Model aircraft are flying or non-flying models of existing or imaginary aircraft, often scaled-down versions of full size planes, using materials such as polystyrene, balsa wood, foam and fibreglass. Designs range from simple glider aircraft, to accurate scale models, some of which can be very large.
Models may be built either as static non-flying models, or as flying models. Construction techniques for the two are usually very different.Static model aircraftStatic model aircraft
(i.e those not intended to fly) are scale models are built using plastic, wood, metal or paper. Some static models are scaled for use in wind tunnels, where the data acquired is used to aid the design of full scale aircraft.
Collectors can buy models that have already been built and painted, models that require construction, painting and gluing, or models that have been painted but need to be snapped together. Snap models require minimal construction and are becoming increasingly popular.Promotional use
Most of the world's airlines allow their fleet aircraft to be modeled as a form of publicity. In the early days, airlines would order large models of their aircraft and supply them to travel agencies as a promotional item.Scale
In static models, the most popular scales are 1:72 and 1:48, followed closely by 1:32. 1:144 is popular for civil airliners, and there is a growing range of military subjects. More detailed models are available at 1:32 and 1:24. Some manufacturers introduced 1:50 scale and 1:30 scale. Japan offers 1:100. The French firm Heller SA is the only manufacturer to offer models in the scale of 1:125. Herpa and others produce promotional models for airlines in scales including 1:200, 1:400, 1:500, 1:600, 1:1000 and more. A few First World War aircraft were offered at 1:28 by Aurora.
Other less popular scales are 1:50, 1:64, 1:96, and 1:128; however, old models are often revived in these scales. Many older plastic models, such as those built by Revell do not conform to any established scale. They are sized to fit inside standard sized boxes. These kits are often called "box-scale" and are often reissued in their original, unusual scales. Some helicopters used to be offered in 1:32 scale, similar to some fixed-wing aircraft models. The trend is to issue helicopters in 1:35 scale, similar to most land vehicle models.Media
The most common form of manufacture for kits is injection molded polystyrene plastic, using carbon steel molds. Today, this takes place mostly in China, Taiwan, the Philippines, South Korea, and Eastern Europe. Injection molding allows a high degree of precision and automation not found in other manufacturing processes. Smaller and cheaper runs can be done with cast copper molds, and some companies do even smaller runs using cast resin molds, but the quality and precision is of a lower standard than carbon steel.
The next most common form of manufacture is cast resin, using silicone rubber molds placed in vacuum chambers to reduce the incidence of bubbles in the castings. This form of manufacture is labor-intensive and involves a degree of waste because the resin attacks the silicone and the molds can only be used about 20-30 times before a new mold needs to be made. The flexibility of the mold does allow shapes and undercuts not possible with any other manufacturing method. This sort of manufacture is reserved for unusual or esoteric subjects in relatively small production runs, and are consequently far more expensive than injection molded plastic kits.
Vacuum-formed polystyrene kits are still being made, but a greater amount of effort is required by the consumer to produce an acceptable model compared to the aforementioned methods. There is a handful of photo etched metal kits which allow a high level of detail but can be laborious to assemble. Specialized kits cast in resin are available from companies such as Anigrand, Collect Aire, CMK, and Unicraft.
Scale models can be made from paper (normal or heavy) or card stock. Commercial models are printed by publishers mainly based in Eastern Europe. Card models are also distributed through the internet, and several are offered this way for free. Card model kits are not limited to just aircraft, with kits being available for all types of vehicles, buildings, computers, firearms and animals.
From the World War I through the 1950s, model airplanes were built from light weight balsa wood and often covered with tissue paper. This was a difficult, time consuming procedure that mirrored the actual construction of airplanes through the end of World War II. The Cleveland Model and Supply Corporation made the most challenging kits, although many model makers became adept at creating models from actual aircraft drawings.
