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PPM, PCM ?

Old 08-25-2005, 08:49 AM
  #1  
SJN
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Default PPM, PCM ?

I just recently lost my Hotspot due to failsafe condition with my Futaba 9c and futaba 149dp.
Its a chilling feeling when flying your jet at high speed, and it just no longer responds to your input, and you just watch it go in.

I never tried this before, but Im bummed that there is no waring or anything.....just total lockout.
It makes my want to install a dual conversin PPM reciever in my next jet, so I at least maybe could get some warning by glitches before total loss of controle



Are you guys using PCM only, or are you using PPM ?




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Old 08-25-2005, 09:02 AM
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Miguel Santana
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Default RE: PPM, PCM ?

Sorry to hear about your loss....I know that feeling too [&o]
I think is important to determinate what caused your radio going into fail safe condition? voltage problem, interference from other radio, etc, etc??
Old 08-25-2005, 10:49 AM
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Default RE: PPM, PCM ?

That pic of your avatar HS sucks, Sonnich. Sorry for your loss. I would prefer to use a PCM receiver in a jet, althought you can use a PPM one without any problems. (I am actually using a PPM futaba in the roo, in a very busy R/C club without concern) ... as you said , the PPM receiver forewarns you of some problem making the servos glitch, but IMO this is not as good as the possibility of activating the landing gear, speed brakes and flaps in case of interference as in a PCM receiver...

A good thing of the brand of turbine I prefer, is that there is a log of the interference hits
on the receiver after each flight... a good way to check how good is your radio link or enviroment. Most times, at the end of a flight session I plug the data terminal on the plane just to see this info...

Enrique
Old 08-25-2005, 12:14 PM
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rhklenke
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Default RE: PPM, PCM ?

Sonnich,

Since moving to jets, helis, and giant scale stuff, I have made it a policy to never again fly PPM (except in my little electrics, but I have a PCM RX in my little TRex electric heli). The PCM stuff is much more bullet proof and having the failsafe option to shutdown the turbine is a must (although you can get add-on stuff to do that with PPM). The PCM transmission/reception mechanism is inherently so much more noise resistant that it can fly in noise environments where PPM is unflyable (been there and seen it). If you really had a lockout on your PCM radio (i.e., it wasn't a power failure, total RX failure, etc.) then I can almost guarantee you that you wouldn't have been able to fly it at all with PPM. Also, in my experience, the failure mode with PPM, is AT BEST, a series of fast glitches followed by a total loss of control - all so fast that you couldn't have done anything but watch anyway, just like you did with PCM...

Bob
Old 08-25-2005, 01:08 PM
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GrayUK
 
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Default RE: PPM, PCM ?

SJN

Really sorry to hear the news, but i would never fly PPM in a JET.[]

Can you imajin a 'glitch' ..say full up elevator... at 200 mph?

Paul
Old 08-25-2005, 01:25 PM
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Default RE: PPM, PCM ?

Sonnich, lot´s of interesting info there... take a look.

http://www.rcuniverse.com/forum/m_22...tm.htm#2231175

BR, Enrique
Old 08-25-2005, 01:25 PM
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Gerald Rutten
 
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Default RE: PPM, PCM ?

AAouch!!! That must have hurt, sorry to see this Sonnich!

I have made it a policy to never again fly PPM
....hmmmm, my policy since flying with jets is never to fly with PCM again after some experiments back in 1997 when I started jet flying. Of course a lot has changed since, but I also witnessed lots of holes in the ground due to PCM usage. True, PCM should be more bullet-proof, filtering a lot of noise out but if the user went to PCM after experiencing some glitches on PPM he makes a terrible mistake!!! Fact is that there where some disturbances and he now suppresses them, not good!

My rough guess is that I have accumulated around 2000 flights all together on 4 different models and never had a crash related to glitching or lockout. The PPM warns you when there is a problem where PCM doesn´t until it´s too late. Than you can sit back and see how the model goes straight ahead into.......[]

.....and having the failsafe option to shutdown the turbine is a must
All proffesional ECU´s have this already incorporated. I use Orbit and after RC signal lost it automatically shuts the turbine down for me, still having a controllable model thereafter (When there is still some signal available) permitting me to still land the thing.

This is for years a large discusion and for sure has come up here on RCU several times. Curious on how this one goes though

Regards,
Gerald

Old 08-25-2005, 03:14 PM
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afterburner
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Default RE: PPM, PCM ?

