Is this a good spot for the CDI?



[link=http://www.flickr.com/photos/25908573@N05/4137363484/]hi def[/link]

Not the best place, that's probably the hottest area on the whole plane that isn't part of the cylinder. Most ignitions won't take that very long before developing problems. Would be better to move it up (or down) on the firewall as far as possible, or better yet, on the other side of the firewall.

You're not only going to cook your CDI but you're going to overheat your engine if it's going to be cowled in with no air outlet, Are those servos behind the firewall close to the ignition battery? They should be 12 inches away, at least. Dan.

Yes, the servos are only a few inches away from the fire wall. I planned to put the ignition battery right next to the servos. Do you mean the servo should be 12" away from the battery or the CDI?

If you do things right, with good connections and good equipment, the servos could sit on the battery. Not recommended, but it could be done. There's a fair number of us with some time in the hobby that locate throttle and choke servos next to ignitions and ignition batteries all the time. Even running a 72 mHz system, which is what I normally fly with.

Odds are likely that when he gets to doing the weight and balance on the plane he'll have to move something back anyway.

This one has the rudder/elevator servos right behind the firewall by looking at the picture?

Yes, 12 inches away from any part of the ignition system including the batteries for the ignition module. Dan.

My plane is a CMPro SPitfire. It is tail heavy as is. If I have to move servos aft, a lot of nose weight would have to be added.

I should have bought a magneto engine instead.

You did the right thing by buying an engine that uses an ignition module instead of a magneto. The spark generated by the module is much more vigorous and leads to improved idle, transitions and higher performance. In fact, you may have noticed that Zenoah is starting to make their engines with modules. That 12 inch guideline applies to magneto run engines just as well so you would not have gained much by using one. By the way, those 12 inches is just a guideline. Eight inches would be safe but anything less would make the installation less reliable. I know there are a lot of guys out there who have gotten away with much less but they are living on borrowed time by trying to defy the laws of physics. Suffer with the need of adding ballast as a compromise to gaining a more reliable airborne system. Dan.

Is this true? I recall numerous threads where tests were conducted and there were no performance differences between magneto versions and electronic ignition versions of engines. The idle, transition and top rpm were all the same. The only reported difference was ease of starting. I recall the test included some Zenoah engines. Many of the warbird guys I fly with use Zenoahs in their warbirds and they say that the performance is the same between the magneto version and electronic ignition version.

-Ed B.

If it's ease of starting only, that's good enough for me. But I don't know how the performance can be the same if the timing isn't advanced at high speed. The timing of the magneto engines is usually a compromise between the high and low speed requirements. Were the tests performended with the same engine with and without the magneteo? Dan.

The general performance between a magneto equipped engine with the timing correctly set and having the correct air gap is no different from an electronic ignition. A mag equipped engine will be off a little bit in the low rpm band but the vast majority of modelers are not experienced enough with gas engine operation to notice the difference, and they don't operate in that low of an rpm band.

The big difference is that an electronic ignition is much easier to alter the timing for specific uses. Meaning retarded for running on hot engines, or advanced for running fuels other than gasoline, or setting a curve that enables best performance on an engine that runs both hot and exotic fuels. Only a select few individuals have the capability of performing those alterations accurately. EI is also much easier to start since the flywheel (propeller) does not have to travel as fast to activate the spark generation mechanism. However, there are a couple of ignition manufacturers that have designed their EI systems to require a fast spin to excite the spark, negating much of the benefit in having EI.

TOM, very well worded. I hope you (and others) had a good Thanksgiving day! Take Care, Capt,n

I hope you and yours did as well. Thanks for the flowers

The manual states, "for the ignition unit...don't wrap the unit by the sponge, cloth or the like...the temperature of the bottom surface of the unit will rise...make sure to leave sufficient space (a gap)."

How am I suppose to keep the vibration from killing it if I don't wrap it?



[link=http://www.flickr.com/photos/25924477@N07/4177413081/]hi def[/link]

You don't need to wrap it, all you need is some foam between it and the firewall.

Will this work?

If it can't go on the other side of the firewall I would fold your piece of foam in 1/2, put all of it under your module,and use 2 zip ties.

Barely, but yes. It's the best you've done so far.

Better would be to get the ignition out of the engine box completely. The engine gets hot, and radiates a lot of hot air. This has been relayed to you before but you seem to be dead set on putting the ignition next to the engine. The life (and reliability) of your ignition will be reduced by the amount of heat it is subjected to. This is a well proven and long standing fact of life with our ignition systems. What you have will work, but don't anticipate a long lifespan for the ignition.

I'm not going to use the cowl at all. I figured there'd be plenty of airflow, without the cowl. And, I moved the CDI to the carburetor side, away from the muffler; my thought is the carb side is cooler. Did I think wrong? I could move the CDI to the inside if I have to, but it's crowded in there.

Also, I am having to add over 30 oz. of lead weights on the firewall, in order to get the C.G. within 1/2 inch of the location specified by the manual. This is really bad. I heard that Spitfires have nasty stall tendencies. All this weight is going to make it even worse. Well, I guess this plane is not going to be a lazy Sunday sport flyer after all.

No true warbird is a lazy Sunday flier. Balance by the manual and keep the nose down in the turns

I would not add 30 oz of lead...try to move engine & other ignition parts forward to get balance! Use hard wood dowel for longer stands-offs. The longer bolts you will need will add weight forward too. Capt,n P.S. A spinner & a heavy prop will help forward balance too.

Oh my. The engine weighs 35 oz. The lead weigh almost as much as the engine!

Let me do the math here for moving the engine forward. The distance from CG to lead is 7". So, 30 oz times 7" is 210 oz-in, the amount needed to balance. The distance from CG to the center of the engine is currently 10". So, 35 oz times 10" is 350 oz-in. In order to balance w/o lead, I need 560 oz-in (210+350) from the engine. That means 16" from the CG (560oz-in divided by 35 oz). So, I need to move the engine 6" forward in order to balance. That's a lot of distance to move!

I think I'm going to try to add lead before move the engine forward. I built the plane as light as I could; so maybe 30oz lead is not going to be too bad. Where can I find that much lead for cheap?

You could try moving the engine forward about 1 1/2 inches with round or square wood standoffs and the see how much lead you need to add for actual balance. Another Idea is ask the same questions in a forum that deals with airplane balance...instead of this gas forum. Best Regards Capt,n