GMPheli
Veteran
Location: W. Bridgewater, MA USA
| I feel compeled to interject one thing here. There is a negative to PCM that has not been discussed. PCM will mask small glitches. These small glitches can be an indication that something is going wrong (generating r/f that is interfering with your r/f link). PCM is designed to do this. PCM receivers receive frames at about 50 hertz. (a frame is a bundle of information that tells the receiver where each servo is supposed to be, plus some sort of verification code, a checksum I think it is called). As soon as a PCM receiver receives a bad frame, it knows it, and goes into hold (ie it holds that last known good position of all servos). If you have programmed the receiver for hold, then it will stay in hold until it receives a good frame. If you have programmed your receiver for failsafe, then it will stay in hold until a predetermined amount of time passes (.75 sec for Futaba, .25 to 1 sec adjustable for JR) and then go to the preprogramed failsafe positions. If at any time during this process the receiver receives a good frame, it will command the servos to go to those positions. If the next frame is bad, it will hold, and start the failsafe clock all over again. So what can happen is you can be flying around getting glitched and not know it. There is no way to know until you start getting glitched long enough so that you can feel the sluggisness of the controls due to all the hold time, or it goes into failsafe. If you can feel sluggishness, then you are getting hit very frequently, and are close to lockout. Granted, a lot of times the r/f noise is vibration related, and when failsafe kicks in and drops the throttle, the offending r/f is diminished, and contol is resumed. But again, it depends on the particular situation. A PPM receiver under these circumstances, will generally start glitching randomly, letting you know something is wrong. Don't get me wrong, I am not saying PPM is better than PCM. They both have there strengths and weaknesses. I just think in order to make an informed decision, one should have all the facts. I recently had a situation where I was getting random glitches with a PPM receiver on my just rebuilt from a crash, PHI Hurricane. It took me a while to figure out what was causing it, and I contemplated going to PCM. Turns out that when I rebuilt it, I installed a new metal tail rotor pulley. The pulley was rubbing on the outer race of one of the tail shaft bearings. I know this because I found a ring around the pulley when I dissasembled it. I shimmed the pulley .005" and the glitch went away for a while. Turns out .005" was not enough, as the tailshaft bearings wore (the tail rotor is almost always pushing one way), the pulley started rubbing again, and the glitch returned. Now I have one tail side plate and bearing shimmed .035, it won't rub again! I have not flown it since the .035 shim job, waiting for Spring!
Alan Angus |
