dieselized Black Widow

I haven't worked with dieselizing any of the .049 engines from Cox, so I cannot really give much advice there, but I'm just wondering if you have tried engines with and without SPI?

The Cox engines that I have converted, a .020 PeeWee and the .07 Queen Bee, both behaved much like traditional diesels. I.e. they produce a high amount of power already at low revs with large props. With the large props it is also much easier to stay on the safe side, i.e. a little rich and undercompressed. It is on smaller props that I have been close to getting a runnaway situation. So in general I prop the engines to to about 10000-12000rpm maximum, if I prop them to run higher revs I have the risks of getting the runaway situation.

So, I would suggest getting you tach out and try a few props to see which prop will make it spin at around 10-12krpm. Going from a 7x4 to a 5x3 prop is a really big jump, why not try with a 6x4 and 6x3 as well?

As a comparison the PeeWee is very happy with a 6x3 prop, while the QueenBee will swing a 7x4 to 8x4 prop.

fredvon4 wrote:
I think I will bypass any of the conversions and just get tried and true engines from the get go to play with. Hoping to bypass much of the frustration I see you and others relate to a lot

Yes, for a novice to diesel engines I would certainly recommend to begin with dedicated diesel engines and commercial diesel fuel.

Once you have the experience though, it is part of the fun to see which engines that one can convert and which engines that are suitable for conversions. It is the timid glow engines that will make the best conversions, total power might be similar but the prop size will be much more useful and they will run very smooth on diesel with a soft exhaust note. Take the Queen Bee engine for example, it will only tolerate a 6x3 prop on glow, anything larger and it will overheat. Very frustrating and impractical. On diesel it will instead easily spin an 8x4 prop and provide much more trust, perfect for an old-school slow-flying plane.

I found that the best way to get "diesel" performance out of a Bee was to reduce the venturi size - until I did that it gobbled fuel.

I am not a fan of the Davis Diesel heads but have done quite a lot with them and because of their construction - they don't dissipate heat very well. To compensate for this I jacked up the ether to 40% and reduced the paraffin content and the oil from the traditional British mix.

I ended up with 40% ether, 30% paraffin and 30% castor (about 1% ignition improver).

It worked well on this but when I knocked the castor down to 20% and upped the paraffin to 40% - the head went into melt down. I have not used a DD head since.

You are probably right about using these for control line - I have only experience of these being used with free flight RC assist but have seen them run happily on 8x4s.

I like the phrase "Runs like a dog" - I like the sound of diesels barking!

The best Cox "diesel" I have is still the Valentine TD build but that has a purpose made cylinder with contra piston. The venturi is reduced size as well.

Any diesel is a trial and error from running setting to flight. Keep it on the rich side with the compression set back slightly with little tweaks following flights  but I think the problem you are seeing maybe the engine overheating with the DD head.

Kris, I'm thinking the same as yourself. Here's why I didn't go for the 6x4. I don't have any because I gave them all away. They're useless for control line flying so I don't own any.I pretty much use 5" props for almost all of my use. However, I recognized very quickly that this is a prop that I readily need. I can pick some up from my fellow flyers but I haven't been to the field in months. The problem I found with trading a 5x3 to a 7x4 is that they require totally different starting procedures such as needling and compression. It's somewhat of a two handed starting procedure as your turning the contra piston and flipping at the same time. Once I get a pop I stop and start opening the needle. As for purpose built diesels. If I was to get serious in purpose built diesels I would probably go with the PAW. I like the looks and quality. They're a bit of money so as if I don't have enough stuff I figured I try these due to having them given to me. I have two different kinds of fuel which is the old Red Max and a old quart of Davis Diesel. The Red Max knocks you on your rear when you open it but it starts far easier. The Davis fuel hasn't been too reliable for me. It could also be old but it was in a sealed can. As for fuel, I can't comment on oil or ether content as they were both commercially made. The Red Max though fired everything up even in 20 deg temps so I'm somewhat favoring this brew.

I think with all my experimentation, I will conclude even based on your experiences as well Kris that this engine will be much happier in the middle with a 6" prop. My flying experience has been pretty poor with them using a 7" prop. As Ian mentions though the Davis head does tend to overheat or over compress. I don't know what ot make of it other than the engine begins to sag. I never found any .049 to ever overheat as I feel you can never get enough heat in them. So I will say over compress. This is pretty cool stuff and I see Bob aka Fit90 has had similar fun in doing so lately.

Why not just fit a glow head to the Coxs and make sure they run as expected that way first? Once you're satisfied that the engine is running as it should as a glow and you're sure it's run in correctly switch back to the diesel head. That should eliminate everything else apart from the head, fuel & compression setting.
I really don't see why the 7X4 should cause too much of a problem as a Texaco runs fine on 7", although they do have more cooling fins on the head so the larger prop must heat them up.

Ken,

Glad to see you joining the diesel fun.  I have only had mixed luck with Cox bee type engines using the RJL head.  I also used the head on a Valentine speed bee with a die cast case.  The performance was quite good at a little over 10,000 rpm with an MAS 7x4, but the head got too hot to adjust by hand.  The three best Cox conversions, for me, have been the queen bee, a straight medallion and putting an aluminum carb body and cox carb on a medallion.  All these started easily and performed well.  If memory serves me correctly, I think they were giving slightly over 10,000 rpm on an 8x4, just not completely certain of that.

Bob