[NSRCA-discussion] RX choices long club testing results

[Jay Marshall]

Wayne, that is just some fantastic info. Was it ever published?
I have designed and used a lot of PCM systems but never for R/C and
therefore I don't know the details of the design specs. I do know, however,
that in theory and practice, a good PCM system is better than a pulse-width
modulated system. There are also additional advantages to PCM such as error
correcting codes, higher resolution, better data processing, etc. All PCM
systems don't use these of course.

NASA switched to PCM on the Gemini project and has been there ever since.

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Wayne
Sent: Thursday, September 14, 2006 4:31 PM
To: NSRCA Mailing List
Subject: Re: [NSRCA-discussion] RX choices long club testing results

Bob, I understand how it works. The Carrier signal is still FM and I
understand that. The fact that the signal is digitally encoded and how it is
encoded rejects bad signal or noise or what ever you call it better than
straight PPM. I spoke to a semi local Team JR guy at a contest and he told
it was like a little email package with PCM to simplify it. If the email was
formatted properly then the RX said OK you are my daddy. If the signal was
not formatted properly it would reject it in favor of the proper format and
if none was found it would do what the Failsafe setting told it to do. In
some cases that is Hold the last input on the channel and if you program it
to go to a specific position it would. In the case of straight PPM if the RX
sees the signal it passes it through after its been filter and so on. This
is why you see a glitch in FM(PPM) and not in PCM

Am I right?


I know that F/S doesn't right the model but in reality if you are getting
radio interference would you rather have the model do something predictable
like low throttle and hold other controls or do something totally
unpredictable. I have played with F/S in the air by turning my TX on and
off. The return of the signal is very very fast like a second or so. My
thinking is that if you have enough interference to make the model go into
hold "Lockout" for long enough to crash then I think its reasonable to
assume this interference is bad enough that the model may very well not be
controllable?

We did some testing last fall at my local club as guys had questions about
this subject. This followed a serious accident in which a 40% model at a
Giant Scale fly-in was controlled by a little $60 FM RX and went to full
power and cut through the pits and into another modeler. This guy almost
died. A tragic event for sure but a $3000 or $4000 model controlled by a $60
RX? one of our club members was a witness to the event and came home with a
major agenda for our club to implement. Our club was  looking at a PCM or
aftermarket F/S device requirement to help mitigate issues. Our big concern
is as with all flying sites the homes are getting closer and we don't want a
model heading off to meet the local neighborhood.

We flew some trainers and foamy models. We took models high and simulated
PCM F/S and shoot down situations. For our tests we only used Futaba and JR
radios. These were higher end TX's as we used the synth capabilities on some
of the systems to create the issues (JR 9303, 8103, 10X and Futaba 9C'S and
9Z's also a 14MZ). We tried lots of different frequencies throughout the
band. We used everything from little hitec foamy size rx's to the top of the
line 14MZ 2048 stuff. In the cases of the PCM system we found that in most
cases the Futaba and JR PCM stuff was pretty bullet proof even in a shoot
down. It took a very close TX of the same brand to cause a PCM HOLD
condition (our test had engine go to Idle so we could hear it go in and out)
If the radio brands were mixed JR flying and Futaba as the shoot down it was
a rare blip of throttle and the longest hold was on the order of maybe a
couple seconds. In the case of same brand shoot down the model would go into
PCM hold for longer until the shoot down TX was off. Then immediately the
model was in full control again. Sometimes we could not even tell the shoot
down TX was on until its antenna was extended or the primary antenna was
collapsed partially. If the primary was PCM and the shoot down TX was in FM
mode it was almost a non existent problem. A very very rare blip of power
when the shoot down TX was closer to the model than the primary. And this
was only with the shoot down antenna extended more than about 1/3 to 1/2
way. In other rare cases the shoot down TX actually became the primary and
started controlling the model. This was rare but we did se it happen. The
2048 didn't do it but the 1024 JR and Futaba stuff could easily. In fact we
had two 9303's and copied the program between them. We just swapped off the
TX that was off, then the pilot that was flying would shut his TX off and
the next guy turned his on. Man it really was a cool experiment and proved
the reliability and consistency of the systems.Pilot saying ready off, #2 Ok
I'm on, got it.

Our club was very impressed with the PCM rejection of the signal. In fact I
felt the model was in many cases better in control with the PCM holds going
in and out than the FM responses. The model never flipped upside-down, never
snapped or went wild. It just held its path and input. If that happened to
be a roll or loop it would continue to roll or loop at the given rate but
that was all. The other note was many club members felt PCM hold was a
forever lockout thing meaning the hold meant the model was going to crash
period and they wanted to have some control. Well we never had that. Once
the interference (shoot down TX) was off every single time the primary got
immediate control. And I mean right now as if it never had the issue coming
back.

