LJ-7 - 2006/05

approximately 15 years ago. I produced composite kits for the
advanced modeler. The fuselages were molded fiberglass and the
wings were sheeted foam cores. I produced three kits at that time: an
Ultimate biplane, an Extra 300, and a Sukhoi.
One day I had the idea to make something from wood that could
be covered with iron-on material, thus be built by the average
modeler. My youngest son Jordan, who was roughly 10 years old at
the time, was good at drawing, so I enlisted his help.
I asked him to come up with a few drawings of what he
considered to be a sleek aircraft. I said I would turn his drawings into
working blueprints and we could build the aircraft; hence the birth of
the LJ-7.
As for the name LJ-7? My wife’s name is Linda and her middle
initial is J. I added the 7 for good luck.place. Set the fuselage assembly and bellypan
assembly aside until the wings are ready
to be installed.
Wing: Obtain foam wing cores from the
supplier listed at the end of this article or cut
them yourself. Lightly sand the cores’
surfaces, being careful not to change their
shape.
Build two servo boxes from 1/16 sheetbalsa
stock. Make the boxes approximately 1/8
inch taller than the thickness of the wing at
the forward side of the core. See the wing top
view for the location. Make the boxes the
proper size to accommodate the servos you
will be using. Install 1/4 x 3/8 aircraft plywood
servo-mounting rails in the boxes.
Cut a hole in the foam core at the location
indicated on the plans that is the size of the
boxes you just made. Cut the hole completely
through the wing core. Dry-fit the boxes in
each wing.
Mark the location of the servo-wire
channel on the side of the servo box using a
1/2-inch-diameter dowel rod with a piece of
1/2-inch-diameter brass tube fastened to it.
Remove the box and cut the hole for the servo
wire.
Glue the boxes into the wing-panel halves
using white glue. Be sure to let approximately
1/8 inch of the box protrude above the surface
of the wing core at the forward part of the
wing to allow it to be sanded flush with the
core’s surface after the glue sets. Be sure to
mount the servo boxes with the servo rails
facing the bottom surface of the wing.
Locate the outboard end of the aluminum
tube spar. Cut a hole in the wing that is
approximately 4 inches front to back and 2
inches wide, with its inboard side centered on
the end of the tube-spar socket. Save the foam
scrap to be reinstalled later.
Slide the tube-spar socket in place and
make a capture plate from 1/8 poplar plywood
that is 4 inches long and as deep as the core is
thick at its thickest point along the 4-inch
hole. Drill a hole in the capture plate the size
of the tube-spar socket and slip the plate into
the wing and over the tube-spar socket. Mark
the wing-surface contour on the plate top and
bottom surfaces, remove, and sand to shape.
Glue the tube-spar socket into the wing
and glue in the capture plate. Leave enough
tube-spar socket protruding past the root of
the wing core—approximately 3/16 inch—so it
will pass through the light-plywood root rib
later.
After the adhesive sets, glue the scrap
piece of foam from the 4-inch hole back into
the wing core. You will have to remove 1/8
inch from it to allow for the capture-platesocket and up against the core with the wingbolt
blocks touching the wing core. Line up
the outside of the root rib with the surface of
the wing core and press down firmly on the
root rib to indent the foam core with the
wing-bolt blocks.
Remove the root rib and remove the foam
from the core in the impressions left by the
wing-bolt blocks to a depth of 1/4 inch. I use
my Dremel with a router base for this.
Glue the root rib in place with epoxy or
white glue and adhere in place with masking
tape until dry. Using a sanding block, sand
the root rib flush with the surface of the foam
core. Repeat this process on the other wing
core.
Prepare the wing sheeting from 1/16 x 4 x
48 balsa. Edge-glue three sheets together
with the grain running parallel to the wing’s
LE and then glue one sheet with the grain
running parallel to the TE. Make two right
panels and two left panels.
Make two 1/8 x 3/8 balsa strips for the
wing TEs. Glue these strips to the inside TE
of both bottom wing skins. Set the bottom
wing skins in place in the bottom foam cradle
for each wing half and place the wing core on
top of them. Sand the strips to the slope of
the wing core while putting downward
pressure on the wing core with everything
sitting in the cradle.
You are ready to install the wing skins to
the cores. Purchase slow-cure laminating
resin for this purpose. Mix the resin
according to the directions. For extra wing
strength, purchase some carbon-fiber strips
or buy some fiberglass screening from a
hardware store. (Yes, the type used in window
screens.)
Cut the screen into 11/2-inch strips. Place
one strip directly over the wing spar from
roughly 1 inch in from the root and take it to
approximately 2 inches from the tip. Place
another roughly 16-inch-long strip 1 inch in
from the root end and ending near the servo
pocket. The aft edge of this strip will be
approximately 1 inch aft of the rear side of the
servo box.
Do this to the top and bottom of each
wing. Glue the strips in place with laminating
resin just prior to gluing the wing skins.
