2. Simple Rubber-Powered Stick Models: The SRPSM
From Ron Williams' Building and Flying Indoor Model Airplanes
The
logical place at which to begin is with the simple rubber-powered stick model
(SRPSM). There are those builders who will fly in the face of logic and attempt
to begin with more sophisticated craft: so be it; this chapter will be here
waiting and they are welcome to return to the joys of these delightful, sturdy
flyers.
The
SRPSM is the ideal introduction to the building and flying of any type of model
airplane. It can be flown indoors or out, and it is simple to build.
SRPSMs can vary in size from three- or four-inch mites to three-foot giants
and, if they are built with a fair amount of neatness and accuracy, they will
fly well.
The
plane selected to be shown here is useful as a graphic and simple demonstrator
of the laws of aerodynamics. It will introduce the beginner to the subtleties
of adjusting free-flight aircraft, first as a glider and then as a
rubber-Powered craft.
Before
trying SRPSM #1, I recommend trying a beginner's kit such as the AMA Cub
offered by Sig Manufacturing Company (see Appendix 1). This plane is also
known as the Delta Dart. These kits contain all the required parts except pins
and glue. The wire hook supplied for the propeller shaft is not very good. A
new hook should be bent to the shape shown in the side view of figure 2-1. It
can be bent from a paper clip or a piece of 1/32"-diameter music wire as
shown in figure 3-22.
The
AMA Cub is an exciting flyer that is built according to the very simple
instructions in the kit. It takes only an hour or two to build and most of that
time is spent waiting for the glue to dry.
SRPSM
#1 will require little in the way of tools and supplies; the list is as
follows:
1. 1 sheet lightweight bond paper,
8-1/2" x 11".
2. 3 pieces medium-hard 1/16" x 1/8'
balsa, 36" long.
3. 1 piece hard 1/8" x ¼" balsa or
spruce, 36' long.
4. 1 propeller, hook and bearing assembly from
a North Pacific rubber-powered foam-flyer or a Guillow's balsa rubber-powered
flyer.
5. 1 paper clip.
6. Glue: Elmer's white glue, Titebond aliphatic
resin or model cement (Ambroid).
7. A Plasticine (clay)
lump about the size of a jelly bean.
8.
Single-edged razor blade.
9. Pliers: pointed nose preferred.
10. A supply of rubber, 1/8" wide by about 20' long (the extra length for
extra motors).
If
you wish to build SRPSM #1, most of the materials will be available at the
local hobby shop
(see Appendix 1 for a list of suppliers of materials). If the plane is to be
built for small children (two to five years), use very hard balsa, spruce or
bass, to take the rough handling they'll give it.
Figure
2-1 presents a "three-view" of the SRPSM #1. A three-view is an
illustration of a plane's design, showing the builder what the plane looks like
from the top, the side and the front. After studying the three-view and collecting
the materials listed above, start construction by copying the layout (figure
2-2) onto a sheet of bond paper or a piece of lightweight typing paper. The
layout is shown step-by-step in figure 2-3. Cut the balsa strips with a
single-edged razor blade, 'making a clean perpendicular cut with a smooth
forward and downward stroke (figure 2-4). The key to a well-constructed plane
is to make joints (places where pieces meet and are glued together) that fit
well and require a minimum amount of glue. Constructing the SRPSM #1 involves
gluing pieces directly to the layout. The layout itself thus becomes the
covering of the wings and tailpieces. Lay each strip over the layout and cut it
to fit against each adjacent strip. The strips are glued to the layout with a
thin stream of glue applied to the strip and smoothed over with a finger. When
using water-based glues like Elmer's and Titebond, wet the side of the strip opposite
the glued side with a little water or saliva. The balsa swells when it's
wet and warps away from the glued side. Wetting the opposite surface balances
the swelling and keeps the strip straight. Glue each strip to each other strip
as well as to the layout except at the center of the leading and trailing edges
of the wing (see figure 2,-2). These joints will
be glued later, at an angle, to create the wing's dihedral (the upward
tilt of the wing toward the tip which gives the wing, when seen from the front,
a "v" shape).
