I'm very new to R/C's. What is the advantage of diesel? Also, from what I see, people take regular 2 stroke glow engines and convert them to diesel. Am I correct on this, or did i misread?

There are a couple of advantages, much better fuel economy, it will turn a larger prop at lower rpm, they rarely deadstick, as long as fuel is available they will run, they have a real slow idle in most cases. There are two basic kinds, converted glow engines and purpose built, in general the converted engines are more powerful, they are more work oriented than hotrods.

A long time modeler, I am new to diesels. I have a PAW 19 that I currently fly and I am forming some opinions based on first hand experiences. I mounted the PAW on a bench mount and spent a good bit of time getting familiar with the differences in diesel operation and breaking in the engine. I then mounted it in an airplane and have flown it a good bit. Here are some of my observations.

After a lot of frustration. I finally got the starting procedures down to where I can start it easily most of the time. The engine likes to start pretty "wet" compared to most glow engines and priming is more difficult since there is no pressure connection to the muffler yet since there is a muffler installed, you can't squirt fuel directly into the cylinder. This requires priming through the intake which creates the possibility if a hydraulic lock if over done. Always turn the prop through by hand several times after priming before attempting to start.

There are two running adjustments that must be made (needle valve and compression) rather than one (needle valve) on a glow engine. The two interact. For instance a leaner mixture requires less compression, and visa-versa. I end up usually setting the mixture, then compression. Usually the compression has to be increased slightly to start a cold engine, and as it warms up the compression is backed off and the needle valve adjusted, then the compression adjusted again slightly. Idle then must be checked. As the engine cools down during idling, and the throttle is again opened, it may be under compressed momentarily causing a little burp. As it warms up again it smooths out. It is possible to find settings that results in a smooth idle, good transitioning, and good high speed performance, but it takes a little fiddling.

Diesels stink. I don't mean the fuel. I rather like the smell of kerosene and ether. However the exhaust residue is dark brown/black and smells like an 18 wheeler thats wet stacking. It is also messy. After installing the PAW in my nice new orange airplane and watching the black oil completely cover the right wing I was ready to give up. Instead I fabricated an exhaust tail pipe that directs most of the mess overboard. Fortunately diesels are pretty tolerant of back pressure and I can't tell any difference in operation or power.

As far as performance in flight I am quite pleased. Running is consistant, I have had no dead sticks, throttle response is prompt, the noise level is low compared to a similar size glow engine. It will turn a little larger propeller, and the fuel consumption is less.

Will I buy another diesel? Probably not. I will continue to fly the PAW and continue to learn, but there is no doubt that the advantages of the diesel come at the cost of increased complexity in operation. If you love to play around with machinery by all means give diesels a try, but If you just want to just crank up and fly, the glow engine is hard to beat.

If the exhaust residue is black the engine is either over compressed or lean or both. The exhaust residue should be amber in color. The Davis conversions seldom need the compression tweaked after the initial setup unless there is drastic change in ambient temp or you change props. What fuel are you using?

Lou w I have paws very good runners but the newer design 2 strokes such
as irvines with davis heads are a dream once the needle is set and left all I do
is increase compression a bit to start back it off when warmed up less than a
minute and go of course OS MMVS and other modern design 2 strokes are
in the same class. try one of these an you will be a happy camper yes I
still fly my paws .but you can not beat the new stuff for power and transitions
I spend very little time fiddling at the field with the new conversions start and fly

Forgot one my OS10FP davis head last 6 times out have not touched compression
or needle just bump and start pretty easy no fuss its in a 54" vmar cub

I started out with Aerodyne standard diesel fuel from Carlson engines. I am now using Red Max diesel fuel. I just finished running the PAW a few minutes ago. It is not overcompressed. If I back off on the compression it starts missing. It is only advanced enough to run smoothly, and idle well. The needle valve is not set too lean. I set it to peak and back off a little. While not exactly black, the oil is certainly darker than honey, more like cane syrup. I am really giving it a good try, but cleaning off the stinky mess before putting the airplane away, I had serious thoughts of replacing it with the K&B 20 that I removed for this diesel. When it was installed, I just went flying, and when I returned, I wiped off a little clear oil and that was that.

I just looked at the price list for Davis conversion heads, and for the life of me I can't see why one would pay almost the price of a new engine to convert. If I need a little more power I'll go to a little more displacement and stick with the simplicity of a glow engine.

A few opinions to share. Your mileage may vary.
A diesel conversion should work well. Davis does (or did) advise running-in the engine BEFORE conversion. Don't expect miracles from a worn out engine though.
A diesel can stand a pretty long extender tube to carry off crud without losing power.
If a black exhaust is not caused by over compression, perhaps it is not fullt broken-in. Otherwise, I would suspect something is wearing.
Breaking in a PAW or other iron/steel engine should consist of one two minute rich run followed by a series of peaked out two minute runs. Allow full cool down between runs.
You should run a diesel at peak, only richening enough to allow for prop unloading. Compression should only be high enough to make it run smoothly. You have to play around a bit because of the throttle.
An excellent online source for full procedure of break-in with thorough explanation of what it does to the engine can be found on the Canadian Nostalgia Combat site.
An excellent book for beginners is "Dr. Diesel's Diary" from and by Eric Clutton.
Hope this helps someone and does not offend anyone [&o] . As said, this is only ONE opinion.

George

I haven't tried any of the conversions but I do run a number of PAWs, DCs, Irvine Mills, and MP Jet.

If you want a real 'set and forget' experience and you like small models, try and get hold of one of the MP Jet Classic .040 side-ports. These really are one flick starters after break-in. I have one on a 1/2a Powerhouse (42 inch) which it pulls up to a couple hundred feet in the minute or so engine run that the attached tank (2cc) gives. Much joy

Is that the one that was derived from the Letmo, only with beam mounts? If so it should run very sweet. The Letmo does also.

George

Yep, that's the one. I also have the 2.5cc Letmo and it runs much the same, but instead of a 7" prop it swings a 12" prop. Slowly. It's eligble for SAM antique events too.