RunRyder - PCM Failsafe

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Main Discussion > PCM Failsafe

Taipan
Key Veteran
Location: Sydney, Australia

Not sure if PPM/FM has f/s (too lazy to check!

)

I don't use it & know there is a big debate on it - my mostly planked club has had discussions on it.

Wasn't the recommended settings to have all controls on neutral with the pitch set at like 5 degrees & the engine on a fast idle? For planks I guess it would be all neutral, engine on idle & slight rudder to prevent runaways.

I didn't start this topic to debate failsafe or not. I was thinking, if it is set & a glitch activates it, does that mean it is out of control for good? As I think if it failsaves, you have to turn the Rx on & off to deactivate it. That right? If so, that means if the glitch passes, you can't control your heli as it's locked out. W/o f/s, you would get control back & land it safely.

Last thing, that means the Rx has the f/s software in it? It has to be has how else can it f/s if the Tx signal is lost?

01-24-2002 Over year old.

Phil Cole
Veteran
Location: Redwood City CA

Most PPM receivers don't have failsafe. Multiplex IPD receivers are the only ones that I know of with failsafe.

All PCM receivers that I know about have failsafe.

Failsafe only activates when the receiver decides that it's not getting a useable signal.

The moment the receiver gets the signal back you have control again. There is no delay in getting the control back.

01-24-2002 Over year old.

SemiArticulate
Veteran
Location: On Location

Found this:
http://www.torreypinesgulls.org/Radios.htm
its pretty good

01-24-2002 Over year old.

winger
Senior Heliman
Location: Indiana

Fail Safe

Ron Lund has a good explanation on why one may want to consider using this feature. He even gives recomendations on settings.

One advantage is if you think you're getting glitched, you'd have your throttle set to idle. If you're hovering an it glitches, you'll hear the engine drop out. Can be a great T/S aid. Plus if you're going in you don't want the engine stuck at speed and the heli doing the funkey chicken on the ground.

Got to http://www.ronlund.com/index.html

Good Luck,
Winger

01-24-2002 Over year old.

Maxx
Key Veteran
Location: Shreveport Louisana

I personally set the throttle hold switch and move the collective to around 0 degrees pitch and then hit the memory key! As Winger said; with a setup like this a glitch bad enough to pollute the signal to the receiver will bring the throttle to idle...I has a frame on my Futura SE crack and I didn't see it, flew most of a tank of fuel and was doing some max. preformance climbouts when the engine went to idle by itself...then came back up (the reduced throttle reduced the metal frames viberating together) and I got it down OK and found what I SHOULD have found during pre-flight! To me PMC failsafe is like the American Express Card...I don't leave home without it! Had this heli been flying PPM I don't think the interferance would have stopped because the engine (without the FS setting) would have stayed at the same throttle setting as the glitch occured at in the first place! JMO Chris

01-24-2002 Over year old.

GMPheli
Veteran
Location: W. Bridgewater, MA USA

Had you had ppm, you would have known about the cracked frame long ago. It would have started showing as random glitches. PCM masked those random glitches. You finally got to the point where the glitches lasted long enough to get you through the hold period and into failsafe. I doubt you would have crashed had you had ppm. Just my opinion.

Alan Angus

01-24-2002 Over year old.

BladeRunner
Elite Veteran
Location: Ontario Canada. Member of "some sort" s

PCM Failsafe?

In my humble opinion, we all are aware(or should be) of the fact that an out of control flying object with high energy rotating parts can and will do a lot of damage to a person, or worse.

ANYTHING that can eliminate or at least minimize that is worth consideration. I believe that the issue is not IF the aircraft will crash or land safely, it's more of what we can do to ensure personal safety. If PCM can aid in that end then it's a no brainer-just use it, as well as any other tools that can accomplish the same goals.

I find comfort in the fact that having lost radio control of the aircraft-for whatever reasons, that the engine can be set to low idle clearly reducing the risk of injury to anyone in it's path. That's just my opinion,aircraft damage and resulting costs are a secondary consideration.

