RC Helicopter Buying Guide
What are RC Helicopters All About?
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, of which some (such as those with collective pitch, meaning blades which rotate on their longitudinal axis to vary or reverse lift) are more maneuverable than others. 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[citation needed] of the same maneuvres as full-sized helicopters, such as hovering and backwards flight, and many that full-sized helicopters cannot.
The various helicopter controls are effected by means of small servo motors, commonly known as servos. 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 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 are now more common and provide improved efficiency, performance and lifespan compared to brushed motors, while decreasing prices bring them within reach of hobbyists. Gasoline and jet turbine engines are also used.
Types of R/C helicopters
Common power sources are nitro (nitromethane-methanol internal combustion), electric batteries, gas turbines, petrol and gasoline.
Mechanical layouts include CCPM (cyclic/collective pitch mixing) in all power sources, fixed-pitch electric rotors and coaxial electric rotors.
Practical electric helicopters are a recent development but have rapidly developed and become more common, overtaking 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 (glow fuel)
Nitro or glow fuel helicopters come in different sizes: 15, 30, 50, 60 and 90 size. These numbers originated from the size of 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.
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 (£28). The next smallest is the standard Picooz 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 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. More advanced coaxial constructions with two swashplates and/or pitch control - common for the big coaxial helicopters like Kamovs - have been realized as models in individual projects but have not seen the mass market as of 2009.
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.
PCM
Pulse Code Modulation. A scheme in which the commanded position for each servo is transmitted as a digitally encoded number. Manufacturers use their own proprietary system to encode this number with various levels of precision (i.e. variable number of bits per servo position). JR use Z-PCM (9 bits, 512 different values: 0...511) then S-PCM (10 bits, 1024 values: 0...1023). Futaba use PCM-1024 and G3 PCM (11 bits, 2048 values: 0...2047). With PCM not all positions are broadcasted at one time (each frame) to save time. The odd numbered positions are sent as absolute in one frame, with the even sent only as differences from their previous values. The next frame the opposite is done. PCM includes a checksum at the end of the frame to check the signal's validity. Hence, if there is interference and the signal arrives distorted at the Receiver, utilizing the checksum it is able to know if it is the original. In case it is not, a feature called Fail-Safe is implemented to set servo positions to a predefined position, or to hold them at the last valid position.
PPM
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
