3. The EZB: The First Step Toward Serious Building And Flying
From Ron Williams' Building and Flying Indoor Model Airplanes
The
EZB is basically a "novice"-type design in that it's a simple balsa
SRPSM. Its maximum wing size is 3" x 18" (chord by projected or
tip-to-tip span) and the propeller must be entirely of wood except for
beginner events where plastic is permissible. Other restrictions have been left
up to contest directors, but this will probably change as soon as the rules
firm up. This chapter and chapter 4 explore EZBs from the beginner's to
advanced levels, showing the wide variety possible within very simple rules. A
beginner's EZB can be expected to fly for three or four minutes with reasonable
attention to detail and light weight and with a little help from more
experienced flyers at the site. Advanced EZBs have flown for more than 20
minutes in high-ceilinged sites.
The
EZB's boom is offset, which "builds in" its turn. The rudder on # 1
is below the stabilizer so it can be more easily attached, using the boom as
its upper edge. The stabilizer tilt helps the plane to turn and the offset wing
(larger on the left side) keeps the turn fiat--that is, without a bank. The
wing is attached with posts to make it easier to handle and adjust, giving room
for fumbling fingers.
The
four variations of EZB plans presented here and in the next chapter show
increasing degrees of design sophistication. EZB #1 (figures 3-1A and 3-1B) is
simple and straightforward, EZB #2 explores geodetic flying-surface
construction, EZB #3 employs round tips for the flying surfaces, and EZB #4 is
a biplane that requires very careful alignment.
The
process described for building the first EZB begins with laying out drawings on
which to build the flying surfaces, then goes on to stripping balsa for spars
and ribs; building the flying surfaces (wing, stabilizer and rudder) and covering
them; building the propeller; bending wire parts; selecting and preparing the
motor stick; making washers, wing posts and paper tubes; assembling the
airframe; and, finally, test flying. Figure 3-2 illustrates some of the tools
and supplies used for building the EZB.
Occasionally
a reference will be made to a figure in another chapter; this will refer to a
more or less sophisticated version of the technique described.
The
first step in building the EZB is to
lay out the parts of the flying surfaces on a suitable fiat building board.
One-quarter-inch foam-core board is recommended; however, 1/2" Homasote,
¼" Upson board or a soft pine board are all satisfactory. An 18" ×
24" board is sufficient for the EZB and Pennyplanes. It must be flat.
Lay
out the stabilizer and rudder to the dimensions shown on figure 3-1A. Note
that when laying out the wing, the two inboard panels are different lengths.
One is 5" long while the other is 4-1/2". Each is divided up into
three equal spaces by the ribs. The 4-1/2" panel is equally divided into
three 1%" spaces. Often, as with the 5" panel, a space will have to
be divided up into equal sections that do not divide readily into the size of
the panel. The parallel line technique, as shown in figure 3-3, is used to
divide a space in such a circumstance.
The
layout for the flying surfaces can be done right on the building board;
however, it can also be done on a piece of drafting or layout paper taped to
the board.
The
next step is to lay out and make a
template for the wing and stabilizer ribs. For the EZB #1, these ribs are cut
in the shape of the arc of a circle. This ,curved rib design creates an airfoil
that provides lift. The arc of the airfoil has a height that is 4% of the wing
chord, or about 1/8" (0.12"). Make the arc by drawing a circle with a
9-1/2" radius and then marking off 3" along the circle. This 3"
segment of circle can be laid out on a piece of stiff cardboard, 1/32" plywood
or thin aluminum. Cut the template carefully and sand the curved edge, making
it smooth and even.
Mark
off a 3¼" length of l/32" "C"-grain balsa.
Lay the rib template at the top of the sheet and begin cutting and moving the
template down, stripping off ribs about 1/32" to 1/16" deep,
using the broken double-edged blade (figures 4-6, 4-7) for the cutting (figure
3-4). Knife blades such as the X-Acto are unsatisfactory because they are too
thick and crush the balsa as well as cut it.
