Dustflyer

My Feedback: (13)

This thing's probably been beaten to death before but I don't know, maybe it's time to bring it up again.

Today I went to pick up my 30% Carden Extra which I had retrofitted with a ZDZ 60 and canister muffler, a quiet setup previously impossible until the release of the new small ZDZ canisters. The gentleman who did the work normally has three airplanes in his garage: three Carden CAP 232's; 30, 35, and 40 percent. I happened to notice the 30 percenter wasn't in there. "Where's your 30% CAP Fred", I said. "The 30% is no more!" he replied.

"What happened?"

"PCM lock."

"Whaddya mean?"

"Well, I was letting my buddy fly the plane. He lined it for a pass and as it came toward us he yelled 'I got nothin!' so I grabbed the box and he was right. Nothing. Absolutely nothing. It's the worst kind of crash you can possibly have because you just sit there and watch the plane go in. I had no control whatsoever. There was nothing I could do. I hate PCM!"

So there you go. What's the deal? I've got a half dozen plain old PPM receivers, one Futaba and the rest FMA Fortress/ Quantum in everything from a 1/4 scale CAP to a hyper-fast Diamond Dust and all have been flawless. Never, ever a glitch!

Why the PCM? Sounds like when one of those locks up you are screwed, plain and simple!

Any comnments?

Woketman

My Feedback: (6)

Yeah, I got a comment. My brand F 9Z PCM rig destroyed my first Isobar. Went into lock out and cruised straight into planet Earth. Get a Multiplex IPD receiver. The marriage of the best of both worlds. Go here to read about it: http://www.multiplexrc.com/ipd_info.htm

I've been flying IPD (9 channel for 1.5 years and the 12 channel for almost a year) and so far, not a single issue that I did not cause by myself. The technology has, so far, been flawless and with the added bonus of absurd long long range!

TonyF

My Feedback: (92)

Well, I'll try not to bore you with comments about how great my sponsor's stuff is, but I have not had a single issue in 3 years and over 900 turbine flights with PCM equipment.

I truly suspect that many so-called "PCM Locks" are battery or power system failures. I truly wish more people would use redundant systems in these models. I have no way of saying for sure what happened to your friends model, but it could have been a battery failure.

Another PCM issue is the use of fail-safe. Too many are simply leaving things set to "hold" rather then programming something into fail-safe. My models are set to idle, in the case of the P-120, the shut-off procedure, and a little up-elevator. The elevator is not there to try to save the model, rather to let me see a visual cue that the model did go into fail-safe. If you don't do that, then you may never know of any problems until the situation deteriates to the point where you do have an extended hold.

I much prefer the ability of PCM to do some programmed thing if it loses link then some scattered control movement.

Bob_Wilcox

My Feedback: (1)

I can not comment on the IPD but there is no choice if you fly turbines. PCM only!

Forgues Research

My Feedback: (7)

But you do have a choice, you can use the IPD receivers from Multiplex.

First if you have problems with PPM, you should fix the problem first, because the problem would still be there with PCM, only you wouldn't know about until it locks up.
The main reason to use PCM, was because of the fails safe, but now the IPD's have one of the easiest to program Failsafe or hold.

I have a bunch of PCM receivers in a drawer gathering dust and will never go back to them.

BTW, Multiplex which invented the PCM's, stopped producing them in favor of the evolutionnary IPD's..

Gordon Mc

My Feedback: (11)

This topic has been covered many, many times, and a search through RCU or other newsgroups will get you a whole load of opinions, old wives tales, urban legands and some facts all mixed together. e.g. try:

http://www.rcuniverse.com/showthread...813&forumid=27

The fact that many people totally misunderstand how PCM works unfortunately does not in any way lessen their enthusiasm for spreading myths.

Without going into huge details (coz you can find that elsewhere), here is the core:

With plain PPM (lets ignore IPD for now), the receiver positions the servos according to whatever incoming signal it receives. If that signal is "clean", then the aircraft does what you told it to. If the signal is an amalgalm of the signal from your TX and some other source (i.e interference) then the receiver does not know that the signal is now garbage, and continues to position the servos accordingly. This is what causes the "glitching" that you frequently see when a PPM system receives interference.

