starting center section

[flyingtodd47]

I have finished the wing ribs. I have also cut most of the ribs and various parts for the cs. I have cut the end ribs and marked the spars for ripping.
The measurement for spar height doesn't agree with the end ribs, they are too tall. Can I use the end ribs as drawn or not.
Just need some guidance. Thanks for any.
Todd

[Bitshifter]

Todd, Are you asking about the Center Section end ribs, Upper Wing end ribs or Lower Wing end ribs?

[flyingtodd47]

The center section end ribs. I have cut them to the plans, When I assemble the spars, bottom and end ribs, The marks for ripping the spars are well above the end ribs, 3/8 on the rear and 1/8 on the front.

[HC85]

Hello, I think this subject has been talked about before on the thread, but I did was to cut the spars down to match the center section end ribs. Also, after the fuel tank is installed in the center section, I made the upper wing end 1/4 ply end ribs to match the center section end ribs perfectly, so there were no noticeable differences in alignment. From what I remember, there were very slight differences between the shape of my truss ribs, verses the center section end ribs with the tank installed. Not much mid you , but I didn't want even a 1/64th difference. Just info for you,... your situation may be different, just something to look for in the process.

Rick (HC85)

[Bitshifter]

I am not home so I can't check my plans but if I remember correctly the spar heights for the center section are incorrect on the plans. I did what Rick did, I cut the spar to fit my ribs. I checked my center section end ribs with my wing ribs, made sure that I understood where the center section floor was and cut the spars. Ultimately you want the center section to be the same thickness as the wing. Do a search on the site for "center section" and make sure that you understand the hole location for the wing mounting plates. If you follow the plans the wing will either be to high or two low by about 1/4 inch. If you build the center section as per the plans then you will need to correct the wing and vice versa.

[mmarien]

centersparsize.png (122.46 KiB) Viewed 4453 times

I also was just looking at building the center section and going through the plans. I don't think the spar measurement of 5.188" on sheet 04 is correct. The spar measures 4.85 (by hand) on the sheet 02. The bottom of the spar is nearly identical to the center section. The rib on sheet 02 has a 1/4" capstrip on the bottom. The center section on sheet 04 has 1/4" plywood. Mind you the plywood is shaved a bit to blend to match the wing. On sheet two the rib has a 1/4" capstrip in the top. The center section has the top of the fuel tank shown in Detail H. Probably 1/8" or less. But 4.84 + 1/8" (0.0625) only equals 4.9" not 5.188" as shown on sheet 04.

Since every build is unique, here is a good way to figure out the centre section spar:

Place a straight edge along the bottom of one of your ribs. Measure the distance along the edge of the spar from the straight edge to the top of the rib and subtract the thickness of your fuel tank material and the 1/4 " for the plywood along the bottom. The 1/4" plywood at the bottom is assumed to be straight and blended to match wing so it is straight when assembled.

[flyingtodd47]

Thanks
Could I place the spars in place on the bottom ply, place a finished wing rib against them with the bottom of the rib on the table , Mark the spars from the rib then subtract the tank thickness?

[mmarien]

That will work. Just remember the ribs have a 1/16" relief at the top of the spar for the plywood leading edge. So you will have to add the 1/16" then subtract the tank thickness.

[flyingtodd47]

Thanks again. I hope to start trial assembly this weekend.
Todd

[orchardair]

I am wondering if this is yet another reason to bring up the question of "does it make more sense to start with the wings, and then continue with the center section".