Model Forum / Radio Controlled / Helicopters / September 2007

Thanks again Kevin, I already have a spare PPM receiver and just wanted to
save a few bucks.

Jack

There have been many, many, conversations/arguments through the years about
the pros and cons of PCM vs PPM.  Dingo is right that PCM is better at
covering up such problems.  Many think the problem should be identified and
corrected instead of relying on the technology to cover it up.  That's all
well and good but I've seen a number of instances through the years where
perfectly good models were lost because of interference problems with PPM
system that couldn't be identified in time.

OK, so PCM just covers it up and doesn't correct the interference.  If
you're not having any control issues, who cares?  I've been flying PCM
systems since the first Futaba 512 PCM 8's came out in the mid 80's and I've
had very few interference issues.  I did have one instance with my
helicopter locked out on me and crashed.  It was determined pretty quickly
that another guy on the field turned on his transmitter, on my frequency,
but didn't have the frequency pin.  "I" had it!  I've also had a time when
the system locked out for no discernable reason.  To this day, I don't know
what caused the problem.  It could have been a legitimate interference
problem like the first example or I could have simply had a receiver
failure.  I'll never know.  BTW, these two examples happened over a 20 year
period!  I don't think I can complain about that!  :-)

As for the fail safe on PCM not saving the model in the event of a lock out,
it's true that you shouldn't count on that happening.  However, I've had two
instances where it worked for me.  The first was a situation where I had a
main shaft bearing going bad and it was making enough RF noise to trigger a
PCM lock-out.  Fortunately, this was happening right about the time the
model was lifting off the ground.  I fail safe everything.  Cyclic and tail
controls stay neutral, throttle goes to idle, and collective goes to a +1.5
to 2 degrees.  As a result, the engine simply cut to idle and the model set
back down as 2 degrees isn't enough to maintain flight.  Granted this is an
unusual situation but it saved them model and gave me a chance to figure out
what was wrong.

The second situation involved a guy I was teaching to fly and we were buddy
boxing his Stinger 30.  We fail safed his system like I describe above.  He
was doing so good that I wasn't having to do anything but stand there and
hold the trainer switch.   I'd told him he didn't need me anymore but that's
another story.  Anyway,  we flew for right at 10 minutes.  His model was in
a stable, stationary hover when all of the sudden, it just descended
straight down and landed.  We didn't have a clue as to why that happened
until we looked down at my transmitter.  It was totally shut down.  I didn't
understand what had happened as we'd gotten no low battery warning buzzer
until I realized that I hadn't moved the sticks on the master radio for
quite some time and the transmitters own time out timer had turned the
system off.  I had no idea that could happen.  Needless to say, that timer
got inhibited right then and there.  As for the model, it sat there on the
ground, happily idling away until we cycled the transmitter, regained
control, and shut everything down.  Again, it was a freak situation but the
fail safe actually saved the model.  Even if he'd been flying around and a
crash would have been inevitable, just idling the engine and reducing
collective to a point that won't support flight but still cause the rotor to
tend to slow down, it probably would have helped to minimize the damage.

FWIW  :-)

Fly Safe,
Steve R.