I was asked that question and I gave one of the usual responses. "Why its heat of cause!". I then started to think about this answer. Why should heat cause the problem? The only answer I could figure was that head temperature went up and caused the fuel to ignite earlier. So with the "ignition" advanced the engine speeded up and got hotter and advanced the ignition some more and so on.
Is this the only answer or is it more complex than that? I don't normally like a 4-2-4 break, but I have seen a runaway with an engine two stroking with a fine pitched prop, so is the answer to load the engine more, add more oil to cool it or just richen up the needle valve? I don't normally have the problem, so I have never really thought through a cogent answer.

Andrew.

That thread is more about how to get controllable power.  Brett had another post somewhere that had a very reasonable explanation for "runaway", which is the engine starting out in a nice 4 cycle run and then at some point it speeds up and stays running fast and hot.

The reason appears to be the rpm's where the peak torque and peak power occur.  The good old 4-2-4 Fox Stunt had a peak torque way down around 6000 rpm.  The peak power was at around 13,000 rpm.  It would swing a 10/5-6 prop at about 9000 rpm in a solid four cycle.  In the air it warms up a bit and settles down slightly higher rpm.  Do a maneuver and it will break into a two cycle while the plane is above 45-60 degrees.  If everything is right as soon as the maneuver finished it drops back into a 4 cycle.

Lots of people tried to make Schneurle engines, such as the FP 40 do this.  The torque peak on the FP is about 10,000 rpm, and the peak power at about 14,000 rpm.  On a 10/5-6 prop it will run rich at about 9000 rpm on the ground.  In the air it will stay at this setting, and may even do a decent 4-2 break for part of the flight.  But usually at some point the motor warms up and switches to running in a fast 2 stroke at 11,000 rpm and goes way to fast- the runaway.

What appears to be happening is that the motor can run at a lower rpm and 4 cycle, or a higher rpm, past the torque peak and two cycle.  But once it warms up, for whatever reason, it can't cool off and switch back to the 4 cycle.  The torque increases at it tries to slow down and prevents it.

Lots of mods can make it into a 4-2-4 engine.  The simplest thing is to run it like the Fox Stunt.  Choose a prop/plug/fuel combination that lets it run at a 4 cycle above 10,000 rpm or so withe the airspeed you want.  That usually means a 10/4 ish prop.  The motor will run more like a Fox, but will 4-2-4 at a higher speed than a Fox.  To keep it under control a smaller venturi will help.  A .250-.260 venturi with the standard OS needle valve will provide plenty of power for a Nobler sized plane.  The small venturi makes the motor run slower and keeps it from running much over 12,000 in the air, so it can keep cool enough not to run away.

If you want/need more horsepower a larger, low pitch prop will work, but the motor won't run 4-2-4.  It has to run in a "fat" 2 cycle.  The larger prop keeps the speed down, but requires more power, so the motor can't be run in a 4 cycle.

The LA 46 will swing an 11 in. prop in this way, right out of the box.  Depending on the plane size, the prop, and the fuel it can either do a 4-2-4 or run a bit larger prop in a fat 2 cycle.

Thanks Brett.

Phil C

This seems to also be the reason behind running big motors.  A big motor loafing along is much easier to control than a smaller motor working its heart out.

Andrew,
The short answer is - Yes it is the heat on the LA 46 that causes the problem. Look at Brett's explanation that Phil posted and that is what he is saying. Some other things that contribute to runaway are: low oil/high nitro in the fuel - if you take a Fox 35 and run it on 15% nitro with 16% oil (RC fuel) on a wide blade 10x6 prop it will do the same thing. Change to 5% nitro and 25% mostly castor and it settles down.

One quick way to check if you have to much load is to reduce the prop diameter 1", same pitch and see if it calms down, then look at other factors like fuel and plug.

Many RC engines are timed to run at the higher rpm to pull the ship at faster speeds, they have a throttle to adjust that. We rely on hitting the sweet spot on the pitch/rpm point of the motor to get our desired speed and still have something left to do the maneuvers (particularly in the wind) or use a pipe or magic muffler to regulate.

