Tuesday, March 5, 2013

W-1202-L Skin Inspection Hole & AOA Port

Due to an extremely late afternoon start in the shop decided it best not to rivet the W-1202-L skin and chose to work on the inspection port and the air port for the angle of attack plumbing. The inspection hole required a little attention so figured may as well finish it up now instead of waiting until later as in the plans.

Started with drawing the positioning line for the AOA port. Joe the Michigan builder who did the math for the AOA port location gives measurements based on the leading edge rivets. Since his airplane used LP4-3 rivets, I temporally placed LP4-3 rivets into the rivet holes and then made a line tangent to the rivet heads per his instructions. The AOA port may be drilled anywhere on this line. I’m choosing to locate the hole for the port about an inch or so from the rib outboard of the stall warning micro-switch … there will be no interference with the operation of the stall warning micro-switch and it is also far enough away from the access port that making inspections or adjustments to the stall warning indicator should not pose a problem.

A line was drawn tangent to the aft edge of the two leading edge rivets. Anywhere on this line is where the AOA port can be located. For those interested in the math behind this location, it's approximately 30 degrees off the chord and 60 degrees off vertical. As a stall is approached, the port becomes more aligned with the air stream and thus the pressure in the tubing will increase. This pressure increase is analyzed by the Dynon Skyview and displayed on the screen as a bar graph.
                                                     Joe’s 30 degree AOA hole location verified by a protractor.
                                                                   Drawing the pilot hole line for the AOA port.

At this point I’m torn as to which method I want to use to connect the AOA plumbing to the W-1202-L skin. Builders have used rivets with the mandrels removed as ports and others have used a sport ball inflating needle with the threads protruding the wing skin and Tygon tubing slipped over the needle (the original method pioneered by Joe a builder in Michigan). Both methods are reported to work great and both have advantages and disadvantages. Of course, a rivet looks great and is virtually unnoticeable … however the small hole could easily become clogged with dirt, paint or wax and there is really no good way to “protect” the port when the plane is on the ground, plus it is harder to connect the Tygon onto the rivet because an "adapter" tube will be needed. On the other hand, the inflation needle requires a ¼” hole, needs to be set in epoxy to keep it in place and the threads do  stick out of the skin …. however, a valve cap can be screwed onto the threads to protect the port from dirt, wax, paint, ect. when the airplane is not in use. For purely cosmetic reasons, I’m leaning towards just using a flush rivet with the mandrel removed and if there is ever any issue with the port clogging, I can always drill out the 1/8” rivet hole to ¼” and install the inflation needle. An example of the inflation needle method used by Joe is in the concept photo below. The right needle’s threads protrude through the wing skin and the tubing would be run 15 foot or so to the left needle which is screwed into the AHARS for the Dynon Skyview in the fuselage.
                                                        Joe's example of the inflation needle method for AOA.

While pondering which AOA method to use, went to work on the inspection access hole in the W-1202-L skin. The ten inspection plate screw holes require being dimpled for #6 screws and the rivet holes for mounting the ten nutplates also require dimpling along with the nutplates themselves. I suggest builders dimple the #6 holes first, then use a small diameter female dimple die to dimple the nutplate rivet holes for flush rivets. If the nutplate rivet holes in the skin are dimpled first, the large head of the #6 dies will likely be up against the rivet hole’s dimple and possibly distort it.  The rivet holes in the K-1100-06 nut plates require dimpling as well.
                                            Dimpling the ten screw holes in the inspection hole for #6 screws.
                                                                   Dimpling a nutplate for flush rivets.
             Using the pneumatic squeezer to rivet nutplates onto the W-1202-L wing skin's inspection hole.
                                                 Finished inspection access hole in W-1202-L bottom wing skin.
                                                   AOA hole location is on the bottom left and circled in red.