This question of model props going supersonic being the mechanism behind an increase in noise level has nagged at me.......found the following post in another RCU forum that gave me some data to work with.
your right the 55 is perfect for scale flying. i finally got a radar gun on my 47 with an 85 in it. ½ throttle 92mph 3 passes at full and they shot 119, 119, and 122mph. I find it interesting how little speed I gain from ½ to full. I am swinging a 24X12 prop on those runs and it was ripping it. I am going to switch over to a 24X14.
Read more: [link=http://www.rcuniverse.com/forum/m_8936563/anchors_9767997/mpage_23/key_/anchor/tm.htm#9767997#ixzz0sUcTELxM]HERE[/link]
At 120 MPH a prop with 12 inch pitch is at "pitch speed" at 10,500 rpm. The angle of attack of the prop blade at this speed is 0 degrees. Talk about prop unloading with airspeed - you can't reduce the load more than this.
A 24 in dia prop at 10,500 rpm travels 2 pi feet/rev, so at 10,500 rpm (175 rev/sec) the speed at the prop tip is 1100 ft/sec. The vector sum of 1100 ft/sec in rotation plus 176 ft/sec on axis yields a resultant along the helical path of the prop tip of 1114 Ft/sec. Close enough to make the case for the prop tips going supersonic (>1125 ft/sec); with a prop of zero effective pitch I wouldn't hazard a guess at how much faster a DLE85 might turn, but if the reported figures for prop geometry and airspeed are accurate, it is at least enough to get the prop tips into the transonic realm.
The fix appears simple enough, assuming one wants to fix it: (1) if using a large dia/low pitch prop for 3D flying, throttle back when flying level or diving, or if speed is the objective (2) do as the author of the cited post said and switch to a prop with more pitch.
In prior posts I inferred a general rule that model airplane props don't go supersonic. As with most general rules, somebody will demonstrate an exception to it. So 804, it appears that indeed "supersonic prop-age makes for a good story."