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Norvel .061 with RJL conversion head

Old 10-18-2015, 04:09 PM
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Default Norvel .061 with RJL conversion head

https://www.youtube.com/watch?v=CFfz...ature=youtu.be

I like the RJL head much better than the Davis head.
I blew out several of the teflon discs
Old 10-20-2015, 07:03 AM
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What prop size is that?
I've always found it better to use a larger prop as a diesel and settled for top revs around 10000rpm.

The Teflon disc are a textbook example of poor engineering, it is the wrong material for the job...

RJL heads are much better, with the same design as the DDD uses in their larger conversion heads.

The RJL head for the Cox 09 works well in the Queen Bee too:

http://www.youtube.com/watch?v=KLFfH1vQKSY
Old 10-21-2015, 12:25 AM
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Originally Posted by Mr Cox
What prop size is that?
I've always found it better to use a larger prop as a diesel and settled for top revs around 10000rpm.
I dunno, I've found that a good 1cc diesel (MP Jet, PAW, or converted Norvel) does really with an APC 7x3, running in the 16000-17500 range. Agreed about the Teflon discs, though - what might work fine in a Cox 049 really doesn't seem up to the job in a Norvel.
Old 10-22-2015, 10:11 AM
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Another thought on the DDD Teflon disk...

When Bob Davis introduced these, they were intended for reed Cox .049s. The changed pressures and loads in diesel conversions too easily overstressed the parts designed for glow use. We didn't have the bulletproof shafts that came available later.

The Teflon disk was like a fuse in an electric circuit. It would burn through before the engine hurt itself. They were and are inexpensive, so the slight inconvenience of swapping-in a new one was worth it.

Small props let the engines rev too high on diesel. A 7-4 or 7-3 prop restrained RPM and kept the compression/power loads down. The newer O-ring sealed conversion heads stand the higher loads pretty well, but I think I heard that that, too, can be overdone.

Bob, if you're listening, did I get that about right?
Old 10-22-2015, 11:20 AM
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I have heard that story many times before and I don't agree with it. The way I see it the teflon is a very poor gasket material and the head will always leak a little. This in turn forces you too increase the compression further in order to get it to fire, and once it does fire it runs well overcompressed and risking damage to the engine...

So the teflon disc is fact more dangerous for the engine than a proper diesel head, sealed either with a traditional pressfit counter piston or an o-ring seal on the counter piston.
Old 10-23-2015, 01:40 PM
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How does the RJL head compare to the Norvel head? The .061 seems to be a popular diesel.
Old 10-24-2015, 10:13 AM
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Mr, C, (posted here; a partial copy from an email to Mr.Cox)

I'll cordially differ with you, if I may.

The Teflon disk is not a gasket, as such. (It is however 'softer" - more compliant - than the stock copper Cox head gasket.) Without a 'seal' across the full diameter, there is NO sealed chamber allowing combustion. The loose slug above the disk WILL leak through the adjuster screw and through any burnt-through segment around the periphery.

And, yes, I have tried to start such DDD converted reed Cox engines with a burnt disk. I will not try to run a diesel with the comp screwed down to piston contact with the head. I could not find a comp adjuster position that fostered combustion with a 'failed' disk.

Perhaps you have found such a setting?
Old 10-25-2015, 07:17 AM
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Hi

My limited experience with the davis diesel .049 head is the teflon disc. When ruptured the engine will not start, so the teflon is required for a seal.
the teflon disc needs babying, run on lowest compression, start and reduce compression quick.
BTW I also reduce compression soon after start then after warm up tune for desired performance.
My MP jet will run with the compression backed all the way out after warm up.
My 90 supertiger with a DD head needs to use an electric starter due to low compression then after some warm up reduce compression. runs good when fully warmed up. Usually for start I need to turn the DD in almost 2 turns to start unless warm. To get the same RPMs on diesel I get a lot of black goo.
In my experience a good diesel will take compression with correct mix to the over compressed point.
You can then dial in the desired performance. Max power needs more fuel, richer mix to control temps.
You guys that run diesels understand this.
That is the barrier to more common diesel use, I think.
The combination of setting two controls right confuses people.
The glow engine by comparison is less complicated.
I used to pick up the running plane and lean with the plane vertical. That way when the plane is horizontal the mix is rich enough to run safely.
Dennis
Old 10-25-2015, 04:50 PM
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Dennis,

