Question: The 4-2 break is sweet. Any hard and fast rule for keeping it under varying temperature conditions? E.g., cooler temperatures would require a bit of needle adjustment in what direction? Or am I over thinking this?
Generally, cooler air is thicker, with tends to make it go lean, so you would need to open the needle to get the same "setting". Hot air is thinner, generally requiring the needle to go in.
However, two other things are also operating at the same time - thicker air gives you both more power, and better propellor "bite", both of which make it go faster, and hot, the converse. If you hold the same "setting", that is, the same place on the 4-2 break curve, it can go substantially faster when it is cool, and slower when it is hot. To hold the same speed, you have to exaggerate the needle motion to run further toward the 2-stroke "setting" when it is hot, and deeper into a 4 when it is cool.
If you are using a tach, it gets even more complex, because if you set it to the same RPM, and ignore where it is relative to the 4-2 break point, you *usually* have to set it to a *lower* RPM when it is hot, and "faster" when it is cold, because depending on the prop load and how it responds, it will unload (speed up from ground to air) more in hot weather than in cold, meaning that you need to set it slower on the ground to get the target in-flight RPM.
Depending on how the engine is set up, these effects can be pretty strong, or weak. There are a variety of ways to deal with these issues, this is a big part of learning to run them. Depending on what a particular combination does, you might change:
prop pitch - generally more when hot and less when cold, to get the same "bite" (but maybe change the break characteristics since this also varys the load feedback during maneuvering)
compression - more when hot, less when cold, to mitigate the power variation from varying air density OR, to make the break stronger or weaker (stronger when hot, since you need more when it it flying slower and less than you are flying faster)
venturi - bigger when hot, smaller when cold, to mitigate the power variation to retain the same "setting"
nitro - less when cold, more when hot, to maintain the same run time and same power level, and "setting"
muffler restriction - more restriction reduces power, less increases power, also changes pressure feedback into tank.
There are a wide variety of things like this to do, they all interact, and which one works or doesn't for a particular set of conditions can only be determined by experiment. Your guesses get better as you have more experience. There are a wild array of obscure things you can do to props to make the system work differently.
All these things are also present for tuned-pipe systems, the generally the variables are far less important because the pipe regulation RPM doesn't really change much with conditions, and the performance is generally so much higher, that even "off" a little bit you have abundant power. With 4-2 break systems you have to try to optimize them all the time, because the performance is low enough that you have to make it work nearly perfectly to just get in a good flight.
Of the adjustments above, by far the easiest to execute, with minimal chance of screwing up, is the nitro. The last one you need to be dealing with is compression (which, unfortunately, seems to be the FIRST thing people do). If you are nominally running 10% in the cold, and it's hotter than normal, then try using 1 ounce of 15%, the rest 10%, so you get 12% net or something like that. My general rule is to change the nitro until the run time comes out the same at the same needle position. The fuel needs to be otherwise identical, because changing the fuel viscosity (by mixing fuel with 18% oil and fuel with 22% oil also changes the fuel viscosity, thicker, the further out the needle needs to go).
I would suggest that to make any real sense of the problem, you are going to need to get a reliable stopwatch and learn proper techniques to time laps. You might make a change, and it feels more "powerful" or pulls harder, but you need to know whether that is because of the way you changed the characteristics, or it just sped up 2/10 of a second a lap - which will have a dramatic effect on what you feel.
I am not trying to intimdate or discourage you, this is the reason it takes decades to become a genuine expert at all this stuff, and we are all still learning on just about every flight. You have to be very careful to control your conditions, and have a good grasp of the underlying principles. It's also why people went to tuned pipe systems as soon as it was practical, and now electric, because they are far less prone to these variations.
Brett
Topic: Fuel line OS 46LA (Read 2809 times)