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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 balsa wood, foam and fiberglass. A vast array of designs are possible, from very simple gliders, to highly accurate scale models, some of which can be very large.
Models may be built either as static non-flying models, or as flying models (also known as aeromodelling). Construction techniques for the two are usually very different.
Static model aircraft
Static model aircraft, those not intended to fly, are scale models commonly 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.
Aircraft models can be bought already built and painted, as well as models that require construction, painting and gluing, or snap fit models (most of which come with decals or paint already applied).
The collector can choose from plastic and diecast military and commercial helicopters and planes; and for the less skilled collector, snap together military and commercial planes. Snap models are becoming increasingly popular because of their ease of construction.
Static plastic model plane kit manufacturers include Revell-Monogram generally recognized as the most popular in the US, Airfix, whose name is synonymous with the hobby in the UK, Heller, similarly synonymous in France, Academy, Hobbycraft, Dragon/DML, Italeri, Minicraft, Fujimi, Hasegawa, Tamiya, Trumpeter and Testors. The peak of the plastic modelling hobby was most probably the 1970s, and while it is still very popular today, at that time the hobby could support a considerable number of competing, large companies. During the 1980s, many of them were forced to radically downsize, restructure, merge, or go out of business. Some attribute this to the rise of computer games over the more traditional type of hobbies. Another consideration is that kits have generally required considerable skill and patience to achieve good results, and that ready-made or more quickly constructed models have taken over the market for those simply looking for a toy to play with.
Well-known static airpane model manufacturers who are no longer in business include Aoshima, Aurora, Crown, Faller, Frog, ESCI, Jo-Han, Hawk, L&S, Matchbox, Otaki, Pyro and VEB Plasticart. Many of their molds have been bought by surviving companies, but original kits are often available from specialists.
Plastic scale model aircraft kits usually come in scales such as 1/144, 1/72, 1/48 (also known as quarter-scale), and 1/32. This scale indicates the relationship between the size of the model and the size of the actual aircraft. For example, in 1/48 scale, 1 mm on the model represents 48 mm on the actual aircraft. 1/72 has been the most popular scale world-wide, but 1/48 offers more scope for detail and is a close second. Thousands of kits are available in these two scales. 1/144 is the most popular scale for scale airliner kits, with a selection of military kits available in this scale as well. A minority favor 1/200 airliners. 1/32 and 1/24 kits are generally military aircraft, from WWI to present.
Due to the high initial costs of producing hardened steel molds, there are relatively few manufacturers of injection-molded plastic models. Smaller manufacturers often produce models of lesser-known subjects, or accessories to enhance mainstream kits, but some very high quality molded kits of mainstream subjects have been released by Sweet, DACO/Skyline, Eduard and others. Most small manufacturers control costs by producing kits using lower-quality or short-working-life molds, vacuformed plastic, or resin casting. A world-wide cottage industry supplies improved and additional parts in resin, photo etched metal, vacuum formed plastic, wire, and printed plastic and paper, to support the kit builder, as well as decals, paints, magazines, books and tools.
Vacuum formed kits are generally for the more skilled builder and are manufactured by small companies such as Koster Aero Enterprises, Welsh Models, DynaVector, and AirModel. Specialized kits cast in resin are available from companies such as Collect Aire, CMK, and Unicraft.
Another category are scale plane models made from heavy paper or card stock. Several card model kit companies exist, smaller even than vacuum formed manufacturers, among them being ModelArt, Halinski, Modelik, JSC, Williamshaven and FlyModel. Many card models are also distributed through the internet, and several are offered this way free of charge. Card model kits are also not limited to just aircraft. Such kits are available for all types of vehicles, buildings, computers, firearms, even animals.
Die-Cast model plane manufacturers include Dyna-Flytes (recognized as the first manufacturer of that type of model), Schabak, Gemini Jets and Herpa Wings.
Snap Fit plastic plane models include Wooster, Long Prosper (In South Africa, Long & Prosper), and Flight Miniatures of Cottonwood, Arizona.
