Greetings guys from a drizzly Montignac le Coq .

Can an engine expert explain please why some 'stock' un-modified engines don't run-away and others do ?

For example ..... My Magnum/ASP 36's , OS46la's and TT 36's don't run away.
Also I am very reliably informed by Mr. Andrew Tinsley ,  GP42's run fine 'stock' as well.

On the other hand , my FP's have all needed modification.

Is there a 'common denominator' that distinguishes some 'well behaved' stock engines ?

Much obliged as ever for any contributions , my engineering education is sadly lacking.

Kind regards ,

Robin [ an ex-pat Brit' in the Charente full of ex-pat Brits' ].

Give me a break! Can someone also define what "Run Away" is? I've seen several references to this term that leaves me assuming it's a screaming engine that will only run in fast 2-stroke mode without a favorable 4-2-4. Is there more to it than that?

Thanks!

To make a comment on my post.  I run uniflow tanks, so have only experienced runaway a couple of times, under exceptional circumstances.  However, I am well aware of wind up, where an engine runs faster and faster as you go through the loops, for example, and have eliminated that with prop selection. 

                  Lets not forget the reasons for the runaway. Runaway doesn't always start in the beginning of the flight. As Tim stated using the FP as an example, the way to tame this engine is by proper prop selection. For example, the Philly Flier setup uses the 10.5x4.5 APC as the prop of choice. This prop is allowing the engine to run as if its in 2nd gear but at a constant speed. When A heavier pitch prop is used its loading down the engine causing not only speed to increase but also a heavier load on the engine. This will more than likely make the engine lean and go off on a tear on which its not going to return to until it cools. This is where proper venturi size comes into play. The muffler being a very important part of the equation as well . A restrictive muffler is going to retain heat and therefore going to make the engine hot and lean out as well. In the event the engine is cowled, air holes of sufficient size are needed to keep things cool. The faster the engine runs the quicker you need to get the hot exhaust out and more fresh fuel has to be delivered into the engine. SO runaway isn't some ficticious word it happens quite frequently. The FP.40 when properly tuned will give terrific results. I have several and when used based on the Philly Flier instructions I have never encountered a runaway problem. Another problem that plagues several engines is the vibrations in profiles. The aerated fuel causes runaway problems probably more than any one example I've given. Our club has been having difficulties trying to resolve some of the bad vibe issues, but were gaining on it. Most recently someone on here posted about using a Dubro clunk pickup. A fellow flier just tried it the other day and sure enough this tamed a plane he was playing with for almost a year now. We've tried everything from bubble wrap around the tank to internal bladder tanks such as the Tetra versions. It seems that once the fuel is foaming its next to impossible to get a setting on the ground as its all over the place and the needle is all but responsive. As I was told when I returned to the hobby a few years back, pay special attention to what works. Fuel itself can make just the difference as I've found and exclusively use Powermaster fuels. I've used fuels with the same oil content as Powermaster but the Powermaster always seemed to give a slight edge on power and steadiness. This simple test was immediately noticed on flights back to back where I had to change the needle when using one fuel to the other. There is more to getting the engine to run in a stunt like condition than just changing a prop. Some of these engines need to be slightly decompressed using head shims. Ken

        Tim, Armor does work and it works quite well. The problem is that its not a cure-all. If you have a gallon I only recommend a squirt or two as it doesn't take much. Put some fuel in a plastic see through jug and shake it. You can see the bubbles form and foam up. Spray a squirt in the container and then shake it noting what it does to the fuel. You will see it doesn't foam at all. Others claimed this makes the plugs burn out prematurely but I've never seen this happen. Ken

To make a comment on my post.  I run uniflow tanks, so have only experienced runaway a couple of times, under exceptional circumstances. 

      I generally agree with the use of uniflow, but the kind of runaway most people talk about happens just as much with uniflow as it does with suction. I believe they mean that the engine takes off screaming fast at some point in the pattern and just stays that way.

   This is more a function of the way people used to run them - trying to run a 40FSR in a 4-2 break with a 6" pitch prop at 8000 rpm - than it is the plumbing.  Of course it doesn't like that, the first chance it gets, it cranks itself up to about 11,000-12000 rpm. Which, not coincidentally, it about what you need if you use a regular stunt prop, i.e. 4" of pitch.

     Brett

I had extremely bad run away with a G51 that was totally cured by adding more oil to the fuel. It appears that (possibly amongst other things) too much heat is the enemy.