CB model designs Mark II Instructions for use

Mark II
Construction Manual
Kit No. CBMD-001
Copyright CB Model Designs 2009

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Thank you for purchasing a Boomer kit. We hope you will find building and flying the
Boomer one of the pleasurable things in life and achieve many hours of enjoyment from
it.
Every effort has gone into making the Boomer one of the lightest high performance P-30
models available. Each kit has been fabricated from select wood for a light airframe that
can meet the minimum 40 gram airframe weight for this event. The resulting model is
suitably strong for the intended purpose of soaring in light lift. It is an excellent transition
model for free flight competition and just plain fun sport flying .
Adhesive and general assembly process recommendations
Cyanoacrylate adhesive (CA in this instruction) is should be used with discretion and
caution. Assembly of the basic structures is best achieved with white glue or cellulose
cement (Duco, Testors, Ambroid, etc.). Use of these types of adhesive offer the best
chance for any minor adjustment during assembly, and minimizes the chance of adhering
the structure to the plan or building board by thin glue wicking through pin holes in the
wax paper covering the plan. This in turn reduces the chance of part damage when
removing a structure assembly from the plan when complete. All parts should be pre-
assembled dry before any bonding occurs to check for proper location and fit. Proceed
carefully and take your time with the assembly process. A Boomer can easily be
constructed and made ready to fly in one week of evening work sessions.
Wing Assembly
Separate all the wing parts from the laser cut sheets. Note that three of the W-1 ribs
include a spar notch near the trailing edge. These ribs are installed at the wing center and
to either side to support the short spar installed there that resists covering damage from
the wing installation bands. Keep these items separate to avoid installing them in the
wrong part of the wing during the build.
Start wing construction by building leading and trailing edge assemblies. The drawing
shows a 1/32 X 3/32 hard balsa cap bonded to the forward side of TE-1 and TE-2. This is
shown as a method of providing some additional stiffness for the trailing edges after they
are final shaped. Glue them onto the forward edges of TE-1 and TE-2 in one piece
segments. Once the glue has dried, sand the cap flush to the trailing edge thickness and
carefully cut the caps at the notches for the wing ribs.
Cover the construction drawing with wax or parchment paper to prevent parts from
sticking during assembly. The use of balsa strips 1/8” thick x ¼” or wider is suggested
as boundary control to the wing plan form outline. Pin this material at the edge line for
TE-1 to start. The wing tip panels have washout built in during construction, and TE-2
will not be held against the building board during assembly as done for TE-1. Use thicker
balsa to provide a boundary edge that still controls the plan form location of TE-2 while
it is rigged for washout during assembly. The LE-1 and LE-2 parts can be controlled for
plan form alignment in the same fashion as done for the trailing edge segments. Position

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LE-1, LE-2, TE-1 and TE-2 parts on the plan for alignment. Note that the LE-1 and LE-2
parts have ends cut at an angle for dihedral. Be sure to locate these parts with the angled
ends up at the dihedral joints. Pinning through these parts is not recommended; use scrap
balsa blocking to press the parts against the boundary strips, and pins through these to
secure part locations.
Assemble the tip gusset T-2 to the outboard end of TE-2 with the parts against the plan
for alignment. Use a piece of wood blocking pinned to the plan at the inboard line of the
T-1 rib to provide a reference surface for the forward and aft gussets. Glue the T-2
gusset against TE-2 with the T-1 side against the blocking. Similarly, bond one T-3
gusset against the aft side of LE-2 against the plan for alignment, and against the T-1
blocking. Bond the second T-3 gusset on top of the first to make up the 3/16” thickness
in this area for T-3.
At the inboard ends of TE-1 and LE-1, glue in the WG-1 parts to complete the
subassemblies for the wing edges.