--------BladeRunner.

01-24-2002 Over year old.

GMPheli
Veteran
Location: W. Bridgewater, MA USA

Bladerunner, I totally agre with you! However, until manufactures start putting counters on PCM recievers, to count how many times per flight the receiver goes into hold, I think PCM is more dangerous than PPM. A few years ago, I saw 2 planes try to fly on the same frequency. The one that was in the air was on PPM, the one taking off on PCM. The PCM plane took off, went into hold, and stayed there untill it crashed. The one on PPM was glitching but landed safely. I'll stick with PPM thank you.

Alan

01-24-2002 Over year old.

GMPheli
Veteran
Location: W. Bridgewater, MA USA

David, I am not talking about failsafe, but hold. Most PCM receivers stay in hold for about a second, then switch to failsafe if they still have not received a valid signal.

Alan

01-24-2002 Over year old.

GMPheli
Veteran
Location: W. Bridgewater, MA USA

Sorry Dave, as soon as a PCM receiver gets a bad frame, (it receives frames at 50 hz) it goes into hold until it either receives a good frame, or it reaches the predetermined failsafe time limit (usually around one second), then it goes into failsafe. As soon as it receives a good frame it will signal the servos to go to the positions in that frame. It does not take a second to go into hold. It takes 1/50th of a second. There is no way of knowing how many times your receiver goes into hold.

Alan

01-24-2002 Over year old.

DanW
Senior Heliman
Location: Baldwyn, Mississippi

PCM Failsafe

A friend of mine was flying his heli at a FF with a PPM receiver when some one turned on another transmitter on his channel. The heli went out of control servo's glitching everywhere the heli was headed toward a group of bystanders but thank God it plowed in just before it got to them. If he had be flying PCM with failsafe set to Idle the throttle and hold the rest the heli would have crashed but it would not have been nearly as dangerous. He now flies with PCM only and is much safer than PPM.

Thanks!
Dan

01-24-2002 Over year old.

HOMEPAGE

Maxx
Key Veteran
Location: Shreveport Louisana

GPMheli, I don't know what radio you use but my JR 10SXII is programmed to go to either hold OR failsafe (selectable on a per-channel basis) and the time can be set between .25 and 1.00 second! My receivers are set to go into FAILSAFE if they don't get a useable signal within the 1/4 second window I have set! I know this from personal experience! If you put your bird on the bench and set the Modulation to PPM and then turn off your transmitter; the servos will either drive off in some direction of stay exactly where they were when the signal from the transmitter ceased. If you do the same experiment with PCM the servos go to the preset Failsafe positions in the blink of an eye! To me, the main difference between PPM(FM) and PCM(FM) is : will the bird crash closer and with the throttle at idle or will it crash farther away and at whatever throttle setting it had when the signal was no longer being received! Chris

01-24-2002 Over year old.

fritzthecat
Key Veteran
Location: New Orleans

I think there is some confusion on the definition of 'Hold', 'Failsafe', 'Move' or 'Awaiting Valid Frame'.

Lets say your tooling along, doing the latest inverted double backflip pirouette roll when the local Taliban rep decides to turn up a tx on the same freq.
1- The rx will notice the bad frame. It will go into 'Awaiting Valid Frame' mode and starts to count down from the preset 'Failsave Timer'.
At this time the rx will not send a new signal to the servos, it will repeat the last good frame contineously.
2- If the interference continues past the allocated 'Time to Failsave', the rx will look into it's 'prereceived FAILSAVE settings' and decides to either:
HOLD the servos at the last known good frame position or:
MOVE the servos to a predetermined position iaw the 'prereceived' info. (Or a combination thereoff)

So, there is a very short timeframe (0.25 to 1.0 sec for JR, 0.75 sec for Futaba) when the rx is waiting and counting down. That is not the HOLD, MOVE or FAILSAVE mode. HOLD or MOVE come after the time runs out and FAILSAVE activates.
3- Now, as soon as one (1) good frame is received, the rx sends that one good frame to the servos and resets the failsave timer back to the beginning. For glitches that last less than say 0.25 seconds the Failsave will not activate but you can feel the control going sluggish as there can be a continuous delay of up to 0.25 seconds before the rx sends a valid frame and the servos react. If you know the feel of your heli this is a telltale sign of imminent VISA destruction so land and check it out.

