MJD

My Feedback: (1)

Quite possibly but not always true - which .46 two stroke? Some are timed for more power at lower rpm, the OS .46AX is an example. In some cases a 10-7 may give more top speed than a 9-7 or 9-8 and there are folks here that have seen that consitently in certain combinations of airframes and engines. Depends so-o-o-o much on your aircraft and engine. However, the specific recommendations made by the other folks here are absolutely where you would go to experiment.

Think of increasing pitch and reducing diameter as shifting up a gear on a car or bike. You have more theoretical top speed but less pull, so if the drag is so much that you can't get there, then you won't. So often some experimentation is in order, to find the optimum combination.

But yes the general rule is, as already pointed out, reduce diameter and increase pitch. You reduce the diameter to account for the additional load from increased pitch - this is on the assumption that your baseline prop is reasonable, i.e. you're not lugging the engine down and it is in an area of it's powerband where the hp output is decent. Sometime just reducing the diameter one inch but staying the same can do the job, as the engine will unload and in most cases produce more horepower - this depends on the motor, and where you started from of course. For example, suppose you fly your Purple Whatzit with a 10-6 prop and the engine tachs at 13,500k on the ground. Now you switch to a 9-6, and the engine tachs at 14,500. The engine is probably [not definitely, but probably] producing more horsepower in this case based on typical hp/rpm curves for .46 size engines. The higher rpm gives you higher "pitch speed", in other words pitch times rpm, meaning that potential top speed is higher. If this choice works for the airframe involved, top speed will go up. And there may be more yet, you might at that point try say, a 9-7.

Small, clean airframes, i.e. low drag, respond best to higher pitch props and reduced diameter. Draggier airframes will hit a "wall" sooner, and often the best performance comes from a middle of the road choice.

Remember - more speed requires more power. To reach the best speed you need to let the engine develop as much horsepower as it can (look at reviews for performance numbers) and then, your job is to find a prop that best utilizes that power. Not to beat the gear analogy to death (but it does work farily well), your goal is to find the gear that gives the highest top speed - go too high, and you may not get there and "shifting down" might give you higher top speed.

MJD