Most of the world's airlines allow their fleet aircraft to be modelled as a form of publicity, some of the most notable being Delta Air Lines, Air France, British Airways, Aerolíneas Argentinas, Avianca, Aeroméxico, Fed Ex, Polar Air Cargo, Air New Zealand, Qantas, China Airlines, Singapore Airlines, South African Airways, Finnair, American Airlines, United Airlines, Lufthansa, Japan Airlines, Royal Jordanian, Korean Airlines and Asiana Airlines.
In the early days, airlines would have large models of their aircraft made and shipped to travel agencies as a promotional item. These models are considered to be some of the most prized model aircraft collectibles. One of the most expensive aircraft models in the world, that of a Boeing 707 made in the 1/10 scale, is valued at US$18,000.
Flying 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 are designed and built in a manner that allows the craft to 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 are designed and built to be flown using 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 almost as much as static models do, while others are built with no intention of looking like piloted aircraft. There are also models of birds and flying dinosaurs. One company, Flying ThingZ of Stroudsburg, Pennsylvania, makes a line of rather whimsical models in addition to a lineup of conventional aircraft. Their more unusual offerings, produced from laser-cut corrugated plastic include a witch on a broomstick, a flying M1A2 Abrams tank, a flying race car and even a 2/3-scale flying lawnmower.
The construction of flying models is very different from most static models. Flying models borrow construction techniques from 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, very thin paper can be substituted for fabric. Or, heat-curing plastic films ("heat shrink covering" or "solarfilm") can be ironed on — a hand-held iron causes the film to shrink and adhere to the frame. A hair dryer can also be used.
Home-grown 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 often 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. Some of these are made by bringing frames of balsa wood and carbon fiber up through water to pick up thin plastic films, similar to rainbow colored oil films. The advent of "foamies," or craft injection-molded from lightweight foam and sometimes reinforced with carbon fiber, have made indoor flight more readily accessible to hobbyists. Many come ready-to-fly, requiring little more than attachment of the wing and landing gear. See: ParkZone Slo-V.
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 (sometimes elaborate) 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 typically spends many hours assembling the frame, covering it, and polishing/refining the control surfaces for correct alignment. Furthermore, the kit does not include necessary tools, and these have to purchased separately. Finally, a single overlooked error during assembly could compromise the model's airworthiness, leading to disaster.
To address these concerns, and increase the hobby's accessibility to the inexperienced and less interested alike, vendors of model aircraft 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 to assemble the model. 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. Brands associated with these types of aircraft include Great Planes, Hobbico, Carl Goldberg Products, Lanier RC, E-Flite, Hangar 9, GWS, HobbyZone and ParkZone.
Gliders are aircraft with no attached powerplant. Model gliders are usually hand-launched or catapult-launched (using an elastic bungee.) The newer "discus" style of wingtip handlaunching has largely supplanted the earlier "javelin" type of launch. Other launch methods include 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, RC gliders can remain airborne for as long as the updraft prevails. 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 available 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 and rewarding.
Walkalong gliders are light weight 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.
Powered models contain an onboard powerplant to propel the aircraft through the air.The model is either an electric or gas motor
Old and cold
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.
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 quite capable of powering lightweight scale radio control aircraft. Gasparin and Modella are two current 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. Thermodynamically, this means that it stores negative entropy, rather than energy, and extracts heat energy from the surrounding environment.
Steam, which is even older than rubber power, and like rubber, contributed much to aviation history, is now rarely used. Steam power is not now commonly considered in connection with flight. (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.
