FP vs. CP electric RC helicopters
The radio control heli hobby can be very overwhelming to the beginner, and two terms that might get you confused are fixed pitch and collective pitch, or 'FP' and 'CP' respectively.
Essentially these terms refer to the pitch control of the helicopter, and the word pitch refers to the angle of the main rotor blades in relation to the horizontal when they are static (i.e. not turning). OK so far?!
When the blades are spinning, however, the word pitch changes to the term Angle of Attack (AoA) and is referred to in relation to the airflow over the blades, rather than just a static horizontal reference.
It's important to note, however, that when comparing FP and CP electric rc helicopters this pitch name only refers to altitude control - cyclic control has the same method on each type of heli.
Above: Pitch angle becomes Angle of Attack when airflow is introduced over the blades.
Lift generation is the primary result (and goal) of a spinning rotor blade, and lift can be generated in two ways.
The first way is to change the Angle of Attack, and this is done by changing the pitch angle of the blades as they move through the air. A greater pitch angle (greater AoA) results in more lift, so the helicopter will rise, and conversely a lesser AoA will result in less lift generation, so the helicopter will descend.
The second method is to change the speed of the main rotors, while keeping the AoA constant. A faster turning blade will generate more lift, while a slower turning one generates less lift (the basic physics of an airfoil section moving through air, exactly like an airplane's wing).
This leads us on to the big difference between FP and CP rc helicopters; one type (CP) can change rotor blade pitch, while the other type (FP) cannot.
So, FP or CP?
Whether the helicopter is fixed pitch, like the Walkera Dragonfly 4 for example, or collective pitch, such as the Blade 400, is an important factor to consider when choosing an electric rc helicopter and the choice largely depends on what sort of flying you want to do, as well as your budget.
FP is self-explanatory in that the blade pitch is, well, fixed. The main blades do not pivot about their longitudinal axis to change pitch angle, (i.e. change the amounts of lift) and so the altitude (climb/descent) of the helicopter must be controlled by the motor speed - as previously mentioned, increasing the speed turns the blades faster and so more lift is generated, and slowing the motor results in less lift generation.
Generally speaking, FP helicopters are not particularly responsive in the air and can be a bit of a handful to control well, at least until the pilot gets used to the feel of it. They are, however, cheaper to buy, slightly easier to master and have less components and complexity to worry about.
CP means that the pitch angle of each main rotor blade can be moved to change the amount of lift generated, by tilting each blade about its longitudinal axis. The motor is kept at a more constant speed throughout the flight and so the altitude is controlled by changing this pitch angle, and hence Angle of Attack, of the blades rather than speeding up or slowing down the motor.
This collective pitch control makes the helicopter much more agile, responsive and smoother in the air but gives a steeper learning curve for the pilot - and they cost more to buy, and more can go wrong!
Above: left, an FP rotor head compared to a CP one, right.
Fixed pitch electric rc helicopters aren't generally seen as serious models - certainly not by CP heli pilots anyway! It's important to understand, though, that FP radio control helicopters are not toys and even though they are simpler than CP ones they should always be treated with respect; the spinning blades of a fixed pitch heli can do just as much damage as those of a collective pitch one. Don't ever be fooled by the (generally) smaller size of a fixed pitch electric rc helicopter; potentially they're just as dangerous!
The bottom line? If you just want an 'entry-level' taste of flying an electric rc helicopter with circuits and hovering, then an FP (or coaxial) rc helicopter will suit your needs very well. You will be limited in how you can fly it but you'll get to learn all about cyclic and yaw control, and how to fly a radio control helicopter safely.
But if you see yourself wanting to fly in a more demanding way, including any kind of aerobatics, and getting in to the hobby at a higher level then a CP helicopter is much more appropriate for your needs. It'll cost you more and take longer to learn, but will reward you with a much better flying experience. Certainly if you do want to fly aerobatically and progress to advanced 3D flying then a collective pitch helicopter is your only option.
The important thing to remember is that both FP and CP electric rc helicopters have a steep learning curve. Single rotor (i.e. not coaxial) helicopters are inherently unstable and first flights usually end in disaster! An rc flight simulator such as the Phoenix sim is an excellent idea if you're serious about flying radio control helicopters. You can learn in complete safety, without the worry of crashing your actual model.
But if you don't want to or can't afford a simulator, you're in for a lot of practice and some potential crashing which is an inevitable part of learning to fly a radio control helicopter well, so be prepared!
On that note, take a look at The Beginner's Guide To Flying RC Helicopters for a comprehensive introduction to the rc heli hobby.