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My only exposure to model diesels, some 28 years ago, really soured me on that type of engine, they were spastic, cantankerous, very rough on mounts had to crank with extremely high compression in order to offset the ubiquitous leak and literally bit the hand that fed them. Also remember they never fully realized the potential to perform.
All in all, trying to get those small McCoy and OK diesels to do their thing was pretty much a comedy of errors, and would have made a dandy plot for the Three Stooges. Of the dozens of guys I've talked to who also had these engines almost everyone experienced much the same problems and frustrations I did (at least mine got off the ground, whereas a number of diesel owners never even got that far) There was one exception, however, and he had no complaints. I suspect he either never tried to run his, or quite possibly he didn't even know enough to ask any questions or not to be happy.
This negative feeling about diesels was well reinforced throughout the next few decades by the many myths and horror stories I heard; some of the juicier ones told of failing shafts, disintegrating crankcases, holes in pistons, overheating, no idle, tearing up mounts and the ability to kick back and eat your lunch with savage fury when being started. On top of all this, the "experts" were quick to point out that diesel operation was limited to the smaller displacement engines (only up to a 25) because anything bigger just wouldn't perform.
I gleefully accepted these stories as the gospel, and did my bit to perpetuate them by adding my own sordid experiences any time I could find someone who'd listen, you know how it is when you don't like something or someone, you're only too happy to believe anything bad you may hear, even if the source is questionable (like the universally vague "they'').
And so it was that I approached my current flight tests with, to say the least, guarded optimism I just couldn't believe that Bob Davis had been blatantly lying about the goodness of his conversion heads, else he would have been run out of town on a rail a long time ago Yet, hadn't all these stories I'd taken a part in stood the test of time.
My nine-foot Aeronca C-3 was used as the test-bed. Originally this bird flew with a '68 vintage Enya .60 that ran extremely well on a 14x4, even though it was over the hill and far from being able to match the power of a modern Schneurle At the time I installed the diesel converted Eagle Ill in its place, I had something over fifty hours of airtime on the C-3, so I was working with a flying machine that I knew very well.
Before installing Bob Davis' nice looking diesel head, I racked up about three hours with the fox, burning methanol, and found out how much better my bathtub flew with the added power of a Schneurle The engine handled both a 14x4 and a 14x6, although I flew most of those three hours with the 14x4 With the Eagle I could do better loops and stall turns, but even this powerhouse would start to lug with that fifteen pounds of airplane hanging behind her. I tried a 16x4, but found it was just too much for the fox.
Then came conversion time, and it is as easy and uncomplicated as the instructions say it is, you just remove the original head and replace it with the Davis Diesel Head, being careful when handling the gasket I'm certainly not an engine expert, although I do love to tinker, and feel that this head exchange is well within the capability of any modeler. If you can't manage this, then you should definitely bypass stamp collecting and just spend your time watching adult movies on Pay-TV.
And then the fun started I put an hour of ground time on my "Diesel -Engine" (DE), getting used to its characteristic and checking RPM and "thrust" readings on different props. I say "thrust" because I used a fifteen pound fish scale tied to the tail wheel and anchored to the ground, and know that none of my readings were accurate, however they were relative to each other and that's all I was shooting for A Royal Pro-Tach was used for checking prop speeds
On both the 14x4 and 14x6, the DE fell a bit short of the readings obtained with the same props on the glow Eagle: 200 RPM and five percent, thrustwise. However, when the 16-inch props were run, it was like being born again, as the diesel's ability to effectively handle a large piece of lumber was clearly demonstrated Although the 16x4 tached at 9850, almost 2000 less than the 14x4 produced a solid five percent more thrust. And the 16x6, which tached on 7500, outdid the 16x4 by a healthy five percent. I also tried a 1150, which can out to a disappointing 6800 RPM and ten percent less thrust than the 16x4.
In the interest of science, and to satisfy my burning yearning, I snuck a 18x6 on that 1/16x24 shaft; she started on the first flip and idled down to 900 rpm without skipping or missing (how's that for a fanatic flywheel action), but in the 5700 revs could only bring the scale to with 17% of the 16x6 reading. Even though I ran out of the compression adjustment (the bigger the prop, the lower the compression will be), the DE turned that 18x6 without any sign of over heating or distress. I ran some high and low speed taxi tests and throttle response was great, as the engine never faltered coming up from the low 900.
