I became suspicious of the pitot/static system during my first flight in the airplane when indicated airspeed during a stall indicated ~30% lower than the manual (50MPH vs 68 MPH). After spending some time looking at the system and other T-18s I decided the problem was likely the pitot/static probe itself. In order to confirm this we needed to establish a good baseline. I chose to do that using two type of points, stall points (IAS during repeatable stalls), cruise points (IAS at a repeatable power setting). This flight was about getting those baseline points. Part of Dennis' painstaking rebuild of the airplane included many updates to the airplane. Lou Sunderland (builder of this airplane) was a tremendous homebuilder/experimenter, and what a tribute to him that Dennis fell in love with the airplane and decided to bring it to the "Sanders" standard. This included complete rewiring, an overhaul of the flight control system, and most notable to this conversation a new panel. That new panel included a new (more conventional) wing-mounted pitot/static probe and a new engraved panel. The trouble was this engraved panel included the published speeds for the airplane (including stall speed) which drew attention to the error.
A pitot/static (or Prandtl) style probe combines the total pressure and static pressure ports in the same probe. This makes for a super clean install and a great looking probe, however, it is not typical on the T-18. This stock Thorp had the pitot and static probes in the top of the vertical, and this airplane was set up that way when Dennis got it. I assumed the path forward was to a more conventional tailcone mounted static port. So before I asked Dennis if I could drill a hole in the side of his gorgeous airplane I decided we should be super confident in the path forward. So I decided to do some homework, get out my old NTPS notes and get this problem surrounded.
Super stoked!!
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