When
all the strips have been attached to the layout, cut the parts (wing,
stabilizer, fin) free of the layout by cutting along the dotted outline shown
on figure 2-2. Use the single-edged razor blade for this, being careful to
avoid cutting the balsa outlines. Now is the time to add any desired
decoration to the flying surfaces before setting them aside to proceed with
making the fuselage.
The
fuselage (figure 2-5) for the SPRSM #1 is a stick. Because it must take the
stress of the fully-wound rubber motor as well as the many hard landings and collisions the plane will
suffer, it must be made of hard balsa or spruce. The fuselage must be carefully
tapered toward the tail along its bottom edge. This can best be done with a
small block-plane, but can also be done satisfactorily with a single-edged
razor blade or modeling knife (X-Acto, Uber Skiver, etc.) by working slowly and
cutting against a hard straightedge. Two short lengths of the fuselage stock
are cut to brace the propeller bearing and the rear motor hook.
The
braces are "double-glued." To double-glue any joint, apply glue to
both surfaces, mate and pull apart, allowing glue to dry; reglue and clamp
together until dry. Bind the braces to the fuselage with fine silk or cotton
thread. After wrapping, give them a final coat of glue and sand the whole
:fuselage lightly with fine sandpaper.
Remember:
cutting hard balsa or spruce with a single-edged razor blade can be done quite
easily, but it must be done slowly. Forcing the blade can cause it to break
dangerously. Cut along the line with a vertical cut, then cut at an angle to
that cut from the "waste" side of the cut (figure 2-6). The idea is
to whittle the piece to a nice clean cut at the required angle with a
clean, "square" end rather than to chop right through in one single
stroke. The cut can also be made with a razor saw, available in hobby shops,
but that is another tool to buy and use at a later stage of the game.
The
rear motor hook is bent from a paper clip. It should pass through the fuselage
from top to bottom before being bent over and given a few coats of glue. The
hole required to get the wire through the fuselage can be made with a 1/32” diameter
drill or by heating a large straight pin on the kitchen stove until glowing red
(hold it with pliers) and pushing it through. When performing an operation
like this, it is always wise to practice on a scrap of wood of similar size and
hardness; there are so many ways to ruin the final product that you might as
well not court disaster on the first try. The hot wire will tend to burn a hole
that's too large, so practice and be careful. See figure 3-25 for hook shapes.
Raising the wing tips for dihedral is the next step before the final assembly
of the airframe. Make a support (figure 2-7) from cardboard or 1/16" to
1/8" balsa sheet. The main objective is to provide a support longer than
the chord of the wing at the center section and to make sure that the support
is firm. It can be made as shown or can simply be a strip of balsa 1½"
wide glued on edge to the tabletop. The top of the wing is the papered side,
the bottom is the side with the balsa flame exposed.
Cut
partway through the trailing edge of the wing (from the bottom) at the center
and crack the wood slightly. Place the wing upside down on the support. Hold
the wing tips down to the tabletop with small weights (flashlight batteries,
coins taped in stacks, etc.). When the wing is firmly in place (a pin or two
through the center rib into the support piece will help), the joints that were
left unglued earlier are carefully filled with glue. Rub the glue into the
joints and then let it all dry for at least half an hour. The wing may
be removed from the jig when the glue has dried.
SRPSM
#1 can now be assembled. The flying surfaces are attached, each in turn, with a
thin bead of glue. Start with the wing, applying the glue along the bottom of
the center rib and attaching it to the fuselage so that the apex of the leading
edge of the wing is 1" from the nose (figure 2;1). Attach the stabilizer
to the underside of the fuselage, paper covering side up with a bead of
glue down the center top of the stabilizer.
Notice:
when the rudder was cut free of the layout, an %" tab of paper projected
outside the balsa frame. If this tab, as well as the bottom surface of the
outline, is glued, it will make a stronger attachment of the rudder to the fuselage
stick. Make sure that the various flying surfaces are attached so that their
alignments are correct and accurate as shown in the three-view on figure 2-1.
