Part II-Skins

Construction Details Finish&Fly Events and Projects Building from Plans Raw Materials Q&A Tools Construction Log

Part I-Skeleton
Part II-Skins
Part III-More of it
Part IV-Ailerons
Part V - Fuel Tanks
Part VI - Finishing


Everything is pre-measured and pre-drilled...
Once I measured all rib positions (front spar and rear spar), I determined the exact rivet lines on the skin by taking a median distance between the rear and front position (on a few occasion, the rib position varied 1-2mm from front to rear).  The rivet pitch has been determined by transferring crimp locations from all ribs onto a piece of aluminum and by trying various pitch scenarios to avoid as many crimps as possible.  The first and second rivets from the spar (top skin) are spaced 25mm.  Then, there are four rivets spaced 30mm.  The other rivets are spaced 60mm and then 65mm (the plans call for a rivet spacing of 70mm).  On the bottom skin, I was able to have a cleaner rivet pitch (constant on all lenght).

Once my template was ready (drilled with no. 40), I used it to pre-drill the skin for all ribs.

Will I rivet the skin to the worktable?
No, but I decided it would be simpler to install and drill the stiffeners to the skin prior to form them to the shape of the wing...  On this picture, the stiffeners are clecoed to the skin, directly on the work table.  After that step, I disassembled and formed the stiffeners to the right shape.
How more fun can it be?
Once the skin is pre-drilled, it is quite easy to align it on the skeleton and start to drill.  I did drill with no. 40 first and then with no. 30.  One thing that surprised me was the curvature of my spar... To ensure that everything is straight and true, I made a jig to hold the spar and skeleton in position (by nailing pieces of 2X4 on the table on both sides of the spar).  The front spar sits on a piece of 1X2 while the rear Z sits on pieces of 2X4.   With this arrangement, I was sure nothing would move.

The "tack" rivet positions also ensure proper alignment of the top of the spar with the skin.  Before drilling the skin, I verified that the front end of my nose ribs were aligned.  The riveting process that follows is quite fast.  One thing that I did by mistake (on all skins) is to drill the outer fib with no. 30 (I should have left it with no. 40 to ease the installation of the tip skin).

I had a bit of slack on the skin between the ribs... this was solved by crimping the stiffeners a bit more.

Stiffening.... stiffening...
I did not like to have a piece of skin behind the baggage compartment that would pop or move...(I want it stressed).  So, I installed another stiffener behind the compartment for the top skins.  Very satisfied with the result and I also believes it looks better (more consistent rivet pattern can be observed).

Also on this picture... I don't know if this is clear enough, but I drilled the rear baggage compartment wall to the skin from inside the wing (the baggage compartment wall was pre-drilled with no. 40).

Let's go for the bottom!
I must admit that quite a bit of time has been spent between previous and this picture... the problem was that the whole assembly was really not stable when mounted on 2X4's.  So, I decided to make a jig to hold everything firmly in space (see next picture).
Here is my wing support...
I made three units like this from 3/4" plywood (but I used only two to support the wing).  The shape follows the contour of the top of the wing and pieces of 1/4" thick UHMWPE are nailed on both ends (so there are contact points only on the spar and rear Z).

To ensure that the supports are identical, I used a router to transfer the shape of the first one to the two other copies.

Can't have a better fit
The bottom skin was left oversize prior to installation.  After it was all clecoed to the wing, I cut the extra material on the rear end of the wing bottom with a knife (following the rear Z).  The fit with the wing is simply perfect.
Riveting remains the fun part
This is quite like a gift for all the hard work that preceeded this step.  I riveted both wing top skins one after another.  My younger assistant, showed somewhere around this text, was operating the air supply for the rivet gun (as you can see).
The wild wires were wired wright(sic) here
I purchased the wires at a local electronic surplus store (further information may be included on this web site).  To be noticed on this picture is that the strobe cable is routed separately from the other wires.  This is to reduce the chances of interferences (well. I don't really now if there was a potential impact, but it doesn't cost more to do it this way).  On the right wing, I routed the nav light wire with the strobe cable (because there were no other wires to bundle with the fragile nave light wire).

The nav light wire is a 18awg (MS22759).  The landing light wires are 14awg.  12awg (because of availability) wires are attached to the spar cap for local grounding the landing lights.  The strobe cable is a 3 conductors, 18awg (note that Whelen accepts this gauge as minimum gauge--they prefer the 16awg).

Not knowing how the plane will be balanced, I routed trim servo cables in both wings (easier to remove than to install).

Ribs... don't move!
All the ribs are maintained in position with a "tooling rod".  This is a tip that I got while looking at a video about building a Vans' RV aircraft.  Pictures of nose skin positioning are not yet available, but let me tell you that it was really easy to drill the bottom skin with precise hole locations.  I predrilled only a few holes to check that the ribs are square. It was perfect!
Big tool for small job
My first nose skin was quite expensive.  Initially, I tried to form the skin by pushing directly on the steel tube with a hydraulic jack (also pushing on the ceiling).  Well, it was very unstable and finally, my skin was a disaster.
Then, I made this quite interesting press for forming the skin.  It is made of wood and it works great.  The forming die is inserted at the bottom of the press and two jacks are installed in the upper section.
One important note here is to make sure the jacks are held in position... on my first attempt, one of the two jacks fell on the skin, destroying it! :(
The forming die consists of 2X4's spaced 2.75 inches mounted on a piece of plywood.  The tube is a 2.25 (o.d.) steel tube.  The fit of the skin on the ribs is perfect.
It looks out of sequence, hey?
One evening, I had nothing I could really do... so I decided to try making a small aileron to see the fit on the wing.
With the flexible hinge, the Zenair dimensions are OK.  However, if we implement the piano hinge modification, the dimensions need to be reworked because the flex point is different and the aileron is several milimeters farther than with flex hinge.  With piano hinge, the result is an aileron that protrudes below the wing.
I asked a RAA 415 member who is flight testing a CH601-HDS what he thought of the flex hinge.  He had no difficulty to fly with that and as a result, I will go with Zenair design.
Installing the nose skin
The skin is NOT pre-drilled, but I measured rib locations.  For the rivet pattern, I used a template, which I align about 4 inches away from the spar (my skin is cut oversize to ease installation).
Installing the nose skin
The top nose skin drilling operation is quite stressful at the beginning because it is done completely blind. However, with my tooling rod installed, I was confident that the ribs were not moving. The result is great.


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