Model aircraft

A model aircraft is a small sized replica of an existing or imaginary aircraft. It may be flying or non-flying (static), and may or may not be an accurate scale model of a full-size design.

Flying models range from simple toy gliders made of card stock or foam polystyrene to powered scale models made from materials such as balsa wood or fibreglass. some can be very large, especially when used to research the flight properties of a proposed real design.

Static models range from mass-produced toys in white metal or plastic to highly accurate and detailed models produced for display and requiring thousands of hours of work. Many models are available in kit form, typically made of injection-moulded polystyrene.

Aircraft manufacturers and researchers also make wind tunnel models not capable of free flight, used for testing and development of new designs. Sometimes only part of the aircraft is modelled.

Static models for display

Static model aircraft (i.e. those not intended to fly) are scale models 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.

Models are available 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.

Promotional use

Most of the world's airlines allow their fleet aircraft to be modelled as a form of publicity.^[1] In the early days, airlines would order large models of their aircraft and supply them to travel agencies as a promotional item.

Scale

Static model aircraft are primarily available commercially in a variety of scales from as large as 1:18 scale to as small as 1:1250 scale. Plastic model kits requiring assembly and painting are primarily available in 1:200, 1:144, 1:72, 1:50, 1:48, 1:32 and 1:24 scale, often depending on the size of the original subject. Die-cast metal models (pre-assembled and factory painted) are primarily available in 1:400, 1:200, 1:72, 1:600, 1:500, 1:300, 1:250, and 1:48. A variety of odd scales (e.g. 1:239) are also available but less common.

The scales are not random but are based on easy divisions of imperial measuring systems so that 1:48 scale is 1" to 4 feet, 1:72 is 1" to 6 feet. Even numbered scales are similarly based on metric systems. 1:72 scale was first introduced in the Skybirds wood and metal model aircraft kits in 1932. Skybirds was followed closely by Frog which produced 1:72 scale aircraft in 1936 under the "Frog Penguin" name. According to Fine Scale Modeler magazine, 1:72 was also popularized by the US War Department (renamed the Department of Defense) when it requested models of single seat aircraft at that scale. The War Department also requested models of multi-engine aircraft at a scale of 1:144. The War Department was hoping to educate Americans in the proper identification of aircraft. These scales provided the best compromise between size and detail. After WWII, the toy manufactures continued to favor these scales. 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 Revell, such as the Fokker Dr.I and Sopwith Camel.

A number of manufacturers have made 1:18th scale aircraft to go with cars of the same scale. Aircraft scales have commonly been different from the scales used for military vehicles, figures, cars and trains – the armour equivalent of 1:72 was 1:76 and the rail version was 1:87 scale however the difference between the scales can be noticeable and so a small number of kits have been offered over the years that match these alternate scales, while there has been a growing tendency in recent years for smaller military vehicle kits to use aircraft scales. This has resulted in a substantial amount of duplication of the more famous subjects in a large variety of sizes, which while useful for forced perspective box dioramas has limited the number of possible subjects to those that are more well known.

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 as they were sized to fit inside standard commercially available boxes. These kits are often called "box-scale" and are often reissued in their original, unusual scales.^[2] 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.

Materials

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 available in the other manufacturing processes used for models but the molds are expensive and require large production runs to cover the cost of making them. Smaller and cheaper runs can be done with cast copper molds, and some companies do even smaller runs using cast resin or rubber molds, but the durability is of a lower standard than carbon steel and labour costs are higher.

Specialized kits cast in resin are available from companies such as Anigrand, Collect Aire, CMK, CMR, and Unicraft, made in molds similar to those used for limited run plastic kits. Vacuum forming is another common alternative to injection molded kits but require more skill to assemble, and usually lack detail parts that must be supplied by the modeller.^[3] There is a handful of photo etched metal kits which allow a high level of detail but can be laborious to assemble, and lack the ability to replicate certain shapes.^[4]

Scale models can be made from paper (normal or heavy) or card stock. Commercial models are printed by publishers mainly based in Germany or Eastern Europe.^[5] 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.^{[citation needed]}

From 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, while Guillow's made relatively easy kits. Many model makers soon became adept at creating models from actual aircraft drawings.^[6]

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.^[7] Snap Fit plastic plane models are manufactured by Wooster, Long Prosper, and Flight Miniatures.^{[citation needed]}

Flying models for sport (Aeromodeling)

Generally known collectively in all its forms as the sport and pastime of aeromodelling, 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 dinosaurs. The reduced size affects the model's Reynolds number which determines how the air reacts when flowing past the model, and compared to a full sized aircraft the size of control surfaces needed, the stability and the effectiveness of specific airfoil sections may differ considerably requiring subtle changes to the design.
Flying model aircraft used in the range of different aeromodeling activities can be placed in one of three groups:

• Free flight (F/F) model aircraft fly without external control from the ground. This type of model pre-dates manned flight.^[8]
• Control line (C/L) model aircraft use cables (usually two) leading from one wing to the controller. A variation of this system is the Round-the-pole flying (RTP) model.
• Radio-controlled aircraft have a transmitter operated by the controller, sending radio signals to a receiver in the model which in turn actuates servos which manipulate the model's flight controls in a similar manner to a full sized aircraft.