Stripping
balsa for ribs or spars (spars are
the strips that run the length of the wing and stabilizer) is often a
difficult process for the beginner. There are various semimechanical devices
commercially available that are designed to assist in stripping, such as the
Harlan stripper and the Jones stripper (see Appendix 1). These devices work
well but; often require a technique as complicated as the "freehand"
technique described here. The mechanical
stripper gives the advantage of precision measurement of cuts, especially the
Harlan cutter. This cutter uses a micrometer adjustment so that the balsa can
be cut precisely.
The freehand technique depends on the eye for measurement; it can be surprisingly
accurate with normal eyesight and practice. Factors affecting the quality of
the cut are the manner in which the blade is held, the surface on which the cut
is made and the edge which guides the blade. As noted before, a stainless steel
straightedge is hard to beat. The usual edge on an artist's T-square is
1-1/2" wide by 3/64" thick. It is heavy enough to stay in place well,
and has a fiat, smooth surface that "adheres" well to the balsa. Use
it inverted (T-head up) when cutting. For "freehand" cutting the
blade should be held as shown in figure 3-5.
Note
that the blade is held perpendicular to the cutting surface, and that all fingertips
touch the table and the thumb rides the rule. This stabilizes the hand and
blade so that the angle of cut is continuous for its full length. The beauty of
developing the skill of hand-stripping is that the blade can be held at angles
different than 90° for bevels when required.
When
stripping balsa, be sure to have plenty of elbow room and a clean surface to
work on. It is wise to make a practice stroke or two before stripping.
Stripping accurately to size takes practice but can be done. Most business
forms for typewritten information are drawn accurately to a tolerance of .001
by draftsmen; the skills are not beyond human abilities.
Narrower pieces of
balsa (3/4" or less) require the straightedge to be fully supported.
Lay a piece of wood of the same thickness
alongside the piece being stripped so that the straightedge cannot tip during
the cut.
Back
to EZB #1. Refer to the three-view for wood sizes (figure 3-1A) and strip the
wood required for the wing and stabilizer spars. Mark (colored marker spot) or
set aside the spars so that they will not be confused with other parts or with
scraps when they're ready for use. Cut a strip for the rudder outline now, too.
Note that the sizes shown in parentheses are for a lighter plane.
Tape
the layout--drawing of the wing and
tail surfaces to the building board and covet- it with waxed paper or a
transparent food-wrap material like Saran Wrap. Some builders claim that waxed
paper affects glue joints; they prefer Saran Wrap. I have had no bad
experiences with waxed paper. After the layout is covered, pin two long,
straight strips of balsa (1/16" to 1/8" thick by 18" to 20"
long) to the layout, next to the wing pattern, so that their inside edges are
3" apart, and in a position so that the leading and trailing edge spars
will fit next to and inside these two guide strips. Place the spars against
these strips and then add small blocks on the inside of the spars to hold them
in place against the guide strips. Do not put blocks where the ribs meet the
spars. Check the outside dimension of the ][aid-out wing to make sure it is
less than 3" (the maximum chord allowed for EZB). See figure 3-7.
Don't
pin next to the spars or into them; always pin into a block separate from the spar.
This might not be so critical on the wood used for a heavy SRPSM, but you
should get used to the technique for later use with smaller wood sizes. Holding
the wood in place with pins tends to crush it and weaken it.
Begin cutting the sliced ribs to lengths to fit between the front and rear spars.
Note that the tip ribs must be cut on an angle to meet the spars properly. Put
a small amount of glue on the ends of the leftmost rib if you are right-handed,
and vice versa if left-handed.. Touch the glued ends of the rib to the front
and rear spar in the place it is to go, then remove it. Go on down the wing
doing the same with each rib, laying the rib over the spars after touching it
to its place. Go back to the first rib, put another small dot of glue on each
end and put it in place. This double gluing will make the strongest possible
joint. The rib should stay in place without any support. Use only enough glue
to cover the end of the rib (figure 3-8).
