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Scale RC models are a most pleasurable form of this hobby for many of us. Our small replicas sweep by with a drama that other models approach but cannot capture. When flying a Mk Ia Spitfire or an F6F-3 through an afternoon sky, a little bit of beauty, courage and awesome endeavor return to us from an earlier time. They're only models but they invoke the real for us.
We power them in various ways and always seek the perfect one. By far most are powered by methanol-burning two- or four-stroke glow engines. There is hope of electric power someday. Gasoline engines have become widespread. Way down the power plant roster are Diesels. They are particularly attractive as power plants for scale models.
We like them because Diesels throw large props, need modest cooling, use paint friendly fuel, smell great and a host of other reasons. But Diesels and especially large ones (10 cc's) seem to be a breed apart, and a minor breed as well. A lot has been written about the others (glow, gas, & electric) and even quite a bit on small Diesels. Where does one go to learn how to make a 10 cc Diesel behave?
I've learned a lot over the last few years about how to make them go and it would be nice to share the information. It is also very likely that many of you have had a similar experience. So, in the hope of learning and sharing some experiences with fellow travelers, these learnings will be offered.
I have in mind to cover such topics as fuel, prop size, cooling, carburetors, starting techniques, sources for engines and accesories, airplane sizes and weights and so forth. Also, if you would like to add your own ideas, discussion points or links to other websites please visit our discussion page for this topic.
For the past several years I have operated MVVS 10 cc Diesel engines in several scale warbirds. All have retracts, flaps and the like. They are all in the 60-70 inch span range, weights varying between 8-10 pounds. They include an F6F-3, Mk Ia Spitfire and a Nakajima Ki-43 II Hayabusa. All have been rewarding to build and fly. The MVVS engines have been a real experience to figure out. There was no one around here to provide direct advice for these large engines.
Instructions provided with the engines are cryptic. Only recently did I learn that the fuel is supposed to be 35 % Et2O, 42 % Kero and 20% Castor with 2-3 % ignition promoter (they call for 2-ethyl-hexyl Nitrate, although probably others will do). All this time I've been using 50/30/20 made up to 2 % overall with I.P. It apparently didn't hurt the motors and they ran well. This mix is apparently a break-in fuel. At present I'm using up my last order of fuel which is 45/35/20 (50:50::castor:Synth) with 2 % I.P. So far, it has also run well. I often return unused fuel to the bottle, so the extra ether allows for a little loss to volatility with enough remaining to run well.
I've been warned about using so little oil. Indeed one engine of another manufacturer has worn out the clearances in the connecting rod several times using the above fuel (the con rod is not bronze bushed). They warned me so I can't complain. But the MVVS's don't wear out on this mix. The oldest one has now been motivating regularly for about five years with no slop developing in the places one would expect. It has worn out the fit on the contra-piston. This is also the motor that I did all my learning on. Believe me there were many lean runs.
Lean runs were the rule rather than the exception for the first several years of operations. I just couldn't believe how small the exhaust opening had to be in order to supply the throttle with sufficient fuel flow. Two of the engines are equipped with the smaller size carburetor that MVVS makes rather than the one for the 10cc motor. It makes them easier to start and adjust. There is no apparent gain in RPM by using a larger carburetor. This was suggested by Eric Clutton and he was correct.
Carburetors set up for methanol don't work well on Diesel fuel. They don't suck hard enough. One of these days I'll hook up a water manometer and find out what the aspiration differences really are.
Keeping the ether in the can has been a problem. I'm sure all Diesel advocates have that problem. One very wise suggestor in a column said to turn the can upside down. If anything leaks, everything leaks. The fuel never goes flat and if the lid isn't tight on the can you have a tattle-tale. It has worked for me.
The three aircraft above all fly with 14/6 Zinger props. They turn about 7500 RPM and fly the planes at about 50 MPH at full throttle. This turns out to be about 300 MPH of scale speed which is perfect. The Hellcat's prop has been cut to 13/6 because it is a very draggy airplane in dirty configuration (gear down, flaps all the way out and canopy open) and needed a few more RPM.
The planes are all built very lightly in order to stay up at slow speeds. That's another subject, one for later.
Starting a 10 cc MVVS Diesel was an interesting experience for the first few times. It amounted to a series of rough attempts followed by long telephone calls to the supplier. After writing all of the conversation down that I could recall, out I would go out again to tempt the forces of chaos. Eventually, starting became routine even if it was not perfect.
They start a little differently from the PAW 0.35 and some have said that I'm doing it wrong. This might well be the case. So I'll describe the process and hope that someone with experience comes forth with instruction or confirmation. Here's what I do: Fill the tank through the fuel line and connect the fuel line to the carb nipple. Add a drop or two (no more!) of fuel to the carb and set the throttle position to what would be a low RPM if the engine were running. Screw in the comp screw about 3-3.5 turns and flip "smartly" as they say. The engine will usually start with a few flips. Very quickly get behind the plane and begin unscrewing the comp screw as the engine warms and regular firing allows. Sometimes the contra-piston will stick and the throttle has to be bumped to high rpm to get it unstuck. Run at high throttle with the comp screw at its fully unscrewed position for a few seconds then go back in about 1/2 a turn. Then the plane is ready to fly.
After flying, during which the throttle response is very good, taxi back and find a way to stop the engine. It will usally continue to tick over for a long time at low idle. Often someone has to hold the plane while while I cause a drag on the spinner.
That's the drill. I've been told that three turns in is way too much but reliable starts seem to demand it. Am I running with the contra-piston generally too far out? This could explain why so much contra-piston is need to start. I'm inclined to continue using the above process since the starts are so relaible and the engines (now up to 5 years old) have been so reliable.
One little detail that really made the engine easier to live with was a carb change. The MVVS 0.61 comes with a 9 mm carb for methanol operation, but I buy the engines as a Diesel with a 7 mm carb. This follows a suggestion made by Eric Clutton years ago and it works well. After trying both sizes of carbs in a set of ground based run-ups with the same engine/fuel/mount/etc., the tachometer showed no RPM difference. However, starting, idle, throttle response and fuel delivery were much better. Everything went better. It virually ended the burping lean runs that robbed the engine of power. They used to happen frequently. At such times it seemed to take forever for the gear to get down while the plane lost altitude and speed. Your precious replica warbird (that friends and family told you not to fly because it was too pretty) was never a great choice for a glider. Those sputtering engine episodes were scary. Anyhow they are rare now thanks to the carburetor change.
Fuel is 45:35:20::(Et)2O:Kero:Oil (1/2 Castor, 1/2 Syn) made up with 2% ignition promoter overall.