I would like your reaction to two statements in Gordon Jennings’ 1973 book on two-strokes used for motorcycle racing, and then I have a question of my own.

Mr. Jennings states that if pistons are run hot enough, " carbonized oil may lock the ring in its groove after a remarkably short time . . ." He also says that in his experience, the better the oil lubricates, the more it carbonizes. He did not mention synthetic oil. Do you find it more or less prone to carbonize?

Mr. Jennings states that CDI ignitions have a lot going for them, but they must be timed with a strobe light. The Rotax material I have seen only addressed timing them statically. What do you recommend?

I have a 503 engine that I bought without a gear box as I have a B type that I planned to use. Now I am wondering if there would be any advantage in my using a C type instead on that size engine?

Respectfully - Wm. Erickson, Covina, California

A.  Mr. Erickson - We’ll address your questions one at a time.

Ring sticking - At one time, ring sticking was the most frequent problem we would encounter while servicing Rotax UL series two strokes. Today, the condition is found with much less frequency. What has changed? Perhaps the mentality of the user, maybe the fuel, or possibly the oil.

Some folks used to think the recommended fuel/oil mix ratio of 50:1 was too lean. In a misguided attempt to be kind to their engines, extra oil was added. In reality, we very rarely (almost never) see lubrication failures. Too much oil, on the other hand, leaves deposits which can cause problems. Most of those who believed the "more is better" concept have either quit flying ultralights, come to accept the recommended 50:1 ratio as valid, or better yet have become converts to oil injection.

Frequent usage will allow the most hours of operation between services. Fuel quality degrades rapidly during periods of storage. Running stale fuel can result in excessive varnish and ring sticking in as few as one or two duty cycles. Varnish build from stale fuel can hardly be blamed on the oil. Remember, the fuel mix is only 2% by volume, if you are mixing at 50:1. That leaves 98% gasoline. Fuel must be fresh.

Using the wrong oil may result in ring sticking. I didn’t say it causes ring sticking. The difference is that some oils contain additives to fight ring sticking, some don’t. In the normal course of servicing engines, we maintain computer records on just under 1,000 engines. We have seen engines that looked great and some which looked horrible after similar hours with similar oil. The oil is not usually the common denominator when determining problems.

The April ‘94 issue of EXPERIMENTER had a good article on two-cycle engine oil submitted by Charles Kudolis (EAA 299878). I don’t know it was written with an ax to grind or not. Regardless, it is valid information. It may be too soon for the article to be reprinted again in the magazine. I will send you a reprint. (Editor’s Note: Reprints of articles are available at a cost of $3.00 each, plus shipping and handling. This article is on pages 25-28 of that issue.)

CDI ignitions - It is true that Rotax material addresses timing the CDI ignition statically only. By design, Ducati CDI units used on Rotax UL series engines have a waist spark. That is, they fire the spark plug at the top and bottom of each stroke. Two bars, placed 180¡ part on the flywheel, each trigger a spark as they pass a magnetic pickup which is strategically placed next to the flywheel. The position of that pickup determines when the spark occurs. Rotax information gives us a measurement of piston travel before top dead center, where the flywheel bar should align with the pickup. Later units allow the technician to adjust the pickup up and down to get perfect alignment. Minute variations in timing are technically possible through difference in technique from one mechanic to another, or because of the subjective way of determining what is perfect alignment. Remember the point ignition engines specify .072" to .092" BTDC. We set them at .086", approximately the middle of the allowable range, but there is very little allowance for difference in timing between the two cylinders. On the Ducati CDI, difference in timing between the two cylinders is not possible because of the 180¡ placement of the bars on the flywheel. Timing should be exactly the same as for both cylinders. Strobing should not be necessary.

Gear Box ABC’s - The choice between B or C gear box should be determined by a number of variables. From a function standpoint, the major difference between the type B and C gear box is their ability to handle propeller inertia. The type B gear box is designed to handle 3000 kgcm2 of inertia. The type C is rated at 6000 kgcm2. What are you going to use for a propeller? Are you limited in diameter? If you turn no more than 60 inches of diameter, most brands would work on the type B, even 3-bladed props. If you intend to turn more than 60-inch diameter, you really should ask the manufacturer what the inertia is of the particular prop you’re considering. If you wish to check the inertia of a propeller, and your manufacturer doesn’t know how, Rotax issues Service Information 11 UL 91 explains the process.