A
note about glues: The EZB can be
assembled with aliphatic resins (water soluble; trade names: Titebond, Wilhold
Aliphatic Resin) or model cements (acetone soluble; trade names: Ambroid,
Duco, Pactra, Aerolite, Micro-X). Indoor planes may often be damaged in such a
way that repair requires dissolving a glue joint. The above-mentioned glues
lend themselves to easy loosening of a joint with the appropriate solvent
(water or acetone). A similar situation will often arise when it comes to
adjustment of the air-frame where a glue joint will have to be loosened.
Alpha-cyanoacrylate
glues (insoluble in this writer's experience, trade names: Hot Stuff, Krazy
Glue, Perma-Bond, Zap) do not lend themselves to this type of loosening of glue
joints. Though very handy for quick repairs, the joint repaired with these latter
glues will be difficult, if not impossible, to alter later if required.
Therefore, the cyanoacrylates should be used with some reserve; the time they
save may not be worth it in the long run. The other glues are only used in
small amounts, usually require minimal drying and setting times and, as a consequence,
are preferred for indoor building except for a few instances that will be
discussed later.
If
the rib tends to fall over, prop it with a pin or block of balsa; if it is too
short, cut another rib to fit properly. The spar should never have to be pushed
in to close a gap for a rib. Keep the spars as straight as possible. When
installing the outside (tip) ribs, tilt them away from the center of the wing
or stab about 1/16" to 1/8". This will help to resist the pull of the
tissue when the wing is covered.
Assemble
the stabilizer in the same manner as the wing. The rudder outline is made with
straight stock. Remove the assemblies from the plan with care, unpinning all of
the blocks and untaping the protective waxed paper from the board. Lift the
waxed paper and carefully peel it away from each glue joint. If there is any
excess glue at any joints (there shouldn't be), trim it away very carefully. If
any joints are not flush, reglue them, lining up the ribs carefully with the
spars. Trim the excess spar lengths flush with the tip ribs.
Cover
the flying surfaces of the EZB with condenser paper. The paper must be
preshrunk before covering. The way to do this is to wet the paper which has
been laid loosely over the ironing board. Wet one side and let it dry. Don't
touch it until it's dry. Use a sprayer such as the type that comes with
household spray cleaners. When the paper is dry, turn it over and smooth it out
with your hands. Wet it again and let it dry. Repeat the process twice again.
Then, when the paper has been dried for the sixth time, iron it with a steam
iron at medium heat or between two sheets of moistened newsprint paper. It can
also be ironed under a pressing cloth or directly after the sixth spraying,
with extreme care. The object is to avoid wrinkles and to make it smooth.
Cut
the paper about 1/4" to 1/2" over size (figure 3-9), all around the
stabilizer and rudder. A sharp pair of barber's shears are the ideal scissors
for cutting loose condenser paper or tissue.
With
a fine brush, such as a #3 lettering brush (Grumbacher series 177, Langnickel
series 671), lay a coat of thin shellac (60% shellac, 40% alcohol), Micro-X
condenser paper cement, thinned contact cement, sugar water (1 teaspoon to 4
ounces of water) or saliva (it works fine and is free) along one spar of the
stabilizer. (If water or saliva is used, wet the underside of the spar as well
to compensate for warping.) Lay the paper over the stabilizer and rub it onto
the glue-coated spar, so that it adheres smoothly and uniformly. Lay it aside
for a few minutes and do the same with the rudder. Go back to the stabilizer
and lift the tissue from the unglued spars and tip ribs, folding it loosely
back over the glued spar. Coat the other spar and tip ribs with glue (figure
3-10A) and smooth the paper onto the whole outline, pulling it taut (but not
too taut--see figure 3-10B), making sure it's all glued down and wrinkle-free.
To reglue, peel the paper up carefully (this may require thinners--alcohol for
shellac or Micro-X cement, acetone for contact cement--to loosen the paper).
