1. Aerodynamics And Parameters; Materials And Sources; Types, Competition And Spaces
From Ron Williams' Building and Flying Indoor Model Airplanes
The
two types of indoor models one will encounter during a first visit to an
indoor meet will be the rubber-powered plane and the hand-launched glider. The
rubber-powered plane is powered by a wound-up rubber band. It is possible to
turn more than 100 turns per inch into the thinner rubber bands; a rubber motor
for some types will often hold more than 2,000 turns. This means that if the
propeller is large enough, it will turn slowly (say 60 revolutions per minute)
as the rubber unwinds; the plane will fly, theoretically, for 33 minutes on
1,800 turns. There are many factors which may prevent such a figure from being
reached; there are also factors which will help to exceed those figures. Some
of them will be explored later.
The
hand-launched glider is thrown to the ceiling. Its "power" is the
throw that sends it climbing to the ceiling and it; might be said that the
glider is earth- or gravity-powered during its descent. Its forward speed is a
function of the pull of gravity, the drag exerted on the airframe by the
atmosphere and the aerodynamic design of the plane which gives it its gliding
ability.
Indoor
aircraft fly best when they are optimally powered, fly consistently, have
minimal drag and are strongly and lightly built. "Optimally powered"
means, in the rubber-powered plane, that the right size and weight of rubber is
used fi)r the rubber motor. The perfect motor would unwind its last turn as it
touched the floor at the end of the flight. The rubber is lubricated to
facilitate smooth winding and unwinding. If the motor is too short, the flight
will be short; too long and the plane will land before it is fully unwound,
carrying excess unused motor throughout its flight. If the motor is too light
(cross section too small), the plane will be under-powered. If the motor is too
heavy, it will be overweight and fewer turns will be possible. The object is to
balance these factors to achieve the optimum longest flight.
The
indoor hand-launched glider is usually designed to fly to a particular ceiling
height. The glider, when thrown full force, will not touch the ceiling, but
will make a smooth transition from launch to glide. Often a glider designed for
a low ceiling will be ballasted up to a heavier weight when it is to be thrown
to a higher ceiling in another space. The reverse situation involves controlling the strength of
the throw to keep a heavier glider from hitting a lower ceiling. High-ceiling
gliders are usually heavier and larger. The highest ceilings, those of
buildings called "Category III sites," such as dirigible hangars, can
seldom be reached by a glider.
Consistency is the ability of the model to fly the same way
repeatedly and dependably. It begins with the design of a stable airframe and
persistent flying and adjusting. There are a number of factor's involved in the
design of a stable airframe. The principles involved are not difficult to get a
handle on. Basically, a plane in flight is in a state of dynamic balance. That
is, it is moving, up or down, but in its groove. In this situation it
can be said that there are two sets of forces acting upon the airframe. The
first is gravity and the second is the pressure of the air upon the flying surfaces
which resist or balance gravity and keep the plane flying.
The forces of gravity are said to be balanced at a
particular point within the airframe. This point is called the center of
gravity, or CG. A well-balanced plane hangs in a normal flight attitude
when held at that point. The position of the CG can be changed by manipulating
the weight of the nose, tail or wing tip.
Similarly, the forces of the air pressure on the
lifting surfaces of the plane also have a point where they are balanced. This point
is known as the center of pressure, or CP. When the two points, CG and
CP, are in the same place, the forces acting upon the plane are coincident and
the plane, theoretically, is balanced. This balance is relative and the forces
need not be coincident when the airframe is flying. A plane will often fly
very well when the CG and CP do not coincide. The difference between
them is known as the constant margin of stability, or CMOS; this margin
is termed "positive" if the forces involved tend to right the
aircraft when the flight path is disturbed and "negative" if they
tend to upset the aircraft further from the flight path. Indoor models can be
built to have a particular CMOS; the details for this level of the science can
be found elsewhere (see Appendix 2, INAV).
Once a plane is balanced and flyable it is then
adjusted to fly in a circle. The basic turning mechanism is the rudder. This is
the fiat, vertical
surface at the rear of the aircraft. If the rudder
is parallel to the front-to-rear axis of the aircraft, the flight path will
tend to be straight. If the rudder is turned from the axis, the aircraft will
tend to turn in the direction toward which the trailing edge of the rudder is
turned (figure 1-1).
