Hello diesel aficionados......... I need assistance

Ron, the crank was stock. My diesel head is from RJL. What I did find out is that Davis utilizes a disc that will blow out if overcompressed. I was aware that diesels distribute a lot more force on the crank I just didn't expect it to go in 2 flights. I was thinking that a 4 pitch might have been easier on the crank opposed to the 6. I just wasn't certain. It was cool though. Ken

Order a diesel crank and get that engine running again!

Phil

Ken,

Not sure if 6 pitch is too much or not, but a stronger crank is a must. I don't run diesels, from what I have read swapping the crank out is high up on the list.

Take 2.  I had almost finished a reply when my wife called me for dinner.  Somehow it has now disappeared.

Anyway, if I can remember what I was saying, generally your starting compression is a little higher than your running compression on a diesel.  It will run fine on a very wide range needle settings, so now that you know about where it runs, you should be fine.  Once running, back the compression off a little so it slows down a bit.  Then have a shot at the needle.  Peak it out as high as it will go with the needle.  You should be running at least 30% castor so don't be worried about leaning it out too much.  Then try the compression.  Treat it a little like you would a needle setting on a glow engine.  Tighten it down until it just starts to sag, then back it off to peak.  Then back it off some more until you notice it slow very slightly.  If you want, have another shot at the needle but as I said, it will be far less sensitive than on glow fuel so near enough should normally be good enough.
It will certainly sound different to the same engine on glow fuel as the combustion is completed earlier than with glow fuel, so it doesn't "bark" out the exhaust ports so much.  I would think that the prop is just a little too big, so I'd probably come back one or two on the pitch- maybe 4".

Now I remember.  The Davis head is flawed in my opinion.  While some say the teflon disc acts like a safety mechanism, many become frustrated with the regularity of it bursting.  Genuine factory diesels pretty much all use a solid contra-piston, so my choice would be the RJL unit.  Sounds like the Killer Crank is also a good choice if you intend to fly it.

Have fun with it.  We colonials tend to be far more familiar with diesels and many of us had our first ic engine experience on one.  Mine was a Mills P-75.  Never had any trouble starting that thing when I was just 11 or 12 years old.  The ether addiction started early and is still lurking 40 years later.

Rod.

Rod, I was pretty much on par with what you wrote in terms of setting. I was a bit perplexed when I released the plane and it went instantly leaner. Is that the correct term for a diesel? I certainly feel the engine was obviously over compressed. I backed it initially, possibly too soon. The temps here were very cold and I may not have had enough heat in the engine when I adjusted. Is this another possibility? I did have the 4 pitch on hand which has a thinner blade area. I think the weight and pitch of the MA 7x6 was responsible for the problem. I was impressed at how easy this did start using fuel that's ages old. Aside from damage tot he reed everything else appears good. Would a double bypass cylinder yield more power? I just don't know because my diesel experiences are strictly running them on a bench. I did like it though and it was just different. IT STILL STINKS but it really captivated everyone's attn. Ken

Generally compression will increase as it comes up to operating temp, so if it was still under temp when launched it could have been a little overcompressed once warm. It certainly sounds like it was just unloading significantly once in the air due to the prop. Going lean is not usually a problem for diesels so I wouldn't think that's what's happening.
I do think the engine would benefit from a duel by-pass cylinder as well as SPI. Most of my diesels are Aussie Taipans and they generally run SPI. Works well for them. Then again, if you do too much to improve performance then reliability will be compromised, although it is only really the crank that's the weak point. Replace that with a stronger one and it should be good.
Also, I think "stink" is too strong a term. If your wife wore strong perfume would you say it stinks? I think not.
I'm not sure if the aroma just reminds me of the "good old days" or if I actually like it. Maybe it's all just psychological.

I love the smell of diesel fuel and the distinct sound they make.

All the previous comments are correct.

Starting a diesel from cold requires the compression increasing. My usual rule of thumb is a quarter of a turn and then back off as the engine warms up.

This applies to bench running.

However, diesels tend to unload in flight and I would back off another 1/8th before release.

It is true to comment on the fuel mixture and the minimal effect this has but it requires caution.

