Questions about Main Spar - Model B

[Meltro]

I am currently planning on beginning work on the main spar but I have some questions that I had hoped would have been answered studying the plans, but the more I read them the more confused I am about them. Specifically, there is a dimension titled 'length of spar web to splice' that appears visually to be longer that the spar web section itself, going from the splice point to the tip of the wing attach fittings. Is this an incorrect labelling or an incorrect drawing? If it's labeled incorrectly, how do you get an absolute for the width of the spar web? I can infer it inexactly based on rib spacing and combining a few other measurements with the distance from the center of the attach fitting hole to the attach fitting tip but I feel like I'm missing something very obvious here about sizing these cuts, given dimensions were given explicitly for all of the spar caps.

Also, each of the spar caps and attach fittings appear to have octagonal cuts at the end as opposed to a more circular profile. I know those corners can have a tendency to act like a punch against aluminum sheet, so I have to ask, are they that shape for ease of cutting/assembly and a more rounded profile would be preferred, or is that shaped that way for a reason?

[geraldmorrissey]

I'm building a Patrol but my guess is the dimensioning is the same. The "length of spar web to splice" tells you how long the spar web detail part has to be. The web extends between the wing attach fittings and assumes the contour of the attach fitrings as it continues inboard past the wing attach hole. As a practical matter, the wing attach hole should be used as a measuring datum as it is a critical feature. To find that location go to the sheet that defines the main spar attach fitting. You will continue to doubt the validity of the dimensions as you go forward as I did, but be aware it's probably correct as drawn. Ask questions before cutting big expensive parts. Having been through the spar wringer, I would start with the rear spars first. You will be much better prepared for the main spar and will make fewer mistakes. As for the cut ends of the spar cap, build per print. Bob's got his reason's.
Gerry
Patrol #30

[Meltro]

As a practical matter, the wing attach hole should be used as a measuring datum as it is a critical feature. 0

Agreed, which is why I was confused about using the edge of the attach fitting as a datum, since it is never explicitly defined in units, only as a drawn shape.

So, does this mean the spar web is cut into the shape of the attach fittings? That would be the missing piece for me, in the drawing it appears to end at the inner edge of the innermost rib. Is it allowed for me to post a small piece of the plan here to show where I'm getting that? (It's drawing 5B, middle from the top, second leftmost, it appears the web ends well before the end of the fitting)

[sjt]

Like Gerry, I am also building the patrol. But the dimension you are looking at would be similar to a patrol,
sparplate.jpg

Under the black washer you should see 5 layers. There are 4 layers that are spar plates. The most center 032 is the web you are refering to. There is also a black line. I have not finished trimming the web. I did run a flush bit router and trimmed the countour around the attach hole. You may be thinking that the length of the web should be in agreement with the cap strips. This is not correct. The web extends past the attach hole. The 032 layer also shares the load.

There is an important point to consider. The videos from desertbearhawk point out the issue. I will try to describe what you will encounter:

You really need to work back from the splice between the 032 c channels. There is some funny business with the rivet spacing near the splice. Ideally you want to have capstrip rivets an equal distance from the splice edge.

split.jpg

In the picture you can see the web at splice point. I want to make sure that there are no rivets near the splice.

[sjt]

The angle of this picture does not show it, but the rivets are spaced such that both are as far from the edge of the spar as possible.

In the picture above, In red- you can see that Bob has called out the distance from the c-web 032 splice to a rivet location.
The goal is to make sure that you can get good spacing between cap strip rivets and not accidentlly put a rivet close to the edge of the 032 under the plate. That would be a no no.
splice2.jpg


So, for me I carefully marked out all the rivet locations that should not be drilled and worked from the center of the spar out to the attach point of the spar.

The end result is that I have to trim -with a router - the 032 web as the last step.
attach.jpg

[sjt]

Again, I am working on a patrol, yours will be slightly different. But, but, but, there are rivets locations near the splice that should not be drilled, until you check if something else goes there.

There is the 4130 steel that will interfer with the normal rivet spacing, and there is a pulley that alters the locations.

You can see a bunch of rivet locations that are not drilled so that something else goes on top and I will have to drill them a little away from the normal spacing
plate.jpg

[geraldmorrissey]

Well done Stan.
Gerry
Patrol #30

[sjt]

I kind of just layed the 4130 bar in the picture for clarity. It is not in the correct location, It does show rivet that have to be moved.
4130-3.jpg


4130-1.jpg


I recommend starting the 032 at the center and route off the excess at the attach fitting much latter. Just do not drill every rivet location near the center until you know what is suppose to be at the location.

