This comes up often and the answer is always the same. It all depends on several factors. One being the engine/plane combination, weight, wing loading, and all that. The plane will not go faster than it is designed to do.. but you can put a bigger engine on it and get there faster, but it won't help beyond a certain point, and all you do is risk airframe damage, wings getting torn off, and so on.

Then there is how a prop works. Of course it rotates, and pushes a chunk of air through the prop arc. Over-exaggeration, but you get the idea (Gordon, don't say it.. ).

But, consider a prop is like the gears in the transmission in your car. If you want to climb a steep hill, you shift down to a lower gear (same engine, same car, but now a lower gear). So, the car "pulls" you up the steep hill, but at a slower speed AND the engine RPM increases dramatically.

Now, you are on a long straight road and you want to go faster. So, you shift up to al higher gear and the car goes faster, but takes longer to get there. Again, same engine, same car, but now a higher gear. So, the engine is now pulling you along at a higher speed, lower RPM, but is working hard to keep you there.

Relate that to a prop. Take a given prop 12-6, as your original post suggested. The 12 is the diameter, the 6 is the pitch. At 10,000 RPM, the prop is rotating 10,000 times per minute, and with a 6 pitch, that's 6 inches of air that passes through that prop each time it rotates. So, that's 60,000 inches of air in one minute. (actual numbers will vary because of other factors, but this is straight theory and numbers without the added factors which will lower that somewhat). So, again 60,000 inches per minute or 5,000 feet per minute, which is slightly under one mile per minute, or slightly under 60 miles per hour (88 feet per second or 5280 feet per second).

Lower the pitch and those numbers go down, so the aircraft will go slower, but the acceleration and "power" (not really, but you get the idea) will increase. It will pull that plane to it's maximum RPM for that prop at a faster rate. Rate of change = acceleration.

Now, for my last "comment". Do not equate noise to power. When you decrease pitch, the engine will tend to go to a higher RPM. That higher RPM will now give the appearance to more power and more speed. You won't go any faster, but you will get there faster and at the cost of more engine wear AND more noise.

As mentioned, get some props of various sizes and play with them. See what works out better. And, remember my last comment.. noise does not equate to speed.

CGr.