A second factor that will cause rubber-powered planes
to turn is the force of torque. Torque makes a plane in flight tend to
turn in the direction opposite to the direction in which its propeller is
spinning. Imagine holding a model with a rubber motor that has been fully
wound. If you hold the model by the body and release the propeller, the
propeller will spin. If you hold the model by the propeller and release your
hold on the body, the entire model will spin, and in the direction opposite to
the direction the propeller spins. In flight, no one is holding the plane and
torque causes it to tend to turn left if the propeller is spinning clockwise
(to the right) as you look at the plane from the rear.
Usually torque is exploited to make indoor models
follow circular flight paths; for planes set to circle to the left the effect
of torque is enhanced by turning the rudder to the left and tilting the wing
to the left as well. It is balanced (resisted) by warping the wing and tail so
that when the torque is greater, these surfaces resist' the turning
tendency more. Another adjustment used to balance the effect of torque is to
make the left wing larger in area than the right wing. This gives the left
wing, the one on the inside of the turn, more lift, to resist the tendency of
the plane to bank and turn to the left. By balancing all these adjustments, the
plane may be made to fly in the same-sized circles throughout its flight, in
spite of the fact that the rubber motor's torque is at a maximum when fully
wound, diminishing to its lowest on landing.
Other adjustments to the airplane are intended to
keep it flying on an even keel, at its optimum attitude. This is the
angle (nose up or nose down) at which the wing and stabilizer (horizontal tail
plane) will combine to provide the most lift. The stabilizer is set at a
particular angle (usually the trailing edge is raised) and the wing's angle
adjusted during flight testing until an optimal angle is found.
The
difference between the angle of wing and stabilizer is called decalage. The
leading edge raised is called positive incidence. The trailing edge raised is
called negative incidence (when related to the centerline of the aircraft's
propeller).
Sometimes the direction of the propeller's thrust is
adjusted. These movements are called right-thrust, left-thrust, up- or down-thrust,
and cause what they suggest: a tendency to go in the direction of the
change.
Hand-launched gliders are traditionally adjusted as
follows: usually the decalage (figure 7-13) is zero. That is, the planes of the
wing and stabilizer are parallel. The basic adjustment for turn is to tilt the
wing in the direction of turn desired (or tilt the stabilizer opposite) (figure
3-1A) and to use the rudder for further turn adjustment. The wing on the
inboard side of the turn is often warped so that its trailing edge is bent
down: this wash-in keeps the inside wing from banking further into the
turn than desired.
Many model airplanes, especially hand-launched
gliders, employ a warp in both wing tips known as wash-out. This warp is
bent or carved into the tip so that the trailing edge is higher than the
leading edge. This wash-out serves to keep the aircraft on its heading during a
stall. A stall occurs when the nose gradually rises until the plane seems to
stop, then dives and moves forward again. Without wash-out the plane will tend
to dive off to one side and spin nose-first to the floor. The wash-out, because
its angle is smaller than that of the stalling center section, serves to keep
the wing tip flying after the center of the wing has stalled.
Our list of criteria also included minimum drag,
maximum strength and light weight. Minimum drag means that the plane presents
as little of itself to the flow of air as possible. To achieve minimum drag,
each control used to adjust the plane to a consistent pattern must be used
minimally because each control--wash-in, wash-out, rudder, etc.--presents more
of the surface of the aircraft to the airflow and, consequently, more drag.
Perhaps an extreme situation can illustrate this point and get us into the next
one. Every once in a while a plane will suddenly be seen to have its wing bow
up on one side, almost to the vertical. This is the result of weak bracing of
the structure and it results, if not in breakage, in a slow and probably
aborted flight. It is like waving this book through the air like a fan.
Compared to waving it edge-on, there is a great deal of resistance (drag) felt
as the book "fans" the air. So, too, with the overdone adjustment.
Wire bracing of the airframe and carefully selected
light, stiff wood combine for the strength of the indoor aircraft.