The
SRPSM #1 can be flown as a glider or as a rubber-powered plane. Flying it as a
simple glider (SG) is described next, with experiments in adjustment. Flying it
as a rubber-powered plane is described further on in the chapter. The assembled SRPSM #1, as shown in
figure 2-8, is ready for test gliding. Press a small lump of clay (Plasticine)
onto the nose of the plane as shown.
The
first thing to do after adding clay to the nose of the plane is to check its
balance. The plane's fuselage should rest horizontally when the plane is supported
at each wing tip (figure 2-9). Add or remove clay until this is achieved.
For
its first flight, hold the SG by the thumb and side of the forefinger, just
ahead of the wing's trailing edge, pretty much as if you were going to throw a
pencil, point first, across the room (figure 2-10).
Grasping
it lightly but firmly, practice the tossing motion without actually releasing
the plane. This motion should be smooth and crisp, not too forceful and
horizontally forward (see figure 2-10). It is a good idea to practice the
motion over and over without tossing the plane so that you can attempt to
visualize what will happen as the plane is released. Figure 2-11 shows the
glide path to be expected from a properly assembled, weighted and adjusted
SRPSM in its glider mode.
The
rudder, elevator and ailerons (figure 2-12), known together as the control
surfaces, are manipulated to adjust the plane to a given flight path. Practice
with their use can take place indoors or out; a beginner should look for calm
weather outside or find a decent-sized indoor space (from the dining room to
the living room is usually O.K.). The SG is so light that it will seldom damage
anything that it might hit unless it knocks over a particularly fragile object.
When
making the first test glides of the SG, inspect the control surfaces before
each flight to be sure they are not bent out of the plane of their adjacent
surfaces. The glide should be steady and straight ahead. (When launching
outdoors, always be sure to launch into the wind.)
The
first exercise to be practiced with the SG is that of getting the glide to be
as fiat as possible when the plane is launched straight ahead. Raising the
elevators will cause the nose to come up (figure 2-13A) while bending them down
will cause the nose to come down (figure 2-13B). A stall occurs when the plane
pitches up, loses forward momentum and then dives toward the ground.
Occasionally, the plane will recover, stall again and go into another dive. A
straight dive occurs when, after launching, the plane abruptly heads nose-first
into the ground. Use very small movements of the control surfaces to effect or
correct either a stall or a dive. When a plane adjusted for a stall is thrown
hard, it will often loop.
Once
you have launched the plane successfully on a straight, forward glide several
times, it's time to experiment with turns. The rudder is the first control
surface used for turns and the ailerons the second. Rudder and ailerons are
usually used in conjunction with each other to flatten or smooth (coordinate)
the turn. But the rudder is the first surface to experiment with, by bending it
very slightly to the right or left. In keeping with the way most indoor planes
are flown today, the experiments described here will all be in terms of flying
to the left. After making a smooth, straight flight, the rudder is
slightly bent, moving it, when viewed from the rear, about %" to the left
(figure 2-14). Tossing the glider with this adjustment will result in a smooth,
banked turn to the left. The rapidity or severity of the turn will depend on
how much the rudder is bent. A turn of 10 to 15 feet, or even wider for larger
spaces, is the aim of these tests of the SG.
To
make the turn tighter, it may become necessary to provide more up elevator to
keep the plane from diving in. But there is definitely a limit to how
much up elevator can be added before the turn tightens up even more! The answer
to this is not to put more up elevator but to begin using the ailerons to
coordinate the turn.
In
full-scale aircraft or radio-controlled model planes, the objective of aileron
control is to cause the wings to bank in conjunction with the rudder's
pivoting of the plane about the plane's vertical axis. This is made unnecessary
in free-flight models through the use of dihedral, the upward tilt of the wings
from their center toward the tips. As the rudder swings the model about its
vertical axis, the dihedral (or polyhedral) acts as ailerons to cause the
plane to bank into the turn. Consequently, the ailerons on the SRPSM and SG are
used not for increasing the bank of the turn, but for reducing it by
keeping the left wing up and the right down in a turn (figure 2--15) to the
left. Make this adjustment by bending the left aileron down and the right
aileron up.