Construction

The construction of flying models differs from that of most static models as both weight and strength are major considerations. Flying models borrow construction techniques from full-sized aircraft although the use of metal is limited. These might consist of forming a frame using thin planks of a light wood such as balsa to duplicate the formers, longerons, spars and ribs of a vintage full sized aircraft, or on larger (usually powered) models where weight is less of a factor, sheets of wood, expanded polystyrene and wood veneers may be employed. Regardless of the underlying structure, it is then skinned and subsequently doped to provide a smooth sealed surface. For light models, tissue paper is used. After it is applied, the paper is sprayed with a mist of water, which causes the paper to shrink when it dries. For larger models (usually powered and radio controlled) heat-curing or heat shrink covering plastic films or heat-shrinkable synthetic fabrics are applied to the model then heated using a hand held laundry iron or heat gun to tighten the material and adhere to the frame. Microfilm covering is used for the very lightest models and is made by bringing the model up through water to pick up a thin plastic film on the surface made from a few drops of lacquer spread out over several square feet. For a more mass market approach, "foamies," or aircraft injection-molded from lightweight foam (sometimes reinforced) have made indoor flight more accessible to hobbyists. Many require little more than attachment of the wing and landing gear.

Flying models can be built from scratch using plans, or assembled from kits. Plans are intended for the more experienced modeller, since the builder must make or find all the parts themselves. A kit contains the necessary raw material, some molded parts, plans, assembly instructions and has usually been tested. Assembling a model either way can be labour-intensive. To increase the hobby's accessibility to the inexperienced, vendors of model aircraft have introduced Almost Ready to Fly (ARF) designs which reduce the time and skills required. A typical ARF aircraft can be built in under 4 hours, versus 10–20 or more for a traditional kit. More recently, Ready To Fly (RTF) radio control aircraft have been offered however among traditional hobbyists, RTF models are controversial as many consider model building integral to the hobby.

Gliders

Glider aircraft do not have an 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 cord. 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 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, sailplane flying can be challenging.

Hang gliders are composed of rigid & material delta wings with a harness (payload) suspended from the airframe. Control is exercised through the movement of the harness in opposition to a control frame,

Paragliders are aircraft consisting of its parts: wing, strings, and a falling mass. Usually the wing of a paraglider is a flexible, fabric wing whose shape is formed by its suspension lines made taut by the paraglider's falling payload and the pressure of air inflating either a single-skin or double-layered wing. Some wings are ram-air where air enters vents in the front of the wing. Control is exercised through lines that deform the trailing edge of the airfoil or the wing's end regions.

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, a mechanism powering propulsion of the aircraft through the air. Electric motors and internal combustion engines are the most common propulsion systems, but other types include rocket, small turbine, pulsejet, compressed gas, and tension-loaded (twisted) rubber band devices.

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, and the fact that the initial high torque of a fully wound motor drops sharply before 'plateauing' to a more steady output, until finally declining as the final turns are run off. Using this torque curve efficiently is one of the challenges of competitive free-flight rubber flying, and variable pitch propellers, differential wing and tailplane incidence and rudder settings, controlled by an on-board timeswitch, are among the means of managing this varying torque and there is usually a motor weight restriction in contest classes. Even so, a competitive model can achieve flights of nearly 1 hour.^[9][10]

Stored compressed gas, typically carbon dioxide (CO₂), can also power simple models in a way similar to filling a balloon and then releasing it.

A more sophisticated use of compressed CO₂ is to power a piston expansion engine, which can turn a large, high pitch propeller. These engines can incorporate speed controls and multiple cylinders, and are capable of powering lightweight scale radio-controlled 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, whether using ordinary poppet valves or occasionally rotary valves offer superior fuel-efficiency (power-output per fuel-consumption), but deliver less power than two-stroke engines of the same displacement – yet, often because the power they deliver is more suited to turning somewhat larger diameter propellers for lighter weight, more drag-producing airframe designs such as biplanes and scale aircraft models of pre-World War II full-scale subjects, four-stroke model engines, fueled either with methanol or gasoline fuels are slowly increasing in popularity from their generally lower noise output.

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).

Jet and rocket

Ducted Fans

Early "jet" style model aircraft utilized a multi-blade and high pitched propeller (fan) inside ductwork, usually in the fuselage of the aeroplane. 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.

Turbine engines

A major 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 (from companies such as Evojet in Germany) 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 turbine engine or 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, as well as many nations' national aeromodeling clubs (as with the USA's AMA) requiring their users to be certified to know how to safely and properly operate the engines they intend to use for such a model.^[11] 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 World War 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". Due to the noise, the use of these is illegal in some countries.

Rocket engines

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. In some countries, 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 European availability of the 'cartridge' motors seemed to ensure a future, but in recent years the cartridges (known as "Rapier" units)have become difficult to obtain, due to a reclassification from "smoke producing devices" to "fireworks". They are still produced in the Czech Republic, but exporting them is problematic.