Splice? 4130? Pulley?

You just need to alter by some small amount a few of the locations, as well as not drill near the 032 end at the splice in such a way that it violates rivet to edge distance.

[sjt]

As to the rounded end of the capstrips, there is one consideration at the attach fittings.

attachfitting.jpg




At the bottom attach fitting, it is rounded just to make it pretty. On the top, it is a different issue. You want that top cap strip to be load bearing against the attach plate.
I think in my case I drilled and riveted the attach plate. I also had left my top cap strip longer. After repeated file strokes and trial fittings I reduced the top cap strip so that it would butt up against the attach plate, while still allowing the match drilled rivet holes to line up.

Just for clarity, you could put the attach plate on without the capstrip and it would line up with the pre drilled cap strip holes.
Or, you could leave the attach plate off and put the top capstrip on and it would line up with the predrilled rivet holes.

But, by design, you could only get both on by hand filing the top capstrip for an interference fit with the attach fitting.

As to it being rounded, you can see that it is slightly easier to make the load contact of the capstirp against the attach plate if both are slightly rounded. It would be a lot harder to get a straight line fit between the two. It is easier to minimize the surface contact area a little.

Again, the goal is to transfer all of the load from the top cap strip by point contact with the attach plate A wing that is in flight would stretch the bottom cap strip and point contact would not do anyting.
But, the top cap strip of a wing flying is in compression. It will want to push the top capstrip and the attach plate together.

I found a picture that shows the top capstip holes not lined up.

start.jpg
I kept taking a bit off until I got point contact and the holes lined up

trim.jpg

[sjt]

The way I did it was to have match drilled the capstrips that would be a set. In the case of a patrol that might be 4 capstrips. two on the front of the spar two on the back of the spar. The inside cap strips were the correct length. At the web splice I slid the inside capstrip so that the holes split the distance to the 032 web edge. The inside cap strips were the correct length. I then could use the aluminum 6061 spacers , splice plates and rubber bands to hold the top and bottom inside capstrips against spacers to get the right distance apart. I then fit the inside attach plate fitting against the top and bottom cap strips. I made my attach fitting with holes that could be used as a reference to the center of the web which I had already marked. I then drilled the inside attach plate fittings to the spar with a length of 032 web that was longer then needed.


My Outside attach plate that is load bearing was match drilled to the inside attach plate at a few locations. (Not the locations over the capstip - they need to be done latter when everything is done)

The outside top capstrip was left a little longer so that it could be filed down. The Outside attach plate location is already set by having drilled thru the 032 using the inside attach plate. The inside capstip is drilled thru the 032. All that is left is to file the top capstrip so that It lines up with the already match drilled inside capstrip And still being a force fit against the outside atttach plate

inside.jpg

[sjt]

Inside capstrip top and bottom held together with rubber bands and spaced out with precut spacers.

Look carefully, I did not drill the holes near the attach plate fittings.

this.jpg

[sjt]

Only after everything is done can you drill those last locations.

If you had predrilled the locations under the capstrips you would not be able to accurately transfer the holes to the outside attach plates.

If you wait to the end, you can reasonably center the holes so that they will go thru the center of the capstrips and be spaced out.

The outside attach plate can be pinned to the 032 and the inside attach plates. Then by eye center punch a few dimples that would go thru the outside attach plates and reasonably in the center of of the capstrips at about the right spacing.

A number of builders have had to rebuild parts at this point because where they thought the holes would be turned out not to be in the correct location.

You can not go wrong if you drill the last holes thru everything at once. It is match drilled and centered where it should be. You do not need spacing between rivets accurate to the thousandths.



Just look at the holes I put in the outside attach plate. I only knew what would be centered after It was pinned at the attach fitting. You do not have a reference to make them (unless you have a cnc) But you know where they should be once everything is pinned in place.



final.jpg

[Meltro]

Thank you so much, sjt. Those pictures answer the question, the spar web "C-Channel" gets shaped to match the attach points, that's the part that wasn't really clear to me.

[Chewie]

I used the beginning of the cap strips as my datum, so the attach hole was a slight negative value. That also worked well. Just be consistent, whatever you use.