One practice I have learned from others in the gas engines forum, is when setting up a new plane, use a PPM receiver and do extensive range checking at all throttle settings and 360 degrees around the plane. When satisfied that the installation is clean in that there is no signifigant loss in range when the engine is running as opposed to when it is off, now you install your PCM receiver knowing that it isn't just masking a noise problem. Of course this situation can possibly change over time as tolerances change of certain components in the ignition system and new frequencies or higher amplitudes may start to be generated.

Marty
Old 08-25-2005, 03:33 PM
  #9  
Gerald Rutten
 
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Default RE: PPM, PCM ?

[sm=confused.gif].......so why not stick with PPM if it was ok in the first place???

I was always teached that if something works, don´t mess with it!!!

Cheers,
Gerald
Old 08-25-2005, 03:58 PM
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Yann
 
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Default RE: PPM, PCM ?

SJN, never go to PPM, stick to PCM...
Old 08-25-2005, 04:24 PM
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Big feet
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Default RE: PPM, PCM ?

Sonnich, Sorry about your plane. I hate all crashes but unexplained ones are the worse.
This is an interesting debate. I do not fly turbines, wish I could afford to. However, I do fly at
public shows from time to time and always use PCM with the failsafe set to close throttle and controls set for
snap roll/spin. This stops the forward flight path very very quickly so preventing the model going of into the distance/ crowd.
I understand the expense of doing this with a turbine model but at the end of the day, better safe than sorry.
From memory (and I could easily be wrong here), after entering failsafe the Rx should continue to look for the Tx signal (until the ground interrupts) so if you did not get control back PPM would not have returned control either.

I fly at a club with numbers limited by our landlord so we all have our own frequency, radio issues are not a problem.

Would this be an option, say 1 channel for 2 or 3 members, and so on? At least you could rule out other Tx interference.

Regards

Peter
Old 08-25-2005, 10:41 PM
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robert440kts
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Default RE: PPM, PCM ?

Interesting idea with large club members!
Old 08-25-2005, 11:32 PM
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Flyjets
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Default RE: PPM, PCM ?

Hi SJN

PCM works by filtering the signal so noise doesn't get thru. When a Receiver goes into it's programed fail Safe it doesn't just stay there. The Reciever is constantly looking for a Signal so it can return out of Fail Safe.

While it is true that PPM will show u a hit better you could program Failsafe in PCM to warn you as well. For instance maybe have it drop the Gear if it goes into Failsafe. Also flying PPM you won't have your engine automatically shut down or throttle back like PCM will.

My 2 cents is only Fly PCM and do a very good range check.

Ian
Old 08-26-2005, 12:56 AM
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Stig Andersen
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Default RE: PPM, PCM ?

Well, Sonnich.

You know the statistics. No jet was ever lost here in DK due to radio interferance on PPM. Only the fancy PCM stuff crash. NO matter how wise you guys are, the statistics speek for them selves. So you guys just keep spending money on expensive receivers and a new plane once in a while........

Some turbine brands have the fail safe function built inn, and will shut down the turbine in case of radio interferance.

Sorry guys,---- couldn't help it.

Paul, (GrayUK)---- We've had this discussion before, havn't we

Bottom line is. Use the system you're comfortable with.
Old 08-26-2005, 03:16 AM
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specialFX
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Default RE: PPM, PCM ?

Has anybody tried out the Diversity-System from ACT?
More Information is avail here: [link=http://www.acteurope.com/]ACT europe[/link]
A buddy is about to order a system and try it out in his 2,5m Katana. If erverything works as expected I´ll give it a try in my SU-27. Dual receiver setup, fully programmable via IrDA and Palm, dual frequency setup possible etc. Seems to be a promising system.
Its PPM but with dual receivers checking each other for data integrity full operational failsafe is possible. No need for DPSI or powerboxes, up to 20 Servo output channels, each one programmable. Very Promising.
Good discussion of this topic is here: [link=http://rc-network.de/forum/showthread.php?t=18242]RC-network.de[/link] (sorry, it´s german, but good info)

regards
Thomas
Old 08-26-2005, 05:01 AM
  #16  
Gerald Rutten
 
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Default RE: PPM, PCM ?

SJN, never go to PPM, stick to PCM...
@ Yann: Arguments please!

This stops the forward flight path very very quickly so preventing the model going of into the distance/ crowd.
For safety this is a very good argument. Although I rather see some backdoor way of even faster resuming control instead of controlling our heavy loaded kerosene bomb into what I don´t know....

