I have been reading old threads in the 1/2A section and there are many threads of plane and engine performance with propeller size changes. For example, plane flies well level but the vertical part of a loop the engine looses power, change the propeller, how? what happens when a 5.5x4 is changed to a 6x2? What is the affect of propeller diameter in relation to pitch and engine speed? Some threads talk about cutting propellers down to get a specific improvement.
Can someone please explain this and/or point me to an article that can shed some light on this subject.
This is a huge topic, and (for IC engines used in stunt, at least) has many poorly-understood "rules of thumb" beyond the basics.
The real simple basics - pitch controls the speed and the diameter controls the thrust, and both affect the load on the engine, and how the load on the engine changes with speed.
If you ran a *fixed* RPM (and using 1/2A sort of numbers), say, 18000 RPM - more pitch would be faster, and more diameter would be faster. That is, at a fixed RPM of 18,000, a 5-3 might pull the airplane at 45 MPH, and 5-4 might go 55 mph. It does that because more of the engine power is being transferred to the airplane, that is, the *efficiency* is better. It is more efficient because the prop (which is just a spinning wing) flies at a higher angle of attack. Same with the diameter - a 6-3 at 18,000 will go faster than a 5-3 at 18,000. Same reason - it's more efficient, in this case, because it is moving more air (the disc area of 6" prop instead of a 5" prop).
However, you already know, it *will not* be the same RPM, because the *drag* from your prop goes up with diameter as well. This is where it gets complicated, because how the engine reacts to the prop varies and can be adjusted to vary how you might want it.
Full-scale airplanes are generally interested in the most speed for a given power, so they choose props that load the engine give the peak horsepower in flight at the highest possible efficiency. For full-scale airplanes that maneuver and change speed, this means that the best prop for takeoff is not the same as the best prop for maximum speed - so they have *variable pitch* propellors, low pitch for takeoff and high pitch for cruise at high speed.
We don't have variable-pitch props (er, at least not commonly, not yet...) so we pick a prop that gives us the desired maneuvering characteristics that has to compromise various factors like load, RPM, power curve, etc. In stunt, the drag of maneuvering slows the airplane down, and you want to either not lose as much, or recover that speed quickly, so depending on how the engine reacts to load, we adjust various prop parameters and engine parameters to make it work in an ideal manner.
A big problem with explaining it for stunt is that *no one agrees what "ideal" means* so we all argue endlessly over how to adjust it. How the engine/prop combination works is *absolutely critical* to how the airplane flies, by far the most critical factor in the airplane performance, far more important than the design of the airplane, as long as there is nothing to crazy about it. Just getting something that will work is pretty easy, getting something that will work better than your competitors is a lifetime of work that we never completely master.
Brett