Fritz

01-25-2002 Over year old.

GMPheli
Veteran
Location: W. Bridgewater, MA USA

I'm sorry, I called "awaiting valid frame", hold. (even though it is holding the last known good position). But irregardless of what you call it, PCM will mask short little glitches. These glitches are usually the start of something wrong. Depending on the way the component is failing, you may or may not be able to "feel" the sluggishness. It depends on the frequency of the glitching. How many times have you heard someone say that they where getting glitches with ppm, and they installed a pcm receiver and the glitches went away. Do you really think that they are gone? No they are still there, you just don't see them anymore. And they are quick enough so you don't "feel" them either. If these glitches are caused by say RF from a gasser ignition, and always remain the same, then you are OK. But if you have a component failing, getting worse over time, it can fail to the point that you go into failsafe and stay there. This would be avoided with an "awaiting valid frame" (or what I was calling hold) counter.

Alan

01-25-2002 Over year old.

GMPheli
Veteran
Location: W. Bridgewater, MA USA

We can take this one step further. With a frame rate of 50 hz, and a JR receiver set for the minimum failsafe time of .25 sec., it will take 12 consecutive bad frames before it will go into failsafe (or hold). The key word here is consecutive. 11 bad ones, and then one good one, and the clock starts over. With Futaba, you would need 37 consecutive bad frames before failsafe. The reason manufacturers do not incorporate a "waiting for valid frame" counter is probably because they know that the R/F link is not perfect, and there will be counts. Some people will think that this means something is wrong. What it should be used for is comparison purposes. If you find that you hits per flight are steadily increasing, then you probably have something that needs attention. I do agree, that in the event of catastrophic failure (ie someone turning on on your channel) failsafe set to low throttle is a good thing. If you are flying PCM, it is your responsibility to make sure it is properly programed. A teen was killed in the UK because PCM failsafe was not checked, and it was set to go to full throttle. Someone turned on while the plane (a big gas job) was idling. You can guess the rest.

Alan

01-25-2002 Over year old.

DangerousDick
Veteran
Location: Cheshire, England

I think the worst thing about PCM is the general ignorance from people who use it as to what it does and what it is used for (I'm not having a go at anyone or group of individuals particularly).

When people are asked whether they have PCM or PPM it will surprise you how many people ask what PCM and PPM mean - so you can guarantee that they won't have set the mechanism in the Tx to reflect the safety aspect it was designed for.

There are rules in place suggesting that the MINIMUM for failsafe is to get the throttle down to tickover but the rest is up to the individual.

The manufacturers unfortunately set the default setting for this feature to 'Hold Last Position' which means that if you're in full throttle at the time of radio link failure then the model will hold that setting until the innevitable happens. There also seems to be a lack of information presented to the buyer of radio systems to be aware that he/she has this facility and how to utilise it accordingly.

With the days of ARTF and price of equipment it is increasingly apparent that people are coming into the hobby with absolutely no experience and can go the the LHS with the cheque book in the morning to fly in the afternoon completely oblivious to the posibilities ahead.

I will happily fly PCM wherever possible and will fly nothing else on '60 size upwards but cost is such that I regularly fly PPM on the smaller models.

A little knowledge can be a bad thing and is regularly demonstrated by people at various levels in the hobby, the more information you can get on this topic (from people that do rather than those who just say they do) the more you are able to make the right decision for you.

Cheers
Richard.

01-25-2002 Over year old.

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