Ready-made models (desk-top models) include those produced in fibreglass for travel agents and aircraft manufacturers, as well as collectors models made from die-cast metal, mahogany and plastic.. Snap Fit plastic plane models are manufactured by Wooster, Long Prosper, and Flight MiniaturesFlying model aircraft
Flying models are usually what is meant by the term aeromodelling. Most flying model aircraft can be placed in one of three groups:
Free flight (F/F) model aircraft fly without any attachment to the ground.
This type of model pre-dates the efforts of the Wright Brothers and other pioneers.
Control line (C/L) model aircraft use cables (usually two) leading from the wing to the pilot. A variation of this system is the Round-the-pole flying (RTP) model.
Radio-controlled aircraft have a transmitter operated by the pilot on the ground, sending signals to a receiver in the craft.
Some flying models resemble scaled down versions of piloted aircraft, while others are built with no intention of looking like piloted aircraft. There are also models of birds and flying dinosaursThe 'Jaguar' Wakefield model aeroplane
The 'Jaguar' rubber-powered aircraft was the most famous Wakefield model of all time and was designed and flown by Ted Evans. Many modelers built this unusual looking 44.5-inch (1,130 mm) wing span aeroplane and achieved success in competitions from 1947 to 1950. The 1948 Wakefield Cup was won by Roy Chesterton in Akron, Ohio, USA using this design which he constructed himself. Plans of the Jaguar were published in the October 1948 Aeromodeller. The Aeromodeller Annual 1948 then dedicated four pages and its front cover to the Jaguar. A photo of the model then appeared on the front of Model Aircraft magazine, August 1949 and a two page spread was given over to the Jaguar in Model Planes Annual 1949. This indicates the importance of this design. Despite the Jaguar's success Ted Evans felt this model could be improved upon and he set out to design the Clipper, and then the model which gained him the most success, the Vansteed.Construction
The construction of flying models is very different from most static models. Flying models borrow construction techniques from (usually vintage) full-sized aircraft (although models rarely use metal structures.) These might consist of forming the frame of the model using thin strips of light wood such as balsa, then covering it with fabric and subsequently doping the fabric to form a light and sturdy frame which is also airtight. For very light models, thin tissue paper can be substituted for fabric. First it is shrunk by water spraying, allowed to dry, then coated with dope. Heat-curing plastic films ("heat shrink covering") can be ironed on — a hand-held iron causes the film to shrink and adhere to the frame. A heat gun can also be used.
Other model construction techniques consist of using formers and longerons for the fuselage, and spars and ribs for the wings and tail surfaces. More robust designs may use solid sheets of wood to form these instead, or might employ a composite wing consisting of an expanded polystyrene core laminated with a surface veneer of wood, often obechi, which protects the core and provides strength. Such designs tend to be heavier than an equivalent sized model built using the traditional method, and would be much more likely to be found in a power model than a glider.
The lightest models are suitable for indoor flight, in a windless environment. The covering for the very lightest of these aircraft can be made by bringing an open framework up through water to pick up thin plastic films, perhaps just a few drops of lacquer spread out over several square feet (known as microfilm). The advent of "foamies," or aircraft injection-molded from lightweight foam and sometimes reinforced with carbon fiber rods and strips, have made indoor flight more accessible to hobbyists. Many come ready-to-fly, requiring little more than attachment of the wing and landing gear. See ParkZone Slo-V;Slow Stick.
Flying models can be built from scratch using published plans, or assembled from kits. Plans are intended for the more experienced modeller, since all parts must be sourced separately. The kit contains most of the raw material for an unassembled plane, a set of assembly instructions, and a few spare parts to allow for builder error. Assembling a model from plans or a kit can be very labour-intensive. In order to complete the construction of a model, the builder assembles the frame, covers it, and aligns the control surfaces.
