Nitro-powered Thunder Tiger Raptor 60

Electric-powered Venom Air Corps Ocean Rescue

Electric-powered Align T-rex 450SE

This Heli-Max Axe Micro CX model helicopter is an example of a micro-sized coaxial model. Note the size comparison with the cellular telephone at right.

Radio Controlled helicopters (also RC helicopters) are model aircraft which are distinct from RC airplanes because of the differences in construction, aerodynamics, and flight training. Several basic designs of RC helicopters exist, some more maneuverable than others (such as helicopters with collective pitch). The more maneuverable designs are often harder to fly, but benefit from greater aerobatic capabilities.

Flight controls allow pilots to control the collective and throttle (usually linked together), the cyclic controls (pitch and roll), and the tail rotor (yaw). Controlling these in unison enables the helicopter to perform most of the manoeuvres a full-sized helicopters can do, such as hovering and backwards flight, and many that full-sized helicopters cannot.

The various helicopter controls are affected by means of small servo motors. A piezoelectric gyroscope is typically used on the tail rotor (yaw) control to counter wind- and torque-reaction-induced tail movement. This "gyro" does not apply a mechanical force, but rather, electronically adjusts the control signal to the tail rotor servo.

The engines used to typically be methanol-powered two-stroke motors, but electric brushless motors combined with a high-performance lithium polymer battery are now more common, as improved performance and decreasing prices bringing these within reach of more people. Gasoline and jet turbine engines are also used.

Types of R/C helicopters

Power sources include:

• Nitro (nitromethane-methanol internal combustion)
• Electric
• Gas turbine
• Petrol / gasoline

Mechanical layouts include:

• CCPM in all power sources
• Fixed-pitch electric
• Coaxial electric

Electric helicopters are now more common than nitro helicopters. Practical electric helicopters are a relatively recent development but have rapidly developed and become more common, and effectively overtaken nitro helicopters in common use. Gas-turbine helicopters are also increasing in popularity, although the high cost puts them out of reach of most people.

Nitro

Nitro helicopters come in different sizes: 15, 30, 50, 60 and 90 size. These numbers originated from the size of nitro engine used in the different models (0.30 cu in, 0.50 cu in and so on). The bigger and more powerful the engine, the larger the main rotor blade that it can turn and hence the bigger the aircraft overall. Typical flight times for nitro helicopters is 7-14 minutes depending on the engine size and tuning.

Electric

Recent advancements in battery technology are making electric flying more feasible in terms of flying time. Lithium Polymer (LiPo) batteries are able to provide the high current required for high performance aerobatics while still remaining very light. Typical flight times are 4-12 minutes depending on the flying style and battery capacity.

In the past electric helicopters were used mainly indoors due to the small size and lack of fumes. Larger electric helicopters suitable for outdoor flight and advanced aerobatics have become a reality over the last few years and have become very popular. Their quietness has made them very popular for flying sites close to residential areas and in places such as Germany where there are strict noise restrictions. Nitro helicopters have also been converted to electric power by commercial and home made kits.

The smallest remote-controlled production model helicopter made (Guinness World Records 2006) is the Picooz Extreme MX-1 sold at many toy stores (although this is infrared controlled, not radio), electronics stores and internet stores, costing about $30 (£15). The next smallest is the standard Picooz counter-rotating blade helicopter.

Several models are in contention for the title of the smallest non-production remote-controlled helicopter, including the Pixelito family of micro helicopters, the Proxflyer family, and the Micro flying robot.

A recent innovation is that of coaxial electric helicopters. The system's inherent stability has, in recent years, made it a good candidate for the design of small models for beginner and/or indoor use. Models of this type, as in the case of a full-scale helicopter, eliminate rotational torque and extremely quick control response, both of which are very pronounced in a CCPM model.

While such a model is extremely stable and can be flown indoors even in tight quarters, a coaxial model helicopter has limited forward speed, especially outdoors. Most such models are fixed-pitch, i.e., the blades do not rotate on their axes which simplifies the model, but eliminates the ability to compensate with collective pitch input. Compensating for even the slightest breeze causes the model to climb rather than to fly forward even with full application of cyclic.

Radio gear

Radio

Small fixed-pitch helicopters need a 4-channel radio (throttle, elevator, aileron, rudder), although micro helicopters that utilize a 2-channel infrared control system also exist; while collective-pitch models need a minimum of 5 channels with 6 being most common (throttle, collective pitch, elevator, aileron, rudder and gyro gain). Because of the normal interaction of the various control mechanisms, advanced radios include adjustable mixing functions, such as throttle/collective and throttle/rudder.

Radio prices vary from $100-$2,000 USD.

Well-known manufacturers of helicopter-specific radio controllers include: JR, Spektrum, Futaba, Hitec, Sanwa (known as "Airtronics" in North America), Multiplex (a division of Hitec).

Modulation

Radios emit the FM signal in two types of modulation.

PPM is cheaper than PCM and is generally used in low-end helicopters. The lack of a failsafe in PPM makes it more suited to small, less dangerous models. Higher-end radios offer PCM and PPM modulation for better compatibility with all radio receivers.