Hand-launched gliders need no bracing because they are built of wood thick and
strong enough to resist the force of the launch. But rubber-powered indoor models are fragile birds. They are handled slowly and
carefully because they are stressed only for flight. For example, the
Pennyplane is of a format of 18" square; without its motor it must weigh
no less than one new U.S. penny. (A penny will fit into a 3/4" square.)
The object of the design is to get the plane as close to pennyweight as
possible, but not lighter. The lighter the plane, the longer it will take to
fall and the easier it is to fly. Lightweight construction is the central
subject of most of this book. I have mentioned the flight, adjustment and
physical characteristics of the indoor model airplane but not yet mentioned the
reason for all this. Ultimately, indoor planes are built so that they will
stay up for the longest possible time which will, in turn, allow the longest
possible enjoyment of their beauty.
Two
questions I am frequently asked are: do indoor models come in kits and can one
get all of the materials needed to build one from scratch at a hobby shop? The
immediate answer to both these questions is generally "yes," but it
must be qualified, for not all the types of indoor models come in kits and
certainly all the materials are not generally available in hobby shops.
Hobby shops will carry balsa wood as thin as 1/32" and sometimes wood
selected for its light weight. The involved indoor builder will deal with any
or all of a few sources selling supplies in the
U.S.A.,
and, I imagine, one or two others in whatever other country a builder may live.
A hobby shop will not stock tungsten wire, ni-chrome wire, karma wire, hand-selected
indoor wood, various microfilm formulas, condenser paper, special glues,
lubricants, bearings, or the other myriad items that fill the indoor builder's
inventory.
The
materials are usually acquired progressively as one builds up the scale toward
lighter aircraft. The four sources supplying indoor materials, as well as those
listed in Appendix 1, are:
Jim
Noonan, Old-Timer Models, P.O. Box
18002, Milwaukee, Wisconsin 53218 (catalog 75¢); Ron Plotzke, Aerolite Model
Supplies, 36659 Ledgestone Drive, Mt. Clemens, Michigan 48043 (for list
send stamped, self-addressed envelope); Gerald A. Skrjanc (who produces
kits for many types of indoor models), Micro-X-Products, P.O. Box 1063,
Lorain, Ohio 44055 (catalog $1.00); and Indoor Model Supply, Box C, Garberville,
California 95440 (send stamped, self-addressed envelope for list). Special
tools and materials can be obtained from other sources listed in Appendix 1.
The four sources mentioned supply a general selection of the indoor
modeler's necessary materials; local hardware and hobby shops will supply the
rest. A mail-order source for general model aircraft supplies is Sig Manu-
facturing
Co., Montezuma, Iowa 50171 (catalog
$2.50). Always include an envelope--stamped and self-addressed--when writing
for information.
What are the
materials used in indoor aircraft building? Except in a few instances they are
the same materials used in other types of model construction, but they are
different in size, weight and certain other properties. Balsa wood is the main
material used in indoor building. Because great efficiency is required of the
wood, only the lightest balsa is used. It weighs from four to seven pounds per
cubic foot. Some balsa is cut to sheets as thin as .008" (eight
thousandths of an inch). Wood this thin would be used for the tail cones of the
lightest indoor aircraft. Most of the wood used for planes up to the FAI
(Federation Aeronautique Inter-nationale) size will be less than 1/32"
thick. Spars and ribs are cut from sheets this thin, and fuselage tubes are rolled from
it. But thinness and light weight are not the only properties important in the
selection of wood.
The manner in which sheets of balsa are cut from the
balsa tree determines the type of grain the sheet will have, which in turn
determines its stiffness or flexibility. Balsa grain is generally described by
three types: "A," "B" and "C" grain. Figure 1-2
shows how the typical cuts are made from the balsa log.
"A" grain is very flexible both across the
grain and along its length. It is used where sharp curves are required as in
spars and outlines. "B" grain is stiffer than "A" grain and
usually remains straight and true. It is usually used for straight spars,
sometimes for ribs. "C" grain is very stiff in both directions and is
excellent for ribs and for rolling body tubes (which have to support the
compressive force of the fully-wound rubber motor) and tail cones. It is not
suitable for spars. Combinations known as "BA" and "BC"
grains have their uses as alternates to "A" or "C" grain
respectively. An ideal and useful resume of wood grain characteristics and uses
appears at the end of the Micro-X catalog.