Diesels run hotter than glow engines and a bench run doesn't cool the engine as much as in flight. (Of course this also depends on the cowling, some team racers are heavily cowled and so the engine is adjusted to compensate.)

Where was I? Oh yes, the fuel has a cooling effect. The richer the fuel - the cooler the engine runs. The cooler the engine - the less fuel is burned. The less fuel burned - compression increases.

A simple analogy is that you can back the compression off, squirt fuel in the exhaust ports, flick it and it will fire.

The usual cause for a bust crankshaft is flooding causing hydraulic lock. I have broken many a Cox crankshaft with my dieselising experiments. I have heat treated them and used the "stronger" crankshafts but a lock then will bust the conrod. In a few instances, this has poked a hole through the crankcase.

I therefore leave the crankshaft as the weak link. I do believe though that, properly set up, the standard cox crankshaft is fine. I have couple of bees that have been bench run for over three hours each now while I reduced the size of the venturi to fine tune running. More of my experiments!

Diesel conversions usually benefit from reducing the Venturi size. They run more efficiently and in the air don't drag in excess fuel to be expelled unburnt.

My experience with flight is with Diesel engines and not diesel conversions, me starting, running and tuning but not flying recently.

With a new engine, set up is "best guess" and flight times are limited but the first flights are always cautious with compression backed off. As compression is increased fuel is leaned for subsequent flights.


It is easy to think that the unload in flight is fuel leaning out but it is more likely the opposite.

Diesel characteristics take a bit of getting used to but I admire anyone taking the leap of faith. No plugs, no batteries, nice smelling fuel and fumes and a distinctive sound.

Diesel fuel in a cut also creates new dance steps and a colourful language.

Great thread as I have a davis diesel conversion on the way. It is the diesel head and hardened shaft, so I am good there.
However, from some of your comments it sounds like I need to have some extra teflon discs handy.

Will let you know how it goes with the conversion. I need to build a little something to put it in when it gets here.
This would be nice:
http://outerzone.co.uk/more_pics.asp?ID=2632

I had the diesel back out this past weekend. I have both RJL head and Davis Diesel heads. The RJL in my opinion is easier to operate for me at least and starts quicker. I went with a woodie 7x4. I feel the Master Airscrew was just too heavy for the Bee even with the Davis Diesel crank. Rpm's are just not there for control line. I can see a r/c plane or free flight greatly benefiting from this prop and run but a no go for control line. I switched to a MA 6.3 x 3. A major improvement in rpm's but I didn't get to fly due to the weather quickly turning and the mad dash to roll up lines and dry everything off.
           I setup a Black Widow with hopes of getting better performance. I had not so good results there. I managed to get a decent bench run but starting was very difficult and I really felt the Baby Bee was easier to deal with in every aspect. I was told to cut discs for the diesel head from soda can aluminum. I'm not sold on the Davis Diesel head. I like the adj. knob as the RJL head has the traditional Tommy bar and lock knob. The thing locks so quickly I find my large hands fumbling and finding there way into the back of the prop. I think the key to success for my use here is a smaller prop and run this thing up. The problem here is the temps and it's near impossible to keep heat in the engine which results in screwing around with the compression and also needle setting due to the viscous fuel. Ken

All good advice so far, and yes the RJL heads are much better than the DDD ones...
I would just like to add some of my own impressions/experiences;

1) Needle and compression setting are linked, a leaner setting requires a higher compression setting.
So once you find peak rpm in terms of both needle and compression setting, you can reduce the compression setting a little (engines starts to miss a little) and compensate by turning out the needle.
This to me gives the "safest" running setting in terms of engine load and temperature.

2) Engine heat and compression settings are linked. A warmer engine needs a lower compression setting. This is why it is dangerous to fly engine that has been set right at max rpm on the ground. In the air it will lean out a little => temperature goes up => engines runs over compressed and heat increases further and so on until something breaks. I.e. you have an unstable run-away situation. The solution is the settings in point 1) above.

3) Prop size and compression settings are linked. A larger prop needs a lower compression setting. Therefore in an engine that is set-up according to point 1), a larger prop is actually a lighter load on the engine. The difference between a 8x4 prop and a 7x4 prop is almost a 1/4 of a turn on the compression lever on the Queen Bee (as an example).

