemblies on the drawing and spar, with the S-2a doublers facing inboard to create the bonding
edge for the S-8 closeout. Use scrap balsa blocking against the S-2 ribs to control the location of
the ribs to the drawing. Use S-8 to check the fit of this part into position against the front of S-1 and
S-2a doublers. Sand a 45 degree bevel on the lower edge to allow this part to sit flush to the
building board surface. Once satisfied with fit of these components, bond in place to the spar,
leading and trailing edge segments, and S-8 to the rib ends. Install the remaining stabilizer ribs S­3 through S-6. Install the S-9 stiffeners to the top of the spar against the inboard side of S-6, and to
the trailing edge. This item prevents warp of the S-6 rib during covering and provides stability for
the vertical stabilizer fins attached to either end of the horizontal stabilizer.

Shape the stabilizer by blending the leading and trailing edge to airfoil contour, with a fairly sharp
leading edge radius and a trailing edge thickness of 1/32 as done for the wing. Blend the upper
edge of S-8 flush with S-1 and S-2 ribs. Sand with 220 and 400 grit sandpaper.

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