The larger DDD conversion heads are of the O-ring type. The Teflon disk was. as I mentioned previously, meant for moderate operating loads in Cox reed valve engines. They DID allow more useful power on larger props for mild-flying models - and still do. However, today, we expect to get more raw power from .049 and 1cc (.061 cu in) engines. It HAS been over 60 years since the essential L.M. Cox 1/2A engine solutions appeared.

In a way, they share with the deathless Fox 35 the characteristic that they serve well for the use intended.

As far as dumping loads of gooey sewage out the exhaust... I'm pleased with the cleaner operation of my home-brews using auto #2 diesel, John Deere ether, and castor + synthetic oils.

The exhaust ideally should give an oily portion with the color of light to medium dark honey. There should be little remaining odor of ether. (True, the raw fuel smells strongly of ether.) In flight, there should be a faint visible vapor trail of the exhaust oil.

Glow engines not designed to operate as diesels may need a bit more fiddling to get that exhaust condition, and may never settle down into it.

Dark, gooey exhaust usually means too much compression for the mixture strength. A diesel should merely slow as mixture goes too lean, and sag off unless needle is richened some.

The two controls make things easier, if you grasp the idea that they tell you what you need to do. Too much compression makes the engine hammer hard - a clear sound hard to miss, VERY rough on the engine... Back off until it runs smoothly, then lean the needle a bit. The engine will clearly sound smoother when it is in the 'good' setting range. Try raising compression to where it starts hammering, Back off to smoother running, then try leaning the needle again. Soon - trust me - you will find a combination where further compression goes to hammering, and further leaning causes RPM to sag. Back off about 1/8 turn on compression, and perhaps 1/4 turn on needle. That should do for flying. In air, if it sounds undercompessed, tweak it a bit higher for the next flight. Slight richer needle matches the comp change.


They do warm up in running, and the effect is the same as raising the compression. Takes a few flights to set up initial 'right' settings... The only further compromise may be in finding a takeoff setting that will 'come in' in flight.

When they are right, they are very consistent, and work best at RPM near the engine's torque peak - larger props, more usable thrust unless you need extreme RPM performance, good economy, etc.


Enjoy!
Old 10-25-2015, 05:44 PM
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Hi Lou
Thanks for the articles.
I don't have the experience that you have obviously.

I found with the paw and the other rc diesels need a certain amount of compression to make the transition from idle to cruise without a lot of hesitation.
While running 30% castor it takes a lot of compression to keep the engines running smooth and a good transition.A little light on the compression and the engine hesitate as they get rid of excess oil.
The engines that have a mix adjustment in the idle range do not build up the excess lube oil at idle and hesitate much less transitioning.. The two needle Supertigers can be set to a lean idle. These engines transition with less comprtession than other engines.
The exhaust will under those compression and mix produce an exhaust of black castor. This is not totally black but in a thin layer it appears dark brown.
I haven't tried any mix of synthetic oils yet.
What would you advise trying.
Old 10-25-2015, 07:24 PM
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Dennis,
Replying here rather than to the forwarding notice RCU emailed me....

The mix with synthetic oil is not the key point. The total oil fraction is more important. PAW instructions start with a 1:1:1 mix of castor oil, ether and kerosene. Then, after initial running, the oil fraction is reduced.

For racing use (high RPM, not very high load) with their 2BB engines, oil% can go below 15% total. Only over the recent few years I've been trying blends with less than 20% castor, and the balance felt necessary in oils like KLOTZ KL-100 (the one I think is all-synthetic, where the KL-200 is 75% synthetic, remainder castor.)

I don't think our diesels, unless brutally mishandled, run the combustion chamber temperatures that our glow do routinely. Synthetic oils, back when Dave Gierke was studying such stuff, burn away below 500°F, while castor lasts a bit above 500°F. Many combustion chamber temperatures he measured fell out between those values (?-!) a suggestion, anyway, that castor protects better against harm from a lean run than synthetics.

