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Model aircraft

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. Many designs are possible, from simple gliders, 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 (also known as aeromodelling). Construction techniques for the two are usually very different.

Static model aircraft

Static 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 72 scale and 48 scale, followed closely by . 144 scale is popular for civil airliners, and there is a growing range of military subjects. More detailed models are available at 32 scale and 24 scale. Some manufacturers introduced 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 1:64, 1:96, and 1:128; however, old molds 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 , 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. 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.

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 and plastic. . Snap Fit plastic plane models are manufactured by Wooster, Long Prosper, and Flight Miniatures

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:

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. One company, Flying ThingZ of Stroudsburg, Pennsylvania, makes 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.

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, very thin paper can be substituted for fabric. 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 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. 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 (microfilm). The advent of "foamies," or craft injection-molded from lightweight foam and sometimes reinforced with carbon fiber, 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.

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.

Gliders

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, 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 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.

Hang gliders come in two large 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 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.

Power sources

Powered models contain an onboard powerplant to propel the aircraft through the air. The model is usually powered by an electric motor or small piston engine, but other types of propulsion include rockets, small turbines, pulsejets, compressed gas engines and twisted rubber bands.

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. Even so, a competitive model can achieve flights of nearly 1 hour. http://www.indoornews.com/indoorrecords/record_list.php

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

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.) Initial ignition is from an external electric current but once reciprocating, the engine heat, pressure and catalizing action of a platinum metal coil in the glow plug are sufficient to keep igniting 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). 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.

Not all simple internal combustion model aircraft engines use glow plugs. There are also "diesels" that are, strictly speaking, compression ignition engines. These also are carbureted, not fuel injected. They have an adjustable compression ratio using a threaded T screw on the cylinder head bearing onto a contra piston within the cylinder bore. They 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 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 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 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, introduced by the Stanzel brothers in Texas during early 1940. It was pioneered and popularized 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 counter-clockwise.

Line tension is maintained by centrifugal force and by the flight characteristics of the model. The air drag of the lines tends to yaw the model toward the inside hindering line tension. 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. 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. Anhedral (wings sloping downward to the outside) improves resistance to cross winds, as with the Wright Flyer. Sweep forward has a similar effect. 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 four 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 in Sweden in July 1996. In 2004 they took place in Muncie, IN, USA, and in 2006 in Spain. In addition to the international categories there are also national variants. The international rules are available from FAI.

  • Speed
  • Aerobatics

Team Race

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 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) (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.

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 , and a pitch of . 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 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.

Other

With Ornithopters the reciprocating-motion of the wing structure imitates the flapping-wings of living birds, producing both thrust and lift.

Model aerodynamics

(See also Flight dynamics).

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.

Monitoring of Model Aircraft Performance

The increased complexity of model aircraft power systems has created the need for tools to measure model performance, both during ground testing and in-flight.

As of 2008, the popularity of lithium polymer (LiPo) based electric power systems increased the need for this monitoring, due to the relative fragility of LiPo batteries. Several small and low cost in flight monitors and bench meters designed specifically for RC are available in 2008. One example is pictured at right.

Footnotes

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
  • Model Airplane Design and Theory of Flight, by Charles Hampson Grant, Jay Publishing Corporation, New York, 1941

See also

External links

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