With PCM, the signal is encoded in such a way as to carry not only the information stating where each servo should be positioned, but also a checksum which acts as a fairly good (but not 100% accurate) way of double-checking the integrity of the signal. The receiver receives the "frame" (one cycle's worth of data), extracts the servo position information, and (using the same algorithm as the TX does) calculates a checksum based on that information. Then it compares its newly calculated checksum with the one it received in the incoming frame. If they match, then the receiver positions the servos accordingly. If they do NOT match, then the receiver checks to see how many other invalid frames immediately preceded this one. If that number is large enough to match a pre-set parameter (typically matching 1/4, 1/2 or 1 second depending on what you set in the TX) then the RX will "go into failsafe" - i.e if you have programmed in some failsafe settings other than hold, the servos will be moved to those failsafe settings; if the number of immediately preceding invalid frames has not yet reached that limit, the receiver simply holds the servos in their last set position.

Now, here's the important part... as soon as the PCM receiver receives a valid frame, it responds to it immediately, and resets its count of how many consecutive invalid frames it received. In other words, no matter how many garbage frames there are, as soon as there is a single good frame, it is acted upon.

In other words, both PPM and PCM will act exactly the same if a valid signal exists. There is a lot of misunderstanding about what the failsafe "delay" setting means. A lot of people seem to think that this is a "lockout period" - meaning that the failsafe will stay engaged for at least that long before the RX will allow a good signal to be acted on. I have no idea where this idea originated, but its nonsense. As stated above, the RX will act IMMEDIATELY on a valid frame.

Consequentlky, the idea that "flying through the interference" is more likely with PPM than PCM (as suggested by your buddy's comments) simply does not hold up. Both systems will respond to even a single valid signal; PPM will glitch with even one or two frame's worth of bad data, whereas PCM will filter those out and stay in the last valid position; with continuous bad data, PPM will continue to glitch, and PCM will activate failsafe.

I used to fly just PPM. Then I got shot down, and I watched as my aircraft came screaming vertically out of the sky at full throttle, and dug a huge hole. I thought about what would have happened if the aircraft had pointed itself towards the pits instead of the weeds, and immediately went out and bought a PCM system so that I could program in at least a throttle-kill. That way it will come down with less enegry, and less energy means less killing power. (The failsafe isn't there to save the aircraft - its there to minimize the chance of killing someone with an out of control aircraft).

Also make sure that you choose PCM for the right reasons. I've heard of several people choosing to use PCM in their big gassers because they have so much noise from their ignition system that PPM is unusable. This simply means that you are using PCM to mask the problem, and the problem is still there, just waiting to bite you. Shield the ignition, plugs, boots etc if necessary, instead of just hiding the problem.

Last but not least - using PCM without properly setting your failsafe is irresponsible. My wife was hit by an aircraft that had its failsafe left as the default "hold", and which happened to be at full throttle when failsafe activated. If the pilot had correctly set his aircraft to idle / kill on failsafe, then she would have had several months less of pain to endure. Similarly, the death of 11 year old Adam Kirby in Britain a few years ago was ruled to be at least partly caused by the choice of "throttle hold" in the failsafe settings of the pattern plane that hit him. If you are using PCM, or any other failsafe method, please think seriously about the consequences, and choose your settings carefully.

As for IPD - I'll leave that to someone else, as I don't have any real-world experience of it.

Gordonn

lov2flyrc

My Feedback: (24)

Mark,
Are you saying the multiplex IPD receiver is compatable with Futaba PCM transmitters? I thought they could only be used with mutiplex systems??

Todd

Forgues Research

My Feedback: (7)

If I may Todd,
IPD's are not compatible with any PCM Tx's because they are not PCM receivers.
They are however compatible with any PPM Tx's. But if you use Futaba in North America, they have to be modified for Phase Shift and also with Hitec. No mods needed for JR, or Airtronics.

Chris Z

My Feedback: (24)

Tony,
I too set my failsafe so the turbine goes to idle, but instead of having one of the control surfaces set to move, I set the landing gear to drop.
The gear dropping is a sure sign that the signal was lost, an elevator jerk up is hard to tell sometimes from a windgust.
Just a suggestion.
Chris

TonyF

My Feedback: (92)

Chris,

My feeling is that the landing gear reacts too slowly to short duration fail-safe's. You'd never see the little ones with just the gear fail-safe'd. A quick up-elevator, however, is much more likely to be seen. Of course, the newer ECU's are starting to give you fail-safe counts, and they will be a valuable tool.