For larger and heavier models, the most popular powerplant is the glow-engine, a form of internal combustion engine. Glow-engines appear similar to small gasoline motorcycle-engines, but glow-engines are considerably simpler in operation. The simplest (and cheapest) glow-engines being a two-stroke cycle engine, using a glow plug to ignite fuel, which is a mixture of slow burning methanol, nitromethane, and lubricant (castor oil or synthetic oil.) The heat, pressure and catalizing action of platinum metal in the glow plug coil are sufficient to ignite the fuel. The four stroke glow engines (see below) with alternate intake and exhaust cycles also rely on the same fuel and ignition system. The reciprocating action of the cylinders applies torque to a crankshaft, which is the power-output of the engine. 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). As Richard Feynman mentioned in his famous There's Plenty of Room at the Bottom lecture, the speed an engine can rotate without breaking tends to go as the inverse of the linear dimension (inverse cube root of the displacement). However, the intake air flow improves less quickly than that with small scale, due to decreasing Reynolds number and, eventually, to viscous flow. Under ideal conditions, the smallest .01 engines can turn a 3.5" (9 cm) propeller at speeds near 30,000 rpm, while the typical larger (.40-.60 cubic inch) engine will turn at 10-14,000 rpm.
Not all simple internal combustion model aircraft engines use glow plugs. There are also "diesels", that are popular in Europe and the world over. These also are carbureted, not fuel injected. They have an adjustable compression ratio and burn a more easily ignited mixture of ether and kerosene (with lubricating oil). These are preferred for endurance competition, because of the higher energy content of the fuel.
Internal combustion (IC) engines are also made in upscale (and up-price) configurations. Variations include four-strokes, multi-cylinder engines, and even spark ignition gasoline powered units. All IC 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.
Jet and rocket
A recent 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 pre-existing 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. In the 1950s, a type of model rocket motor called the Jetex engine was quite popular. Today, flyers mount readily-available model rocket engines to provide a single, short (<10second) burst of power. However, government regulations and restrictions have rendered rocket-propulsion unpopular even for gliders.
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 new 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 high-cost prevented widespread adoption within the industry until the early 1990s, where 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 glow-engine) is prohibitively expensive (>$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.
Also referred to as U-Control in the USA, the models are 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. Over the years since its introduction in the late '30:s, control line flying has developed into a high level competition sport.
There are four main contest categories for control line models: Speed, Aerobatics, Team Racing and Combat.
The international rules are defined by the Fédération Aéronautique Internationale (FAI). World and Continental (presently only European) Championships are held with semiannual interleaving. The World Championships were held here in Sweden in July 1996. 2004 they took place in Muncie, IN, USA, and in 2006 it is Spain's turn. In addition to the international categories there are also national variants. The international rules are available from FAI.
Team Race — the Formula One of aeromodelling The international class is F2C. 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 pilot + mechanic teams compete simultaneously in the same circle, and the object is to finish the determined course as fast as possible. The all-out important catch is that 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) (19 g:s). 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. Conduct of both pilots and mechanics is subject to numerous regulations.
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 and highly specialized 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.
Most powered model-aircraft, including electric, internal-combustion, and rubber-band powered models, generate thrust by spinning an airscrew. By far, the propeller is the most commonly used device. The blades of the rotating propellor push against the atmosphere, and by Newton's Third Law, the air's reactionary force pushes the aircraft.
As in full-size planes, the propellor'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 propellors, 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 propellors are usually specified as diameter × pitch, given in inches. For example, a 5x3 propellor has a diameter of 5 inches, and a pitch of 3 inches. The pitch is the distance that the propellor 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 different 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 propellors. On such powerplant arrangements, the transmission serves to match the powerplant's and propellor'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 are propellers 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 rise of effective electric power 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 or large gas engine, enabling accurate modeling of planes such as military bombers and civilian airliners.
The fan-unit is an assembly of the spinning fan (a propellor with more blades), held 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.
In jet-powered model aircraft, the engine is a single-piece assembly with no user-changeable parts. The turbine-wheel spins at extremely high speed (>150,000 RPM), limiting most adjustments to the original factory.
Ornithopters do not use airscrews at all, the reciprocating-motion of the wing structure imitates the flapping-wings of living birds, producing both thrust and lift.
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. 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 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 may have a plugoid with increasing climbs and dives. 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.
Δ==References== 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
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