Position spars WS-1 and WS-2 in place over the plan. Use scrap balsa blocking pinned
into the building board between the rib locations to maintain the spar position forward
and aft. Make sure the spar stand-off tabs are against the plan. Begin assembly by dry
fitting the W-1 ribs over the spars notches and into the TE-1 notches. Adjust the
position of the spar segments and trailing edges to achieve best fit of all parts to the
drawing. Dry prefit the tip panel next. Note that all the tip ribs have standoff tabs to
support the trailing edge of the rib in proper location for washout. TE-2 is shimmed up
until it is flush to the upper edge of each tip panel rib. Position a 1/16” shim on the
inboard side of W-4, another 3/32 shim in between W-4 and W-5, and a 1/8” shim at the
tip of TE-2 to support the trailing edge during assembly.
Prefit the T-1 rib next, with the TE-2 shimmed for washout as controlled by the ribs
already positioned. Cut off the end of WS-2 and LE-2 to match the joint as shown on the
plan and with T-3 gussets respectively. Position the T-1 rib for best fit forward and aft,
making sure there is sufficient material to clean up at the trailing edge when wing sanding
is performed.
When satisfied that all parts are aligned and fitting properly, carefully bond the wing
assembly together. Do not install the W-1 ribs at the wing center and polyhedral joints
yet; these will be installed when the dihedral is added. The W-1 ribs tails include a small
rectangular boss in the rib profile. This controls the position of the rib end for location
relative to the finished shape of the airfoil through the trailing edge. The boss should be
seated against the plan on the lower edge when installed. TE-2 is installed with the upper
side flush to the upper edge of ribs W-2 through W-5 and T-1 with the inboard end at the
dihedral joint touching the plan.
Install the 1/16” square stiffeners on either side of each rib at the trailing edge of the
wing. These are ¾” long, and installed to be a little above the rib profile as shown in

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View D-D on the drawing. Don’t trim the tapers yet-this is done as part of the wing
sanding process. The stiffeners add strength to keep the trailing edge from twisting as
well as prevent buckling of the thin rib sections from the wing tissue covering tension.
Add the 1/16” square turbulator spars next. At the dihedral and polyhedral joints, allow
½” overlap of the spars to allow the scarf joint to be made when the dihedral is added.
Again, no W-1 rib is installed yet in these locations.
Remove the tip panel from the drawing carefully. Using a fine sanding block, VERY
LIGHTLY sand a shallow bevel at the end of TE-2 and LE-2 where they join the inboard
wing panel and form the dihedral joint. The material cross section is so small there is
very little sanding required-you can easily overdo this. Note the color change at the end
of these parts to judge how much material is removed. Carefully rig the tip panel at the
dihedral dimension for the tip shown on the plan and inspect the resulting joint at TE-1
and TE-2 and also the LE-1 and LE-2 for little or no gap. If this area became over
sanded, a small filler of balsa can be added during the bonding process to correct.
Move the tip panel out of the way to install the W-1 rib onto the inboard wing panel still
on the board. Carefully tilt and slip into place under the excess turbulator spar ends and
onto the WS-1 end that is notched to fit. Check the alignment to LE-1 and TE-1.
Position the outboard rib angle gauge against WS-1 and W-1 to provide the tilt toward the
inboard side of the wing. Check alignment of the rib for being on center to the dihedral
joint, and bond in place; do not glue in the turbulator spars yet..
Place the tip panel back into position with dihedral as shown on the drawing. Plan form
align to the drawing using the blocking to trap the tip panel in place. Make sure the end
of WS-2 is correctly against the end of WS-1, and the LE and TE joints are touching.
Once satisfied, bond the inboard end of WS-2 to WS-1 and W-1; LE and TE joints. The
turbulator spars should still be left with the excess on either side of W-1, and nothing
glued into the notches in W-1 yet. Add the 1/16” square stiffeners at the W-1 rib trailing
edge at the dihedral joint.
The next step requires a very sharp, fine razor to perform. Do not attempt with a #11
knife blade. One half of a new double edge razor is recommended, although a extra fine
single edge razor is almost as good. The process requires light pressure and a very fine
blade is best used to perform the cut.
At the dihedral joint, the turbulator spars are overlapped on top of each other, with the
lower one positioned in the W-1 notch. Using the razor , slice down through the stack of
spar material at a diagonal similar to what is shown on the plan, centered on rib W-1.
