People often ask, "which is better, FM or PCM?" The short answer is, it's a matter of personal preference. If someone turns on a transmitter on the frequency you're flying on, a PCM-equipped helicopter will crash over here, while an PPM-equipped helicopter will crash over there.

The long answer is:

PCM	FM, aka PPM
Hides small amounts of interference The advantage of this is that transient interference (brief bursts of static) will cause the servos to hold their last position until the receiver can recognize the signal again. This is definitely a good thing if you're in a contest, because it means your aircraft won't bobble unexpectedly. The only drawback to this is that if your aircraft has a small radio frequency interference (RFI) problem, you won't see it right away. By the time this small problem develops into a big problem, it may be too late.	Glitches with small amounts of interference The drawback of course is that your aircraft will jump around during RFI incidents that would go unnoticed with PCM. However, small RFI problems have a way of growing into large RFI problems. Some folks (myself included) enjoy the security that comes from seeing problems when they're tiny, so they can be investigated and fixed before they become large.
Fail-safe PCM radios allow you to decide what positions the servos go to when the radio signal is lost. With an airplane that has a lot of dihedral, this might mean gliding in a circle until contact is re-established. With a helicopter, this doesn't mean much at all. It takes constant correction to keep a helicopter from crashing, so there isn't a lot you can do. The undeniable advantage to PCM is that you can set the throttle to go down to an idle in the even of interference. Often, just stopping the vibration from the motor (or even just changing the RPM) will be enough to stop the interference and allow you to regain control. And even if it doesn't, a helicopter with a low head speed will do a lot less damage to whatever it hits, as compared to a helicopter at full head speed. A child was killed in the UK recently due to a runaway airplane whose engine was revving away when should have been set to an idle or a complete stop.	Failsafe devices are available for PPM systems, but they aren't quite ready for prime time yet. Futaba makes a throttle-to-idle device, but they suggest using it for cars and boats only - not aircraft. At least one other manufacturer has developed a similar device as well. I'm not sure what makes the Futaba unit unsuitable for aircraft, but I do hope the problem is solved soon, as this would be a step forward for the safety of everyone involved in RC, including spectators.
Myth: Once PCM locks out, it stays locked out. Not true. A PCM system will function normally as soon as it can interpret the radio signal. This may happen a fraction of a second after the system locks out. Myth: If the servos didn't jump to their failsafe positions, there was no interference. Not true. Some PCM systems (definitely the Futaba 8UH transmitter and 148DP receiver I use) will command all servos to hold their last position for a couple seconds BEFORE commanding any of them to their "failsafe" positions (this includes bringing the throttle to idle). The advantage of course is that in competition you won't lose points for glitches - the disadvantage is that you won't know if there's a problem brewing until it's bad enough to cause a couple full seconds of signal loss. This also renders some "glitch counters" rather ineffective. The 'hold last position' delay fools the glitch counter into thinking everthing is OK. The glitch counter won't know there's a problem until the system goes into full lockout mode, at which point you'll know, too.	Myth: PPM is not suitable for helicopters. Not true. Typically you hear this from hobby shop personnel who would rather sell you the expensive PCM setup rather than the less-expensive (but often just as usable) PPM setup. I've been using PPM in my Concept 30 SRX for four years now. Myth: PPM is not suitable for gasoline-powered helicopters. Also not true. While gassers do generate more interference than glow kits, many people have used PPM successfully. Look at it this way: if youre helicopter is generating so much RFI that a PPM system cannot control it, you have a problem with your helicopter, not with your radio. Converting to PCM to get around a noise problem does not solve the noise problem. Rather, it masks the noise problem. If the noise problem gets worse, you may well find that PCM provided only a false sense of security. This is why I have been known to take PPM receiver out of my Concept and put it into my Futura for the first flight after a major rebuild. If there's a loose part somewhere causing RFI, I want to know immediately, while the interference is minor and manageable, rather that later, when the interference has grown to unmanageable proportions.

Package Deals
When you buy a complete radio system, you may have the option of a "PCM" or "FM" version. The PCM version is usually more expensive because it includes not only a more sophisticated receiver, but better servos as well. For example, the Futaba 8UHF (PPM) includes five single-ball-bearing standard-motor S3001 servos, while the Futaba 8UHP (PCM) includes a set of double-ball-bearing coreless-motor S9202 servos. The former cost about $32 each, while the latter typically sell for $55 to $60. The five servos might add $100 to the cost of the package, in addition to the $85 price difference between the PPM receiver (which uses old analog technology) and the PCM receiver (which uses new digital technology).

Failsafe Settings
Much has been written about the 'best' settings to use for PCM failsafe. Following are the settings I use, and the reasons I use them.

Throttle

This goes to idle. This is the single most important failsafe setting. Bringing the throttle to an idle will ensure that the rotor slows down before it hits the ground, which can save blades, property, and lives. If the interference was caused by vibration (and it usually is), there is a pretty good chance that the interference will go away when the engine goes to an idle. This means you get control again. Usually you get an oscillation - failsafe hits, the engine idles, control is restored, the engine picks up, failsafe hits, the engine idles, and so on. If this happens, hitting throttle hold will often restore control - pick a spot (any spot) and auto to it.

Leaving the throttle at its last position, or going to anything above an idle, is dangerous at best and fatal at worst. This is not theoretical - in the summer of 2000, a child in the U.K. was killed by a model that flew away, out of control, in lockout, with the engine running.

Gyro

This goes to heading hold mode.

Cyclics and Tail Rotor

I bring them to center. My thinking is that I'll be better able to regain control if the helicopter's orientation doesn't change while I'm out of control.

If you don't have a heading hold gyro, you should bring the tail rotor to the same "zero pitch" position you use for throttle hold, as this will help keep the heli from yawing when failsafe sets in.

Note that the gyro setting (heading hold vs. standard) and tail rotor setting (centered vs. zero pitch) should agree with each other. If the tail rotor commands zero pitch and the gyro goes to heading hold, you'll be out of control AND doing pirouettes. Ditto if the tail rotor commands "center" (hovering tail rotor pitch) and the gyro goes to standard mode - since there's no main shaft torque, you need zero tail rotor pitch to keep the heli from doing pirouettes.

Collective

Hold last position. For quite a while I was setting this to zero, thinking that it was the only compromise that makes sense - a little positive pitch would help if the heli was upright when failsafe it, but would auger the heli in if it was inverted when failsafe hit. 50/50 odds didn't much appeal me, so I went with 0 collective, which would be mildly disturbing no matter what happened. (Lovely.)

Then a fellow named David Lodge pointed out in the newsgroup that "Whatever collective you have at the time is most likely to decay the rotor speed most rapidly." And when it comes to not inflicting mortal wounds on people nearby, stopping the rotor is the top priority.

As an added bonus, this will keep the heli from embarking on an unexpected change of position or direction when failsafe hits. I had two incidents of zero-collective failsafe while hovering, both of which brought the heli almost to the ground before the control (and hovering pitch) was restored. Nothing will exercise your sphincters like watching a 5-foot flying lawnmower leap groundward from a waist-high hover, then leap back into that same hover a fraction of a second later.