Hatz Biplane Association

Hi Nick,
I mentioned that I used the Finnish birch ply. This is NOT aircraft grade, although I think it runs circles around aircraft grade. The 6 or 7mm I bought has around 13 plies, all birch. I boiled samples in water and there was no delamination.
Al

Thanks for that information, John.

In other news, I've finally concluded, thanks to the Forum Archive, that the classic plans are in error. The plans give incorrect dimensions for the spars, which are too tall by 5/32" at the front spar and 5/16" at the rear spar. The spars should be built to fit the centre section end rib, which makes the front spar 5-1/32" at the high point and the rear spar 3-13/16" at the high point.

For anyone else getting to this point, my considerable reading has uncovered several builders who have had to solve this same puzzle and come to this same conclusion. There has never been a failure of a Hatz centre section spar.

Nick

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

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

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

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

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, 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.

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

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