Indoor models, are generally covered with three types of material: condenser paper, Micro Lite (a polycarbonate film) and microfilm. Beginners' models and flying scale models are often covered with Japanese tissue, a lightweight tissue usually available in colors.
Condenser paper is a tan-colored, nonporous tissue
used in the electronics industry. It weighs from .008 to .011 ounces per 100
square inches and comes in sheets about 18" × 30". It is attached to
the aircraft structure, after being preshrunk on a wood frame, with shellac,
sugar water, indoor cements (acetate type), water-based glues or, yes, saliva.
Shellac or sugar water is preferred where the covering will not be overly
stressed because both dry slowly and allow the paper to be pulled smoothly over
the structure.
Micro Lite is lighter than condenser paper, weighing
about 0.005 ounce per 100 square inches. It is available clear or with an
aluminized (chrome-like) silver finish. It is usually attached with shellac or
a rubber-based contact cement. It is trimmed or cut with a fine brush dipped in
plastic solvent such as MEK (methyl ethyl ketone). The thinness of Micro Lite
makes it very difficult to handle and so it must be attached to wooden frames
which will hold it in position for covering the aircraft surfaces. Micro Lite
is usually limited in its use to Pennyplanes and some specialty craft.
Microfilm is a subject for a book in
itself. It will be discussed briefly here and its use described later in terms
of commercially available formulas. Microfilm is a nitrocellulose film
(similar to acetate) made by pouring a solution composed of nitrocellulose
(lacquer or nitrate dope), thinners and plasticizers (the formulation of these
solutions approaches the occult in terms of the secrecy surrounding them) upon
the smooth surface of a tank of water. The tank is usually only a few inches
deep and about three by four feet wide and long. The film floats on the surface
of the water; once it hardens it is removed from the water on a balsa frame or
wire hoop coated with rubber cement. The film's thickness is determined by the
color of light refracted from it. If the film is colorless it is either too
thick or too thin. The most brilliant colors indicate the lighter films, and
the paler colors are the heavier films. Microfilm is attached to the balsa
structure of the plane with water, distilled Water or saliva. Microfilm is
described in more detail in Chapter 6.
Indoor planes are held together with different types
of glue. Balsa structures are usually held together with nitrocellulose-based
cements, generically known as model cements. These glues are either specially
formulated (Aerolite, Micro-X) or modified from commercial model airplane
glues. Some modelers make their own glues by dissolving acetate in acetone or
other thinners and adding plasticizers. Other builders use water-based glues
such as aliphatic resins (Titebond) or polyvinyl acetates (Elmer's), but these
glues have limited applications. Epoxy is often used in very small amounts for
the attachment of metal parts to wood. The model cements are applied with all
sorts of instrUments from a simple sharpened stick to specially made glue gun's
and hypodermic syringes. Glues are selected on the basis of strength, stability
(non-shrinking), rapid drying and flexibility. Alpha-cyanoacrylate glues, new
to the model building market in the last few years, known by trade names like
"Zap," "Hot-Stuff," et al, are being used experimentally.
They find wide use in hand-launched glider constrUction and in repair work.
They set almost instantly, are strong and light, but possess some serious
potential physical hazards to the user. They should not be used where a joint
might need to be taken apart.
Once the flame is built and covered and the plane is
ready for assembly, the wire bracing on the plane becomes important. Wings and
fuselages for most microfilm planes and some paper covered planes are usually
braced with fine wire. Heavier steel wire is used for holding the rubber motor
at the propeller and motor stick. Occasionally other components are braced and
other bracing mediums are used. Motor sticks are braced with small balsa struts
in compression and tungsten wire (about .0010" in diameter) used in
tension. This bracing resists the tension of the fully-wound rubber motor which
tends to bend the motor stick, often collapsing an insufficiently-braced stick.