The resulting scarf joint should slip into the W-1 notch, straddling the rib. Bond this in
place and repeat for the rest of the spars. Direction of the scarf joint is not important,
only that the joint has no gap and is a shallow angle as shown on the drawing to transfer
wing bending loads effectively. If a gap manifests in the joint after cutting, make a filler

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from thin balsa that can regain contact with both joint surfaces and bond in place. Cut the
excess fill material off after the glue has dried. Resist all urges to add additional doublers
at the dihedral joints. This wing has never had a failure at these joints the entire time this
design was being test flown and used for competition flying, including some very hard
crashes. Good joint quality is a must!
After both tip panels are rigged for dihedral, follow a similar procedure to rig one half of
the wing to the dihedral angle shown. Add the short 1/16” square support spars common
to the three W-1 ribs at the wing center; this completes the wing structural assembly.
Wing sanding and shaping
Begin shaping the wing by profiling the wing tip leading edge to blend into T-1 and
match the radius shown on the plan. Clean up the wing edge profile to remove any
mismatches and inconsistencies. Remove the stand-off tabs from the spars, tip panel ribs
and T-1.
Make a contoured sanding block to use for light sanding of the wing underside. On a
scrap balsa block 1” wide, carefully trace the bottom profile of rib W-1 using the cutout
in the scrap laser cut sheet as a template. Saw this profile into the block with a band saw,
scroll saw, etc. An alternate method is to make a series of balsa templates cut to the same
profile and assemble into a contoured block at least 1” wide. To the contoured surface
bond a strip of 120 grit sandpaper the same width as the block. On the opposite side bond
another strip that would be a flat surface to use for sanding all other areas of the wing.
Also useful for getting into small areas for shaping are small emery boards.
Start the wing sanding and shaping on the underside to start. Sand the lower surface of
the leading edges to be flush with the lower edges of the ribs. Then sand the underside of
the trailing edges to remove the step condition relative to the lower rib profiles and
continue the airfoil contour into the trailing edge. Care should be taken when shaping the
trailing edge not to over sand, as this area becomes fairly thin in cross section when
finished. The best way to sand these areas is to place the wing upside down against a flat
surface that allows one section of the trailing edge to be held flat against it while the
adjacent areas with dihedral are allowed to drape off either side. Typically this would be
the corner of a table or other raised working surface that will allow the other areas of the
wing to be in the clear as you restrain the one section of wing for shaping.
Use the flat side of your sanding block and carefully sand just the trailing edge material
until the step condition is removed at the forward edge and the rib contours are flush to
the surface of TE-1 or TE-2. Hold the wing carefully by lightly pressing against a few of
the ribs near the trailing edge with one hand to keep the wing structure from flexing
sideways as you proceed with sanding. You may wish to color the lower surface of the
trailing edge before you begin sanding to see how much material is being removed. The
aft edge of TE-1 or TE-2 should be left as stock thickness in the resulting sanded
contour. Repeat this process for each of the remaining trailing edge segments

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Once the leading and trailing edges have been blended flush to the rib contours, use the
contoured side of the sanding block to very lightly touch up any areas of the under camber
for flush condition at the lower edge of the rib and wing spar joints. Make sure the wing
spars are flush with the W-1 rib at each dihedral joint. Work the underside of T-1 to be
smooth and flush with WS-2 and TE-2 / T-2 contours. Leave T-1 as a rectangular cross
section for now.
Shape the upper surface of the wing by sanding down the LE-1 and LE-2 contours to be
flush with the rib edges for the upper side. At the dihedral joints the emery board may be
more useful to shape the areas adjacent to this area. Once the leading edges are blended
flush with both the upper and lower rib profiles, finish the leading edge radius to a fairly
sharp blend-approximately .06 radius or what is shown on the wing cross section on the
drawing. Use the flat side of the sanding block to carefully sand down the tops of the
turbulator spars to level these in to the contour of the wing-it doesn’t take much sanding
here. Blend the top of T-1 lightly to complete the necessary profiling to fair this in with
the wing contour. Now blend off the tops of the TE-1 and TE-2 segments until flush with
the tops of the ribs and any of the projecting 1/16” square stiffeners sanded flush as well.