Cover the rudder in the same way, on one side only. After the glue has dried,
cut the excess paper away very carefully with a razor blade all around the edge
(figure 3-11).
Before
covering the wing, the tips must be
moved up into their dihedral angle. Cut the front and rear spars (leading edge,
or L.E., and trailing edge, or T.E.) as shown in figure 3-12.
Make
a small jig with cardboard or balsa (figure 3-13) to, support each wing tip at
3" above the tabletop.
After
the tip is bent up into position and glued (just a bit of glue in the razor
cut), the joint will require trimming as shown in figure 3-14.
Cover
the center section of the wing in the same manner as the stabilizer, trimming
the excess paper when the paper cement is dry. Then cover the tips. The
critical part of covering the tips is cutting the condenser paper to fit the
dihedral break. Begin by cutting a nice, clean, straight edge on the paper. Lay
it into position so that the paper overlaps the ribs at the front and rear
spars. Note that the paper will overlap the center section by a considerable
margin (up to ¼” at the midpoint of the wing chord (figure 3-15).
On
a piece of lightweight bond paper, lay out a straight line with three points
along it 1-1/2" apart. At the center point, perpendicular to the
line, measure off a length equal to the overlap of the covering paper at the
dihedral joint shown in figure 3-15. Sketch in a smooth arc on the bond paper
from the outer points to the top of the perpendicular length of overlap (figure
3-16). Cut along the arc with a pair of scissors and check the fit by laying
the bond paper over the joint at the dihedral. Trim the arc to get a clean,
smooth fit. Trace the arc along the condenser paper edge. Cut along the arc
drawn on the condenser Paper's edge with the scissors. Test the paper for fit
on the wing and trim as necessary. Begin cementing the tip covering along the
rib at the break, on top of the center-section covering. After the paper is
smoothed out and the cement is dry, peel the paper back on the rest of the tip
and proceed to cement; pull it taut and trim it as you did the other surfaces.
Lay the flying surfaces aside in a safe place (model box) and proceed with
the propeller and motor stick.
There
are basically two methods for forming a wooden propeller. We will explore the
first and less accurate method of forming on a cylindrical surface for the
first EZB. We will cover the alternative, the helical block method, for EZB #2.
Cut the propeller blanks as shown on the plan from a piece of .025 balsa; 1/32" balsa (.031") can be sanded down for this. The
balsa should be "C" grain and quite light. The two propeller blades
should always be cut from the same piece of balsa and from similar grain in the
same piece so that they will not be too different.
Next,
cut a sheet of bond or tissue paper 1%" wide and lay it onto a surface to
which masking tape will not readily stick. Lay 4" strips of masking tape
across the sheet of paper and cut the paper with the single-edged razor blade
between the tape strips (figure 3-17). You will use these paper/tape strips
later.
Now
find a cylinder 4-1/2" to 5" in diameter (about half the diameter of
the propeller to be built) and make sure its surface is clean and smooth. A
quart (or larger) glass soft-drink bottle or the metal cans English crackers
come in work fine.
Mix
about 6 to 8 ounces of hot water with 2 tablespoonsful of ammonia in a shallow
bowl about 6" to 8" wide. Soak the propeller blades in this solution
for three to five minutes. While they are soaking, set up a protractor or
adjustable triangle, and draw two lines 1-1/2" apart on the cylindrical
surface (bottle) so that they are tilted at 15° off the vertical
(figure 3-18). A grease crayon/pencil will be useful for this. The Stabilo
Pencil Company's "All" pencil is ideal for this purpose and comes in
many colors.
Remove
the propeller blades from the ammonia solution and rinse them in cold running
water. [,ay the blades, separately, on the bottle at an angle parallel with the
slanted lines drawn on the bottle's surface. Hold the blades to the bottle with
the paper-surfaced masking tape strips as in figure 3-18(3). Wrap the bottle
for the length of the propeller blade with three or four layers of cotton strip
(old bed sheet), holding this cotton binding in place with a few short strips
of masking tape. Place the bottle in a warm, dry place overnight or in an oven
set at 250° for no more than 15 minutes. Remove the binding
and the two blades.