To increase the hobby's accessibility to the inexperienced, vendors of model aircraft have introduced Almost Ready to Fly (ARF) designs. Compared to a traditional kit design, an ARF design reduces the amount of time, skill, and tooling required for assembly. The average ARF aircraft can be built with less than 4 hours of labor, versus 10-20+ for a traditional kit aircraft. More recently, Ready To Fly (RTF) radio control aircraft have all but eliminated assembly time (at the expense of the model's configuration options.) Among traditional hobbyist builders, RTF models are a point of controversy, as many consider model assembly as integral to the hobby.Entry Level
A popular entry level flying model aircraft for beginners is the AMA Cub designed by Frank Euling around 1967 (AKA MAAC Cub and Delta Dart). It can be purchased at most hobby stores and is available through mail order.
Other notable designs are the Squirrel (Darcy Whyte), Denny Dart (Neil Dennis), Canarsie Canarie (Don Ross) and Canarsie Courier (Don Ross).Gliders
Glider aircraft are aircraft with no attached powerplant. Larger outdoor model gliders are usually radio-controlled gliders and hand-winched against the wind by a line attached to a hook under the fuselage with a ring, so that the line will drop when the model is overhead. Other methods include catapult-launching (using an elastic bungee.) The newer "discus" style of wingtip handlaunching has largely supplanted the earlier "javelin" type of launch. Also using ground based power winches, hand-towing, and towing aloft using a second powered aircraft.
As gliders are unpowered, flight must be sustained through exploitation of the natural wind in the environment. A hill or slope will often produce updrafts of air which will sustain the flight of a glider. This is called slope soaring, and when piloted skillfully, radio controlled gliders can remain airborne for as long as the updraft remains. Another means of attaining height in a glider is exploitation of thermals, which are bubbles or columns of warm rising air created by hot spots on the ground. As with a powered aircraft, lift is obtained by the action of the wings as the aircraft moves through the air, but in a glider, height can only be gained by flying through air that is rising faster than the aircraft is sinking relative to the airflow.
Sailplanes are flown using thermal lift. As thermals can only be indirectly observed through the reaction of the aircraft to the invisible rising air currents, pilots find sailplane flying challenging.
Hang gliders come in two categories: hang glider and paraglider. The default use of the term is for the stiffened-wing sort; the paraglider is fully flexible winged.
Walkalong gliders are lightweight model airplanes flown in the ridge lift produced by the pilot following in close proximity. In other words, the glider is slope soaring in the updraft of the moving pilot (see also Controllable slope soaring).Power sources
Powered models contain an onboard powerplant to propel the aircraft through the air. Electric motor and internal combustion are the most common propulsion systems, but other types include rocket, small turbine, pulsejet, compressed gas and tension-loaded (twisted) rubber band.Rubber and gas propulsion
An old method of powering free flight models is Alphonse Pénaud's elastic motor, essentially a long rubber band that is wound up prior to flight. It is the most widely used powerplant for model aircraft, found on everything from children's toys to serious competition models. The elastic motor offers extreme simplicity and survivability, but suffers from limited running time, an exponential reduction of thrust over the motor's operational cycle, and it places substantial stress on the fuselage. Even so, a competitive model can achieve flights of nearly 1 hour.
Stored compressed gas (CO2), similar to filling a balloon and then releasing it, also powers simple models.
A more sophisticated use of compressed CO2 is to power a piston expansion engine, which can turn a large, high pitch prop. These engines can incorporate speed controls and multiple cylinders, and are capable of powering lightweight scale radio control aircraft. Gasparin and Modella are two recent makers of CO2 engines. CO2, like rubber, is known as "cold" power because it becomes cooler when running, rather than hotter as combustion engines and batteries do.
Steam, which is even older than rubber power, and like rubber, contributed much to aviation history, is now rarely used. In 1848, John Stringfellow flew a steam-powered model, in Chard, Somerset, England. Hiram Stevens Maxim later showed that steam can even lift a man into the air. Samuel Pierpont Langley built steam as well as internal combustion models that made long flights.)
Baronet Sir George Cayley built, and perhaps flew, internal and external combustion gunpowder-fueled model aircraft engines in 1807, 1819 and 1850. These had no crank, working ornithopter-like flappers instead of a propeller. He speculated that the fuel might be too dangerous for manned aircraft.Internal combustion
All internal combustion engines generate substantial noise (and engine exhaust) and require routine maintenance. In the 'scale-R/C' community, glow-engines have long been the mainstay until recently.