Best,      DennisT

Using a range of venturi inserts was in wide practice in the day. Enya's came that way. Three venturis right in the box. Plus two different compression heads. FP40s, the engine as sold for stunt, came with extra head gaskets in the box. A small and a large venturi are listed for FP40s even now. Big buck stunt engines are commonly tuned by prop choice, fuel, head gaskets and venturi size. The aim is to get regulated power that compliments a given airframe. In other words tune with venturi size and other factors so that the engine runs the same way, flight after flight, providing extra power where most useful in the pattern. FPs work great when you tune with venturi size as well as the other variables. LA46s as well. Common practice to use a small FP40 venturi (.257, I think) in an LA46. I happen to like a .272 venturi in an FP40 for stunt. A size in between the large and small venturis sold for that engine by OS. FP40s suddenly became manageable and tunable for me, when I started working with venturi size to throttle down. Like I said, I have seen many folks do this with different $400 engines. Beats me why folks do not use the same approach with FPs, LAs and other similar engines. Same principles. Look, I used to go crazy trying to get FPs to run sloppy rich then break like Fox 35. Forget it. If your engine is running away chances are it's seeking the engine speed it was designed to run at given the venturi size, prop, fuel, compression and so forth. A smaller venturi will also improve fuel feed. It won't kill latent power -- the power you are seeking as the engine breaks from a wetter 2-stroke to a leaner 2-stroke. Or in the case of an LA46, a smaller venturi will still allow you to switch off (break) between 4-stroking and 2-2stroking. Usually.

Thanks for all the insight everyone. Lou your description of Bill Netzeband's work is very interesting. I don't have access to the US magazines and if you have a reference for it, I will try to dig it out the article somehow.
So I had already answered my own question, that is to say that because of the heat build up, the ignition timing is advanced and hence the speed increase. I had always assumed that the heat build up was in the head, the bypass heating never occurred to me.
Over the years I have seen claims for the use delrin venture inserts. Because the plastic takes longer to heat, then the charge is not heated so much. Mostly these claims seem to come from diesel powered team racers, although I seem to remember similar claims for stunt engines. Pity that bypass areas can't be made from plastic, it would be interesting to see the effect!
If, as seems probable, the ignition advance causes the runaway, then the use of a colder plug may be beneficial on such engines as the FP 40, which seems to have an evil reputation for runaway. But then in the four stroke mode, the engine will like as not flame out! As with most things, you have to compromise!

Regards,

Andrew.

Andrew,

I'm not sure if Wild Bill ever published his comments, or, if so, where. (can anyone help on this?) I had several opporunities to chat with him at - usually - Tucson VSC's over the years; I may be citing one or more such conversations... Rustler's observations certainly accord with Wild Bill's as I recall them.

On glow v diesel engine heating: unless I mistake things, the strong chill of vaporizing methanol in the glow engine's air intake/lower case path slows the heating from upper-engine pieces. Sitting between back-to-back flights lets that heat 'soak' down a glow engine's metal, but not disperse. Diesels don't heat as much in the upper sections - kerosene does not chill, or evaporate for that matter. It atomizes more than vaporizes. Diesels seem to 'soak' the heating more evenly, but the peak heat is not as high as the combustion chamber pieces in a glow.

(granted: little experience with "at-the-limit" diesel operation, or with the oldest 3-port diesels run on over 40% ether fuels. Those, apparently, ran cooler than ambient temperature occasionally.)

Eagerly anticipate any and all differing experiences!

I corrected a run away problem with my son's plane, and the cause has not been mentioned above. About halfway through the tank the engine would lean out and just scream for the rest of the flight. Sometimes the lean out would occur right during his first inverted flight. I was pulling my hair out trying to find a solution. Finally discovered a ball of solder was bouncing around in the tank, and would apparently wind up at the entrance to the outlet tube and restrict. I hope this might help

Brian Hampton: "The 75/25 fuel raised the oil flow to a level that kept the engine cool enough to run in a steady 4 stroke."

We had this happen with the LA series motors and others.  Not sure if it is the flow of oil or that extra oil changes the force of the charge ignition.  On the LA 46, which wasn't running away, was that more oil made the 4 cycle more consistent.

I also found another cause while trying to get a complete flight in at Brodak's last week.  The load of the prop makes a huge difference.  I had a plane trimmed last fall on a stock Zinger 10/4.  I broke the prop this spring and replaced it with one that looked identical.  I couldn't get a decent run out of it.  The same thing happened with that setup at Brodak's.  After fussing and fretting, I took a couple props into the shop and and thinned them about 1/3 on the front face.  Just a very rough job, thinning the front face from the root to the tip, then thinning the trailing edge to .020 or so and then rounding the front face into a smooth airfoil.  I bolted it on and it immediately ran much smoother and cooler, four stroking easily at a higher rpm.  I figure to try some more oil next time I go flying to see what happens too.