The propeller spar (the
piece that connects the two blades) is made from a piece of hard 1/8"-square
balsa 3" long. Measure and mark off the spar into three equal
spaces and mark the center point all along one side. Cut the face of the two
opposite front edges away, back to a 45° angle as shown in figure
3-19. Drill a hole in the center of the spar with a #78 drill as shown in the
illustration.
It
is accepted as a convention that a propeller pulls a plane forward (or pushes
it) by spinning clockwise when viewed from the rear. Exceptions are always so
noted and occur in very specific situations (such as when two propellers are
used on two motors so that they turn in opposite directions).
The propeller blades should
be sanded with #320 wet-or-dry paper (used dry) so that they are tapered to the
edges from the centerline of the blade and toward the tip. Attach the blades to
the spar with model cement or cyanoacrylate cement (Zap, Hot Stuff). The
centerlines of the propeller blades should coincide and their angles from the
center of the hub should be the same.
A
small jig can be built from 1/16"-thick balsa to align them (figure
3-20). Assemble the jig carefully, aligning the supports (labeled "A"
and "B") using the lines marked on the parts and the dimensions
shown. When the jig is dry, lay the prop spar in the notch on the angular
support "A." When a short piece of 0.015" wire is pushed through
the hole in the spar, it should line up against the edge of support
"B." Tape the wire firmly in place with a short piece of ¼"-wide
masking tape. Be sure, at this point, that the faces of the spar have been carved at a 45° angle. A bit of
wax on the faces of the supports will help to prevent the propeller blades from
sticking to them when the blades are glued to the spar. Use a crayon or a
cotton swab dipped in benzine, wiped on sealing wax or beeswax, and then on the
face of the support. When viewed along its length, the propeller blades should
be lined up (figure 3-20).
Elsewhere
in this book, I will describe the use of many different jigs. Their design and
careful building are at the heart of the assembly of nearly all aircraft. They
are used in varying configurations and degrees of complexity for aligning the
components of the airframe. The way the jig holds these parts will, in turn, determine
the way the plane will fly. Jigs should be kept simple and be easy to
use.
Balance the propeller after
removing it from the alignment jig. Pass a length of 0.015" music wire
through the hole in the hub, and either holding the wire by hand or taping one
end of it so that it overhangs a table edge, begin balancing the propeller. If the blades
are in balance, the propeller will always return to a horizontal position after
being spun on the music-wire shaft. If it is not in balance, sand material away
(carefully) from the blade that turns down until the propeller stops in a
horizontal position. Let me reiterate that the blades should always be cut from
the same piece of balsa and from similar grain in the same piece so that they
will not be too different. When a double bearing is used, as described in
figures 4-22 and 5-15, the propeller can be balanced on its hook shaft in the
bearing.
Bend the prop hook after the propeller is balanced. Using needle-nosed pliers, ]bend a hook as shown in
figure 3-21. Any of the hooks is suitable; each finds a proponent among indoor
builders. Beginning at one end of a long piece of 0.015" wire, bend a
hook. Put a tag of masking tape on the opposite free end to make it visible; it
can be quite dangerous flipping around, and can cause a severe eye injury if it
hits your eye. If you have trouble bending the first hook, cut it off and try
again until a satisfactory hook is made.
It
is one thing to look at the drawing of a hook shape and another to bend it.
Figure 3-22 illustrates the steps
involved in bending each of the three hooks shown above. A fourth type of hook is
often employed for flying scale models, but can be used for other types as well
(see figure 5-1).
The
flat side of the pliers is used for the right-angle bend on the hooks (see
note, figure 3-2). After the general shape of the hook is bent, the ends are
trimmed. Sharp edges should be filed so that they won't cut the rubber. The
hook should be given its final twists and adjustments so that the hook is the
shape shown and so that the bends are all in one plane: the hook should lie
fiat on a fiat, smooth surface.