For larger and heavier models, the most popular powerplant is the glow engine. Glow engines are fueled by a mixture of slow burning methanol, nitromethane, and lubricant (castor oil or synthetic oil, which is sold pre-mixed as glow-fuel. Glow-engines require an external starting mechanism; the glow plug must be electrically heated until its temperature can trigger fuel-ignition, upon which the engine's combustion-cycle becomes self-sustaining. The reciprocating action of the cylinders applies torque to a rotating crankshaft, which is the engine's primary power-output. (Some power is lost in the form of waste-heat.)
Vendors of model engines rate size in terms of engine displacement. Common sizes range from as small as 0.01 cubic inch (in3) to over 1.0 in3 (0.16 cc–16 cc). Under ideal conditions, the smallest .01 engines can turn a 3.5" (9 cm) propeller at speeds over 30,000 rpm, while the typical larger (.40-.60 cubic inch) engine will turn at 10-14,000 rpm.
The simplest glow-engines operate on the two-stroke cycle. These engines are inexpensive, yet offer the highest power-to-weight ratio of all glow-engines. Glow engines which operate on the four-stroke cycle (or six-stroke cycle) offer superior fuel-efficiency (power-output per fuel-consumption), but deliver less power than two-stroke engines of the same displacement.
Internal combustion (IC) engines are also available in upscale (and up-price) configurations. Variations include engines with multiple-cylinders, spark-ignited gasoline operation, and carbureted diesel operation. Diesel-combustion operates by compression-ignition. The compression-ratio is controlled by an adjustable threaded T screw on the cylinder head, bearing onto a contra piston within the cylinder bore. Diesels are preferred for endurance competition, because of their fuel's higher energy content, a mixture of ether and kerosene (with lubricating oil).A very large US Coast Guard C-130J Hercules flying model. The wingspan is 18 feet 6 inches (5.6 m). The crew of five who fly and maintain it are in the backgroundJet and rocket
Ducted Fans Early "jet" style model aircraft utilized a multi-blade and high pitched propeller (fan) inside of ductwork, usually in the fuselage of the airplane. The fans were generally powered by 2 stroke piston engines that were designed to operate at high RPM. Early brands of these units were the Kress, Scozzi, and Turbax, among others. They generally used 0.40 to 0.90 cubic inch displacement engines, but Kress made a model for engines as small as 0.049 (1/2cc). This basic fan-in-tube design has been adopted very successfully for modern electric powered "jet" aircraft and are now quite popular. Glow engine powered ducted-fan aircraft are now relatively uncommon.Miniature jet turbine
Turbine engines A development is the use of small jet turbine engines in hobbyist models, both surface and air. Model-scale turbines resemble simplified versions of turbojet engines found on commercial aircraft, but are in fact new designs (not based upon scaled-down commercial jet engines.) The first hobbyist-developed turbine was developed and flown in the 1980s by Gerald Jackman in England, but only recently has commercial production made turbines readily-available for purchase. Turbines require specialized design and precision-manufacturing techniques (some designs for model aircraft have been built from recycled turbocharger units from car engines), and consume a mixture of A1 jet fuel and synthetic motorcycle-engine oil. These qualities, and the turbine's high-thrust output, makes owning and operating a turbine-powered aircraft prohibitively expensive for most hobbyists. Jet-powered models attract large crowds at organized events; their authentic sound and high speed make for excellent crowd pleasers.
Pulse jet engines, operating on the same principle as the WW II V-1 flying bomb have also been used. The extremely-noisy pulsejet offers more thrust in a smaller package than a traditional glow-engine, but is not widely used. A popular model was the "Dynajet".
