Suggestions to Preston Briggs
This site was last updated on February 15, 2008.
Here's a useful article, written by Henry Nelson and published in the MACA newsletter and reprinted in Flying Models, July 1998 (before he won the 2001 Nats).
It may surprise many users of the N36C that long ago, before there were Nelson engines, I flew combat. Undoubtedly, I never flew well or with great success, but I was then, and have remained, interested in the event. This is why the N36C exists today.
The following describes some of my experiences using my own hardware and information about some of my latest products.
If I wanted to fly Slow Combat, I had to come up with a suction venturi, but there was a problem. When I first designed the N36C, I used the simple rectangular intake because it was light and eliminated some mold problems associated with the offset intake. As all of you who have made your own Slow Combat venturi know, it makes it hard to fit a proper one that is sealed to the case. I knew that too, which is why I avoided the problem by not building one.
Now I have made a venturi insert which uses an O-ring seal to the crankcase and has its own needle valve assembly which uses my plastic seal/collet arrangement. The drawbar which tightly clamps the venturi has its own O-ring seal. I've settled on a venturi size which gives trouble-free performance with the GRW tank.
Second, I needed some motor mounts to fit the Czech planes, but I was concerned about the normal metal engine mount system. Clearly, the mounting surfaces on the airplane are not going to be perfectly parallel both fore-and-aft and side-to-side. However, fore-and-aft does not matter (within reason) because there is clearance between the normally used 4-40 mounting screws and the holes in the crankcase. But, if the airplane is not parallel side-to-side, the mounts will be non-parallel and will pry on the crankcase. Since I try to build the mounting lugs on the crankcase perfectly parallel, I looked for something that would not distress the crankcase.
What I came up with was a magnesium backplate with mounting forks stuck out behind it. Of course, the forks can still be misaligned with the airplane, but at least the bending is further away from the rear bearing. Also, in a crash, the mount hits solidly on the airplane rather than trying to break off the engine mounting lugs. If the mount is too loose on the airplane, I recommend coating the mount with release agent and epoxying it to the plane.
Finally, I needed shutoffs and I chose to use Mejzlik's. I made a needle valve assembly to eliminate leaks and the reported weakness of Tomas's unit.
The result of these efforts have been positive. I had fun, which was the main objective. The biological hardware didn't work real well, but then it never was of top quality and has not improved with age. On the other hand, the engine-related hardware did work as advertised. This was gratifying as there are always contradictory stories floating about and it's best to refute these with personal experience.
Next, I'd like to discuss head spacing. All N36Cs are sent out with the head clearance set at .005-.006 inch. Usually when they are returned for service, we find them at .010 to .020 inch. Why? Well, I'm not sure but I suspect that if users are blowing plugs or having other problems, it's caused by substandard fuel. I mix my own fuel and use a mix of 15% nitro, 20% UCON LB625, and methanol.
All last year, at contests and in test flying both Fast and Slow setups, I used APC 7.8x6 props and never did burn out a glow plug. Most flying was with the above 15% mix, though I also evaluated 10% and 5% nitro mixes and on the test stand I've run 40% and 65%. Now if you are accustomed to using more nitro, I have no complaint. Tipping the nitro can will gain speed at the cost of more critical settings and higher fuel consumption. Unfortunately, if you've decompressed the engine, more nitro may just return performance to the original level.
We have recently seen the following results in a Fast Combat engine using my standard test prop which runs at an appropriate in-air rpm range:
My advice is to make every attempt to run your N36C with the original settings. If it works for me, it can work for you.
The most overloaded component in the N36C is probably the rear bearing. When designing the engine, I was worried that this thin, lightweight bearing might not hold up at all, but if we want an 8.5 ounce engine with a 17 mm crankshaft, the 17x26 bearing is the only game in town. Well, the bearing is adequate but there's no extra margin.
The problem seems to be rust. (Note here that bogus fuel (see above) can be a prime cause of rust.) Rust on the balls (the bearing's most highly loaded component) causes stress concentrations which can cause the balls to break. The least expensive answer is probably to just use pure fuel, thoroughly oil your engines after use, and replace the rear bearings after a season's use.
Help, at a price, is on the way. Ceramic balls are virtually indestructible and I have been able to order the rear bearing with them. The price will be $46. They won't be a cure for sloppy maintenance because the races can still rust, but they'll prevent pieces of bearing balls from destroying a piston and cylinder.