Push the shaft of the
hook through the propeller spar from the rear of the spar. Bend the end of the
shaft over very carefully, as shown in figure 3-23. The distance from the hook
to the first right angle should be 5/8" minimum, maximum 1".
If the hook is too long, the length of the rubber one can use is effectively
reduced, and if too short, it can cause friction with the nose bearing and
difficulty in attaching a fully wound motor. After the last bend is made in the
shaft, trim it off as shown.
Slide
the shaft back through the spar and press the short end of the shaft into the
spar. The end can be pressed in to its full depth, or it can be lightly pressed
so as to make only a small indentation, and then the spar can be drilled
slightly with the #78 drill so that the end can be inserted and the shaft made
flush with the spar. It can also be cemented alongside the spar.
Push
the shaft out again and coat the bend and the part of it which will be in the
spar with a layer of model cement. Pull the shaft back into place and then put
a layer of cement over the bend exposed on the face of the spar. Repeat with
another light coat or two as each coat dries. Lay the propeller aside for the
time being.
One
of the most critical aspects of the EZB is the selection of the motor stick.
The stick should be light but stiff. This combination is often difficult to
find. Sticks can be compared by holding them so that they project over the
edge of a table and then applying pressure on the unsupported end of the
sticks. Comparative stiffness can be measured with weights, but comparing by
feel is usually sufficient. The stiffness should be checked with the stick held
fiat as well as on edge.
EZB
#1 employs an untrimmed and fairly heavy motor stick. In the interest of weight-saving,
the motor stick can be carved down considerably, to a cross-sectional area half
the size of the 1/8" x 3/16" piece used for EZB #1. But, for
the time being, the motor stick should only be cut at one end at a 60° angle
to the horizontal and sanded lightly. The bending of the rear rubber hook is
the next step in building EZB #1.
Rear
rubber hooks vary in design quite a bit. The hooks presented here (figure 3-25)
are shown in order of the preference of the author.
The
hook labeled "A" is preferred when using an "0" ring on the
rubber. An "0" ring is simply a small rubber or wire ring slipped
onto the rubber and used to connect the rubber to the hooks. The "A"
hook is preferred because it presents a smooth, curved surface to the wound
rubber motor. The other types require that the end of the wire be smoothed very
carefully with a file so that they have no sharp edges.
The
rear hook is attached with model cement and reinforced with small tissue or
condenser-paper tapes. Add a coat of cement (thin) as a final reinforcement.
The front bearing is
attached next. This can be a commercial bearing, or one made with a
strip of "half-hard" aluminum .020" thick by .060" wide by 1/2"
long. Drill a hole in one end on the wide face with a #78 drill. The
bearing should be bent as shown in figure 3-26 and attached with fast-setting
(four to five minutes) epoxy to the motor stick.
At this point the few
parts left to make for the EZB are: the propeller shaft (thrust) washers, the
tail boom and the wing mounts. Two materials are required for the washers: a
piece of thin metal and some thin Teflon. The washers are available
commercially (Micro-X, Peck, Old-Timer); however, making them is not difficult.
Metal washers are
made from brass shim-stock or aluminum beer or soda-pop cans (fig. 3-27). I use
aluminum. Cut a strip of aluminum from a can about 3/32" to 1/8"
wide with old scissors. Cut the corners off one end of the strip and then
cut off that end about the same length as the strip is wide. Holding the small
piece with tweezers or pliers, cut off the other two right-angle corners to
make an octagon 3/32" to 1/8" in diameter. Make a
slight indentation in the center of the octagon with a sharp steel point such
as that on a compass. Drill a suitable hole (#78)through the center with the
drill and pin vise. Hold the washer by whatever method proves convenient
(square-nosed fiat pliers work well) and file the washer smooth on all surfaces
and edges. Repeat the process for successive washers.