Also, different ratios are available for the different styles of gear box. Type A and B are available in 2.0:1, 2.238:1 and 2.58:1. C and E boxes are available with 2.62:1, 3.0:1, 3.47:1 and 4.0:1. The higher ratios are generally used in conjunction with large diameter multi-bladed propellers. These large multi-blade props also have the highest inertia. There was a 3.0:1 ratio developed for type and B boxes, but it was never really marketed, one reason being the prop size needed to work well with such a ratio is in excess of the inertia limits of type A or B gear boxes. It’s not possible to say whether one should use a type B or C gear box based solely on the selection of the 503 engine.

Type A gear boxes were used through 1989 on most UL series engines, and are still used on the 277. On the twin cylinder engines, the type A gear box mounts to the crankcase via an adapter plate. The adapter mounts to the crankcase with 4 allen head bolts. These crankcases are referred to as "provision 4." In 1983 and part of ‘84, there was a special 3-bolt case which used a three-bolt adapter, and the type A gear box. To change the gearbox from offset up to offset down required removing the gear box adapter and turning it 180¡. Around 1989, a crankcase change was made. The new cases had 8 holes, thus provision 8. The type B gear box which is internally the same as type A, attaches to the provision 8 case in either up or down configuration without the need of an adapter. They both dampen torsional vibration via a system of Belville washers and a dog hub/dog gear arrangement. Type A and B gear boxes are not interchangeable. "A" fits provision 4 and "B" fits provision 8.

Type C gear boxes are totally different internally and externally. They dampen torsional vibration via a rubber "Hardy Disc" rather than Belville washers. They also weigh about 10 more pounds and are more expensive by nearly $500. Type E gear boxes dampen torsional vibration identically as the C unit. They are similar internally, but not identical. The functional difference is the addition of an electric start unit to the Type E. Type C and E fit provision 8 cases only.

Service Information - Much is available in the form of service information written by Rotax. You may obtain individual copies, or the whole package, from your airframe manufacturer or from any Authorized Rotax Service Center. The set of copies in a 3-ring binder from Green Sky Adventures is $40.00 plus shipping.

Regards - Gerald Olenik, Green Sky Adventures, Inc.

Q.  Dear Q&A - My questions are about jet engines. What sort of sources are there for used engines? How do you determine thrust requirements for an airplane? What can one expect to pay for an engine? How about the TRJ-300 kit. Is this a reliable engine? Does the FAA have any special restrictions for experimentals or jets? How about operating costs? Advantages or disadvantages with a jet versus internal combustion? Reading about different planes, I notice that thrust is usually at least 33% of gross weight. Is this right?

This is a tall order; I hope you can help. - Alan Fitzgerald, Grand Island, NE

A.  Alan - You’re right; this is a tall order. We don’t have much information about jet engines, so again we’ll turn to our mighty readers with the hope that one of them has experiences they’ll share.

Q.  Dear Q&A - I am looking for information on Subaru conversion engines. I am going to be building a Glastar (tail kit ordered) and at this time I am seriously considering a Subaru, either 2.2 or 2.5 liter, but I am beginning to have some apprehensions about them. The only thing I have heard of them are the glowing reports by the companies that are building them, and nothing from the people using them, good or bad When I hear only glowing reports, I get nervous as I don’t believe there are any perfect mechanical products. I wonder if there is a vacuum that is drawing off the selective comments. Either they are fantastic, or the users are having problems but don’t want to admit they were stupid enough to really invest in one. It would be nice to hear some actual user reports. I’ve heard the good; now let’s hear about the problems and bad points. I’m really interested in the new 2.5 liter Subaru that is listed in the Outback model, and is at this point in such short supply that none are available yet. Installed in the Outback its supposed to produce 155 hp and peak torque at 2800 rpm. I would like to find a power curve diagram for it so propeller and other items can be planned.

Sincerely, - Art Fish, Corvallis, Oregon

A.  Art - Okay, we’ll print your letter and ask our readers to respond with their true life, tell all experiences É just like those horrendous day-time talk shows! So, folks, send us your tales of success and woe, and we’ll dedicate a column to reporting what you have to say. Art, we’re also forwarding your letter along to Don Bouchard for further response about the Outback engine and the power curve diagram. Good luck with your Glastar project! - M. J.

Q.  Dear Engine Q&A - The spirit expressed in a letter from Dan Riebs (January ‘96 EXPERIMENTER) is just the sort of deep thinking I like to see among EAA members and reflects one of the reasons why I subscribe to the EXPERIMENTER. In his logical query, Mr. Riebs asks about the possibility of creating a new type of low drag radiator for his aircraft. Using sheet metal, he proposes the construction of a sealed, double-sided, 1/4-inch thick "cooling pocket" (my terminology) within the confines of an engine cowling. Through this would pass hot powerplant coolant to be cooled by ambient air flow.