Your ST 90 may be ringed or ABC technology. ABC should fit tightly enough for a first start as diesel, if the conversion head provides enough compression ratio at hand-propping RPM. Restarts while still expanded-hot , though, the electric starter my do best.

Still 2 turns in on compression for starting seems a lot. It may just be the natural adjustment for your engine, though. Most 'designed-as" diesel instructions urge as little change to compression as we can get away with. The compression screw is attacked by each combustion, I. e., rotation. Hammering against a not fully locked-down thread is not kind to the threads. That's why the T-bar locking levers are often seen.

Also, it is accepted practice to run throttles diesels a bit above happiest non-throttled compression and a tad leaner. The compression difference speeds RPM change and the leaner needle reduces build-up of unburnt fuel in the 'case.
Old 10-26-2015, 04:56 AM
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Hi Lou

My supertigers are ringed. If they were abc or aac or any other none ringed type they would diesel much better,
The contra piston in the DD head is pretty small .680 compared to the bore 1.1" so two turns is not excessive.
From cold it takes a lot of spinning to warm the cyl/ compression chamber so it will fire. Even then it is a put put mis kind of run until things warm up.
Most authorities recommend a lesser percentage lube oil in ball bearing engines.
I don't want to damage the engines so I haven't experimented too much with less oil. Most of my mixes are 30%ether and 30% Klotz benol oil, 3% amsoil cetane booster with the remainder kero
Old 10-26-2015, 11:13 AM
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Originally Posted by Lou Crane

The Teflon disk is not a gasket, as such. (It is however 'softer" - more compliant - than the stock copper Cox head gasket.) Without a 'seal' across the full diameter, there is NO sealed chamber allowing combustion. The loose slug above the disk WILL leak through the adjuster screw and through any burnt-through segment around the periphery.

And, yes, I have tried to start such DDD converted reed Cox engines with a burnt disk. I will not try to run a diesel with the comp screwed down to piston contact with the head. I could not find a comp adjuster position that fostered combustion with a 'failed' disk.

Perhaps you have found such a setting?
But of coarse the Teflon disc has to act as a gasket in order to seal the combustion chamber. It needs to seal well all around its periphery, that's ecactly my point. The Teflon material is not an adequate gasket material, a soft metal is much better for that. The piston and screw above the disc does not seal anything, it merely supports the teflon disc.
I have never been able to burn out a disc, instead they leak around their periphery. If you bring the piston to TDC and hold it there, yuo will see this as a rather rapid loss of compression, much like a worn piston/cylinder would behave.

By replacing the Teflon disc with a metal one, you can in contrast get a fully adequate seal around the periphery and a much slower loss of compression when held still at TDC. The compression setting on the head screw will now be lower for starting the engine, compared to the Teflon, hence also more gentle running for the engine.

Around 30 years ago there where also other conversion heads for Cox engines, with better performance but at a higher cost. There was for instance a Swedish product which consisted of a classical iron contra piston running in a steel head bore, much like in a classical diesel, that outperformed the DDD ones.

So 30 years later I see no reason to continue to fiddle with those silly Teflon disc that never did the job well. Most engineers would get this without even trying to run an engine with it....

Last edited by Mr Cox; 10-26-2015 at 11:16 AM.
Old 10-26-2015, 11:37 AM
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Correction, the head was made from a high silicon alloy and they where made by Mats Böhlin in the eighties.
Below is an engine test from August 1980, comparing a TD on glow with the two conversion heads. The DDD heads where too sensitive on the compression screw (possibly due to a poor heat conduction and the large contra diameter) and didn't give any stable setting. The disc had to be replaced very often, in fact on every start when a "smaller" prop was used...

