I have two different fuselage air extractors on my LS8-18. The reason I did this is because I knew my cockpit was being pressurised because the canopy vent would pop open at 100kts and no air was entering the cockpit thru that now open vent.

The first one I installed was the then "holy grail" of extractors, the Mandle. This was installed as instructed by DG on the lower fuselage just aft of the gear doors. This was a hastle mainly because I had to enroll in the DG maintenance program to be able to buy the installation kit. Bottom line...NO CHANGE IN NOISE LEVELS IN THE COCKPIT AND NO CHANGE IN CANOPY AIR VENT POPPING OPEN AT 100KTS.

I then had Rex install the locally sourced airvent that is placed on the top of the fuselage just behind the canopy. Immediately quieter cockpit. Now I could feel air moving past my face when the forward vent was opened. The canopy airvent stays closed right up to redline.

Did it improve my L/D by 10 points? Can't tell you.