BIG BIRDS
By Al Alman (Model Builder, June 1986)

Converting a Schneurle-ported sixty, ninety or larger two-stroke glow to diesel is an extremely viable power alternative for BIG Birds... So if you have any of these engines not earning their keep, you might want to contact Bob Davis (Davis Diesel Development, P.O. Box 141, Milford, Connecticut 06460) and find out if one of the 50 diesel heads he makes for Schneurles will fit your mill.

Diesels haven't been popular here in the US of A, and one of the reasons is the abundance of old wives' tales perpetuated by many older modelers. These guys remember that the early diesels ran rough as a cob, were tough to keep in mounts, and wouldn't idle... which is pretty much the way things actually were.

However, Davis' diesel heads have completely eliminated those problems because now you start with a smoothrunning, Schneurle-ported glow engine and then simply put a Davis head on it. And what you get for your money is more power and torque with the ability to swing a much larger prop, the same smooth running, a great idle and twice the economy.

Another reason for the diesel's lack of popularity has been the fuel; it's not readily available at local hobby shops and, according to some, it really stinksl
It is true that at the present time you do have to mail-order Davis' diesel fuel (the fact is that many modelers are mail ordering their glow fuel by the case, also) ... but it's now in a concentrate form that stores easily, and by mixing together equal parts of this concentrate and kerosene you end up with premium diesel fuel (it's always best to mix only what's needed for a week or two so that your fuel is fresh). And as with most fuel, you realize a savings by buying by the case.

And as far as the "stink" goes... keep in mind that we're used to (only) glow fuel so that anything else is going to seem and smell different... and in this case "different" equates to "worse" in most folks minds. I happen to like the smell of kerosene/diesel fuel, but still haven't gotten used to the smell of nitro... even though I've been mixing and using my own homebrew for almost 20 years. And by the way, if the diesel fuel smell is a problem, just spike it with 3-4 ounces of one of the many different scented lantern fuels.

And a really BIG plus included in these new comprehensive instructions is practical information about selecting the right prop for your diesel. Most of us have tunnel vision when it comes to propping engines... because the two-cycle glow types we've been living with all these years leave little room for experimenting. So, typically, we stick a 10x6 on a forty or an 11x7-8 on a sixty and let it go at that...which is not the way to prop a diesel (or a 4-stroker, for that matter).

Here's part of what the new instructions say:
"If your engine was, for example, a .40 before you converted to diesel, you ran it on a 10x6. As a diesel, however, you may now run it on an extraordinarily wide selection of propellers. To help you choose the right prop, we first need to know a few things about its intended use.

"For BIG light planes that want to fly slow, use props with larger diameters and lower pitches. Conversely, for smaller medium weight planes that you want to use to burn up the sky, use smaller diameter props with higher pitches.

"You must always use a propeller with a load number that falls within the range limits listed for your engine displacement. Remember, as you select props with higher load numbers and retard the timing, your engine will run slower and thus performance may not be suitable to the needs of the aircraft. In the end, only you and your aircraft will determine the best propeller and engine speed that tills the bill. We hope this data will serve as a useful guide'

Davis' instructions also include the load factors for 67 different props, up to and including a whopping 110,592 for a 24x8. But since "Old Bill" only allows me just so much space for my monthly meanderings, I can't include that list here. Although it's duck soup to figure out because all you have to do is crank Diameter and Pitch into this formula: DxDxDxP- Prop Load Factor.

As an example, you can see by looking at the above Load Range chart that a 24x8, or any other prop with a factor close to 110,000, like 20x14 or a 2200, is the most prop recommended for a 2.6cid engine.

Of course all this data is only a guide, and shouldn't be considered as the absolute gospel. . . but it is solid info that's based on ten years experience and was carefully and conservatively averaged out by Mr. Davis.

Hopefully this should dispel the misunderstanding about (propping) diesels, and, perhaps, get you to try one. They do work extremely well. I have five, from a 40 up to a Super Tigre 2500 (1.5cid)...and have never had to use an electric starter on any of them. Also, in contrast to the usual starting and idling problems common to inverted glow engines, diesels LOVE to run upside down and in that position can idle for hours and still respond without dying if the throttle is suddenly jammed too high. And there's no starting battery to mess with, either.

Of course having yet another "thing" to adjust, in this case the head, has also turned many guys off... because it's something they've never had to mess with before, plus the instructions that came with the early diesel heads were not as specific and detailed as they might have been.

Well, being aware that his instructions were not quite up to snuff, Davis did a careful rewrite and now, in effect, leads you by the hand, from start-up through flight adjustments.

As an example of what to expect from a dieselized schneurle, my converted Fox Eagle III now swings a 16x6 at 7500 which is the same RPM I get with the same prop on my Magnum .91 4-stroker. Guys, don't overlook this Eagle Ill! It's a superior, Iongstroked brute on glow... that outpowers everything in its class as a diesel. Or, how about a dieselized O.S. .90 that turns an 18x6 at 7450... and burns hardly more than a half ounce per minute while doing it. If you happen to need, or want, more power, the relatively new Super Tigre 3000 (Davis is prototyping a head for mine at this time) should come very close to equalling the power of a three cubic inch gas burner with a 2000 or a 22x8... and it'll be lighter and much smoother.

Also... you're bound to like the diesel's sound and C-O-O-L running ....