Since diesels don't need as much air coming through the carburetor as glow engines do, the larger props tend to reach their max RPM before the barrel is fully opened. On the 16 inch props, this seemed to be at about ninety percent, and opening the carb that last ten percent had no apparent effect. However, when swinging the 18x6, max RPM was reached at fifty percent open, and pushing the throttle beyond that point really unbalanced the fuel/air mixture and caused severe bucking. I was afraid the poor engine was going to do itself in with a double-hernia. Davis does cover this salient point in his instructions and recommends that you readjust your servo throw (move your connection one hole in at the servo or one hole out at the throttle arm).
After I got through playing the numbers game on the ground, it was time to evaluate the DE the only way that really counts . . in the air, pulling that 15 pound airplane I started with the 16 x 4 and, in keeping with my static readings. The engine performed very much like she did as a glow with a 14x4, which was very well, indeed. Three flights later I switched to the 16 x 4 . . and couldn't believe the difference; it was like having a brand new birdie, especially in any kind of-climb attitude, where the diesel refused to lug or sag. That 16x4 worked the same kind of magic Cinderella's Fairy Godmother did, only I didn't have to sweat the midnight curfew. Both ''Bouncing Bertha'' and I really got turned on by that engine/prop combination, and I was riding such a big high by then that I flew till it got too dark to see. I'd have to say, unequivocally, that the satisfaction I got from those last three hours of flying time would have easily rivaled any other kind of erotic stimulation (I do follow my own advice and always wear dark trousers when flying.)
Every other diesel I know of has an extended arm on the compression screw that is bent ninety degrees and acts as a handle for adjusting compression Bob Davis feels that this encourages tinkering, so his compression screw is a 1/4 inch socket-head type that accepts a 3/16 inch Allen wrench. I've got to admit that this nicely machined and personalized head looks very neat with the socket-head bolt instead of a lever, but I'd recommend attaching some kind of orange or yellow streamer to the wrench otherwise you'll lose it the first time out.
Also, as with gasoline, you've got to replace all your silicone and surgical tubing with neoprene or whatever else won't be affected by the diesel fuel And while mentioning fuel, let's talk about cost' Davis' fuel is currently fifteen bucks a gallon, however, he'll let you have a case (4 gallons) at a 5 percent discount considering this saving and the economy of a diesel, actual fuel costs shouldn't exceed the price of a gallon of glow fuel. And don't forget you won't have any more plugs or batteries to buy or mess with.
My DE idled beautifully at 1600 (Duke's recommended low needle valve setting is very much in the ball park. and Diesel-Eagle hand starts on one or two flips and does a lot of typical diesel burping at first, then when warmed up, she smooths out and runs and sounds great. At this point you open the throttle and, after leaning the needle valve till she starts to miss, back off a tad to achieve smooth running and you're ready to go.
I put in over a dozen flights that afternoon and found that I was consistently getting at least 15 minutes out of the newly installed 8-ounce tank. Since I never did run out of fuel and was able to taxi back every time. Mr. Davis' claim that the diesel will average 2 to 21/2 times more minutes per ounce than a glow engine seems to have a lot of credibility.
A few days later I located a 16x8 and found that it couldn't match the 16x4 for pull and since the flight tests seemed to correlate pretty well with my ground readings on the other props, it's quite probable that the 16x4 is optimum for my BIG Bird In fact, it would also he the best prop for any other similar size and weight airplane (using the same or comparable engine)
According to Davis, you really don't have to back the compression off to where she misses, and then increase the setting till she's running smoothly la la the instructions, which he'll be changing appropriately) As you reduce the compression after that first start, you'll find that there are a few areas of compression setting that allow for maximum RPM. If you leave the headset for one of these highest RPM peaks. you won't even have to hardly mess with making compression adjustments too often My throttle response with a 16x6 was amazingly good. A prolonged idle of two minutes or more would allow for a burp two if the throttle was jammed to maximum go, but even then the engine never came close to crapping out
The bottom line on the Diesel-Eagle is at it impressed the hell out of me; Bob Davis' conversion head converted me from diesel-hater to diesel-lover, because it did everything Bob said it would which is a lot more than you can say "out most other products available today. My diesel delivered good usable power on large props, and did it while being a fuel miser. She started easy, had outstanding idle, and ran as smoothly is the glow version Between the different sound of the diesel and the prop spinning at lower RPM (7500). My DE became hard to hear at about 250 feet. She ran cooler than the glow did,and the variable compression head allowed the ignition timing to be set for whatever size prop was being used on any particular day, regardless of the temperature and humidity And since two of any engines worst enemies, heat and friction, are less, it's reasonable to assume that a diesel will last longer than a glow engine.