It
is worthwhile, on becoming familiar with the techniques outlined here, to
experiment with a turn to the right, for there are times when a turn to the
right is appropriate. For instance, many outdoor free-flight planes are adjusted
to turn right as well as some indoor hand-launched gliders and flying scale
models.
With
the SG adjusted to fly in a 10- to 20-foot circle to the right or left, it can
be thrown as high as possible--a heavy version should reach 30 feet or so--and
it can be flown in a gymnasium or outdoors. Be careful of the trees and always launch it into the wind[
Flying
the plane as the SRPSM #1 means adding the propeller with its bearing and the
rubber motor to the SG. Remove the clay and slip the propeller-bearing assembly
over the nose of the plane. The propeller should be 5" to 5%" in
diameter. If the box of the bearing assembly fits too loosely, the nose must
be shimmed up with pieces of hard balsa or cardboard so that the nosepiece
slides on snugly. The binding that holds the lower piece to the main stick
(figure 2-5) should provide all the bulk required. It is important that the
bearing does not slip or twist about when the motor is fully wound.
The rubber strip required for flying the SRPSM #1 is usually obtainable from any model airplane hobby shop. See also the lists in Appendix 1. The rubber should be 1/8" wide for best results. Rubber of other widths will work. The next size smaller (3/32") will under-power the plane, but will provide longer flights because more turns can be put into it. The next size larger (3/16") will over-power the plane, providing spectacular flights of short duration--great fun out of doors.
The
first loop of rubber for testing
should be about 10" long and 1/8" wide. All sizes of rubber
can be tied as shown in figure 2-16, with an overhand knot first and then a
square knot pulled up tight against the overhand knot. Be very careful that the
second knot is a square knot and not a granny, as the granny will not hold when
the rubber is lubricated and fully wound.
The
rubber should be wet when the knots are tied---chewing lightly on the ends of
the rubber strands and wetting the rubber with saliva works fine. After pulling
the knots tight, trim the rubber's ends to no longer than 1/4" nor
shorter than the width of the rubber. To tie rubber that has been lubricated,
the rubber must first be washed in soap and water to insure a tight knot. The
lube can be "chewed" out of the rubber, but; as a flavor
combination, rubber and lube will never be found on any menu.
What is lube? Basically,
it's an oily or slippery liquid which enables one to put more winds in a rubber
motor and which reduces wear and tear on the motor. Any commercial lube is
satisfactory (Sig, Micro-X, Aerolite, etc.), but, in a pinch, Johnson's K-Y
lubricating jelly mixed with a little green soap and/or glycerine (all
available from the drug store) will work fine. The lube is rubbed sparingly
onto the length of the rubber with the fingers.
Test flying the
rubber-powered SRPSM #1 is best done in a large, open space, especially if it
is covered, knee-deep, with soft grass. The hard floor of a gymnasium is not
going to have any adverse effects on the plane, but soft grass will encourage
confidence.
Hold
the plane in one hand, 'by the fuselage, about halfway back from the wing's
leading edge. Using the other hand, turn the propeller in a clockwise direction
(see figure 2-17). A flight or two with 100 to 150 hand-winds on the propeller
will determine what is required in the way of adjustments. What held true for the
plane as a glider will hold true for it when powered. If the plane dives in,
add up elevator. A small amount of left rudder and bending down the left
aileron slightly should result in a rapid climbing spiral to the left when the
motor is more fully wound. As the limit of the motor is reached or larger
rubber is used, use more left aileron with even a bit of right aileron (bent
up).
To
wind the motor more, it is taxing, to say the least, to use one's fingers. The
answer to this is to employ a winder. A simple, inexpensive 5-to-1 winder made by Kyosho and sold in the U.S. by
Sterling Models, Inc., is ideal for the beginner. The only other winder
commercially available at this writing is a 16-to-1 winder, also inexpensive,
sold by Midwest Products Company. Both are usually available in hobby shops. A
10-to-1 winder is described (for scratch building) in Chapter 10.
