From RC Helicopter Wiki
Helicopters all generate lift and directional control via one or more large main rotors. Within this pattern there are a number of variations, both in design and how they are powered.
 Power sources
Lipo batteries are much lighter that NiMH batteries (for the same amount of power), but must only be charged with a special LiPo charger, or they may explode. They must also not be over-discharged, as they will be permanently damaged.
Electric helicopters are very quiet, but the batteries can be very expensive as the helicopters get larger. Recharging a battery takes about an hour, so regular fliers will want at least two or three battery packs.
Nitro helicopters run off a liquid fuel containing methanol and nitromethane (for power) and oil (for lubrication). Nitro helicopters are very loud and in general much larger than electric helicopters.
Nitro helicopters can be flown almost continuously as it is very easy to refill the fuel tank and fly, compared to electric helicopters where you may have to wait for your batteries to recharge. The fuel is relatively cheap compared to the up-front cost of LiPo battery packs, but of course you have to keep on buying it.
 Petrol (gas) and other power
A small number of very large helicopters use a two-stroke petrol engine, or gas turbine engine. These are strictly specialist machines. Some (unwitting) retailers advertise nitro machines as 'gas powered'; it is probably best to avoid these people.
 Helicopter designs
 Toy electric helicopters
Toy helicopters are any helicopter that does not have full directional control, which for all intents and purposes means it does not have a swashplate. Toy helicopters are often quite robust and easy for the beginner to fly, but time spent learning to fly them will not translate well to any more advanced helicopters. Spare parts are often not readily available, meaning that when the helicopter is damaged in a crash, it may be impossible to get it up and running again. Examples of toy helicopters are the PiccoZ.
 Electric Coaxial helicopters
Coaxial helicopters ("co-ax") have two rotors mounted one above the other on a common axis. The rotors rotate in opposite directions, cancelling out each other's torque reaction and therefore not requiring a tail rotor.
Electric coaxial helicopters are the easiest to learn to fly as they are designed to be self-stabilizing to a large extent; when trimmed correctly, if the pilot stops making control inputs, they will tend to return to something resembling a hover. Coupled with their relative robustness it makes coaxial helicopters an excellent inexpensive introduction to helicopter flying, especially for those who do not wish to invest the necessary time to learn to fly a conventional helicopter. The controls are similar enough to larger helicopters to make a coaxial a useful first step for someone with no helicopter experience.
Electric coaxial helicopters weigh in at from around 50 grams (2 ounces) to 250 grams (9 ounces), and are ideal for flying inside in a good sized room or squash court; they can be flown outside in absolutely still conditions. The low amount of energy stored in the lightweight spinning rotor blades limits the amount of damage that occurs when you crash, both to the helicopter and whatever you crashed into.
 Electric Fixed pitch helicopters
In helicopters generally, a fixed pitch ("FP") design refers to the lack of collective pitch control. Vertical control is achieved by turning the main rotor faster or slower, thus generating more or less lift. Model coaxial helicopters are typically fixed pitch.
When referring to model helicopters, a fixed pitch helicopter is typically one with a single main rotor and a smaller tail rotor for anti-torque control (also often fixed pitch, and driven by a separate motor). Fixed pitch helicopters have few moving parts; coupled with their light weight (50g (2oz) to 300g (10oz)) they are quite robust to crashes, as long as the throttle is closed before the crash occurs. They can be flown indoors or outside in a very light breeze (1-3mph).
In undemanding flight, a fixed pitch helicopter flies very similarly to a collective pitch helicopter, making it a useful stepping-stone to a more expensive to buy and run collective pitch machine, but they require substantially more practice to fly than coaxial helicopters.
In collective pitch ("CP") helicopters, the rotor speed ideally remains constant, and lift is varied by changing the pitch of the main blades throughout their entire rotation. This has a number of advantages over fixed pitch helicopters, including better control response, the ability to autorotate, and the ability to fly inverted. The price is a much more complicated mechanical design that is much more fragile than a fixed pitch helicopter.
 Electric collective pitch helicopters
Electric collective pitch helicopters generally start at about 450g (1lb), and the smaller models can be flown indoors in sports halls, but they are really outdoor helicopters, perfect for flying in large empty parks---local regulations permitting. The low mass also reduces the amount of damage done in crashes, and the helicopter can often be back up and running again quickly. The disadvantage of small collective pitch helicopters is that they are strongly affected by wind, limiting the wind they can be comfortably flown in to around 10-15mph. They also tend to have much more sensitive controls than larger helicopters, making them harder to learn to fly.
Larger electric helicopters should not in general be flown in public places because, like nitro helicopters, they can be extremely dangerous.
 Nitro collective pitch helicopters
Nitro powered collective pitch helicopters generally start at around 3kg. Due to their size and noise, they really require a dedicated flying site to fly. The extra size and weight does make them easier to fly, but the crash costs are typically more. Due to the weight and energy stored in the spinning rotor, in the hands of a new pilot they can be extremely dangerous and should only be operated with training.
 Unusual helicopters
Very rare are tandem rotor helicopters, where two counter-rotating rotors are mounted longitudinally on the helicopter, and the majority of the rotor torque is thus canceled out. Due to the general complexity, model tandem rotor helicopters are usually collective pitch, although there is no reason why fixed pitch versions are not possible.
 See also
- Helicopters on Wikipedia.
- Demonstration of fixed pitch helicopter robustness on YouTube.
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