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Last edited by Mr Cox; 10-26-2015 at 11:40 AM.
Old 10-26-2015, 11:40 AM
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Hi

I always thought the cox engines were cute light weight short lived engines. Well made but light and short lived.
Old 10-26-2015, 11:46 AM
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With plenty of castor in the fuel they can live for pretty long.
Old 11-01-2015, 01:23 AM
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Mr. C,

Thanks, again, for good info! Agree mostly, and only regret I was too otherwise busy in the early 1980s to hear of Mats Boehiin's conversion heads. They sound good!

The hi-temp O-ring heads do very well on engines designed for hi-RPM glow operation, AND also seem to enjoy turning heavier props at lower RPM ( a lower compression setting condition, too.)

A comment above I couldn't locate quickly enough to quote, mentioned considering Cox early, inexpensive reed versions as of short life expectancy - Kleenex engines, IOW. They weren't, but in their price and performance range, many didn't expect more than that. So they handled their engines in a quickly destructive manner.

Start with a prediction then make sure the results match your prediction. Warp and skew the facts until they do, IOW. We have too much of that today in supposed "science," in too many other areas. The precision of Cox engine production then, and probably even now, was/is excellent. Abuse is abuse.

E, g., today, some high precision engines do better with NO castor in their fuels - ever. Many traditionalists still insist on some % castor, then blame the engine when it runs poorly... I'm uneasy running an engine with NO castor in the fuel, but I'll take the word of the guy who spent the time, money and effort to develop his engine, then stake his reputation on it.

You may have noticed a few posts here, or in other engine-related forums (ACK, that should be fora - I know, I know...) people complained that a highly regarded engine did not work well for them. It blew bubbles around the head and the backplate, wouldn't idle, was hard starting and short on power. Did they ever think to check that the factory assemblers actually did tighten the assembly bolts correctly? It's better today than in days of yore, but still rates a check to confirm...

When all else fails, read the instructions...
Old 11-01-2015, 02:52 AM
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Originally Posted by Lou Crane
Mr. C,

Start with a prediction then make sure the results match your prediction. Warp and skew the facts until they do, IOW. We have too much of that today in supposed "science," in too many other areas.
So you're an expert on Global Warming as well Lou?
Old 11-03-2015, 03:57 PM
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qaz,

Where did I mention Global Warming in my post?

IF ( capitalized, bold, underlined) there areas from which we hear conflicting doomsday findings at frequent I intervals, I take no "position" on that.

Coffee is good for you - NO, bad!! No, good!

Red wine?

Meat proteins?

Many other things...

Why did you instantly think I aimed at any one aspect? I'm not focused on any one argument, I simply notice the frequent changes of ABSOLUTE final scientific word. At least some of which seem more for publicity than anything generally good for most others.

I am mildly troubled by what I observe, that's all.
Old 11-08-2015, 06:09 PM
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Fashion based science Lou?

I deal with that on a daily basis - it seems that the truth is where the focus is.

Anything else is deemed unpopular.

Anyhoo, surely a flexible disk or O ring is not the best solution when one wants to fix a small volume and compress it accurately?
Old 11-10-2015, 04:11 PM
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I think I replied to a forwarded, separate notification of your post, that did not show up here..

In essence, yes, 'popular focus' seems to determine too much of what is announced and supported as the ultimate, final scientific TRUTH. Wait until next week, film at eleven...

In another such comment, I expressed my acceptance of O-ring seals in a moderately close fitted contra-piston, with an O-ring that can withstand the pressures, chemistry and temperatures of combustion chamber sealing for our model diesels.

For the pressures, they act like metal rings - sealing more due to the pressures pushing the ring towards the cylinder wall than does simple compression due to assembly. As long as they can handle the chemical and temperature conditions, what remains as a problem?

Creating a metal-to-metal interference fit between contra and cylinder is a very demanding - expensive - process effort. It is simpler - and less costly - with an appropriate O-ring solution. Of course, it is less than a theoretically ideal solution, but appears to be a successful alternative.

I have run RJL conversion heads on NorVel (or subsequent) engines to over 20,000 RPM without damage, failure or problem.

It does work, if done right - and seems to hold up in such near-extreme use. The larger engine DDD heads use this solution and are very highly thought of...

I appreciate your comments, and consider them when I try to understand results I've observed. Thanks!

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