To
use a winder, it is necessary to have
a "stooge," i.e., a frame that holds the plane (figure 8-12) or a
hook that holds the rubber (figure 2-19), or a friend to act as stooge. The
rubber can be wound off the plane and then placed, wound, onto the plane. To
wind on the plane, wind from the rear hook end of the rubber as the plane is
held by the propeller and fuselage by a friend. The friend should grasp the
propeller and its bearing between the thumb and forefinger of one hand while
encircling the rubber motor with the thumb and forefinger of the other hand.
This will tend to guard the plane, should the motor break and fly back toward
the plane. The person operating the winder stretches the rubber by moving away
from the plane; the rubber is stretched three to five times its normal length
as winding is begun. After one half of the winds are turned in, the winder is
moved slowly toward the plane as the last turns are added to the
motor.
Different
sizes and types of rubber will allow
different amounts of turns in any given length of motor. Smaller cross sections
of rubber (narrower strip) will take more turns than larger cross sections.
When I encountered the question of how many turns a rubber motor would take, I
found graphs and tables and widely varying answers. The most useful advice was
to practice winding a test piece of rubber until it broke. At the time it
seemed senseless .... Wind a motor until it ]broke? What could be learned from
that? But as I began, in desperation, to make up short motors of similar
lengths and to master the counting of turns (this requires some concentration,
especially as people wander up to converse or ask questions at turn number 53
or 64 or 86 · · .), I began to gain a sense of what different winding
techniques and different rubbers were capable of. I found that periodically
feeling the rubber, checking its resilience or hardness, told me much about
what to expect from what I was winding. With practice and patience, I found
that there was a technique to winding rubber; that my technique improved
with practice; and that I could get much more out of rubber that was considered
inferior than I was supposed to.
The winding is begun by placing the knot end of the rubber on the stooge hook and the loop
end on the winder. After winding, remove the loop end from the winder and,
holding the plane by the propeller and its bearing, slip the loop over the prop
hook. Remove the knot end from the stooge hook and slip it over the rear hook.
After inspecting all adjustments, the plane is ready to launch.
When
winding off the plane, a firm place
to anchor one end of the loop is required. A "quick and dirty" way to
accomplish this, which frees the flyer from the need for an accomplice, is to
put a screw hook (or cup hook) into a piece of wood, and glue, tie, clamp or
tape this piece of wood to a firm place or heavy object (figure 2-19) or put
the hook into the side of a willing, heavy toolbox. It's a good idea to inspect
whatever hook is used for sharp edges and to file or sand them off if
necessary. Such flaws can cause a tightly pulled or wound motor to be nicked
and to break.
Launching the SRPSM #1 is just like launching the SG, except that, while one hand holds the
plane's fuselage, the other holds the propeller to keep it from turning and
then releases it so that the propeller is spinning just before the plane is
launched. When the rubber motor is fully wound or of wider rubber, launch the
plane upward (rather than straight ahead as in the gliding exercises).
The launching motion should be free and relatively effortless: just support the
plane as the motor takes over and "guide" it into its flight path.
Observing
beginners, it is amazing how many failures precede a solid success--but what is
more amazing is how quickly those failures are left behind. Perseverance is the
most valuable part of one's effort in most things, and doubly so with building
and flying. It may take only one or two tries to get the knack of launching a
simple rubber-powered stick model, or it may take dozens.
What matters is to keep at it; to try to repeat what went well and to forget
what went wrong. Most of the best builders and flyers I know are those who
experienced the most difficulty at the beginning, but put it behind them as
quickly as they could.
The
three-views which follow are for other simple rubber-powered stick models.
Figure 2-20
illustrates a plane from the 1930s, the decade considered the golden age of
indoor flying. There were thousands of clubs and hundreds of thousands of
indoor flyers during the late twenties and up to World War II, The Beginner's
Tractor was actually quite sophisticated, but a beginner of the time had immediate
access to advice and information from local modelers. Department stores and
newspapers in many cities sponsored clubs providing meeting places, newsletters
and continuous support. High schools and junior highs sponsored flying as well.
Alternative SRPSMs are plentiful in kits (from most hobby shops) or from plans
that appear regularly in model magazines and newsletters.
