Trim Tab

by Frank Justice

Alignment Notes:

None of the alignments of the trim tab are particularly critical or difficult to achieve; however, keep in mind that the closer the trim tab conforms to the extension of the shape of the elevator the better performance will be.
Fabricating the Skin
  1. Bend a piece of posterboard (thin cardboard) in the middle, using the trailing edge of the elevator to shape the bend. Cut it to a size about two inches larger than the trim tab.
  2. Mark it to the exact shape of the trim tab opening in the elevator as follows:
  3. Mark lines on the elevator that are extensions of the trim tab opening, far enough so that they will be visible past the edges of the cardboard.
  4. Position the cardboard over the opening. Lay a long sraightedge along the trailing edge to align the rear edge of the cardboard with the elevator edge and tape the cardboard to the elevator.
  5. Mark the exact shape of the trim tab opening on the cardboard using the marks on the elevator as a guide.
  6. Subtract 3/16" from the leading edges and from the outboard edge (the hinge allowance and the gap to the elevator edge) and mark these lines to cut.
  7. Cut out the trim tab pattern and tape it in place on the elevator to verify its shape. Cut out two triangles of posterboard that are the same size as the opening at the large end of the trim tab. Remove the pattern and cut out two more triangles that are the same size as the end of the elevator that corresponds to the small end of the trim tab. Tape these triangles to the pattern in the proper positions. Cut off the large end of the triangles so they are 1/2" short of the leading edge of the trim tab pattern.
  8. Lay this pattern onto the outside of the trim tab skin and mark it onto the skin. Cut out the skin, leaving a 1/8" radius on the inside corners.
  9. Using the pattern less the triangles mark the bend lines at the ends of the trim tab. Cut out a radius from the triangles along the bend line for about 3/16" and round the corners.
  10. Bend the top end tabs down 90 degrees with a hand seamer. Bend the bottom edge tabs up in such a way that they will fit inside the top edge tabs when the trim tab is closed. (The top is bent first because it is more difficult to make a perfect tab when the other one is in the way. The top tab is the one that has to look nice.)
  11. Complete the trailing edge bend.
Attaching the Spar and Hinge
  1. Clamp the trim tab spar to the inside of the upper side of the skin with the flange edge even with the skin edge. Verify that the lower flange of the spar is flush with the lower skin; if not, bend the lower flange as necessary.
  2. Drill with #40 the lower skin to the spar.
  3. Make the trim tab control horn using drawing 5a.
  4. Drill with #30 the holes that hold the end tabs closed and the control horn on. See drawing 5a.
  5. Clamp the hinge in place under the upper flange of the spar and drill the upper skin to the spar and hinge with #40.
  6. Disassemble. Countersink the holes in the spar. Deburr. Dimple the skin.
  7. Prime all parts.
  8. Rivet the lower skin to the spar.
  9. Cleco the upper skin to the spar and hinge. Clamp the other side of the hinge to the elevator. Verify that the trailing edges of the trim tab and elevator form a straight line. Remove from the elevator and rivet the upper skin to the spar and hinge.
  10. Rivet the ends and control horn.
  11. Bend up the leading edge of the lower skin just enough so that when the trim tab pivots down the skin clears the lower edge of the elevator.
Attaching the Trim Tab to the Elevator
  1. Clamp and/or tape the hinge to the underside of the elevator rear spar flange. Verify that the trailing edge of the trim tab and the elevator form a straight line.
  2. Drill through the elevator holes into the hinge.
  3. Remove the elevator by pulling out the hinge pin; deburr the holes.
  4. Rivet the trim tab hinge to the elevator.
  5. Trim the end of the hinge even with the trim tab and elevator edges. Cut the hinge pin about 3/8" shorter than the hinge and round the ends slightly.
  6. Put the hinge pin back into the hinge. If it does not go in easily, realign any parts of the hinge that may have been distorted by the riveting operation.
  7. Captivate the hinge pin by drilling holes for running safety wire through each end of the hinge.
trimtab.doc 5/15/93 Frank Justice

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