More engine questions

First of all what is "runaway"?

Most of the modern engines are set up to run rich until going into tricks?

It is the tendency of many Schnurle engines, that if you try to run them in a "traditional" stunt run (4-2-4), they will start out running fine in the 4-cycle mode that you set them, but when you begin a maneuver they will break into a hard 2-cycle, and from there on you are saying "Hi Ho Silver, away!" for the rest of the flight. They break from the 4-cycle to the 2-cycle, but don't come back, hence the term "runaway." There are several ways to solve this, including lowering the compression and choking down the venturi, retiming the sleeve, blocking off portions or all of one or more of the ports, running the engine intentionally at a higher rpm to begin with and using a lower pitch prop to control the lap speeds--or, sometimes, a combination of these.

Mr. Neumann is right on with his advice on your runaway problems. I would like to go one step further and suggest that you consider the possibility of low oil content. The oil in our fuel does more than lubricate the engine. It is also a major factor in keeping the engine cool. An over heated engine is much more likely to run away. Check with someone familiar with the engine you are using to see what they recomend.

> First of all what is
>"runaway"?

It's a characteristic of a lot of modern engines have when run in a very rich/slow needle setting with too large a prop. Some older engines would stably run that way, but most modern engines will not due to internal differences. What happens is that the it will stay rich until loaded or the fuel pressure drops for whatever reason, the engine will pick up speed, and then never come back to running slow. If you have too big a prop, or too much pitch, this will result in a screaming fast flight.

All the engine is really doing is running as designed.

An entire cottage industry has sprung up to modify these motors to run like they did back in the "good old days", including our host. Many people still prefer this, for reasons not entirely related to performance. If this is what you want SSW and several other reliable detuners are available for a nominal fee.

> Most of the modern engines
>are set up to run
>rich until going into tricks?

It depends on how you classify modern. 50ish years ago, and until about the mid/late-80's most people set the engine to be on the lean side of the 4-stroke, so that load would cause it to break into a 2-stroke and retard the speed loss caused by maneuvering. From the late 70's on, most modern engines didn't/don't like doing this with reasonably-sized props. Most modern engines with conventional mufflers require modification/detuning to do this.

In the mid-late 80's, some guys from Jersey (Dean Pappas figured it out, and Hunt turned it into a competitive system) started using tuned pipes not as power-boosters (as in speed), but as regulators to prevent the runaway phenomena. This allowed the tremendously improved power that the modern engines intrinsically had to be harnessed. At the same time, almost as a side effect, they also tuned them to use lower pitched props that had tremendous speed control properties even without the engine helping. The combination also has the very favorable property of being very easily adjusted to get the desired characteristics. This system is now almost universally used in serious competition. Initially, they were run pretty close to peaked-out, and since then have been backed off a bit to run a little richer, depending on the individual preference. Mine runs in a 4-stroke the entire flight, for instance.

Along the way, some of us realized that while the hot-rod piped system were really good, but that most of the good things about them could be gotten with just the low-pitch and high rpm, and an unpiped engine. Early attempts at this were spectacular disasters, mostly because we were trying to fly old "35-sized" airplanes with modern 35-40's with 3 times the power. It would probably work, but would require far too low a pitch than is commonly available. An unmodified modern 40 is way too much motor for these airplanes, and will result in very unsatisfactory performance. They must be detuned to be able to fly the airplane properly.

The real solution is to use an engine about half normal size. Run as it was intended (relatively fast) with a small diameter 4" pitch prop. For example, a modern 20 or 25 is a pretty good replacement for for an old or a detuned modern 35. It also runs much better on a profile mount, which is something the older engine have a real problem with sometimes. It can't run away, since its set to "runaway" speed on purpose.

This approach has been used widely on the West Coast for the last 6-7 years with pretty universally good results. The engines run a little off-peak and then you adjust the prop load to get just the right speed. These little motors typically end up with props like:

Os15FP - 8-4 or 9-4 APC (small "35" planes like the Ringmaster)
OS20FP/Veco 19BB - 9-4 or 10-4APC, 9-4 Tornado Magnum (larger "35" planes like the Flite Streak and the Skyray 35, Fancy Pants)
OS25FP/LA, TT25- 10-4 or 10-3 APC, 10-4 Tornado Magnum, 10-4 Rev-Up. (flapped planes around 500 squares like the Twister, Banshee)

These types of systems are much easier to set up, and much easier for beginners to adjust and fly than old-style runs. Basically, you take them out of the box, bolt them on, and go fly. If they have an R/C carb, just wire it wide open. These engines are also much less picky about fuel than some older engines, some of which can easily be destoyed by normal hobby-shop fuel in minutes.