I've written about this several times before, but here goes again.

Consider the pitch of a prop as you would the transmission in a car or truck. Lower gears give you more power but at a higher RPM. Higher gears give you more speed at a lower RPM. If you are climbing a steep hill, and you have a standard transmission, you will probably shift down to get more power. An automatic transmission will do it for you. Then to go faster on the highway, you can then shift up in gears and it will go faster at the same RPM as you were using for the climb.

The pitch is the same as the gears in a transmission in the car or truck. If you want to climb that hill (go vertical), then reduce the pitch. If you want to go faster, then up the pitch. Example: Climb or go vertical = 10-4 Higher speed.. 10-8.

Do the math. I will use the 10-4 prop as an example. The 4 is 4 inches per revolution. AT 10,000 RPM, a 10 pitch will move through the air 4 inches for each rotation, or in one minute will travel 40,000 inches or .631 miles per minute or approximately 37 miles per hour. Double the pitch and, seeing as it's all linear, well, you will double the speed. But, with a given amount of power, you will take longer to get there (to 74 MPH). With the 4" pitch, you will accellerate faster, and have pulling power for a stronger pull to vertical.

Also keep in mind that some engines won't have the power to turn a 8 pitch prop at 10,000 RPM (the 40 LA may be one of those) so you will never see that RPM. This will show up as sluggish operation and a real lack of oomph when flying.

There are other drawbacks too. Decreasing the pitch will cause RPM problems with the engine. Your engine that normally would run at 10,000 RPM with a 6 pitch prop will definitely increase in RPM with the 4 pitch prop. This makes more noise and will cause problems with the engine if run that way to long. So, there is a trade off.

Bottom line: Yes, you can change the pitch (decrease it) for more vertical at the cost of speed and more noise and possible engine issues if the engine is not in good enough condition to handle the increase in RPM. Don't take the specs as gospel for engine RPM capabilities. Not to many off-the-shelf engines will really do 16,000 RPM and run reliably.. or run at all for very long for that matter. Stick with what the manufacturer recommends for prop size and experiment.