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.

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