Rocket engines are sometimes used to boost gliders and sailplanes, such as the 1950s model rocket motor called the Jetex engine. Solid fuel pellets were used, ignited by a wick fuse. Flyers mount readily-available model rocket engines to provide a single, short (less than 10 second) burst of power. (US?)government regulations and restrictions initially rendered rocket-propulsion unpopular, even for gliders; now, though, their use is expanding, particularly in scale model rocketry. Self-regulation of the sport and widespread availability of the 'cartridge' motors ensures a future.Electric power
In electric-powered models, the powerplant is a battery-powered electric motor. Throttle control is achieved through an electronic speed control (ESC), which regulates the motor's output. The first electric models were equipped with DC-brushed motors and rechargeable packs of nickel cadmium (NiCad), giving modest flight times of 5–10 minutes. (A fully-fueled glow-engine system of similar weight and power would likely provide double the flight-time.) Later electric systems used more-efficient brushless DC motors and higher-capacity nickel metal hydride (NiMh) batteries, yielding considerably improved flight times. The recent development of lithium polymer batteries (LiPoly or LiPo) now permits electric flight-times to approach, and in many case surpass that of glow-engines. There is also solar powered flight, which is becoming practical for R/C hobbyists. In June 2005 a record of 48 hours and 16 minutes was established in California for this class.
Electric-flight was tested on model aircraft in the 1970s, but its high cost prevented widespread adoption until the early 1990s, when falling costs of motors, control systems and, crucially, more practical battery technologies came on the market. Electric-power has made substantial inroads into the park-flyer and 3D-flyer markets. Both markets are characterized by small and lightweight models, where electric-power offers several key advantages over IC: greater efficiency, higher reliability, less maintenance, much less messy and quieter flight. The 3D-flyer especially benefits from the near-instantaneous response of an electric-motor. As the size of a model aircraft increases, the cost of electric-flight increases much more rapidly than traditional glow-engine flight. As of 2005, an electric-flight conversion for mid-large scale-models (above 0.60in3(10cc) glow-engine) is prohibitively expensive (greater than 400 USD.) Most such models remain powered by the venerable glow-engine, as their pilots prefer the sound and smell of a genuine 2 or 4-stroke IC-engine.Control Line
Also referred to as U-Control in the USA, it was pioneered by the late Jim Walker who often, for show, flew three models at a time. Normally the model is flown in a circle and controlled by a pilot in the center holding a handle connected to two thin steel wires. The wires connect through the inboard wing tip of the plane to a mechanism that translates the handle movement to the aircraft elevator, allowing maneuvers to be performed along the aircraft pitch axis. The pilot will turn to follow the model going round, the convention being anti-clockwise for upright level flight
For the conventional control-line system, tension in the lines is required to provide control. Line tension is maintained largely by centrifugal force. To increase line tension, models may be built or adjusted in various ways. Rudder offset and thrust vectoring (tilting the engine toward the outside) yaw the model outward. The position where the lines exit the wing can compensate for the tendency of the aerodynamic drag of the lines to yaw the model inboard. Weight on the outside wing, an inside wing that is longer or has more lift than the outside wing (or even no outside wing at all) and the torque of a left rotating propeller (or flying clockwise) tend to roll the model toward the outside. Wing tip weights, propeller torque, and thrust vectoring are more effective when the model is going slowly, while rudder offset and other aerodynamic effects have more influence on a fast moving model.
Since its introduction, control line flying has developed into a competition sport. There are contest categories for control line models, including Speed, Aerobatics (AKA Stunt), Racing, Navy Carrier, Balloon Bust, Scale, and Combat. There are variations on the basic events, including divisions by engine size and type, skill categories, and age of model design.
The events originated largely in the United States, and were later adapted for use internationally. The rules for US Competition are available from the Academy of Model Aeronautics. The international rules are defined by the Fédération Aéronautique Internationale (FAI). World Championships are held semiannually throughout the world, most recently in 2008 in France, with a limited slate of events - special varieties of Racing (F2C or "Team Race"), combat (F2D), and speed (F2A), all limited to engines displacing 0.15 cu. in (2.5cc), and Stunt (F2b) which is essentially unlimited with regard to design and size.
