Last we learned from Rudi Kurth of Switzerland, Ed Hamlin and Bruce Tifft, that a longer prop extension spool would reduce the cockpit noise level and possibly increase performance. We collected accurate baseline data, then removed the standard 3-inch extension and installed longer ones.
We have tested 4-inch, 5-inch and 6-inch extensions. Most of our data was for the 6-inch extension. Results are as follows: As compared to 3-inch, sound level at pilot's station and rear seat was reduced two and a half to three DBA at high speed cruise and climb conditions. Oil temperature on all three airplanes tested with the long extension was increased. Cylinder temperatures on one of the airplanes increased. There was no measurable change in the performance of any of the airplanes due to extension length. We have been reluctant to recoemiend the long extensions because any change in the propulsion/drive system of an aircraft must be thorougly tested for long-term durability.
We now have a total of 300 hours 6-inch time on Long-EZs with no indication of problems, and, since the two 8-inch extensions on the Defiant have run 600 hours, it appears that no mechanical problems are indicated. Thus, if you can stand a 20 to 30 degree rise in engine temperatures you can, with a 1.5 lb. weight penalty, achieve a significant noise reduction by using a 6-inch extension. Note that we have tested this only with Lycoming engines and cannot predict the durability of a Continental application. Also note that since the Brock spinner mates to the front flange of the 3-inch extension, it is not as well supported when using the long extension. Brock is now developing an aft bulkhead for his spinner to provide firm centering support regardless of extension type.