Dimpling Skins

Flush riveting the first 30% of a flight surface can effectively reduce the majority of parasitic drag created by riveting with the remainder of the wing providing little drag reduction for the additional work.

Begin located in Australia C press riveting frames were unavailable and due to cost and size airfreight was not a option. It was decided to examine the use of a low cost option of using a dimple die and pop riveter.

Dimpler - click link 

Aircraft Spruce lists a 120 deg dimpling tool to suit the Cherry G-28 pop riveter. This unit is designed to screw into a Cherry pop riveter tool but as luck would have it did not suit my unit. With no alternative available it was decided to modify the die and the threaded portion was cut off. 

Note: There are other dies available that are universal but this one created the result illustrated

A trial on one rib indicated that the tool created a usable countersink in the rib and the decision was made to proceed to dimple the leading edge of the elevator stab to the face of the spar. If you were planing the next grand champion then it would pay to use better equipment but this is beyond my ambition and skill level. 

Modified Dimpler

The final result is dependent on technique and this builder will describe his method as clearly as is possible below. 

The nails provided have about a 0.25 mm clearance in the pre-drilled hole so the best method is to install the female die at the rear then place the male at the front of the component. 

For ribs use a thumb and forefinger to support the female die and too center the male on the hole. Mount the puller and while holding it about 1/3 along the handle close the two dies to hold the dies in potions and apply light squeeze, next place the other hand at the end of the handle  and apply firm pressure clamping the dies closed.

Update 11/5/16:  A new tool has been found to create a better dimple - Click 

For the skins use the same technique but fit the rivet gun, apply LIGHT pressure and wriggle the male die until it "Clicks" into center off the hole then repeat the technique above.

The nail will require light dressing to remove the ribs created by the tool from time to time - this becomes less with use and this builder used two nails for the elevator stab. When correctly centered there is a very short stroke required to form the dimple. The factory supplied rivets fitted neatly into the assembled countersinks.

Footnote: The blue painters tape covers the holes that fix the skin to the spar. They are in place to prevent the builder accidentally applying a dimple to any of these holes as time passes working and the builder enters a derrrrrrr..!!!! zone. Knowing this a series of NO RIVET was also added with a felt tip pen. A story for a future blog will illustrate how errors are created by human performance during assembly.

Completed Elevator Stab

Stab fitted into the Assembly Jig

Comment - 21/6/18
I purchased a CherryMax riveter to fit the die as it was easier and helped when the nail broke as the two components just took off - Tried another similar product from another supplier and this one produces the best dimple.