The FM systems in the same test didn't care about brand of TX, and didn't
care about distance or antenna extension on the shoot down TX. In fact most
cases the models became uncontrolled and while it would still respond to the
primary TX in the middle of some of the glitching the models were just
basically all over the place. It seemed like the RX was passing just any
signal through. Of big note if the shoot down TX was FM mode also the models
were glitchy but the primary could still have an effect not really to fly
the model in a save situation but it had an effect. . If the Shoot down TX
was PCM there was no hope. The models were literally all over the place and
yes some primary control was getting through but you could not rely on the
input to fly the model.

We conducted this testing on several weekends and with lots of club members
present. Yes we took some risks but we really wanted to see if there was a
difference. The result of this test made my local club hit the hobbyshop
with a huge order of PCM RX's. I think he sold like (25) JR R770's over the
course of the next week. The old guys in the club found out their model was
better protected using the PCM over their FM and it was a black and white
comparison. Besides most sport models they guys are flying are in the
$400-800 range and what is the extra $40 from the R700fm to the R770pcm. Now
both of these are Synth so that is cool too.

Our club now has requirements for a F/S device or PCM on all models over
7lbs. Why 7lbs? well we felt that we didn't want to discourage trainers and
new folks starting out, but we did want basically anything 60 sized and
bigger to have some F/S capability and the club rule is a safety officer can
ask you to show it before you fly. I know most of the GS fly-ins in our area
and the Joe Nall now require this. Before you take off the flightline
official asks you to turn off your TX to prove a low throttle setting. The
groans came from the old guys for sure, but in the end they were the ones
that really felt the clubs responsibility to our neighbors and to our selves
was more important. The next thing was a phase in period of 1 yr. We are now
coming up on that 1yr and the rate of crashes has dramatically reduced at
our field. The concern was older FM TX's that could not use do PCM well H9
answered the call with a  $20 F/S module for use on FM systems. Now I think
there are 3-4 different brands out there. So for the cost of a gallon of
fuel and a few props you can have PCM or a FS device for FM.  We have a log
book that you are suppose to log any crashes or mishaps. We use it to razz
the guys and also to keep track if we have an issue on certain channels or
with a  specific brand or RX or the like. Well the 2005 log book was always
an issue of radio quit, glitched or what ever. Since the testing and mass
exodus to PCM the crashes are greatly reduced and top it off the crashes we
are having with radio issues are the type of stuff than you would see like
dead battery or bad switch not the mystery glitch or I got hit. Guys now
know what radio problems look like and they are diagnosing crashes better
and preventing them. Now with everybody flying PCM its really rare to see
radio problems at the field. Our field is mostly JR thanks to our semi-local
Team guy. The most common radio now is the 6102, 8103 and the 9303.

Seems that pattern guys tend to spare no expense in the TX department lots
of 9Z's and 10X's as well as the 9303 and 9C systems yet I'm seeing many use
a PPM RX. My curiosity was peaked as I would have expected nobody to use FM
in this group. Before the NATs the young man was having issues with his
system and when he told us what he was using for servos, leads and radio
gear I was shocked. A competition level pilot using aftermarket extension
leads and FM Rx'. I was always taught to stay within the brand. Servos are
OK to mix brands with but for switches, extensions, and RX's stay in your
own yard the OEM stuff is just a dollar more you have less trouble with it.
The models are expensive and we spend hours and hours building and trimming
them up. The real cost in loss of a model is the time trimming and setting
up a new plane to be just perfect. The Money is one side but to me its all
the time and passion working toward the perfect flying model. It sucks when
you are 1/2 way there or even 99.99999% and the model is gone hit the CLR
key in the TX and start over with a new one.

Just passing on some experience we had with it. We proved to ourselves that
PCM not only protected our people and neighbors better but it saved more
airplanes too. I understand we really only tested a single condition shoot
down situation and real interference is more random and rare in the real
world. We felt that the shoot down was the worst case situation for our
field as we are our away from industrial areas and other noisy environment
stuff. We do have some homes about 1/2 mile back behind our pit area to the
south but models are never flown back there. We have a rule to keep
everything North of our runway.

By the way lots of guys locally are using the Spectrum DSM as it has a
failsafe like PCM and it works great in those small parkflyer models. One
guy is even flying it in his 40 sized trainer converted to electric.

thoughts?

Wayne

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