Again, this is best performed by holding one section of wing at a time against the
workbench edge and carefully sanding just the top of the TE- or TE-2 to locally shape and
refine until the wing contour is represented. The trailing edge will be fairly thin-
approximately 5/32” thick at the forward side when all shaping is completed. Finish
profiling the 1/16” stiffeners at the ends of the ribs to the tapered condition shown on the
drawing, using a sharp razor and sanding to final plan view profile.
Using a small piece of 120 grit sandpaper for fine shaping all areas of the wing and
blending corners. The lower outboard edge of T-1 is radius blended to the upper
outboard edge to remove all boxiness to T-1 and provide a streamlined shape to the
wingtip. Continue to finish sand the wing with 220 and 320 grit paper until you are
satisfied with the smoothness of the result.
Finish the wing substructure with one coat of full strength nitrate dope, lightly sanded
with 320 grit when dry. Follow this with an additional full strength coat of dope to seal
the grain where the covering is to be adhered. This means the entire framework on the
lower surface of the wing needs to be edge sealed, as the tissue covering will be adhered
to each rib and spar to maintain the under cambered airfoil contour. Seal the entire
leading and trailing edge, top and bottom. Seal the T-1 rib completely and edge seal the
tops of the ribs at the dihedral joints. Any additional tissue color breaks based on the
wing structure should also be sealed to allow tissue attachment at these locations. If you
plan to cover with MicroLite or other heat shrink covering, follow the manufacturers
recommendations for preparing the surfaces for covering.
Horizontal stabilizer assembly
Construction of the horizontal stabilizer begins with assembly of the 1/32 ply S-2A
doublers with ribs S-2; make sure these are assembled as opposites. Use center rib S-1 as
a template to establish location of S-2A. Align the spar slots in S-1 and S-2 to index

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together and lightly trace against the forward edge of S-1 to mark the forward edge
location of S-2A. Bond S-2A in place to the line and flush to the rib bottom. Install the
filler SF-1 at the top of S-2A at this time as well. Bevel the forward edge after
installation to mate with the back surface of the S-7 closeout as required.
Use the same method to control the plan form boundary of the stab assembly as done for
the wing. Position the STE-1, SLE-1, ST-1 and SS-1 parts on the plan. Dry fit all the
ribs and adjust everything for best fit to the drawing. Use blocking to control the inboard
faces of S-2 that project forward of the S-7 closeout to maintain good alignment with the
fuselage stab platform. Glue in the gussets S-2 and S-3, and the ends of SS-1 into the
notch in ST-1 on either end of the stab.
Glue in the ribs starting with S-1 followed by S-2 on either side. On S-7, sand a bevel on
the bottom edge to allow this to sit flush to the building surface. Glue S-7 to the front
edge of the S-2A doublers and front edge of S-1. Glue in the remaining ribs.
When dry, remove from the plan and shape the leading and trailing edges to shape, etc.
Do a fine sanding to cleanup overall, then apply two coats of nitrate dope as done for the
wing. The stabilizer is covered before the filler SF-2 and plywood strike plate are
installed. Do not cover until you have done a pre-fit of the stabilizer with the stab
platform on the fuselage.
Vertical stabilizer assembly
The fin assembly is straightforward-just assemble over the drawing using the laser cut
parts. When dry, sand the F-1 segment to a tapered shape as shown in the plan view of
the fuselage, and round off the rest of the fin outline. Apply two coats of dope to the
outline parts after fine sanding and then cover with tissue provided or the covering
material of your choice.
Fuselage assembly
The fuselage assembly is largely complete as received. As noted previously, perform a
prefit of the stabilizer assembly on the platform. Check for freedom of movement to
allow the D/T pop-up to have complete range of motion required. There should not be a
snug fit to the platform-some very slight side to side freedom of movement should be
present when in the flying position. The stab covering overlap onto the inboard face of
S-2 will take up some of this gap so be mindful of this when fine tuning the fit, if
required. If the gap is tight, remove material from the ends of the stab platform using a
sanding block until the proper fit is achieved. If desired, the upper surface of the filler on
the stab platform can be sanded down to be flush with the stab contour at this time.