The international class of racing is referred to as F2C (F2 = Control-line, C=racing) or Team Race. A pilot and a mechanic compete as a team to fly small (370 grams)(13 oz.) 65 cm (25 in.) wingspan semi-scale racing models over a tarmac or concrete surface. Lines are 15.92 meters long (52.231 ft).
Three pilots, plus mechanic teams, compete simultaneously in the same circle, and the object is to finish the determined course as fast as possible. Tank size is limited to 7 cc, thus 2-3 pitstops for refueling are needed during the race.
The mechanic stands at a pit area outside the marked flight circle. The engine will be started and the model released at the start signal. For refuelling, the pilot will operate a fuel shutoff by a quick down elevator movement after the planned number of laps so that the model can approach the mechanic at optimum speed, around 50 km/h (30 mph). The mechanic will catch the model by the wing, fill the tank from a pressurized can by a hose and finger valve, then restart the engine by hitting the carbon fiber/epoxy resin propeller with his finger. Ground time of a good pitstop is less than three seconds.
The race course is 10 km, corresponding to 100 laps. Flying speeds are around 200 km/h (125 mph), which means that the pilots have to turn one lap in 1.8 seconds. Line pull due to centrifugal force is 85 N (17 lb). A faster model will overtake by the pilot steering it above the slower one while he moves his handle with lines over the opponent pilot's head.
After two rounds of elimination heats, the 6, 9 or 12 fastest teams enter two semifinal rounds, and the three fastest teams in the semifinals go to the final, which is run over the double course.
Maximum engine size is 2.5 cc (.15 cu.in.). Diesel, i.e. compression ignition engines are used. They are single cylinder two-stroke, designed for this purpose. At the world championship level it is not uncommon that the competitors design and build their own engines. Their output power is approaching .8 horsepower at 25,000 rpm.Airscrew types
Most powered model-aircraft, including electric, internal-combustion, and rubber-band powered models, generate thrust by spinning an airscrew. The propeller is the most commonly used device. Propellers generate thrust due to the angle of attack of the blades, which forces air backwards. For every action there is an equal and opposite reaction, thus the plane moves forwards.Propellers
As in full-size planes, the propeller's dimensions and placement (along the fuselage or wings) are factored into the design. In general, a large diameter and low-pitch offers greater thrust at low airspeed, while a small diameter and higher-pitch sacrifices thrust for a higher maximum-airspeed. In model aircraft, the builder can choose from a wide selection of propellers, to tailor the model's airborne characteristics. A mismatched propeller will compromise the aircraft's airworthiness, and if too heavy, inflict undue mechanical wear on the powerplant. Model aircraft propellers are usually specified as diameter × pitch, given in inches. For example, a 5x3 propeller has a diameter of 5 inches (130 mm), and a pitch of 3 inches (76 mm). The pitch is the distance that the propeller would advance if turned through one revolution in a solid medium. Additional parameters are the number of blades (2 and 3 are the most common).
There are two methods to transfer rotational-energy from the powerplant to the propellor:
With the direct-drive method, the propeller is attached directly on the engine's spinning crankshaft (or motor-rotor.) This arrangement is optimum when the propellor and powerplant share overlapping regions of best efficiency (measured in RPM.) Direct-drive is by far the most common when using a fuel-powered engine (gas or glow). Some electric motors with high torque and (comparatively) low RPM's can utilize direct-drive as well. These motors are typically outrunners.