Woketman

My Feedback: (6)

Roger is correct. IPD receivers will work with any JR or Airtronics transmitter set to send a PPM signal. But I thought that you could purchase IPD receivers that already had the proper shift for Futaba & Hitec and not have them modified, but perhaps I was wrong. If you want to get one that way, simply go to the Multiplex web site and e-mail Karlton: http://www.multiplexrc.com/

Chris Z

My Feedback: (24)

Tony,
your right. My retracts drop really quick, I guess for the more scale slower extensions, it would't work.

mr_matt

My Feedback: (10)

As far as I know ALL of those "ECUs" come from JetCAT USA :-)

Modelman

My Feedback: (7)

SimJet D-1 ECU has built-in failsafe parameters as well.

Craig

BMT

Gordon,
I regards myself as pretty well qualified to speak on the subject of PCM and PPM. But I am not going to as your explenation is exactly spot on. I hope folks take note of your very valid comment of PCM masking interference problems. PCM is my choice but not knowing when them glithes start to appear is a downside for me. Our BMT controller not only counts failsafes but also logs the exact time when they occured and how long each one lasted and is displayed in real time together with parameters like airspeed, throttle position, RPM, EGT, Batt Voltage and Rx voltage etc in a graphical form once downloaded from the ECU. This may very well give early warning and clues as to the origin of "locks"
Andre Baird

mr_matt

My Feedback: (10)

did not know anyone had that feature, I have never actually seen anyone with a BMT.

Gordon Mc

My Feedback: (11)

Andre,

I appreciate the comments, and the explanation of the BMT system. Seems like you're continually coming up with neat ideas.

Baie dankie,
Gordon

Veritech

Mr_Matt,
I actually seen a BMT-80 at Winimac this year. The guy was flying it in a white Roo with yellow or orange accents I believe. The motor looked like any other KJ66 design other that the tail cone was not open in the middle where it meats, like the Ram 750 is, if that makes sense.

I asked the owner about it and he seemed very happy with the motor and he did not seem to have any problems with it. I believe that the BMT start box also charges his on board batteries when it is hooked up to the ECU, not sure if it charges the turbine pack or the RX pack but it seemed like a nice idea.

Now back to your regularly scheduled discussion,
Randy

BMT

Jis Gordon,
Dont tell me you are another of them brainy Sa's that left us all alone here..
Sies man,
Is dit lekker daar?
Cheers
Andre

Gordon Mc

My Feedback: (11)

Goeie more Andre - hoe gaan dit met u vandag ?

Ek het 7 1/2 jaar lank in Suid Afrika gewoon - van 1971 tot 1979. In die lasste 23 jaare het ek baie van die Taal vergeet :-( conseqently I'll proceed in Engels ;-)

I'm actually originally from Scotland, but my parents decided to emigrate to Australia and ended up in RSA instead. I loved it there, and somehow fluked it so that the SADF was willing to pay me to learn to fly while attending university - but just when I thought I had everything organized my folks decided to move back to Scotland - and dragged me with them since I was 4 months too young to be allowed to stay on my own.

Scotland didn't particularly excite me though, so as soon as I amassed enough experience to be able to escape, my wife and I set out to emigrate to Australia, and somehow ended up in California instead. I must admit, I love it here - in many ways it reminds me of RSA... great weather (unlike Scotland), extremely friendly people (unlike Scotland), and the ability to be financially compensated comensurate with how hard you actually work (definitely unlike Scotland).

I frequently think about coming back to visit SA (and show my wife where I grew up - Springs, in the Transvaal), but a lot of people have warned me to stay away from those areas. Pity. Maybe one of these days I'll still make it over tere, and just stick to the Cape, Natal and OFS.

Later,
Gordon

BMT

Hi Gordon,
My origins are also from the highlands of Scotland but my great grandfather escaped as far back as 1806.
Keep in touch and remember to visit us Cape Town when you return.
Groetnis
Andre