If you are planning to use a Gizmo-Geezer or Ikara P-30 front end, check the fit and make
any adjustments necessary to install these items. The drawback on these are the extra
weight they add, but they do offer ease of thrust adjustment.
Decide which timer you are going to use. The Ikara timer option is simply glued over the

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hole in the pylon at the location shown. If you plan to use one of the other two timer
options, a 1/32 ply adapter plate is provided for you to glue in place by aligning to the
hole in the pylon. Install the Ikara timer after finishing the fuselage.
Install the D/T trip wire assembly as shown on the drawing. Use the laser cut fillers to
fair in the tube and strengthen the joint. The filler can be tapered to a feather edge if
desired to reduce weight and offer some streamlining. Install the timer spring tension
adjustment tube as shown in a similar manner as done for the trip wire.
Do any fine sanding you feel is necessary on the fuselage and apply two coats of thinned
nitrate dope over all the exposed areas, including the entire stab platform. The inside of
the motor tube was sealed with dope before forming, so there is no need to do anything
on this area. Sand with 320 grit between coats to remove any fuzz that stands up in the
finish. Apply an additional coat to the pylon area to seal the grain a bit before applying
color.
It is suggested that the fuselage motor tube and tail cone be covered with tissue to
improve strength of the wood and improve durability. Cover these areas using two pieces
of rectangular tissue for the straight section, and one piece to wrap the tail cone area.
The rectangular pieces are installed so they overlap slightly at the approximate center line
of the motor tube, top and bottom. Use dope thinned 50% to attach tissue and minimize
chances of warping the motor tube. Prefit one piece against the side of the motor tube and
pylon, adjusting forward or aft until the aft edge of the tissue is at the aft end of the
straight section. Make sure the piece is also evenly distributed up and down to allow the
edges of the piece to be on or past the center line on top and bottom. Secure the tissue in
this position temporarily using a few brush strokes of dope applied near the front and rear
of the pylon area to tack in place. Now make a cut in the tissue at the front of the pylon
and another at the widest point in the pylon, just enough to allow the tissue to continue to
drape around the motor tube forward and aft of these cuts. This also allows the paper to
be tailored to match the pylon edge where it joins the motor tube by lightly rubbing the
tissue against this area with your fingertip until a crease appears in the tissue that has been
formed against this seam. This is your guideline to trim the pylon cutout in the tissue for
an accurate fit on the motor tube wrap. You can also use a soft pencil to lightly trace this
joint while pressing against the pylon edge seam (very light pressure please!).
Remove the tissue from the motor tube temporary dope dots using dope thinner or just
pulling away if not adhered too strongly. Cut away the tissue based on the pylon cutout
marking. You can then use this tailored piece of tissue as a template to cut the opposite
side covering.
Install the tissue by applying dope on the motor tube in the area just below the pylon
joint. Re-position the tissue based on your trimmed edge and adjust location while the
dope is still wet. Continue to add dope along the side of the motor tube, working forward
or aft from the starting point, pulling the tissue taught to keep wrinkles to a minimum.

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Don’t try to work down the upper or lower edges until the side area is adhered and as
smooth as you can get it.
Now go back and start working down the upper edges, starting with the pylon area again
and working forward and aft. Finish the bottom edge last. Work the tissue toward the
upper and lower edges to remove wrinkles as you go. Repeat the installation process for
the opposite side. It is recommended you do not use a razor against any tissue overlap in
an effort to create a neat butted tissue seam. It is too easy to accidentally cut along the
grain of the motor tube and create a flaw that then turns into a ridge during the finishing
process.
Apply the tail cone tissue covering by starting along the bottom center of the cone,
aligning the tissue edge to match the joint in the motor tube. Adjust this edge prior to
doping in place to get a neat overlap or butted joint. Any mismatch can be covered with a
contrasting tissue stripe or strip of base color tissue to resolve.
