Centre Section problem

[Nick]

Good day Hatzers. Some advice, if you please.

I built my centre section several years ago, now that I'm looking to assemble my upper wings, I see that I may have made an error.

The front spar measures 4-7/8" high at the highest point as per the CB-1 plans, but the Classic plans call for a height of 5-1/8", so the front spar appears to be 1/4" too narrow. This is confirmed by the fact that my front spar doesn't reach the top of the centre-section end-rib by about 1/4".

Now the weird part. The rear spar is 3-11/16" high as per CB-1 plans, but the Classic plans call for it to be 4-1/8" in height. That means mine is 7/16" narrower, which doesn't sound good, but it fits the end rib quite well. On the other hand, if I make the rear spar the size called for in the plans, it will be too wide to fit the end rib. The end rib is taken from the Classic Plans. If I make the rear spar to the height on the Classic plans, the profile won't match all the ribs I've made.

I really don't remember what I did back then, and I may have to start from scratch with my centre section, which would be...annoying...however I'd like to hear if anyone else ran into any problems like this during assembly of the centre section from Classic Plans.

Thanks in advance for your time and thoughts.


Nick

[Nick]

Perhaps it would be easiest if some of you who have finished your centre section could simply tell me the measurements of your two centre section spars.

Thanks guys.

Nick

[Nick]

OK, well I've pretty much decided I need to build my centre section from scratch. I don't know what I was thinking back then building it so out of whack. I guess it was the first thing I built. So here's a new problem.

The centre section is 54" by about 26 ", which means I can't buy the typical 48"X48" Birch aircraft spruce plywood which is about a hundred bucks. I'd expect to pay double for a 4'X8' sheet to cut it out of, but guess what, the 4'X8' sheet is $450.00!!!. What? Four and a half times the price of the 4'X4' sheet? That's at ACS&S BTW.

I'm hoping someone out there has a better (more affordable but of equal quality) option to suggest.

Thanks folks.

Nick

[johnkerr]

I think I order a 48x60 or 48x72 for a "slightly" better price.

[dougm]

Nick, sounds like you were looking at the price for 45 degree plywood... you can use 90 degree for the c/s bottom and it's "only" $239

If you are buying plywood give some thought to shipping... 4x8 sheets ran around $120 to ship as I recall, so I bit the bullet and ordered all the plywood for the entire plane in one shot as the shipping was the same whether it was one sheet or 10.

[alvinsager]

Hi Nick,
I used Finnish birch ply for the bottom of my c section. If I remember correctly, it was sold in 61 x 61" sheets. Make sure what you buy is the waterproof variant. Check out AIT (Anderson International Trading) on the west coast. They cut two sheets to my rough sizes with the grain direction I specified, saved lots on shipping that way.
Al

[Nick]

Thanks for the tips, gents. Yes, I remember there used to be 61" square finnish birch ply. Can't seem to find it now.
I'll check out AIT.

Nick

[Nick]

Hey Al, i contacted AIT and they said I could certainly purchase their AC grade plywood --- from ACS&S.
Oh well, sometimes you just have to buck up, I guess.

Nick

[Nick]

Good Day gents.

In rebuilding my centre section spars, I think I've come across an error in the Classic plans. The plans call for the front spar to be 5-3/16th" at the highest point, and for the rear spar to be 4-1/8" at the highest point. But no matter how I measure it, I can't see the front spar as higher than 5-1/32" and the rear spar 3-13/16". If I make the spars according to plans, the Centre section will be about 5/16" thicker than the top wings, which is obviously wrong.

If anyone out there has their centre section handy, I'd be obliged if you could tell me your centre section spar dimensions.

Also, the Classic plans centre section end rib is notched along the top and bottom edges. The top edge notch appears to be there to accomodate the top aluminum sheet of the fuel tank, and the bottom to accomodate the 1/4" plywood on the bottom of the section. I have seen photos of other folks centre section, but nobody seems to have made the top edge notch in the end rib. So I'm wondering how you all went about it.

Any help on this is most appreciated. I don't want to build the centre section wrong AGAIN.

Nick

[johnkerr]

I notched the end rib of the center section as you described. In final assembly there were a couple of places needing cutting and fitting to get eveyrthing lined up. It will be helpful if you have your fuel tank to include in the test alignment. I don't recall that anything was more than 1/8" out and I was able to get it back to award winning condition.

John