Microfilm wings and lighter paper-covered wings are braced with nichrome wire
or a variation of nichrome called karma wire. This wing bracing is
usually .0007" to .0010" thick (thin?). Lighter
wire, .0006" nichrome and polyester filaments are used for smaller,
lighter structures, and tailplane bracing. Polyester filaments are doubled up
and used for wing bracing by some flyers. Occasionally, on EZB and Pennyplanes,
balsa bracing is used.
Steel wire .020" and finer is bent
to make the hooks used to hold the rubber motor on the motor stick and to
connect the rubber to the propeller. This same steel wire is sometimes used to
make nose bearings which carry the propeller shaft; however, small, light
strips of aluminum are more popular as nose bearings.
When
one first sees an indoor model or visits an indoor session, the aircraft itself
is there: a matter of fact. It brings forth questions of the simplest sort: why
is this one bigger, that one smaller, or the other one covered so? Why are they
different one from the next? Though probably built primarily for their beauty,
it can be said that indoor aircraft are built and flown according to class
specifications for the purpose of competitive (or comparative) flying. The
exception to this would be aircraft that
are built experimentally. Classes of aircraft are coordinated nationally by
the Academy of Model Aeronautics (AMA) and internationally by the FAI. These
classes will be described in the rest of this book in the order of a suggested
sequence for the building of indoor models. The rules for indoor model classes
are delineated in the Official Model Aircraft Regulations of the Academy
of Model Aeronautics, 806 Fifteenth Street N.W., Washington, D.C. 20005.
The
hand-launched glider is limited for the sake of indoor competition only by a
maximum wing area of 100 square inches. Wing areas that large are seldom built,
for they involve an increase in weight. Hand-launched gliders have a tendency
to move rather fast and, on striking the floor or a wall, heavier gliders can
be expected to suffer from the encounter. A serious "glider-guider"
making a launch is a study in concentration. He will use meditation,
breathing, counting and rigorous training like throwing heavy weights into the
air or clay lumps against a ceiling. Some flyers will have many gliders and
each will have its own groove; to get each to perform at its optimum requires
long sessions of patient work.
As
we move into powered flight, we must consider the source of that power. It's a
rubber band! Some motor, eh? Wind it up in 60 seconds
and
it takes 20 minutes to unwind. Rubber motors are sized to unwind at an optimum
rate for the plane and its propeller. The rubber is about .046" thick,
sliced from wider strips into a width suitable for the particular model it
powers. The rubber used in Indoor today usually comes from a producer known as
Pirelli, in Italy. Manufacturers in the U.S. produce rubber which does not, so
far, come up to the Pirelli with comparable power characteristics.
Stripped
rubber (i.e., rubber precut into ready-to-use strips) is supplied by Micro-X
and a few others, or it can be purchased in larger strips (up to 1/4") and
stripped by the flyer. Rubber is stripped in a range of widths from 1/8"
(Pennyplane) to .020" (smallest microfilm craft) by a variety of methods.
Winders and strippers and a Variety of other accessories are produced by small
machine shops throughout the Indoor world. The winders turn the rubber at a
ratio of from nine to 20 turns of rubber to one turn of the crank handle. The
rubber is lubricated with a solution composed of substances like glycerine,
green soap, castor oil and surgical jelly. "Lubes" are sold
commercially and quite often are individually formulated.
The
EZB has the characteristics of a wing span maximum of 18", a wing
chord maximum of 3", a propeller constructed entirely of wood and
built to other requirements specified by local contest directors. The local contest director
must announce variations in design requirements prior to the meet. The usual
restrictions involve paper covering, wood bracing and solid, unbraced motor
sticks. The EZB is a simple design to build and fly with broad appeal to both
beginner and expert. A light model would weigh about a gram; the average EZB
probably weighs about 2 grams.
The Pennyplane is a limited class originated by a
club called the Chicago Aeronuts. It requires an airframe (less motor) no
lighter than one new U.S. copper penny (3.10 grams), a wing span and body
length of no more than 18" excluding propeller, a maximum motor stick
no more than 10" in length and a single direct-drive (ungeared) rubber
motor and propeller. A new class called Novice Pennyplane has been established,
limiting the design further with a maximum 5" wing chord (width
from leading to trailing edge), 4" chord x 12" span
stabilizer dimensions, solid motor stick and maximum 12"-diameter propeller.