Apply dope and wrap the edges up around the cone. Slit the tissue at the front and rear
edges of the stab platform to tailor this area. The tissue can wrap up onto the stab
platform pylon and onto the underside of the platform. The areas forward and aft of the
platform just continue to wrap up the cone until the seam meets or overlaps at the top
center of the cone. Trim the aft end of the tissue to the profile of the tail cone filler and
overlap over this edge and seal with dope. Clean off any excess on the stab platform
edges and seal all seams with dope. Shrink the tissue covering using denatured alcohol
applied with toilet paper over the entire covered fuselage. DO NOT use water to shrink
the tissue, as this can soften and release the motor tube joint and result in a ridged seam
down the length of the fuselage when dry. There is a reinforcement strip applied down
the seam to help prevent this from occurring, but don’t take chances. Install the scrap
3/32 sheet tensioning stops at the front end for holding the nose block in place, and also
the one at the fuselage center line on the bottom of the tail cone to locate the D/T
tensioning bands installed for the D/T pop-up.
When the tissue is dry apply one coat of thinned nitrate dope (50/50) over the fuselage
and pylon seam to seal.
Install the D/T line guide tube at the tail cone end using CA and micro balloons to
strengthen the joint. The stop on the D/T line hits against the front end of this pretty hard
each flight, so make sure it can’t get knocked loose due to a weak glue joint.
Next, install the vertical stabilizer fin. Use a pin to peck some holes through the tail cone
tissue in the area the fin will be installed-this is defined by alignment marks on the tail
cone. Install the wing hold down dowels in the pylon. Use a piece of light straight wood
about 12” long strapped to the pylon top with rubber bands. Place the fuselage with the
wood plank installed onto a level working surface and block up the ends of the wood
plank evenly to make this parallel with the working surface. Block the fuselage sides so
it cannot shift when rigging the vertical fin. Now make a jig using blocks clamped to a

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drafting triangle or other tool that you can square to the working surface. You need to
provide a gap at the bottom such that this temporary jig can be positioned with one edge
over the fuselage to control the side of the fin for alignment. Set up so one edge is
perpendicular to the working surface-this edge will locate the side of the fin for vertical
location.
Using the alignment marks on the tail cone, adjust the location of the jig inboard and
outboard until it contacts the fin on an area ahead of the tapered section and positions the
fin onto the reference marks. When satisfied with the setup, apply cellulose cement to the
bottom of the fin and position to the reference marks for forward and aft location (the
forward most marks are the location of the fin leading edge), and against the jig vertical
edge for vertical alignment. Place a thick balsa stick against the fin on the opposite side
of the jig contact line and against the working surface in a lean-to position. Place a small
weight against the bottom of the stick to maintain light pressure against the side of the fin
and hold in place against the jig edge. Allow the setup to remain undisturbed until the
glue has dried.
After the fin has been bonded in place, add the tissue gussets at the base of the fin on
either side using dope to adhere.
Finish the fuselage by applying Design Master floral spray colors to fade in to your base
tissue colors or provide contrast. Typically the end of the tail cone and stab platform get
dusted with color, as well as the pylon. The nose area with the tensioning stops is usually
dusted with color that will also be used on the nose block assembly to match. Go easy on
the colors to avoid weight build up on the finish.
Nose block assembly
Assemble the provided nose block using the 5/32 thrust bearing housing tube as an
alignment pin. Assemble the 1/32 ply nose bearing doubler with segment ‘A’ using the
5/32” OD alum tube as an alignment pin. Use thin CA to bond, but remove the tube
temporarily to avoid bonding it in.
Re-install the alum tube into this assembly, and slide insert ‘B’ on next, with the grain
orientation 90 degrees to ‘A’. Wick thin CA into the joint-remove the tube or use the
glue sparingly if you don’t want the tube to be bonded in place. Slide ‘C’ onto the tube
and orient to ‘B’-the key slot in the bottom of ‘C’ should be in line with the slot in ‘B’
for insert ‘K’.
Continue to build up the nose block in this manner-you can add the alignment key ‘J’ to
help with the clocking of the remainder of the discs. The diameters of ‘E’ and ‘F’ are
made slightly larger than necessary to allow for some cleanup and fairing into the motor
tube outside diameter during the shaping step. Do not install the tensioning band posts
‘K’ until the nose block has been profiled to shape. Check fit of the assembly into the
motor tube. You will need to make a slot in the motor tube to capture the alignment key