So If you want to treat your engine nicely, put on a large prop and tune the engine on the rich and under-compressed side. The reward is a smooth runner with a soft engine note that swings big props.

The exhaust residues are not nice though, so try to clean the plane thoroughly at the airfield and leave all the paper-towels and rags there. Then wash all you cloths at home before the "boss" finds out...

The QB is a real charm on diesel, spinning an 8x4 prop with ease;

Kris, I've been around diesels, but I haven't had hands on. I have to agree with all of your statements as I've discovered exactly what you mention by trial and error. I always try and back off the compression as much as possible. I found the engine does unload and thus becomes over compressed. I just don't have sufficient rpm's to do stunts with the plane. It flies the plane but it doesn't have enough umphh to fly it with authority. I didn't see any gain in using the 7" prop. The smaller prop however had a very different exhaust note showing some promise. I'm curious if a muffler would be helpful in heat retention. I need to get back to this asap or when the snow clears. I'm running very low on my diesel fuel. We had somewhat of a vintage diesel running day a few weeks ago. The Drone .29 and some ED diesels. It was really cool. Ken

Hi Ken,

Lot of good info above so far. The Davis Diesel crank can work but only if you have a "good one". Inasmuch as std cranks are plentiful not to mention dirt cheap, I prefer to go to a stronger crank like that found in a Killer Bee. I was am surprised at how fast I could break a std crank & how easy it is to burn thru the DDD teflon discs. I was also surprised by harder starting in my experience. When the fuel & plug was swapped back to glow, then instant starting & all performance returned to normal. Obviously, there is MUCH to learn about running diesels & it's tough without a Brit around to help! Good Luck!

Cheers, Paul

you could also temper a normal axle
i once did it whit a car axle and it is still holding
(dont know for how long)

I have Tee Dee, Babe Bee and Pee Wee with head diesel conversion.

I only test Tee Dee and work so good. Diesel is powerful and move bigger props.

Kris,

Your video finally got to me. I ordered two RJL .074/.09 heads and dug out my Queen Bee. I will do the carb mod along with the diesel head. I hope I can get mine to run as well as yours. Thank you for the inspiration. I ran a dieselized medallion .049 with a cox 8x4 today and was really impressed with it. It ran between 7900 and 8000 RPM for two ounces of fuel. It was rock solid the whole time. I also couldn't believe how long it ran on that amount of fuel. It felt like forever compared to what I am used to.

Where you buy RJL .074/.09 heads ?

I have had very successful runs on the bench. The problems arise when it comes time to fly.I get very mixed results from run to run and it really is becoming more of a hobby in itself. The temps here just made it into the 40's yesterday. I strongly feel the weather isn't helping my situation as I just can't seem to retain heat in the engine. That seems to be a major problem and the compression changes as it heats up during flight.

RJL heads are available at mecoa.com

So far I have only had bench runs and I am definitely only at the beginning of the learning curve. Temps shouldn't be an issue for me. I doubt the engines will ever be run below 70. Most of the time I get to fly is in the summer with temps well above 90. I am still trying to figure out what I should do different when installing a diesel engine instead of a glow engine. I am enjoying tying something new and am hopeful that it will provide good, usable results.

Fit90 , what kind of flying are you intending on with the diesel conversion ? Are you scratch building or kit ? getback

Mostly kit built R/C. I will probably go with something a little bigger and slower for my first diesel plane. I am still trying to figure out what should be my first diesel plane.

Putting a pylon onto a sailplane is probably the easiest way to run any Cox .049 if you already have a sailplane.

Phil

With an RC diesel it can pretty much go anywhere you otherwise would have used a glow engine. The advantage is that one can run a larger prop at lower revs, thus giving lower noise and an overall nicer behaviour. It is true that they are very well suited for the old-school slow flyers but there is nothing wrong with it in a more modern design. Here is one example, with a MVVS junior 2cc, doing inverted low flights along the field with a diesel in the nose will put a smile on most people faces...

Nice looking plane