It is interesting to note that flyers considered expert are already submitting
record attempts for approval in this new class.
The Pennyplane is the ideal transition plane from
EZB to more sophisticated, lighter construction. The traditional construction
techniques for light weight are used, but the small, heavier format allows for
sturdier sizes of material for inexperienced eyes and fingers. The fuselage tube
and the tail cone are rolled of balsa, and curved wing and tailplane shapes are
often employed. The class has also been a base for experimental work with low
aspect-ratio wings (wide chord relative to span), biplanes and tandem
configurations. Penny biplanes have frequently flown for more than 15 minutes
at Category III sites (100 feet or more high).
The international class of indoor aircraft is known
as the FAI class (within the FAI it is known as FID). This class is built to no
less than one gram nor more than 65 cm. (about 25-1/2'') wingspan.
A plane of this size with 150 to 200 square inches of wing area is
very lightly built; it involves all the techniques of lightweight construction
its builder can bring to bear. It can take years to learn all of the tricks
needed to get a plane light, strong and consistent; it can also take many
aircraft.
Other classes of a similar nature which attract
building are the "D" class which limits wing area to 300
square inches, the "A" class limiting wing area to 30 square inches,
and the Paper-Stick class. The Paper-Stick is limited to paper covering of all
flying surfaces and propeller, and 100 square inches of wing area, but is built
to weights similar to those of the microfilm-covered "D" and FAI.
Bracing of surfaces and minimal weight are hallmarks of these stick classes.
Among the more esoteric indoor models are the
smaller classes ("A"-ROG: Class "A"-Rise-Off-Ground) and
the largest ones (unlimited by maximum size or minimum weight). The
"Cabin" class is a class limited by a certain amount of drag in the
form of a required fuselage cross section and a functional takeoff (landing)
gear. The techniques of lightest building are used in these classes as well as
for the helicopter, gyrocopter and ornithopter.
The helicopter is commonly two propellers rotating
in opposite directions. One revolves in normal fashion relative to the other,
which is attached to the stick or tube supporting the rubber motor. The
gyrocopter is pulled forward by the propeller in the usual manner but is
supported in flight by a horizontal flee-wheeling rotor or two. It may have
wings but they must not exceed the rotor blades' area. An ornithopter gets its
power from rubber powered flapping wings.
This type of aircraft generates a great deal of excitement when flown. The
sight and sound of an aircraft flying by its flapping wings is so powerfully
evocative of the flight of birds (or bats) that one is drawn into rapt
attention as the flight begins. As the flight ends, a certain pathos accompanies
the aircraft to the floor as one sees
the
end of its short effort. Those who build and fly the ornithopters do so
infrequently; the craft are technically complex and the poor things tend
to
be torn to pieces in the event of a major component failure. Once the
flapping mechanism is sent into a state of imbalance, the resultant eccentric
forces tend to wind the plane up on itself.
As the Ornithopter inspires excitement when it is flown, so, too, the flying scale classes draw rapt attention. Indoor scale involves two classes of scale models: AMA scale and Peanut scale. Both are based on models of heavier-than-air, man-carrying aircraft. Scale models are judged for their fidelity to scale and then flight-timed for a total score. Points are awarded for both to determine a winner. AMA scale models are judged on a complete and detailed guide which specifies what points the model will receive for certain features. Peanut scale is judged to a similar guide (see AMA rules) or, in some local events, judged and scored on a basis which compares the entries in the event against each other. rather than against a set of detailed standards.
Scale
models are attractive for their reflection of realism. In flight they draw the
eye to a focus related to their size and movement, so much so that the interior
background tends to fall out of focus. At the point where one becomes conscious
of the whole situation4.e., miniature airplane, flying indoors--the spectacle
offers great pleasure. The lineup of models for judging always draws
attention, for the planes vary from the crudest approximation to
highly-detailed masterpieces of the model-builder's art.
In
mentioning a scale model's "reflection of realism," one must remember
that the other classes of indoor models are realities in and of themselves;
they cannot be models of larger or other-sized aircraft. They present reality
on their own level. The scale model will often be called "more real";
this is evidence of the strength of the illusion they create.
