And quite accurate. Normal rule of thumb dictates that to allow the engine/motor stay in the same RPM range (a desirable thing) that you need to increase pitch if you reduce diameter roughly one number or if you need to reduce pitch you increase diameter the same number.

The problem of this is when you reduce the diameter even though you will still move a similar amount of air by increasing the pitch but what you have done is increase what is called propellor disc loading, an important performance parameter and also applys to rotorcraft as well. The disc load is the aircrafts total weight divided by the disc area of the propellor. Increase diameter and decrease pitch or reducing diameter and increasing pitch will impart exactly the type of subtle performance changes described by Rick above.

By the way the second number 'pitch' is the exact distance in inchs that the prop would move forward in one revolution if it were operating in a theoretical solid.

Hope you found a little theory entertaining.

John