.....so if you did not get control back PPM would not have returned control either.
Wrong, if PCM didn´t reacted at all there may be a slight chance that you could have had a jittering signal from PPM. Not ideal of course but maybe the last tiny bit to bring the model home or at least bring it to an area where it´s safe to plumb it into terrain, away from crowd.

Also flying PPM you won't have your engine automatically shut down or throttle back like PCM will.
As I mentioned before, most (If not all) turbine brand ECU´s have this function also incorporated.

The Reciever is constantly looking for a Signal so it can return out of Fail Safe.
It can, but most TX do this with a certain delay. With a 200+ mph aircraft this is naturally unacceptable....

I agree with Stig that the statistics do have the tendency to bend towards the PPM pro´s, although I also agree that you have to use the system where you have the most confidence with.
Thomas system looks interesting though.

Regards,
Gerald
Old 08-26-2005, 06:01 AM
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Default RE: PPM, PCM ?

Sorry for the loss SJN..

but the point is, at least for me: to know what caused the black out..

an interference, or something (battery) be disconnected on the plane?? (another lock out cause...)

why i had a loss (not jet) on a lock out and the cause was a faulty battery!!
Old 08-26-2005, 06:44 AM
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SJN
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Default RE: PPM, PCM ?

Thanks for all the very good comments guys.
I still have some time to make up my mind until my next project is done.

I do not know the cause of the failsafe.
Im pretty sure that the plane still had power. I had 2 batt wires from the same battery going into the RX First flight of the day, and the Batt pack was just fully recharged. The plane was at very high speed in a bank, so the elevon servos where deflected quite a bit. Im sure if I had a power loss, that the control surfaces would have been pushed to neutral, or at leeast moved a tiny bit and the plane would have reacted to that.... But it just kept its final track, and the engine shut off.

We where only 2 pilots at the flightline that mornig, so its unlikely that someone shot me down.
Old 08-26-2005, 06:48 AM
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Default RE: PPM, PCM ?

Sonnich, I was in your shoes about five years ago. (ch Futaba systems on PCM destroyed one of my jets, seriously damaged another. I had it! That's when I tried the 12 ch MPX Profi, first with the 9 ch IPD RX, then the 12 ch. I have NEVER looked back. Rock solid reception since with NEVER an issue (that I did not cause). The MPX IPD is it! Hands down, in my experience. Forget PCM, move up!
Old 08-26-2005, 06:49 AM
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GrayUK
 
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Default RE: PPM, PCM ?

The point is you will never know why it locked out, it could have been battery, or xtal failure or any number of other reasons that were not interference.

So we need to put this into perspective.
Most interference is transient, even sun spot activity, local hospitals, electric cables etc.. can cause problems.

Transient interference is the 'glitch' you see when flying, could be anything, could even be skip from another country.
I can remember sitting at my local field one hot summers day listening on the monitor to Italian taxi's!

So, transient interference is what your RX has to deal with most of the time. In this situation the PCM wins, it just ignores it, you do't see it and the 200mph model does not get a sudden 'glitch' sending it who knows where, or worst still, due to loads, having a structural failure

Bear in mind that the frame rate on most systems is about 50 hz (50 per second), so 50 updates to the Rx every second.
My PCM needs 1 complete second of no good signal to activate failsafe. This means you can lose 49 frames of data and still have control, even if response is sluggish.

When 1 second is reached the failsafe mode engages, in my case, shut throttle and hold all other settings.
The RX is still looking for a 'good frame' and on my system the return of control is almost instant.

So, in a transient interference condition (99% of all interference) the PCM RX will do its job.
What happens on total signal loss?
Neither system will help [&o]

PPM will send this energy full projectile hurtling in all directions as the RX tries to make sense of the interference and pass it on to the servo's.
The PCM will (in my case) shut the throttle, hold the settings and the model will continue on that line until signal is restored or the ground comes up and hits it.

So, both are terminal, but in the PCM case you have a projectile on a set course which makes the chances of getting out of its way higher.

I have had lock outs, over the years, it is frustrating, but even with that i would NEVER fly a jet on PPM.


Try a little experiment on the ground.
Get 2 sets of gear in 2 models.
Put them both on the same frequency, 1 on PCM 1 on PPM.

Switch on the PPM Model and TX, with it working now switch on the PCM TX. All hell breaks loose, controls go every where.

Now switch every thing off and put the PCM set on, once running switch on the PPM TX.

You will still have control, as i said before the response may be sluggish, but you will have control.