While this sounds like an excellent idea, I submit that it would most likely prove ineffective because of the relatively small heat exchanging surface offered by such a design. In fact, even if the entire cowling of a small homebuilt aircraft were somehow able to be used for this purpose, the resulting cooling area would still be less than that of a traditional finned heat exchanger (radiator).

Conventional radiators, of course, are built using very thin, closely spaced, convoluted copper or aluminum fins. Through these are directed one, two, three or more rows of small tubes to carry engine coolant. This construction technique creates a heat dissipation surface area many times greater than the actual dimensions of a radiator. For instance, in looking over a two-row radiator before writing this, I noted that it had 22 fins per inch - each 1-1/2 inches deep. This calculates to 33 square inches of cooling fin surface for every surface square inch of radiator core dimension. Based upon these numbers, a 12 x 16-inch radiator core (192 square inches) would expose approximately 6,336 square inches of cooling fin surface to the slipstream. This equates to 44 square feet, not including the heater tanks and fluid tubes. Obviously, it would be difficult to free up this kind of surface area on a small homebuilt aircraft.

Beyond this consideration, there is another problem associated with incorporating the suggested heat exchanger pocket in an engine cowling (or any other part of an aircraft). You see, modern liquid-cooled engine designs rely on pressurized cooling systems to prevent evaporation losses and boil-overs. Once an engine comes up to operating temperature, a special radiator cap maintains the system pressure at 14 to 16 psi.

Let’s assume for a moment that a 3-foot by 3-foot cowling area (a randomly selected number) could be made available for an engine cooling pocket. This 9-square foot area would actually wind up being 18-square feet when both interior and exterior surfaces of the pocket were considered. The result would be a low pressure boiler. Imagine the effect on 2,592 square inches of unreinforced heat exchanger area (18 square feet x 144 = 2,592) when subjected to 14 pounds per square inch of cooling system pressure! The cooling pocket would bulge like a can of spoiled tomatoes.

When one considers the reinforcement and structural weight required to design a cowling cooling pocket capable of containing system pressure, AND the likelihood that any resulting cooling area would be less than needed, it would appear that Mr. Rieb’s heat exchanger idea is not feasible for installation aboard small aircraft.

Sincerely, - Richard K. Mater, Santa Maria, California

Q.  Dear Editor - This is a comment you might like to use in your engine column, or elsewhere. The Canadian column in a recent issue of "Sport Aviation" mentioned a report done in Finland on two-stroke airplane engines that was available there. I sent them a letter asking how I could get a copy and they sent me one, for which I thank them. One could, however, take issue with a number of their conclusions and recommendations. My comments are confined to one point. First they say, in effect, that the problem with two strokes used in aircraft is that they suffer from a thermal overload. I’ll not argue that point . Then they say that the problem is made worse when people over prop their engines. That has not been my experience.

I recently decarbonized a friend’s 447 Rotax that has an inflight adjustable Ivo Prop which he uses aggressively to obtain a faster than normal cruise at 5,000 rpm in his miniMax. He had 114 hours on it at the time and it was in excellent shape with very little wear. Certainly less wear that my own 447 at 101 hours.

I have a friend who is an instructor and uses a 503 Rotax in a Maxair two seater. He purposefully over propped it to hold down the engine speed. I gave it a complete overhaul at 300+ hours. It was the first time it had been apart. It was in great shape except for the lower rings being stuck and all four having rather wide end gaps. It is now back in the air with new rings and new crankshaft seals.

The Rotax engines put out a lot of power for their size and weight. The authors of the "Finnish Report" recommend Rotax make their engines in a lesser state of tune in order to enhance reliability. I would like to see Rotax offer that as an option.

Sincerely - Bill Erickson, Covina, California

A.  Dear Bill - We’ve obtained a copy of the Finnish report here at EAA Headquarters as well and after reviewing it, I’m in agreement with you that not all of their recommendations are good and a couple certainly don’t jive with what the factory Service Centers recommend. We’ll check with Eric Tucker, an independent consultant on Rotax engines, and with the North American distributor of Rotax, Kodiak Research, and see what their reactions are to the report and your suggestions, and we’ll include their responses in a future column.

Thanks for writing! - M.J.

 
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