PCM or PPM for gasoline/petrol engine

Both work equally well, since both use essentially the same receiver sections.

What must be kept in mind is that PPM and PCM differ only in their decoders, not the receiver sections necessarily. So, in fact, the PCM receiver does not reject glitches. The PCM decoder simply masks them by ignoring codes it does not like.

I fly both types of systems. I like to set up my gas engine powered models with a PPM receiver, so that any problems are obviously evident. Then, once the system is running well, I'll switch over to PCM for the other advantages that such systems offer, such as fail safe, etc.

Can you start out with PCM? Of course, but it can mask problems that may be minor on the ground, but serious once airborne.

Ed Cregger

Understood.

I have, or will have (pickup PCM this weekend), both types of receivers and may take the advice to start out with PPM and see how things go.

Have you experienced much problems using the standard unshielded magneto ignitions?

No, I have been very fortunate in this regard. I have seen and helped others with problems that could make you pull your hair out, but it is rare and is usually self inflicted by metal-to-metal (not always readily apparent), or forgetting to use a non conductive throttle cable.

Ed Cregger

In general, the type of radio used in gasoline powered models is not the problem rather the installation is. There are several specific issues you MUST address correctly or risk loosing the aircraft while at power and in the air. The first rule is to make sure your engine is using a resistor type of plug or the RF splatter will be too strong. The second rule to follow is that absolutely no electronics (batteries,receivers, servos, or radio switches) be within 8 inches of the engine for magneto driven engines. I use 12 inches as a general rule because some of my engines use electronic ignition and they swamp our radios worse than the spark does. The reason that exclusion includes battery packs used by the receiver is that it acts as an antenna when too close to a spark source and will send the bad signal down the power line to your radio. Someone else may wish to get more technical and explicit on the subject.. As someone else said, use a plastic rod to actuate your throttle and make sure you have no metal to metal connections that can vibrate causing RFI.

| No, I have been very fortunate in this regard. I have seen and helped others | with problems that could make you pull your hair out, but it is rare and is | usually self inflicted by metal-to-metal (not always readily apparent), or | forgetting to use a non conductive throttle cable.

... and note that these problems are not restricted to gas powered engines. They happen to glow engines too, and electrics too. (Though electrics don't usually have throttle cables, so that's not an issue.)

Any time that metal rubs on metal (good connections are good, but metal rubbing on metal is bad) it makes all sorts of noise. The way to resolve it is to make sure that 1) the metal bits don't rub on each other (best) or 2) make sure that the two pieces of metal are well connected (like solder a piece of wire between both pieces somehow) and that will stop the intermittent connection from making so much noise. It's intermittent connections that cause problems -- not good ones.

"Six_O'Clock_High" > be powered by a Homelite 25cc "weed eater" engine. I will be using the

NGK type R plug. Yep.

Definitely plan on maxing out the distance between receiver and engine etc.

Yeah, throttle connection is a pain given the Walbro positioning. Throttle plate is at an angle so I'll probably have to mess around with bell cranks or some such.

I'm using the standard manifold that came with the engine but slightly ported to suit the larger Walbro I'm using (WA-167). Engine is running right on the 8000rpm threshold. I have a little transition problem but expect to resolve that with some better tuning.

I still need a "velocity" stack, which is more a trumpet to stop the prop wash pulling the charge out the front of the carb.

I occaisonally see this when I use a plug wrench or hot pocket on the glow plug. Minor twitching.

Definitely will be using a non-metal to metal throttle connection. Bulk of plane is foam so it should be a bit easier to insulate.

My personal view is that I will never use pcm. Both are equally susceptible to interference. The difference is how they handle it. PCM can potentiallhy lock you out for several seconds. At least with ppm, you generally have intermittant control. So PCM goes to preset control settings. How is that going to help if you aren't flying nice and level at the time? What if you are on the backside of a loop? Just read a thread in RCU about a guy with a brand-new high-end pattern plane. Was flying and had some type of radio interference. It went into lockout and stayed there all the way to the ground.

That said, the most important thing is to do all you can to eliminate the possibility of interference. I use plastic pushrods for throttle. I don't mount a servo in the engine compartment. No metal to metal. The same things as everyone else has said.

John VB

The longer you are without any control the more dangerous things become.

As I stated to a PCM fanatic "I'd rather fight the plane to the ground than have no control at all".

Well, you can cut the engine and go into a gentle turn. Of course, if you are NOT nice and level that gentle turn could be worse.........

Seen that happen to several club members. By comparison I once had massive glitching when running a new channel (which turns out is very bad in the area) but was able to fight the plane to the ground. Considering I had only recently achieved my solo rating I was lucky to recover from glitches that literally took the plane from level flight into an inverted dive.

Common sense is what I want to apply in minimising potential interference. I'm not against PCM but believe it should not be used as a method to avoid interference. Reduce intereference by all means possible before relying on some electro-wizardry to save you.

So, my plans are to start with PPM and minimise any interference I note on the ground. Then once flying watch for interference and attempt to deal with that. Once relatively stable, change to PCM for that fraction more assistance.