Just Another Cheap Speed Mod, or Cabin Breeze-Reduction Too?

The objective is to make covers for the two holes pictured above [think blue].
The photos above were taken during the wheel well refinish project on my Comanche 400.

Also make covers for the kidney-shaped opening in the root rib.
These covers will benefit cabin-draft reduction.

For reference photos see top two at top of this page. Airflow creates high pressure under the wing and consequently in the wheel wells. There are 3 openings located in each wheel well to be blocked. One is at the wing-root rib, a kidney-shaped cover; the other 2 are round covers. One is just outboard the MLG strut; the other is behind the wheel-well splash shield. If you give some thought to this issue you’ll discover this approach is also a speed-mod. Remember Bernoulli’s principle? The airflow through these openings entering through the wheel wells to exit points is a total waste of energy; except possibly to vent odor from mouse droppings.

The primary interest here will probably be cabin draft-reduction, although I prefer the speed-mod benefit. The kidney-shaped cover will be the most effective in this cabin- draft reduction approach. There is also a cover that is supposed to be in place where the main spar and conduit penetrate the fuselage. If they are missing there will be a very large draft in the cabin. See the photo above outlining the cover that should be found.

Another point, although quite small, is the small area around the conduit entry/exit thru the fuselage; plug those areas with some sealant. I use “duct seal”, a product of Gardner Bender – Google™ it – about $2 from various sources; sorry to say Home Depot no longer carries this stuff. Or possibly purchase at an air conditioning/heating establishment. See the sealer photo above.

Now let’s proceed. Use a piece of heavy poster paper and cut to the approximate size, of the cover for the root rib, plus some margin. Measure the conduit diameter [7/8”?] and mark the hole location; use a circle template to draw the hole. Slit from an edge of the pattern to the hole; then accurately cut the hole.
Position the rough-cut pattern over the conduit and using your fingers make an impression of the edge of the root-rib opening onto the poster paper. Now you almost have a pattern. Using scissors, cut on the outline of the rib-opening impression, then lay this pattern onto  another piece of poster paper and add a 3/8” margin; now you have a new pattern. Test-fit the pattern and trim as necessary before making the final product. The hole for the conduit should leave a bit of room to avoid contact, or make the hole larger [1-1/4”] like my photo for a grommet AN931-14-20. Note – the rib cut-out covers for the left side and right side
are identical pieces, just mirror images.

Two options; if you plan to install this cover with an installed conduit use option 1.

Option 1 – make a 2-piece cover split diametrically through the conduit hole; add 1/2″ overlap at the split. Use pop rivets at the overlap to secure.

Option 2 – for installing with new conduits, make this a one-piece cover as shown in the photograph on the bottom of page 1.

Use 2024-T3, 0.032″ aluminum for all the fabricated covers. Use a hole saw for cutting the hole for the conduit opening in those covers. Install the covers using “3M Scotch-Mount double coated acrylic foam tape”. You can find this stuff on-line, Google™ “3M ScotchMount, or try an automotive store [NAPA]. It comes in 1/2″ width which is too wide to apply neatly; cut to 1/4” width. This stuff has an acrylic adhesive which is tenacious and thusly works well on clean surfaces. Nix the silicone-adhesive method here.

Another hint – adhere some baffle material at the openings where the aileron cables enter/exit the fuselage; likewise flap cables [conduits on the 400] and step-lock cable.