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 cases surpass that of glow-engines – however, the increasing popularity of the much more rugged and durable lithium iron phosphate-celled batteries is increasingly attracting attention away from LiPo packs. 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 and electric power technologies, with the increasing adoption of brushless motors powered with better battery chemistries and controlled with a electronic speed control in place of a throttle servo 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.

Starting around the year 2008 the entry of Chinese direct-to-consumer suppliers into the hobby market has dramatically decreased the cost of electric flight. It is now possible to power most models weighing less than 20 lb with electric power for a cost equivalent to or lower than traditional power sources. This is the most rapidly developing segment of the hobby as of end of year 2010.

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.

Team Race

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.

Propulsion 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, both 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 liquid-fuelled 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 petrol or methanol 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. Speeds of up to 200 mph have been recorded on electric-powered ducted fan airplanes, largely due to the high amount of RPMs produced by ducted fan propellors. 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.

Models in manufacturing

Aircraft manufacturers and researchers make models for various purposes. Besides static display for marketing purposes these include models for aerodynamic research and engineering manufacture.

Aerodynamic research

Research models are made for wind-tunnel and free-flight testing. For wind tunnel research especially, it is often only necessary to make part of the proposed aircraft.

Engineering mock-ups

Full-scale static engineering models are constructed for production development, often made of different materials from the proposed design. Again, often only part of the aircraft is modelled.

Model aerodynamics

(See also Flight dynamics).

The flight behavior of an aircraft depends on the scale to which it is built, the density of the air and the speed of flight.

At subsonic speeds the relationship between these is expressed by the Reynolds number. Where two models at different scales are flown with the same Reynolds number, the airflow will be similar. Where the Reynolds numbers differ, as for example a small-scale model flying at lower speed than the full-size craft, the airflow characteristics can differ significantly. This can make an exact scale model unflyable, and the model has to be modified in some way. For example drag is generally greater in proportion at low Reynolds number so a flying scale model usually requires a larger-than-scale propeller.

At higher speeds approaching or exceeding the speed of sound, the Mach number becomes important (the speed of sound is Mach 1). At these speeds the air becomes compressible and its characteristics change dramatically, with shock waves forming. Fast jets are often inefficient at low airspeeds, so a model unable to fly at the speed of sound will also be inefficient. In particular, the swept wings and pointed noses common on fast jets tend to increase drag at small Mach number.

Manoeuverability also depends on scale, with stability also being more important. Control torque is proportional to lever arm length while angular inertia is proportional to the square of the lever arm, so the smaller the scale the more quickly an aircraft or other vehicle will turn in response to control or other forces.

One consequence of this is that models in general require longitudinal and directional stability, resisting sudden changes in pitch and yaw. While it may be possible for a pilot to respond quickly enough to control an unstable aircraft (such as a Wright Flyer), a radio control scale model of the same aircraft would only be flyable with design adjustments for stability, or with avionics providing artificial stability. Free flight models need to have both static and dynamic stability. Static stability is the resistance to sudden changes in pitch and yaw already described, and is typically provided by the horizontal and vertical tail surfaces respectively, and by a forward center of gravity. The three dynamic instability modes are pitch (phugoid) oscillation, spiral and Dutch roll. An aircraft with too large a horizontal tail on a fuselage that is too short may have a phugoid instability 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. 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.

See also

Footnotes

1. ↑ These include Delta Air Lines, Air France, British Airways, Aerolíneas Argentinas, Avianca, Aeroméxico, FedEx, 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.
2. ↑ Revell's Wright Flyer was reissued in the original and unusual scale of 1:39.
3. ↑ Koster Aero Enterprises, Welsh Models, DynaVector, and AirModel manufacture vacuum formed models.
4. ↑ "Photoetching At Home" by Andy Slater, The Model Makers Resource," accessed 30 March 2007
5. ↑ Card model kit companies, smaller even than vacuum formed manufacturers, include Schreiber-Bogen (one of the largest), ModelArt, Halinski, Modelik, JSC, Williamshaven and FlyModel.
6. ↑ http://www.jitterbuzz.com/navion.html
7. ↑ Die-Cast model plane manufacturers include Dyna-Flytes, Schabak, Gemini Jets and Herpa Wings
8. ↑ http://web.archive.org/web/20091028062404/http://www.geocities.com/aerohydro/mfm/model.htm
9. ↑ Testing Commercial Rubber – R.J. North, Model Aircraft magazine, Feb, 1961
10. ↑ http://www.indoornews.com/indoorrecords/record_list.php
11. ↑ AMA. "AMA Documents – Turbines". AMA. Retrieved 5 October 2012. 

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

External links

Wikibooks has a book on the topic of: RC Aircraft

Wikimedia Commons has media related to Models of aircraft.

• Model aviation on the Open Directory Project
• Database of diecast model aircraft

v t e Scale modeling Miniature arts Architectural models Building models Brass models Diecast models Model aircraft Model cars Model commercial vehicles Model construction vehicles Model figures Matchstick models Model military vehicles Dioramas Model robots Model trains Model rockets Model ships Gundam model Miniature wargaming Room box Dollhouse 1:6 scale modeling List of scale model kit manufacturers