Carlson Skunk Works

OK. The whole month of July has come and gone since my last post. AirVenture is over with for this year and I did not attend it, but I watched it on the webcams. The yard seems to need mowing continuously and the garden needs regular attention. Then there are issues with parents, kids, my wife and a potential dog. All this does not even take into account needing to go to work on a reasonably regular basis and doing my part at church. HOWEVER, progress has been made!

The leading edge skin has been applied to the right wing! Well, most of it. The last section of this is the root end of the wing where the leading edge angles down and back. However, I don’t see that this will be a big problem.

Yes, I took pictures of the finished product and I think it turned out pretty nicely. I did not take pictures of the steps that got things this far. Maybe I will take a few pictures of the process when I do the other wing, so that I have it all documented. There are a few details to work out, but I am not going to peal back what I have applied just to make it look perfect. I am going to have the chapter TA come over and give a formal opinion before I haul the wing to the hanger.

Over the past few weeks I have been working on all of the hardware that needs to be installed on the wings. These are things like the brackets for attaching the struts, jury struts, the spar mounting plates and the control cable mounting brackets. As I installed each of these and the bolts that hold them in place, I applied wood sealer in quite liberal amounts. I wanted to be sure that the wood was fully sealed in all the areas that would be inaccessible once the leading edge skins were in place. I also wanted to be sure that the insides of the holes where the bolts went through the spars were fully sealed as well. This resulted in some excess sealer covering the bolt threads, so the nuts are “sealed” to the bolt as well as being the standard AN365 elastic stop nuts. I think they should stay on quite well.

The internal drag bracing needed to have its final adjustment as well. The Maranda uses plastic strapping material for the internal drag bracing. This is the type of strapping that is used to secure large boxes and to strap down equipment to pallets for shipping. There are brackets on each end to secure the straps to the wing spars. The wings have four sets of bracing each.

The front spar needed to have filler blocks attached between the ribs so that the leading edge skin will have something to be glued to. These had to be fitted to both the top and bottom of the spar. When the glue had dried they needed to be sanded down to make a nice surface for the skin to attach to.

Once all the hardware was in place and the wing was fully sanded, I needed to seal the front side of the front spar and the parts of the nose ribs that will end up inside the leading edge skins. The sealing process starts by applying masking tape to the areas where sealer is not wanted. My wife had some standard tan masking tape, but I went to Menards and got some green painters tape to use for this instead. My reasoning was that the tan tape might blend with the wood and it would be easy to miss removing some of the tape when I was doing the glueing. The green tape stands out nicely and I don’t think that I missed taking any of it off at the appropriate time.

The next thing was to prepare the plywood for the leading edge skins. This is the thing that I have been dreading since I first purchased the plans about seven years ago. The skins are made from 1.5mm birch plywood which is reasonably flexible, but not to the point that it will readily wrap around the radius of the front of the ribs. I researched lots of ways to bend wood, but found very little information regarding how to bend plywood. Many people use steam to bend wood and my research indicated that the steam is primarily a means of conducting heat to the wood. I had purchased a heat gun for the purpose of bending the plexiglass that I will be making the windshield from, but since I already had that and since my research indicated that the bending of wood relied on heat, I put two and tow together and came up with the idea of heating the plywood in the area where I wanted to bend it. This worked great!

I marked the plywood where I wanted it to bend around the nose of the ribs. Then I used my belt sander to scarf the ends where the plywood sheets would meet. Then I clamped one side of the sheet to a piece of 1/2″ plywood (for stability) and turned on the heat gun. I pulled the other side of the plywood toward me and heated the area along the line that I had drawn and before long I was able to bend the plywood back. This allowed me to achieve a bend diameter of 1 1/2″ to 2″. When things started to get close on the back side I switch the heat to the outside of the bend for finishing. Then I clamped each sheet to ensure that it retained its position while it cooled.

I used 2′ x 4′ sheets of plywood. The leading edge of the wing has a reinforcement piece of wood that runs the full length of the wing and connects all of the ribs. This piece was at 10 3/4″ from the back side of the bottom of the front spar. That is where I wanted the bend to be centered. Therefore that edge of the plywood ended about 2″ from the other side once the sheet was bent. I clamped the plywood so the one side was 2″ from the other side.

When the skins were cool I marked where the ribs would be and where the spars would meet them. Then I applied masking tape to these areas and applied sealer to the rest of the inside of these skins. I let everything dry and cool overnight.

Now that everything was ready it was time to apply the skins!

The tape was removed, glue was mixed and applied, the skin was held in place and staples were driven home! I used plenty of staples to ensure that the skins would stay in place and not pop out.

Here is what things look like from the front. I have a couple of small gaps where the skins meet. I scarfed the plywood on the ends that would come together and put plenty of glue on them, but glue doesn’t pull things together by itself.

From the back you can see (or imagine that you can see) how the skin fits to the spar.

The root end of the wing still needs to have a skin fitted and attached. This will require that I sand the area where the current skin is attached to the rib so that I develop some area on the rib that will provide area for the new skin to be glued to the rib. The area between the two ribs at the root end of the wing will have plywood skins both top and bottom. The wing tip area will have similar skins applied. However, the root end has some interesting angles that will need to be dealt with. But that is later.