CONTROL LINE SPEED
Glenn Lee, 819 Mandrake Drive, Batavia IL 60510
The deadline for this column arrived too soon! Instead of working on my Speed engines like I should be, I'm building a couple of model engines. No, not model airplane engines—just models of four-stroke twin-cylinder antique engines, and there are an awful lot of parts involved. I've always been fascinated by the horizontal opposed twins—always wanted one, OK, but never had a chance to get one.
I've promised in past columns to write about newer engines if I obtained useful information. There are several relatively new engines available, so I'm sure some of you are interested. Some of the older ones are still competitive; K&B still has the .65s for sale, and remember, Bill Nusz used one of them to win the '93 Nats.
The latest ones have some minor internal modifications and a larger-diameter minipipe. Bill also used a Nelson .40 to win Formula 40, so both of these engines are still in there, even though they have been around several years.
Dub Jett, a past champion Speed flier and World Champion in Pylon Racing, is making a variety of .40s, with versions that have front or rear intakes and side and rear exhausts. All are strong-running engines, but they can still use some tuning for all-out Speed flying. They have massive, well-braced, solid crankcases and have been performing well in Pylon, FAI Pylon, and Quickie 500.
Dub sent me a rear-intake, rear-exhaust version for trial in Formula 40, so I built a new airplane for it last summer just before the Nats. The rear intake is a huge opening, the same size as the cutout in the rotor. I don't know if such an opening is really necessary, but it does run. A remote needle valve is supplied, and a fuel nipple injects fuel into the side of the big intake. The needle valve assembly is designed for a pressure tank; it won't work with a pen bladder because it doesn't shut the fuel off and it is too coarse to set.
I test-ran the engine and completed the airplane just before the Nats, but I didn't have any time to check performance. The piston fit was really tight, and on the second flight the airplane torqued in and bounced off the pavement, splitting the wing and ripping out the leadouts. If your F40 engine is running properly, the airplane usually takes off so fast that it doesn't have time to roll in!
I repaired the airplane for the Dayton contest in September, and disassembled the engine to see what was wrong. An examination of the piston revealed little or no taper at the top of the sides, which gave an indication why it was so tight at the top of the stroke. In production, Dub machines the taper at about .0015 at the top as the piston is turned with a diamond tool. The piston is then lapped to assure roundness and size, so sometimes the taper can be reduced or eliminated during the lapping.
In the old days, we would have run the engine at high rpm to obtain a good fit and taper, but it takes too long to do this with an ABC or AAC sleeve and piston—that silicon alloy just doesn't wear against a chromed surface! So I took a chunk of cast iron and bored it out with a taper of .00375 per inch per side. For every inch of length, that equals a total of .0075 on the diameter. Like so many other Speed innovations, this dimension came from Bill Wisniewski, who measured it from a good-running, well-broken-in engine.
The cast iron was about one inch long and was bored out until the piston would go in about halfway, so I could keep it concentric when lapping. You only want the tapered area to be about 1/10–1/8 inch long, and it takes very little lapping to achieve that—be careful!
I used some 900-grit aluminum oxide powder with light oil for lapping. If you try this, don't use emery compound or diamond compound; be sure you get aluminum oxide or some of Fox's Lustrox powder. The size of the grit should be around 1000; don't use coarse stuff like 500 or lower. These powders can also be listed by micron sizes, and 1000-grit is roughly equivalent to 3 microns. Lapping compounds are available from lapidary shops, astronomical supply shops, metallurgical labs, and some industrial supply houses. The laps can be made from brass, copper, or steel, but they aren't as good as cast iron.
The tapering of the piston really woke the Jett engine up! It needed a hotter glow plug than the Nelson insert-type that it came with (to allow a richer setting at takeoff), so I have to get some of the little GloBee inserts. Even so, it was right up near the winners at the Dayton contest.
I'm also going to open up the exhaust opening in the crankcase (not the sleeve), and use a larger-diameter minipipe to try for more rpm. You want the exhaust gases to expand and get out of the way as soon as possible with a minipipe. With a tuned pipe you need compression at the exhaust port, but not with a minipipe.
I might try a trumpet-shaped head, too. The low-nitro fuel doesn't need the high turbulence you get with a squish-band head. I'll have to try some other props: maybe less pitch, less diameter, maybe different types. Things to do!
The Jett engine is an AAC type utilizing a chrome-plated high-silicon-alloy aluminum sleeve and a similar alloy piston—a combination that can give as good or better performance as the ABC brass sleeve, and is lighter. I learned years ago how to fit the ABC engines to go fast, but never worked enough with the AACs to know much about them. The brass sleeves expand from the heat a little bit more than the aluminum-alloy pistons, so they have to be very tight at room temperature.
Testing an AAC engine for fit
Dub told me how to properly test an AAC engine for fit, so I'll try to explain it to you:
- Run the engine on a test stand with a slightly smaller prop to allow the engine to rev up to flying rpm. In other words, run it hard—using the flying fuel, of course.
- Let it run long enough to get up to maximum temperature.
- Pinch off the fuel line to stop the engine.
- Immediately—before it gets any chance to cool down—turn the engine over and feel the "bump" at the top of the stroke.
What you want is a perfect seal when the engine is at operating temperature. If the prop turns over with little or no compression—if it "squishes" rather than "pops" over—the piston and sleeve fit is too loose. If it binds when turned over, then the fit is too tight. Simple, huh? I never would have thought of it!
Transcribed from original scans by AI. Minor OCR errors may remain.