Pulse Code Modulation. A scheme in which the commanded position for each servo is transmitted as an encoded number. Manufacturers use their own proprietary system to encode this number with various bits of precision. JR use Z-PCM (10 bits, 512 values) then S-PCM (11 bits, 1024 values). Futaba use PCM-1024 and G3 PCM (12 bits, 2048 values). PCM also includes a feature called Fail Safe to set servo positions to a predefined position, or to hold them at the last valid position, should a valid radio signal be lost.

Pulse-position modulation. A scheme in which the commanded position for each servo is transmitted as the duty-cycle of the transmitted pulses 1 per servo position.

Spread spectrum

This new application of an established technology uses frequency hopping on the 2.4GHz band, instead of the various frequencies in the lower MHz ranges, the real advantage of this system is that the frequency hopping means that radios are no longer using just one frequency all the time during the flight, but a multitude of frequencies.

Many radios can be on at once, without interfering with each other, as they change frequency approximately every two milliseconds, so even if two transmitters are on the same channel, they are not on for long, so the pilot is not going to notice any abnormal behaviour of the model in the 1/500th of a second that they are interfering. This gives one the advantage of turning on the radio transmitter without having to worry about causing interference with other pilot's radios.

The down sides to this technology at the moment are mainly legal, as there is some question as to whether they will be banned in Europe due to the pressure from Wifi providers, as it uses the same band they do, as some claim that spread spectrum interferes with these networks.

The other down side is that precautions have to be made in the installation in models, certain materials in a models construction such as carbon fiber can mask the signal. In some cases, "satellite" receivers with secondary antennas need to be used to maintain better line of sight with the transmitter radio.

Controls

RC Helicopters usually have at least four controls: Aileron (Roll - Cyclic Pitch), Elevator (Fore-Aft Cyclic Pitch), Rudder (Yaw) and Pitch/Throttle (Collective Pitch/Power).

For simple flight, the radio is usually configured such that pitch is around -1 degree at 0% throttle stick, and somewhere around 10 degrees at 100% throttle stick. It is also necessary to modulate the throttle in conjunction with the pitch so that the model maintains a constant 'head speed' (the rotor's RPM). This is beneficial for consistent and smooth flight performance.

If aerobatic '3D' performance is desired, then the 'idle up' mode of flight is used. In this mode, the collective pitch ranges from its negative limit at 0% throttle stick input, up to its positive limit at 100% throttle stick. The throttle, on the other hand, is modulated automatically by the radio transmitter to maintain a constant head speed and is usually at its lowest value when the throttle stick is centered and the pitch is zero. This mode allows the rotor to produce a thrust 'upwards' (by using negative pitch) which, when the model is inverted, allows sustained inverted flight. Usually a more advanced computer radio is used for this kind of flying, which allows customization of the throttle-collective mix.

The cyclic and yaw controls are not by definition different in these two modes, though 3D pilots may configure their models to be much more responsive.

Construction

Construction is typically of plastic, glass-reinforced plastic, aluminium or carbon fiber. Rotor blades are typically made of wood, fibreglass or carbon fibre. Models are typically purchased in kit form from one of about a dozen popular manufacturers and take 5 to 20 hours to completely assemble.

These model helicopters contain many moving parts analogous to those on full-size helicopters, from the swashplate to rotor and everything in between.

The construction of helicopters has to be more precise than for fixed-wing model aircraft, because helicopters are susceptible to even the smallest of vibrations, which can cause problems when the helicopter is in flight.

Additionally, the small size and low weight of R/C helicopters and their components means that control inputs, especially cyclic (pitch and roll) can have a very fast response, and cause a rotation rate much faster than the equivalent input might produce on a full-size aircraft. In some cases, this quick response can make the model unnecessarily difficult to fly. For this reason, most model helicopters do not use the (simpler) Bell rotor head design, but instead use the Hiller design with a flybar, or Bell-Hiller mixing, the former providing a much greater degree of stability, and the latter mixing the quick response of the Bell system with the stability of the Hiller design. Some models use the simple Bell design, but this is limited mainly to scale models that are more challenging to fly, or models using advanced electronic stabilizing equipment.

To reduce mechanical complexity and increase precision of the control of the swashplate some model helicopters use Cyclic/collective pitch mixing.

Competition

Aerobatic helicopter flying has historically followed the Fédération Aéronautique Internationale rules, which for helicopters are labelled F3C. These include a predetermined routine of hovering and aerobatics.

An advanced form of RC helicopter flying is called 3D. During 3D flying, helicopters perform advanced aerobatics, sometimes in a freestyle form, or in a predetermined set of moves drawn up by the organisers of the competition. There are a number of 3D competitions around the world, two of the best known being the 3D Masters in the UK and the eXtreme Flight Championship (XFC) in the USA.

Commercial applications

Although RC helicopters are generally used by hobbyists for recreational purposes, they are sometimes used in applications such as low altitude aerial photography, filming, policing, and remote observation or inspection. Some companies make RC helicopters specifically for these uses.

Remotely piloted or autonomous helicopters powered by electric motors instead of by gasoline or diesel internal combustion engine, with electricity provided by fuel cells or batteries include a type of small unmanned aerial vehicle (UAV) which can carry 10 pounds weight of payload.