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Cement Blocks Don’t Fly!
Like many of you, I like things that are different. Many of us who fly R/C seem to have inquisitive minds. We seem to be able to do (and have fun at it) things that non-RC’ers simply can’t enjoy. Some of us are primarily R/C fliers, other are attracted to R/C by a basic love of aviation. Most of us are intrigued by engines, and maybe a few of us are “the high scientific theorist-type” in electronics and/or aerodynamics and R/C is our fulfillment, and some of us love time on the building bench.
I particularly love the sound of a model engine. But I also like things that are different. For some time, I’ve been planning on designing and building a big, slow, easy-flying R/C model along the lines of the PT-40. It will be easy to see, high-winged, have a round cowl, maybe have big fat wheel pants, and be a taildragger, too. Maybe it’ll be capable of dropping rolls of toilet paper, or dropping a couple of pounds of kid’s candy at the field and airshows. It’ll be pure Sunday fun to fly, and it’ll be built on a small budget. I’ve been gathering parts for it and I now have the wheels landing gear, wing spars, servos, battery pack, all the R/C stuff, etc., and the design is now pretty firmly fixed in my mind. I recently bought a Super Tigre .90 for the model and it’s one sweet-running cost-effective load of power in a .60-sized crankcase. It’s the “G.90 Ring R/C version with the Silent muffler, PN#1428.” I carefully broke it in using my metal test stand that’s safely bolted to a cement block. It turned 13,600 rpm with a 12x6, 11,800 with a 13x6 and 10,400 with a 14x6 that would be more in keeping with the needs of my planned model, but the prop load was too much and the pre-ignition “took out” the K&B #4520 idle bar plug. A second plug failure quickly proved pre-ignition. The 16x6 was too much and I suspect maybe the 14x6 was a bit too much also. The first photo shows the S.T.90 mounted on the cement block. I’ve removed the standard glow head and I’m about to replace it with a Davis Diesel “converter” head. Like many of you, I like things that are different!
Let me tell you about the “pluses/positives” of converting a good running glow engine to diesel. The primary flammable ingredient in glow fuel is methanol. The primary flamable in diesel fuel is kerosene, and kerosene actually has much more energy per unit of weight/measure than methanol (and more than gasoline, too.) The dieselized model engine is much quieter, both on the ground and in the air. You can easily run a larger prop that’s more efficient and that generates less sound. The combustion cycle of the model diesel engine is different in that the fuel burn rate in the cylinder is so fast that the burn is completed before the piston comes down and opens the exhaust port. Methanol (with nitro) burns slower and sometimes the still-burning glow fuel put bunches of sound (and heat) out into the muffler. The muffler of a dieselized R/C engine runs much cooler and it does far less work. In fact, you can put a length of clear plastic flex tubing onto the muffler’s exit with a tie wrap. The tubing won’t melt and the black exhaust (yes, the exhaust is black just like a 18-wheeler’s) will be vented clear of the model.
When I build for my new S.T. .90, I’ll probably route the flex tubing through my fuselage, then exit out the aft end. Simple and clean, huh? Normally, using a flex tubing exhaust extension severely hampers rpm on a glow engine, but due to low heat expansion after the diesel’s burn cycle, you’ll usually see no rpm loss. Ten or twelve ounces of model diesel fuel will run about 25% longer (sometimes even more!) than the same tank of glow fuel and, as a side note, instead of using silicone fuel line, you use Neoprene fuel line like that in Klett fuel tanks. Since there is no glow plug, it can’t burn out and lean diesel engine settings just cause it to quit running. Diesel fuel has natural lubricity that tends to extend engine life. The model diesel fuel is available in hobby shops, which can order it from Hobbico as well as other sources. Davis Model Products, who advertise in RCM every month, stock diesel “converter” heads for a wide range of engines.
The diesel “converter” head adds one more control to your engine. The added control is a hex screw where your glow plug would normally be. The hex screw positions what’s called a contra-piston that fits on the bottom of the Davis “converter” head and moves it up or down. When the contra-piston is screwed down a slight bit, it makes the combustion chamber slightly smaller and increases the compression ratio, which advances the ignition timing, and, when screwed up slightly, it retards the ignition timing. This second control is what yields the flexibility to run bigger props at quieter speeds with greater fuel efficiency…and to do something different in our Sunday fun. I find that with the same prop and with the same fuel, I seldom adjust the contra-piston from Sunday to Sunday. The model diesel fuel has a medicinal-clean odor (it contains ether) and the burned fuel also has a non-glow fuel odor of burned kerosene. This is not a problem.
My new S.T. .90 with the Davis Diesel Products “converter” head now very comfortably turns a 14x6 at 9,850 rpm (idle is 1750), turns a 16x6 at 7,450 rpm and turns an 18x6 comfortably, too. I think the 14x6 will be perfect for my plans, but cement blocks don’t fly.
So, for quick fun, I’ve put a Davis “converter” head on my great running Thunder Tiger .42 that’s currently flying a bunch with an 11x5. As a diesel, it’s turning the same rpm with an 11x7 but, for added fun, I’ve put on a Bolly 4-bladed 9-1/2 x 6-1/2 that is shown in one of the photos. The plane is “like a rocket” in flight with the added power of kerosene. The 4-blader (there are plenty of 3-bladers in our hobby shops) is turning a bit over 11,000 rpm. Does that mean it’s turning two regular 9-1/2 x 6-1/2 props at 11,000? Or does that show the power is like turning one 9-1/2 x 6-1/2 at 22,000 rpm? What do you think? I’m not smart enough to figure that one out…I’m just enjoying the fun of something different.