Making Teflon washers requires a length of brass
tube with the same inside diameter as the outside diameter of the required
washer. This tubing is usually available from hobby shops. Sharpen a piece of
3/32"-inside-diameter (inside diameter - i.d.; outside diameter = o.d.)
brass tubing by filing away the outside edge back to about 1/8” to 3/16"
(figure 3-28).
Twist
the brass tube while pressing it into .005" to .010" Teflon sheet.
This is often available at plastics supply stores in sheet form and sometimes
in tape form from hardware stores. As the Teflon is drilled (against a piece of
soft wood), the disks will be pushed up into the tube. They can be pushed out
with a piece of wire or a smaller size of tubing. Sharpen the tube after every
six to 12 washers on a hard Arkansas stone. While the plug of disks is in the
tube, drill holes in their centers with a #78 drill. (Punching the center hole
with a pin or hot wire does not work.) A small flap may be pushed out by the
drill; pull it off with tweezers. Thread an aluminum washer, a Teflon washer
and another aluminum washer over the hook onto the prop shaft, and you have a
nice, smooth bearing assembly.
The tail boom is made from stiff but light "B" grain 1/32"
sheet balsa. Cut a strip 6" long, 1/8" wide at one end, tapering to
1/32" square at the other. Slice the wide end vertically but on an angle
so that when the end is pressed to the table the thin end is 1/2" from the
tabletop (figure 3-29).
The
wing mounting posts are made with
medium (hardness) 1/16"-square balsa. The balsa is rounded into a dowel by
first shaving off the corners (a razor plane works well for this), and then
spinning the dowel between two surfaces of #320 sandpaper (figure 3-30). Make a
length of balsa about 6" long into this dowel shape.
To
make paper tubes, cut four or five strips of Japanese tissue into about
3/4" x 7/8" to 11/8" (rectangles). With a crayon, lightly wax a
length of 1/16"-diameter brass tube or the shank of a 1/16"-diameter
drill bit or a length of 1/16"-diameter music wire. With the bottle of
aliphatic resin or tube of model cement handy, we are ready to roll tissue
tubes. Make sure that one end of the form (tube or rod) is smooth and
snag-free. Wet one end of the rectangle of tissue with saliva, stick it at
right angles to the form and roll it onto the form one turn (figure 3-31(1)).
Next, deposit; a glob of glue where the paper comes around upon itself and
continue rolling the paper around the form. When using model cement, it is
helpful to wet the paper just before rolling it. This can be done by licking it
with your tongue or spraying it with an atomizer.
Roll the tube on the form so that the excess glue coats the paper tube. Then, using your thumbnail, slide the paper tube off the end of the form. The tube should not be touched again until it is dry. The end may be crumpled, but it can be trimmed off later. Just let it lie there and dry until it can be safely handled. When it's dry, trim both ends with a sharp razor blade.
It's
time to assemble the EZB. We will
start with the wing. Draw a line 12" long with a 3" line
perpendicular to it from its midpoint on the work board. Lay a few pieces of
1/16" sheet scrap along the line, and then lay the wing on its trailing
edge so that the edge is parallel to the 12" line and the center rib is at
the perpendicular 3" line. A few small blocks of balsa pinned to the board
will help to hold it in place (figure 3-32).
Cut
the top of the balsa dowel so that it has a small, fiat face as shown in the
detail of figure 3-32. Then cut the dowel off at 2W' overall. Put a spot of
aliphatic resin on the fiat face and slip it under the wing against the
trailing edge at the center rib. Remove it, let it dry, reglue and
replace it parallel to the 3" line (perpendicular to the wing). The
joint should be lightly pressed together with the tweezer points. Pin
balsa scraps in place to hold the wing post and let it dry.
When
dry, remove the wing and repeat the process for the leading edge with
the following exceptions: prop up the tips of the wing with 1/8" scraps
(the tips were not propped for the trailing edge) and the center section with
1/16" scraps. This will "jig in" the correct angle of incidence
for the wing. Cut the wing post to 2-3/4" overall. Place the leading edge
of the wing post so that it is 1/16" to the right of the
perpendicular line at the end of the post and right on the perpendicular
line where the post joins the wing (figure 3-33).
