My 2 cycle chainsaw does the same. Run it rich (4 cycling) and when I put it into a log it breaks into a two cycle providing more power and RPM. I don't understand why.

There have been some pressure measurements on model airplane two cycle engines. They do four cycle, with a bang every other rotation. The power is about 60% of two cycle, probably because there is a denser charge when it does fire. If properly set, a four cycling engine will go to two cycle when load is applied. Why that is, I do not recall.

I never thought my old Fox actually four cycled, that is fired every other time piston passed thru TDC. Rather, it was run rich, at a "steady" lower RPM. When the manouver tightened, it leaned the mixture while the manuver was in progress. This was sometimes called a "break", IIRC, and provided extra power thru the manouver(s). Not all engines were cabable of running like that. Last Stunt contest I attended(Visalia, CA 20 years ago?) I noted no one was runing "four cycle" like I remembered and had gone to larger engines.

It's pretty much agreed that an engine 4 cycles with a rich mixture because of incomplete scavenging from a power stroke contaminating the next charge enough to stop it igniting. This certainly does happen on every other cycle as Jim mentioned and I have the graph of the pressure measurements (converted to torque) here.

The reason why they switch (4-2-4) in flight is still open to question but it seems to have something to do with an added load on the engine giving a sudden increase in internal temps at the combustion chamber making it easier for a contaminated charge to burn. It's all still a bit of guess work though.

Well, learn sumpin new all the time! I'd always figured it was just a discriptive term given to the rich running. Looking back to the early 60's thru '65, I don't believe it was ever explained that way to me. It was just somthing we strived for. I had an old Red head McCoy that would run like that, but my models, mostly Sterling profile ships didn't like to be flown slow. I was just learning to fly my Yak 9 inverted when I got to fly a buddy's Ruffy with a Fox 29. It made me want to build a full stunt ship. But the Yak and the McCoy went straight into the pavement (I got confused) right after that flight.
thanks
charlie

Just to add a bit more to this 4 stroking thing which Jim alluded to before. Even when running in a clean 2 stroke not all of the exhaust gets out so there's always a bit of contamination to the fresh charge which means the engine can't develop quite as much power as it should. When the engine is 4 stroking and misses a beat then the cylinder is still filled with a fresh mixture plus what remains of the contamination which then has a chance to escape through the exhaust port. This gets rid of almost all contamination so the next fresh mixture that gets compressed is virtually uncontaminated (more oxygen available) so burns with a much higher pressure than it can ever do even when running in a clean 2 stroke so that firing stroke is very powerful. That's why there's very little change in revs when an engine blips into a 2 stroke from a 4 stroke.

Originally Posted by downunder

It's pretty much agreed that an engine 4 cycles with a rich mixture because of incomplete scavenging from a power stroke contaminating the next charge enough to stop it igniting. This certainly does happen on every other cycle as Jim mentioned and I have the graph of the pressure measurements (converted to torque) here.

The reason why they switch (4-2-4) in flight is still open to question but it seems to have something to do with an added load on the engine giving a sudden increase in internal temps at the combustion chamber making it easier for a contaminated charge to burn. It's all still a bit of guess work though.

In control line Id always associated it with the tank suddenly being BELOW the engine rather than behind.. so the mixture leaned out in the climb..

And in my experience moving to two stroking increases RPM dramatically. I have a vidi memory of years ago watching a competition speed event and seeing the tuned pipe (Rossi IIRC) burbling around te ring being shamelessly whipped until the rpm rose and the 4 stroking vanished and the thing ;leapt forward, and then after a few seconds RPM got into the tuned pipe range..and the poor pilot had a second to slap his handle in the yoke while the model screamed round..

I was amused for almost ten minutes

Originally Posted by vintage1

In control line Id always associated it with the tank suddenly being BELOW the engine rather than behind.. so the mixture leaned out in the climb..

:

This is one of the theories behind the motor breaking and increasing RPM, the other being the "loading" one mentioned earlier.

If you are sitting on the ground and point the nose up the motor will speed up.
This will be true of almost any motor that has been set slightly rich, not just those that 4-2 break.
if you apply more load to a motor it will generally slow down. It might go from 4 stroking to 2 stroking, but only because it has slowed down.

In a loop in any attitude the nose will be pointed in towards the centre of rotation due the the angle of attack, so part of the G load is always facing aft relative to the datum line.
(At high G's the rule of thumb for structural loading is that the aft G load hence the forward load on the wing is 20%-25% of the vertical one)

So which one of the two main theories makes more sense?

I know which one I believe to be true

Pat MacKenzie

We've all held our running planes nose up and noticed that they lean out, and RPM increase When we hold them nose down they richen and RPM decrease. This is about gravity and fuel pressure changes. A 2 cycle chain saw is set rich, 4 cycling, and when you put it into a log it breaks into a more powerful 2 cycle. Apparently, there are at least 2 dynamics causing a 2 4 2 break.

I don't have experience with chain saws, but do they actually increase in RPM when you load them, and then slow down when the cut is done? Without you touching the throttle?

In video number 4 here, at about the 5 minute mark, it sounds like they slow down when the cut is engaged, then speed up when the cut is completed.

http://www.husqvarna.com/us/support/...os-how-to-use/

I agree that the switch from 4 cycle to 2 cycle can be caused by leaning the engine. However I don't think the reaction is quick enough to explain what happens in flight. I don't run a 4-2-4, but rather a rich 2 on my Fox stunt 35. It runs to suit me, and the discussion of what happens with four cycle is moot.

I believe it is a combination of both tank height and engine load. The RPM change between 2 and 4 is dependent on many things. Intake and exhaust porting along with compression ratio and venture size all make a difference. Many engines will jump 12 to 14 hundred rpm when going to a 2 from a 4. Others like the Fox 35 will only gain about 500 rpm with a 10-6 prop.

FWIW, I found the break too strong with a Fox on a 10x6.
Mine always ran better on an 11x5.
With the 10x6 the model had to be run too rich to get it to fly slow enough, so when it broke it tended to jump. With the 11x5 the break is much softer, just enough to keep the airspeed up in the climb.
This was in a Gieske Nobler and a SIg Chipmunk, both fairly light.
In my BiSlob the 10x5 was about right, but you are after a different kind of break with the 'slob.

Originally Posted by pmackenzie

FWIW, I found the break too strong with a Fox on a 10x6.
Mine always ran better on an 11x5.
With the 10x6 the model had to be run too rich to get it to fly slow enough, so when it broke it tended to jump. With the 11x5 the break is much softer, just enough to keep the airspeed up in the climb.
This was in a Gieske Nobler and a SIg Chipmunk, both fairly light.

In the heyday of the Green Box Nobler, the props of choice were Rev-Ups, and their pitch numbers were off at least a full inch, same as the Top Flite props. Thus, a 10-6 Rev-Up was really just a 10-5, anyway. I was running Tornadoes more than other types, and repitched them to 10-5s from the delivered 10-6.

I had some overweight Skylarks later, which I put rebuilt McCoy 40s in (same engine that Ed Southwick used, himself), and used 11-5s in those. My own Chipmunk was less overweight, but I tried using the same ST G21-40 that Jim Van Loo used, although without success. I ended up with a G21-46 in it (11-6 props).

If you can get them, I found the Taipan 10-5s, 11-5s and 11-6s were really good props for stunting with 35s, 40s, and 46s,


Kiwi

I usually ran Rev Ups, the 11x5 were made by cutting down 12x5 to get 11x5 wide blade. So as you say, probably really more like 11x4.