Apply the laminating resin to one of the
bottom skins of one core. (Do not apply the
resin to the foam core!) Place the glued-up
skin in its corresponding bottom wing cradle
and set the core on top of it. Glue up the top
skin and set it on top of the core.
Set the top cradle in place and weight the
entire assembly down with heavy objects. I
use concrete blocks—enough to cover the
entire wing panel. This whole process takes
place on your straight and true building board.
As soon as everything is in place, measure
from the wing LE and TE centerline to the
surface of the building board. The
measurements should be the same all over. If
they are not, shim the wing where needed to
make them the same. Repeat the entire
process for the other wing panel and for the
horizontal stabilizer/elevator piece.
After the laminating resin has set, cap the
LE with 1/2 balsa stock and carve/sand it to
shape. Cap the wingtips with 1/4 balsa stock
and sand to shape. Repeat this process on the
horizontal stabilizer/elevator.
Mark the aileron hinge line on both wing
panels. Put additional lines 1/4 inch on either
side of the hinge line; these are the actual cut
lines. It pays to mark all these lines on both
sides of the wing; then you can cut halfway
through from both sides. Slice between the
cut line and the hinge line, and block-sand
down to the actual cut line after the aileron
has been removed from the wing.
Cap the aileron LE and the wing TE with
1/4 balsa stock. Put a hinge line on the center
of the aileron LE and block-sand the LE to a
bevel from the rear side of the 1/4-inch cap
material at the aileron surface to the hinge
line. Do this from the top and the bottom to
the hinge line.
Install a piece of 7/16-inch-diameter dowel
into the aileron for installation of the control
horn. See the wing top view for the location
of the dowel.
Mounting the Wings: Mark a line on both
sides of the fuselage, in pencil, 21/2 inches
down from the thrustline (if the fuselage is
upside-down on the table, the line is 21/2
inches up from the table), and return the
fuselage upside-down to the building board.
Weight it in place to keep it from moving.
Slide the aluminum tube spar in place in the
fuselage.
Install your wing-mounting kit in bothplywood on the table that is approximately
the length of the spar and the width of the
wing panel. (This acts as a shim while setting
the wing panel.)
Place the wing panel in the top foam wing
cradle. Tape the cradle fast to the wing panel,
making sure the LE of the cradle is 1/2 inch
back from the capped LE so it is in its
original position with relationship to the foam
core.
Slide the wing panel onto the spar and
push it up close to the fuselage. Check to see
if the centerlines on the LE and TE match up
with the line drawn on the side of the
fuselage. If they don’t, adjust them with the
wing-mounting kit adjusters.
When in alignment, slide the wing away
roughly 1 inch and place a 1/4-inch-diameter
dowel center (available at most hardware
stores) in each of the fuselage wing-bolt
holes.
Press the wing panel against the dowel
centers to mark the location of the wing-bolt
holes in the panel. Remove the wing panel
and drill and tap the wing-bolt holes in the
wing for 1/4-20 nylon bolts. Repeat this
process for the other wing panel.
It’s time to complete the stabilizer/
elevator. Mark the hinge line on each
stabilizer half according to the plans. Mark an
additional line 1/4 inch on either side of the
hinge line on each half of the stabilizer.
These second lines are your cut lines to
separate the elevator from the stabilizer.
Cut the elevators from the stabilizers.
Place the stabilizer halves in their respective
bottom cradles with approximately 1/2 inch of
the root hanging over the cradle. Glue the
stabilizer halves together with slow-setting
epoxy. Tape across the joint until the epoxy
sets.
Cap the LE of the elevators with 1/4 balsa
stock. Mark a center hinge line on the LE and
taper the edge to a bevel from the surface of
the elevator to the hinge line. Cap the TE of
the stabilizer with 1/4 balsa stock that is a
continuous piece from one side of the
stabilizer to the other.
Completing the Fuselage: Install the landing
gear. It has to be split in half at the center.
Each half is slipped through its respective
side through the slot provided in the side of
the fuselage.
After inserting the gear, you must remove
additional aluminum so that the end of the
gear ends up against the center support beam
on the bottom of the LGP and the forward
part of the angled mark on the underside of
the gear just touches the outside of the
fuselage.
Drill the gear for two 8-32 socket-head
bolts each. Mark the bolt holes in each
landing-gear half and install blind nuts in the
LGP from the top side.
Glue all the top formers into their
respective positions and then install the 1/4-
inch square spars. Install the cockpit floor
made from 1/8 balsa, with the grain running
from side to side. Cover the turtledeck and
foredeck with 3/32 balsa sheet and then sand
to final shape.
It is suggested that you soak the
turtledeck and foredeck balsa sheeting in a
solution of ammonia and hot water before
trying to bend it. This technique makes the
wood more pliable.