Move the PPM TX closer to the PCM RX and slowly you will lose control, the RX will start 'framing' as it loses more and more good frames.

Closer still and it will go to failsafe.
Step back and immediately you have control.

If that does not convince you...nothing will [X(]

I have demonstrated this many times to 'doubters' who usually go out and by a PCM set after wards.

At the end of the day you fly what you are happy with, i love PCM, never had an issue with it, had far more crashes due to interference before i used it than i have since.
I will stay with it....until something better comes along....

Stig
Yes we have had this conversation before
Old 08-26-2005, 08:03 AM
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Gordon Mc
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Default RE: PPM, PCM ?


ORIGINAL: Gerald Rutten
The Reciever is constantly looking for a Signal so it can return out of Fail Safe.
It can, but most TX do this with a certain delay.

Sorry, but that is just plain wrong. See http://www.rcuniverse.com/forum/m_19.../tm.htm#199627
Old 08-26-2005, 08:37 AM
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Default RE: PPM, PCM ?

ORIGINAL: Gerald Rutten

[snip]

As I mentioned before, most (If not all) turbine brand ECU´s have this function also incorporated.


Regards,
Gerald
You're the second guy to mention that, but the problem is, if the ECU is still getting pulses, even though they may be erratic from a PPM glitching episode, the turbine will continue to run. I know of no ECU that will monitor the incoming pulses and filter them like an IPD receiver will, so the only way to get reliable shutdown on loss-of-signal is to use PCM.

BTW, the point still stands that if you truly had a PCM failsafe due to interference, then the aircraft would have been unflyable with PPM as well. I would suspect that most of the crashes that guys attribute to PCM "lockout" are really, either battery failure, switch failure, servo failure, or RX crystal failure...

Bob
Old 08-26-2005, 08:49 AM
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Default RE: PPM, PCM ?


ORIGINAL: rhklenke

...
I know of no ECU that will monitor the incoming pulses and filter them like an IPD receiver will, so the only way to get reliable shutdown on loss-of-signal is to use PCM.
...

Bob
That´s not exactly true, the ACT diversity system provides reliable programmable failsafe with PPM. The system monitors signal integrity of both receivers and is able to establish safe transmission even if one RX completely fails. The integrated data-logging can be downloaded after each flight into a Palm for further investigation and storage on a PC. It logs signal strength and how often during a flight one (or even both) RX´s lost signal and in how many cases the diversity sys was able to recreate full connection. Very useful. Seems to be a well engineered system.

regards
Thomas
Old 08-26-2005, 09:29 AM
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Default RE: PPM, PCM ?

i'll start by saying that I agree 100% with everything that Stig posted. His post deserves to be read over and over. I have a gas engine background so I will comment on the idea that an airplane should be set up to with ppm and then go pcm. there might be a few guys that do this, but it really is a waste of time. one of the posts mentioned spurious noise from various sources, and how pcm ignores it and ppm results in a glitch. so imagine i have a 40% airplane with a ppm receiver, and I am doing all sorts of tests and the engine ignition creates this spurious noise - which some of them do. there will be occassional glitches, and one can spend a very very long time trying to isolate the problem. Which is why very few even bother. Start with PCM and the receiver ignores the noise, because as Stig pointed out, the noise isn't sustained. The "masking" argument is not new, but it falls on deaf ears. If you walk up and down the line at an imac event, and have this dicussion, you would be hard pressed to find anyone who start with ppm and goes to pcm. you start wtih th equipment you will be flying.

neither system is going to prevent a crash once the gods align themselves for that to happen. i would never fly a jet on ppm. as much as i would hate to see mine heading for the ground, if the interference is strong enough to cause a fail safe, safety, not the well being of my aircraft, is my first priority.

Paul
Old 08-26-2005, 09:56 AM
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Default RE: PPM, PCM ?

I don't know which term is more misleading, "failsafe" or "lockout".

To me this is where most of the problem lies, with these terms. Generally speaking, the PCM system is a type of "noise" filter, and as soon as the noise is gone, the control comes back in about 1/50th of a second.

Now if the claim is that while PCM filters all of the data (good and bad) and PPM lets some of the good data through...well I have rarely seen a slightly corrupted PPM signal.

And the PCM we are using really does not hide anything, quite the contrary, on the JetCat ECUs, you can see how many failsafe events (shorter in duration than the shutdown interval) occured during a flight. It is great to check this flight after flight. Usually, it is zero, then you might see a spike. This is a good advanced indication of a problem. Seen it many times.

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