After a thinking I was over the hump of the learning curve a new hump popped up. I had been all set going out several weekends and putting in a few flights each time (I'm not an early riser so I get to the field just before the Socker Kids and only get in a couple flights before they start their games). Anyway, a couple weeks before the fall contest everything goes wrong with a very bad vibration - rough running that would not settle down. Well I miss the contest and a few weeks later I decide to do some tests, fire up just as it was and it shakes like mad. I changed the prop to one that worked early on and poof problem goes away. I fly for several weeks and think well maybe for the King Orange meet we will be ready. Then three weeks ago it starts again, this time changing the prop made no difference, talking to Eric Clutton he suggested that maybe the prop washer got pushed a little to close to the case and once the engine warmed up it started rubbing which increased the heat which etc. etc. etc.. Anyone have this happen and what kind of clearance do we need?

You need no more than the thickness of a playing card, but if you have say .005 - .010" when the engine is cold and with the prop fully tightened, this is not your problem. The case doesn't expand that much.

regards
David Owen

Well the little PAW 15 that I put in the Ringmaster did a fine job at the King Orange meet here in Florida were it place 2nd in OTS. Now there were not a lot of entries (four in total) but the victory wasn't so much the 2nd place but the fact that it performed spot on for both rounds and allowed me to fly two solid flights. There was lots of interest in the way it ran and pulled the ship in the 10 - 12 mph wind. I met a couple of diesel guys and we had a great time.

Stuntguy, congrats on your 2nd place. I built a number of Ringmasters in the 1950's but none as good looking as yours. I have tried to attach a picture of a Ringmaster Jr. in 1958 for a Fox .15 glow. Painted with a sort of Japanese WWII color scheme. I had the full size Ringmaster with Fox .35 Stunt and a Fox Combat Special on pressure. Also after the Jr. a light weight full size Ringmaster I built with wood lighter than in the kit and a thicker airfoil for a Super Tigre G.30 2.5 cc diesel. Just clear dope and silkspan on the diesel version to keep the weight down. Flew great well into the 1960's. Jack

Ringmaster photo?

Try photo agsin.

Stuntguy,

I've had a few engines which tightened up from case heat expansion. My solution was to pull the thrust washer and flat-file it to where there was a bit more fore and aft 'play'. It doesn't need much, but it does need some... None, by the way, were PAWs...!

Props should be carefully balanced. Our diesels have a different 'push' on the piston than glows. Glows fire and burn most of the fuel/air very quickly. Kerosene burns a bit slower, so at the RPM we use, it is actually more like a push than a whack with a hammer. At a "nice" setting, this push can be pretty well synchronized with the metal parts flailing around. A well-set diesel - with well-balanced prop - can run very smoothly, even without having a hefty counter-balance on the shaft as in most recent glow engines. This applies more to our sport/stunt type of use than all out racing or combat uses... An out of balance prop can mess that up. It's worth a few minutes to get the prop closer to balanced...

Another funny thing, which you noticed in bench tests on various props. Larger props require lower compression than smaller props. Has to do with ignition point timing. That's point in time, shaft position. The larger props won't turn up as much RPM, so start of ignition does not need to be 'advanced' by a higher compression setting. For a smaller prop, start of ignition does need to be advanced - higher compression setting when it has completely settled in. It's one benefit of being able to control both mixture and compression.

There's a lot of good comment in this thread! It is good to review even stuff you think you know completely...

Lou, we are in disagreement on what goes on in a diesel vs glow engine. In a diesel engine there is a burn throughout the combustion chamber as soon as combustion temperature is reached from the added heat of compression. This gives a sharp increase of pressure on the piston. The exhaust from a diesel is expanding and cooling, not burning, gas. This is why you can stick your finger into the exhaust to see what color the oil is. So each firing cycle has a sharp blow included. This is why diesels break cranks and rods and throw prop blades in comparison to the same engines run glow. In a glow engine, the burn starts at the glow plug, and a flame front expands throughout the power stroke and out the exhaust. This is why sticking your finger in a glow exhaust will result in a burned finger. The power stroke peak in a glow is much more prolonged and smoother than in a diesel. It is a matter of universal initiation of the burn in diesel vs localized initiation in glow, rather than differences in burn characteristics of the fuel. Or so it seems to me.

Lou my PAW 29 had endplay I filed a brass washer lapped it a a glass plate and used as a spacer got the play down to abouy 0.01 and no gauling slippery brass aginst aluminium
a little oil come thru the front so no issues martin

Jim on the burn thing I dont think the burn is the "big bang" I think ignition starts in the squish band area and procedes from there yes its fast, and yes there is a complete burn
by the time the exhaust port opens martin

Lou,
your observation on the relationship compression - propellers/rpm is right, but my guess is that you confound the cause with the effect.
Hcci charge starts firing basically at a given temperatore for a given stoichometric ratio. Now two are the reasons for which the temperature of the charge rises up to the ignition temperature. The first and most obvious is that the charge is subject to an adiabatic compression, the second, absolutely not to forget , is that the charge is heated from the hot liner, piston and head and this heating depends from walls temperature and time. As far as this heating time increases due to low rpm, this heating plays a bigger role in determining the temperature and following ignition of the charge. On the other side if you run speedier, for avoiding the ignition delay due to the reduced heath contribute from walls, you have to compensate with an enhanced temperature contribution from the adiabatic compression. This means you have to rise the compression setting.
Ugo

One thing that must be remembered is that when a larger prop is used the throttle is going to be more open for a given rpm compared to a smaller prop therefore the cylinder filling is greater because the throttle is open farther. If running a larger prop at lower rpm there is more time for cylinder filling, either way the compression must be reduced.