Originally Posted by Brenner

Hey Henning,

Typically you will want to run a larger diameter prop in front and a smaller diameter prop in back. This is so that the vortices that are shed from the tips of the front prop won't interfere with the rear prop.

In the setup that you are describing you might hear more noise as the tip vortices from the front prop create turbulence for the rear prop to pass through. Also, the rpm of both props will change slightly as the planetary gearset in the gearbox rebalances the load from front to rear. This is a benefit that comes from using a planetary Drive instead of a fixed gear Drive. With a fixed gear Drive the prop rpm ratio front to back will remain fixed, which will skew the power loading on each prop, and could result in noticeable trim changes to the aircraft.

As far as noise is concerned, just because theory says that the props will be noisier, doesn't mean that a person will be able to actually hear any difference. I've heard that the human ear needs a 3db difference before a change in sound can be perceived, so it could measure noisier, and you still might not hear a difference.

The final test is for you to try it and see if it works in your setup.

Brenner ...