Removing Wing-Skin Dimples
[Created 5/14/2010]
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[ Click on all photos for larger image ]
The problem:
For a dimple-free skin, the skin at the point of each hole must be flush with the rib and the rib flange must be level with the skin surface (and most likely perpendicular to the plane of the rib).
- If there is a gap between the skin and the rib, the .016 skin is easily pulled down to the .020 rib L-angle ... creating a dimple.
- Correct lighting conditions are required to make the dimples easily visible.  The shadow pattern on the right was created by light shining through open wooden steps.
It is easier to avoid dimples on the nose skin than the rear skins.

Chapter 25 EAA Technical Advisorys recommended looking at some Cessna Spam-Cans on the flight line; and thus, lower my expectations for dimple free and dent free skins.
At some point as you remove a dimple, you create a new dimple in an adjacent area that wasn't there.  Thus, eliminate the worst and live with the rest.


   
                                   Plain view                                                           View with shadow pattern

Dimple Tool Set
The Nose-rib profile can easily be reformed through skilled use of a hammer and fluting pliers as the rib L-angle is not supported by trusses along its length.
When the rib camber matches the skin camber ... you are done.

Conversely, the Mid-ribs are supported by trusses created by U-channels; thus, reforming the rib profile is not possible (unless you remove the L-angle ... see below).
- I created a set of tools to move the rib edge towards the outer skin by stretching the rib metal adjacent to the hole..
- The die tool stretches the rib edge outward the correct distance to meet the skin (trial and error is replaced by judgment through practice).


The tool set is simple:
U-Shaped Form, depth control pieces and forming die.
The forming die has a slot to allow it to overlap the rib
 while perpendicular to the plane of the rib.

An alternate forming die required if
the hole is very close to the inner flange.
The rib is pulled to one side to allow room for the hammer.
(Squares-Up the hole perpendicular to the plane of the rib.)

Hole "lifted" through forming.

Side view of the same hole.

Note: This dimple removal process warps the rib.  
A few blows with a hammer on each side of the "raised hole" will stretch the metal and straighten the Rib.
You place the rib on a heavy piece of metal to one side of the raised hole and strike a few heavy blows on rib with a hammer.
Do NOT use the stretcher shown below!

Classic Stretcher/Shrinker tool set with "S" shaped angle.
The angle has been shaped (L to R) Stretcher, Shrinker and Fluting Pliers.

Note: the Stretcher can cause a tear in the aluminum.
The Stretcher/Shrinker both create thin grooves in the aluminum.
Sources of stress to create   cracks.

Dimple Removal Process
- Install Clecos in all the holes which have been reamed with a #30 drill bit.  You will need about 500 Clecos.
- Using the shadow pattern method shown above, mark dimples with a Sharpie pen.
- Additionally, holes can be located with a straight edge (see below).  This also shows the depth of the dimple.
- The depth of the dimple can also be estimated by removing one Cleco.  The skin will pop out.  Sighting through the hole, estimate how much the rib hole must be raised.
- Pressing down on the skin over the hole with the removed Cleco can further indicate that the rib is not flush with the skin.
- Remove the rib and deform the indicate holes.
- Replace the rib.  Check the results.  You may have to remove the rib and make further adjustments.  Tedious?  Yes.

Dimple Tool Set in place.  One hammer blow forms the "raised hole"

Click to see larger image.  One can see that the middle hole
is "raised" more than the two holes on each side.

Metal surrounding hole is checked to ensure it is square to the side.

Close up view ... note Cleco with long nose for deep holes.
(I was running short on normal Clecos.)

While testing different methods, the rib flange was totally mangled.
After drilling out 14 solid rivets, it was replaced with a new flange.

Note, one can test the fit easily since it was an "end" rib.  One can
add the lower part of the rib and drill the holes with all parts in place.

A straight edge reveals the amount of gap at the dimple


Insert truss portion of rib and drill in place using hole finder.

The Result:  OK, this wing is not as dimple- and dent-free as it appears in this photograph.



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Copyright 2008, by S. Steve Adkins, all rights reserved.