While
the wing posts are drying, attach the tail boom to the rear left side of
the motor stick, adjacent to the rear motor hook. Lay the motor stick upside
down on the tabletop and make the top edge of the tail boom flush with
the tabletop as well (top of tail boom parallel to top of motor stick).
An
alignment jig is necessary to mount the stabilizer in proper alignment.
Cut a piece of ¼" sheet balsa 1-1/2" x 7". Cut a
perpendicular notch in the middle of one end (very carefully) 1/2" deep
(figure 3-34). The notch can be cut with a few strokes of a razor saw.
Draw
a line in line with the notch down the length of the jig and over the
end opposite the notch. Draw a line on the building board about 20"
long and pin the jig-piece over it at one end so that the notch and line on the
jig are parallel to it.
Slip
the rear motor hook into the notch and put pins alongside the motor stick to
hold it, centered, over the line on the jig and board. Lay the stabilizer
gently on the tail boom with the trailing edge at the very tip of the boom. If
the boom bends under the weight of the stabilizer, block it up to keep it
straight. Put a small piece of 1/8" balsa under the right tip and put a
small weight on the same tip of the stabilizer to hold it against the board. A
small styrofoam thread spool or a chunk of ¼" balsa about 1" square
should be plenty.
Lift
the weight and stabilizer off the tail boom, put glue on the leading and
trailing edge undersides and reposition stabilizer and weight. Let it dry
until it can be handled without being distorted (figure 3-35).
When
the stabilizer is dry, turn the fuselage assembly over and attach the rudder
along the underside of the boom with two spots of cement. Make sure it is in
the same plane as the motor stick: i.e., vertical, rather than perpendicular to
the stabilizer which, remember, is tilted.
When
the rudder is dry, insert the propeller shaft in its bearing and attach a loop
of rubber between the hooks with a little slack. Lay the assemblage over one
finger and note the approximate point at which it balances. Take a 4" or
5" length of fine thread and suspend the fuselage assembly near the
approximate balance point. Move it until the fuselage is parallel to a level,
horizontal reference line and mark the point. Simple as this may sound, in
practice it can be quite difficult, for the assembly will have a tendency to
move about and make it hard to mark the balance point (center of gravity: CG).
A Small brush with ink or watercolor can facilitate marking the CG (figure
3-36).
After
the CG is located, remove the rubber loop and propeller. Make a mark
7/8" to the rear of the CG and another 21/8" in front of the
CG. Make two more small marks 1/16” outside of each of the first two (in
front of and in back of). Glue a 3/8" length of tissue tube wing socket
between the pair of marks in front of the CG on the left side of the fuselage
so that the bottom end of the socket is even with the bottom of the motor stick
(figure 3-37), Glue another %" tube between the pair of marks behind the
CG in the same manner.
The
EZB is ready to fly. Insert the wing
posts into the wing sockets. It should be a snug fit with the end of the post
flush with the bottom of the socket. The wing should have its left (viewed from
rear) inner panel 1/8" higher at the leading edge than the right, measured
at the dihedral break (figure 3-38). Put the propeller into the bearing and put
on the short loop of rubber used for balancing. The plane when dropped (not
thrown) forward in a slight nose-down attitude should glide forward smoothly in
a turn to the left. If the loop is of about .055 to .056 rubber, wind about 200
to 250 turns and give the plane its living-room tests.
At
a weight of 2.4 grams with 0.065 x 0.046 x 12" to 14" rubber and
about 1,200 turns, the plane should fly with a 20' circle for three
or four minutes. The same plane built to a lighter weight can do much better.
The
EZBs which are described in the next chapter use weight-saving techniques and
other propeller designs. They can be used as a basis for personal experiment
and to develop building skills and a familiarity with the characteristics of
the class.
