A fixed pitch helicopter has blades that are statically set to a given angle. Lift is controlled exclusively through the use of the throttle to raise or lower the speed of the main blades.
Fixed pitch helicopters require only two servos for the swash Cyclic control.
Fixed pitch helicopters are a good choice for beginners, not because they are any more stable or easier to fly but because they tend to be tougher. An FP heli will usually survive a typical beginner mistake, whereas a CP heli will usually break it's blades and bend a shaft or two.
CP helicopters tend to have symetrical blades, so they can fly inverted. FP helis can use cambered blades that are more efficient (but won't work upside down). This means they can generate the same amount of lift at lower speed. FP rotor heads are also simpler, as the blades do not need to rotate relative to each other, which means they can be more rugged. Finally FP machines are usually designed with the beginner in mind, so they are generally designed to sacrifice some speed and agility in favour of toughness and simplicity.
FP helis generally fly quite differently to CP helis, but the only factor that is directly attributable to the fixed pitch is that the only way to descend is to reduce the motor speed - and lower rotor speed means reduced cyclic authority. Rapid vertical descents should be avoided as you can loose all control. Similarly there is usually a slight delay when increasing the power.
Other differences in handling are usually related to the lower head speed.
- The first is that at lower speed, changes in cyclic pitch have a smaller effect. Often quite large stick movements are required to fly a FP heli (which can be a good thing for a beginner)
- The second effect is that the blades form a cone instead of a flat disk because there is less centrifugal force to hold them out and the lift causes them to bend upwards. As the cone of blades travels forward at speed, the blades at the front generate more lift than those at the back, causing the heli to nose up, stop and reverse (or fly in gentle circles). Forward cyclic is required to hold fast forward flight, and top speed may be limited more by the cyclic than by the motor power. Again, for a beginner, this can be useful as it stops the heli from building up too much speed and (given enough space and height) returns the heli to a hover. However this affect also means that the heli is pushed in the direction of the wind and knocked around by gusts.
The E-Flite mSR exhibits all these symptoms but it's handling is also affected by it's 45 degree flybar which gives the heli so much stability that it will stop as soon as the cyclic is released and hover hand free. Normal FP helis are not this stable and still need practice to hover.
Can a CP heli be converted to a FP by setting a flat pitch curve?
This question is commonly asked by people who want to make a CP heli easier to fly, but the above should show that while the answer is 'yes', you would only receive the disadvantages of the FP heli and none of the advantages! You would reduce the collective response (and possibly the head speed) while keeping the symetrical blades and complex rotor head.