Install the 1/8 balsa sheeting to the bottom
side of the aft fuselage and sand to final
shape. Install former Firewall Bottom
(FWB). It is necessary to bevel this former
where it meets the firewall. Install 1/8
sheeting between FWB and F1 and sand to
final shape. Sheet the belly pan with 1/8 balsa
and sand to final shape.
Glue in the 1/4 aircraft-plywood tail-wheel
block with epoxy. Sand to shape.
Mounting the Horizontal Stabilizer: With
the landing gear removed, install the wings
and bolt them in place. Tape the bottom wing
cradle to each wing. Shim the entire cradle
up off the table at least 2 inches on both sides
and weight it down so it can’t move. The
wings should now be at 0° incidence.
Draw a centerline on the horizontal
stabilizer’s TE. Set the stabilizer in its saddle.
Go to the forward end of the fuselage and
stick a pin in the center of the top 1/4-inch
square stringer at the firewall. This will be
your measuring reference point.
With the stabilizer in its saddle and set
to 0° incidence, go to the TE and measure
down to the table from the centerline you
drew. Measurements should be the same on
both sides.
Measure from the pin at the firewall to thesides; these measurements should be the
same. Sometimes it is necessary to trim the
saddle on one side or the other to get the
proper dimensions. When all the
measurements are the way they should be
and the incidence is zero, epoxy the
horizontal stabilizer in the saddle.
Tail Blocks: Make a sanding fixture from 1/4
balsa stock that is the shape of the balsa tail
blocks from front to back, and sheet it on
both sides with 1/16 balsa except where it
goes into the vertical stabilizer post slot.
Tack-glue this sheet in place and tackglue
the rough-cut tail blocks to it. This
allows you to shape the tail blocks without
having the vertical stabilizer in the way.
After shaping, remove the blocks and set
them aside for now.
Installing the Vertical Stabilizer: Dry-fit the
vertical stabilizer by slipping its post into the
slot between the fuselage sides. It should sit
down all the way to the 1/4 plywood tail-wheel
plate and be in full contact with the horizontal
stabilizer and former F6.
When you have accomplished this, epoxy
the vertical stabilizer in place. Epoxy it to the
fuselage sides, horizontal stabilizer, and former
F6. Remove any excess glue. Set a square on
the table and make sure the vertical stabilizer is
perpendicular to the table and the horizontal
stabilizer. Epoxy the tail blocks in place.
Final Assembly: You are ready to install all
hinges on all the control surfaces. I like to
install 4-40 fully threaded rods into the
dowels that I previously installed in the
control surfaces. I leave approximately 1
inch sticking above the surface and thread
in enough rod to almost cover the length of
the dowel. Then I attach a Du-Bro E/Z
Attach Horn Bracket for the clevis to affix
to.
Mount your engine with the 11/2° right
thrust and set your cowling. I mounted the
cowling with 4-40 nylon screws. I mounted
the belly pan with a 1/4-inch dowel through
the rear F4 former and glued it to former
F4B. Then I ran two 4-40 nylon bolts
through the fuselage and into the aircraft
plywood rails of the belly pan just forward
of the landing gear.
I used the new Goldberg steerable tailwheel
assembly. I also used the Du-Bro
light foam tires.
Engine: Power is provided by a SuperTigre
2300 with a Bison Pitts-type muffler. It all
fits well into the cowling. The exhaust
tubes from the Bison muffler were a perfect
length and needed no extension.
The aircraft was designed around the
old SuperTigre 3000, which was fitted with
a cast-aluminum J’Tec muffler. This engine
setup was considerably heavier than the
setup I’m currently using. It also had less
horsepower.
However, this created a problem when it
came to balancing. I had to use a 2200 mAh
battery pack strapped to the firewall to
make the aircraft balance properly. For this
reason I suggest that you keep everything
as far forward as possible.
Test Flight: Before test-flying the LJ-7 I
had run two tanks of fuel through the new
engine, so it was run on the rich side during
the test flights. I used the recommended 18
x 8 propeller and the engine turned a
respectable 7,800 rpm.
Takeoff was uneventful, with the model
tracking straight down the runway with only
a bit of pressure on right rudder. When up to
speed, a little gentle backpressure made the
airplane lift off gently and climb out with
ease.
Once at altitude the LJ-7 needed only
one click of up-elevator trim to fly handsoff.
After several level loops around the
circuit I decided to throw everything on high
rate and see what this model had. I pulled
the nose straight up and it climbed until I
decided to pull the throttle back. Power was
not a problem even with the rich setting.
I rolled the model to inverted flight, and
it didn’t seem to care if it was right-side up
or upside down. Little down-elevator was
required to make it fly level—just a little
pressure on the stick. Snap Rolls were crisp
and fast. I even tried a Lomcevak, and the
LJ-7 did it with no problem. I let go of all
the controls and it recovered nicely. Knifeedge
flight was held for as long as I wanted
it.
Landing went as smoothly as the flight.
On final I reduced throttle to approximately