With the reduction method, the crankshaft drives a simple transmission, which is usually a simple gearbox containing a pinion and spur gear. The transmission decreases the output RPM by the gear ratio (thereby also increasing output torque by approximately the same ratio). Reduction-drive is common on larger aircraft and aircraft with disproportionately large propellers. On such powerplant arrangements, the transmission serves to match the powerplant's and propeller's optimum operating RPM. Geared propellers are rarely used on internal combustion engines, but very commonly on electric motors. This is because most inrunner electric motors spin extremely fast, but have very little torque. Ducted fans
Ducted fans are propellors encased in a cylindrical housing or duct, designed to look like and fit in the same sort of space as a model jet engine but at a much lower cost. They are available for both electric and gas engines, although they have only become widely used with the recent improvements in electric-flight technology for model aircraft. It is possible to equip a model jet aircraft with two or four electric ducted fans for much less than the cost of a single jet turbine or large gas engine, enabling affordable modeling of multi-engine planes, including military bombers and civilian airliners.
The fan-unit is an assembly of the spinning fan (a propellor with more blades), enclosed inside a shaped-duct. Compared to an open-air propellor, a ducted-fan generates more thrust per crossectional-area. The shaped-duct often limits installation to recessed areas of the fuselage or wings. Ducted fans are popular with scale-models of jet-aircraft, where they mimic the appearance and feel of jet engines, as well as increasing the model's maximum airspeed. But they are also found on non-scale and sport models, and even lightweight 3D-flyers. Like propellors, fan-units are modular components, and most fan-powered aircraft can accommodate a limited selection of different fan-units.Other
With Ornithopters the reciprocating-motion of the wing structure imitates the flapping-wings of living birds, producing both thrust and lift.Model aerodynamics
The flight behavior of an aircraft depends on the scale to which it is built. The Reynolds number depends on scale and speed. Drag is generally greater in proportion at low Reynolds number so flying scale models usually require larger than scale propellers.
Mach number depends on speed. Compressibility of the air is important only at speeds close to or over the speed of sound, so the effect of the difference in Mach number between a slow piloted aircraft and a small model is negligible, but models of jets are generally not efficient flyers. In particular, swept wings and pointed noses are used at high Mach number to reduce compressibility drag and tend to increase drag at small Mach number.
Angular momentum also depends on scale. Since torque is proportional to lever arm length while angular inertia is proportional to the square of the lever arm, the smaller the scale the more quickly an aircraft or other vehicle will turn in response to control or other forces. While it may be possible for a pilot to fly an unstable aircraft (such as a Wright Flyer), a radio control scale model of the same aircraft would only be flyable with the center of gravity moved forward, or with avionics. On the other hand, angular inertia, and therefore large scale, generally degrades stability, because it introduces a delay. Static stability, resisting sudden changes in pitch and yaw, is generally required for all models and is usually considered a requirement for piloted aircraft. Dynamic stability is required of all but tactical piloted aircraft.
Free flight models and flight trainers need to have both static and dynamic stability. Static stability is the resistance to sudden changes in pitch and yaw and is typically provided by the horizontal and vertical tail surfaces, respectively, and by a forward center of gravity. The three dynamic stability modes are phugoid, spiral and Dutch roll. An aircraft with too large horizontal tail on a fuselage that is too short may have a phugoid with increasing climbs and dives. With free flight models, this usually results in a stall or loop at the end of the initial climb. Insufficient dihedral and sweep back will generally lead to increasing spiral turn. Too much dihedral generally causes Dutch roll. However, these all depend on the scale, as well as details of the shape and weight distribution. For example the paper glider shown here is a contest winner when made of a small sheet of paper but will go from side to side in Dutch roll when scaled up even slightly.
Source : Wikipedia
References : RCadvisor′s Model Airplane Design Made Easy, by Carlos Reyes, RCadvisor.com, Albuquerque, New Mexico, 2009
The Great International Paper Airplane Book, by Jerry Mander, George Dippel and Howard Gossage, Simon and Schuster, New York, 1967
Model Aircraft Aerodynamics, by Martin Simons, Argus, Watford, Herts, England, 1978
How to Design and Build Flying Model Airplanes, by Keith Laumer, Harper, New York, 1960
The Middle Ages of the Internal-Combustion Engine, by Horst O. Hardenberg, SAE, 1999
Model Airplane Design and Theory of Flight, by Charles Hampson Grant, Jay Publishing Corporation, New York, 1941