The BMFA Specialist body for Radio controlled Helicopter flying is the AHA

Radio-controlled helicopters  are model aircraft which are distinct from RC aeroplanes because of the differences in construction, aerodynamics, and flight training. Several basic designs of RC helicopters exist, of which some (such as those with collective pitch, meaning blades which rotate on their longitudinal axis to vary or reverse lift so the pitch can be altered and can therefore change the angle of attack) are more manoeuvrable than others. The more manoeuvrable 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 same manoeuvres as full-sized helicopters, such as hovering and backwards flight, and many that full-sized helicopters cannot, such as inverted flight (where collective pitch control provides negative blade pitch to hold the helicopter up inverted, and pitch/yaw controls must be reversed by pilot).

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

The engines typically used to be methanol-powered two-stroke motors, but electric brushless motors combined with a high-performance lithium polymer battery (or lipo) are now more common and provide improved efficiency, performance and lifespan compared to brushed motors, while decreasing prices bring them within reach of hobbyists. Petrol and gas turbine engines are also used.

Types of R/C helicopters

Common power sources of R/C helicopters are glow fuel (also called nitro fuel - nitromethane-methanol), electric batteries, petrol and turbine engines. For the first 40 years, glow fuel helicopters were the most common type produced. However, in the last 10 years, Electric powered helicopters have matured to a point where power and flight times have equalled glow fuel helicopters.

There have been two main types of systems to control the main rotors, mechanical mixing and cyclic/collective pitch mixing (CCPM). Most earlier helicopters used mechanical mixing. Today, nearly all R/C helicopter use CCPM.

Practical electric helicopters are a recent development but have rapidly developed and become more common, overtaking Glow fuel helicopters in common use. Turbine helicopters are also increasing in popularity, although the high cost puts them out of reach of most people.

Glow fuel (nitro fuel)

Glow fuel, or nitro fuel helicopters (not to be confused with petrol powered helicopters) have been made in several sizes over the years. These are referred to as the "class" of the helicopter. They include 1/2A class, 15 class, 30 class, 50 class, 60 class and 90 class. These class numbers originated from the size of engine (engine displacement measured in cubic inches) used in the different models. For example, a helicopter with a .30 cubic inch engine is a "30 class" and a helicopter with a .90 cubic inch engine was referred to as a "90 class" helicopter. 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 time for nitro helicopters is 7–15 minutes depending on the engine size and tuning. The maximum height of operation for RC helicopters, be it glow fuel, gasoline, turbine or electric, is effectively limited to the height at which the model is still visible. Most quality radio control systems have a range of over a mile, when the model would be long out of sight.

Electric

Two small electric helicopters emerged in the mid-1990s. These were the Kalt Whisper and the Kyosho EP Concept, flying on 7/8 1200 mah NiCad batteries with brushed motors. However, the `540' brushed sized motors were on the limit of current draw, often 20-25 amps on the `hotter' motors, hence brush and commutator problems were common.

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.

A relatively recent innovation is that of coaxial electric helicopters. The system's simple direction control and freedom from torque induced yaw have, in recent years, made it a good candidate on 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 can have extremely quick control response, both of which are very pronounced in a CCPM model.

While a coaxial model is very stable and can be flown indoors even in tight quarters, such a helicopter has limited forward speed, especially outdoors. Most models are fixed-pitch, i.e. the collective pitch of the blades cannot be controlled, plus the cyclic control is only applied to the lower rotor. Compensating for even the slightest breeze causes the model to climb rather than to fly forward even with full application of cyclic.

Controls

RC Helicopters usually have at least four controls: 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 centred 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 fibre. Rotor blades are typically made of wood, fiberglass 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 often referred to as flybarless. 

Competition

Aerobatic helicopter flying has historically followed the Fédération Aéronautique Internationale (FAI) rules, which for helicopters are labelled F3C and. 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.