Dowty Rotol Ltd, is the best known English factory of propellers and controls.
-To reduce the noise of propellers and engines due to the laws in countries with such problem, they developed propulsion systems such as ducted propellers, but using the same power plants.
-In the shown case of a BN Islander that normally produces 86 dB with its Lycoming IO-540 engines and say "free" propellers, it dropped to 65 dB, which was very interesting.
-But it was not necessary to apply this system because most aircraft comply with the current regulations.
-This was the case of the Cessna Reims Rocket that was overpowered, and with only decreasing the maximum rpm by 100, it entered within the order.
-The same plane in the USA already came out with this small limitation, it was called XP and lowering its power from 210 hp at 2800 rpm to 190 hp at 2700 rpm, it was correct.