I'm very excited that finally, I can move on to the next major subassembly of the plane: the wings !
Only very few things have to be done to the empenage (some rivets in hard to reach places) and I will do that when I have some better technique or better tools to do so.
The work on the wings start with preparation of the main wing spars. These are the most important parts of the entire plane and will take the most of stress and load so you better handle them with extreme care.
You will curse ! each time you drop something accidentaly on the spar (cleco, bolt,...) and create a small scratch. It will mean polishing it out later.
First you need to install the nutplates where the tank skin attaches to the main spar. The fuel tanks can actually be removed so all attach points of the tank are with screw. 8R8 screws (#19) are used to hold the tank skin to the spar.
This means you have to countersink the screw holes so that skin and screw sit flush when installed. Moreover, the nutplates that will hold the screw also need to be rivetted and countersunk to the spar. In other words: lot's of countersinking ahead!
There are about 30 nutplates per flange, 2 flanges per spar, 2 spars on the plane.
When starting the countersinking process, I discovered a serious problem on the main spar of the left wing. I still don't understand how I could have missed this during inspection when doing the inventory.The flange of the left wing on the inboard side is seriously bent in. I guess it must have been pushed by another part during shipping. However, the flange is thick and hard to move so it might also have been something that happened in the plant at Vans.
Because the main spar is so cricital and it creeps everybody out when a defect shows up, I decided to send a problem report to Vans asking for advice.
Their response was - as usual - a typical Vans support answer. But I'm happy I asked and to get confirmation that what I was about to do was the allowed and correct way.
----------------mail to Vans----------------
Ihe flange on the bottom inboard side of the left wing is bent down a bit. It looks like something layed against it during transport or at the factory and has forced the flange in.
I concentrated during inspection on scratches but somehow completely overlooked this. I only notice when I put it straight on my workbench to start countersinking for the tank attach plates.
I added some pictures that illustrate the problem. Can you tell me what I can do to fix this ? I have an idea, but since it is the main spar, I rather follow your advice on this one as it's
too critical to mess up.
---------------response from Vans---------
Jurgen, Just slot a piece of hardwood or even a 2x4 with a saw. Slide the slot over the flange and use the board as a lever to bend the flange back out. We use this technique frequently and it works well. scott/van's
Looks pretty nasty isn't it ?
I followed the advice and clamped some 2 x 4's on both sides of the flange up to about 2 inches further then the start of the bend.
I worked out fine. I added a long clamp grip to the blocks and started pushing. You need quite a bit of force to get some movement in the 0.063 material.
Then measured in different locations the distance between the two flanges and also eyeballed the straightness of the flange using some aluminum angle layed against the flange.
After all this commotion, I can finally start the countersinking. To prepare for this process and protect the spar for chips disappearing in the joints, I taped the joints with some painters tape.
Countersinking for the AD3 rivets is straightforward. First drill the holes to full size using the #40 drill. I always use my drill guide these days so I am sure that the drill is perpendicular to the surface. Then deburr the bottom side of the drilled hole on the inside of the flange.
Next countersink the spar so that the rivet sits absolutly flush against the spar flange. Easy enough as you had already plenty of practice on the empenage doing the same thing.
The come the center holes for the 8R8 screws. This is a bit more complicated. The first step is easy: drill all of them #19.
RVator article states that you need to make an outside diameter hole here for .365 to .375" diameter. Lot's of log websites also show the picture with the measurement. I tested this on one hole but found the hole to be too large. I made a test piece of tank skin and dimpled it #19. I I lay the skin in the .370 countersunk hole, I can move the skin (it wobbles).A bit of experimenting showed that the ideal diameter is about .350". When I later looked up the countersunk military specs, it also shows a .351" reference for the 8R8 #19 holes. So I decided not to follow the .370 from Vans and go with the narrower hole. The more material is left behind, the stronger. The thighter the fit, the stronger. I can still take out some extra material if the skins don't fit perfectly later. If I remove it now, there is no way to put it back. I'll report back later on how well this was when I attach and finish the fuel tank assembly.
Another deviation from the Van's procedure is how to countersink. I'm not a hero in countersinking (as I learned the hard way on the trim tab spar in the elevator) so I want my setup to be as stable as possible.
As you countersink to .370, or as in my case to .350, the #19 hole you initially had will seriously enlarge. That means that your pilot in your countersink will not get any more grip and will certainly chatter. Van's tells you to attach the nutplate, and then use the nutplate as a pilot holder to countersink with a #30 pilot. Oh boy, oh boy ! Only the idea of have a very rapid rotating device spinning in the tap of the nut plate screws gives me goosebumps ! Secondly, the edge of the enlarged hole creates a knife edge that must be removed to avoid the side of the hole in the long run to cut your screw in two by vibration or stress. It is very difficult to do this when the nutplate is already installed and impossible to reach the back of the hole.
Therefore, I used a technique that is slower but is also used by a lot of other builders. I drilled a #20 hole in a thick aluminum bar (I don't have a #19 countersink pilot so 20 does fine).
I deburred the hole and clamped it under the center hole. This part is crucial. insert the pilot in the hole as you clamp so that you maintain perfect alignment.
For the countersinking, do NOT use the air drill. It spins too fast and the control of the speed is difficult.
Use an electric battery driven drill with an RPM of more or less 1000 to 1400 RMP.
When countersinking this size holes: start slow with low pressure and let the pilot settle in the pilot hole. Then the countersink starts settling in, increase rpm and pressure.I tend to remove the drill at about 3/4 through, clean the hole and cutter and then with high pressure go to final size. I had no crazy looking chattered wholes, no ovals.
There are a huge amount of countersinks to make. Remain patient and stay concentrated !
It is not difficult but the repetition of the job might make you sloppy.
Don't forget to deburr those sharp knife edges ! I used the edge deburring tool (the one you also use for cleaning lightning holes) inside and out and then used some 400grit sandpaper to clean up using a small piece of paper wrapped around a lond 4/32" rivet.
It will look like this when you are finished:
Then spot prime all holes using zinc chromate after cleaning the holes with silikon entferner. (note: you should actually wait with this until you also countersunk the 2 K1000-04 platenuts on the inboard web to avoid having to do the priming twice).
Countersinking ready !
My girlfriend helped out in attaching the platenuts to the spar. I'm not sure if this was a necessary step but I just wanted to see how they would like like.
Time to rivet the nutplates in place. This is best done with a pneumatic squeezer. First I slide two AN426AD3-4 rivet in the countersunk holes, slide the nutplate over and then clamp it with a crocodile clamp to keep it flush against the inside of the flange.
Using the squeezer, the rivets are set to final size. Pay attention to maintain good pressure on the manufactured head as you want the rivet completely flush with the spar flange.
And this is how it looks like when they are all finished. It's a hell of a job but very rewarding in the end when well done.
Close up of the nutplates and the inspection plate cover nutplates.
