My Fokker DVII Project

[ECramer]

Man, I wish I could see the photos of this work. I don't know what format you're using, but your photos just appear as a red X on my screen (I think you're probably using photobucket or some similar online storage, and those are blocked for me).

Sounds like your project is really going places.

[Brad]

Originally Posted by Jeff Brooks

I have glued two of the scarf joints together. I scarfed the 12' to the 14' pieces.



It is nice to see the plywood on the spar drying on the left and the beams scarfed together on the right.

Uh-oh. I see a potential problem here Jeff, I think you MIGHT have made a serious error. I could be misinterpeting the photos, and I also could be wrong in the way the plans specify to do this, but...

It looks to me like you have the face grains of the plywood running parallel to the spar caps. That is weaker than the other direction... and even more ideal is 45 degrees. (I based that on the pic showing the plywood with the inspection stamp laying beside the caps.)

The webs are there to carry shear loads between the upper and lower caps of the spars. (upper cap is in compression, lower cap is in tension.) So... if the plywood is applied with the face grain parallel to the spar caps, the forces applied are nearly parallel to the face grain, or in other words trying to split it along the face grain.

Now, it is possible that this was taken into account in the design, and that is the way it is intended to be... however, I suggest you look into that just to be sure, for the sake of safety.

YES, it means a lot of scarf joints in the plywood! But, that is generally the "accepted practice" in box spar construction... either the face grains vertical, or at 45 degrees.

Sorry if I am overstepping courtesy here, or if I have sent up a false alarm... I'm just trying to help keep you safe.

[Jeff Brooks]

Brad,
I think you are right. I must have missed this. I will abandon this one, and correct this with the next one.
P.S. Your input is always welcome ... even the bad news~

[Jeff Brooks]

Wow, I figured that spar was a goner. I assumed this skin would be impossible to remove without tearing out the good wood in the top & bottom pieces. Since I had nothing to lose, I figured that I would tear the plywood off to test the quality of the gluing. As it turns out, it wasn’t too difficult at all. I might be able to recover the spar.

I used a chisel to split the plywood between plys. This gave me a grip on the nails with a pair of needle nose pliers. This got me started, and later I could just pull the skin off with my hands and still pull the nails.


Once the nails were out of the way, I used the hand planer to get the plywood off. The block planer is such a neat tool, I could remove such small amounts of material, that I could see the pencil marks under the glue.


The end result was that I got back to the raw wood, with minimal loss. Once the Finnish 5 ply birch plywood arrives, I can get the skin back on ... and in the right direction!



Oh, and the quality of the glue was great!

[Jeff Brooks]

Hi all,

Not much new to report today. I have completely stripped the plywood skin off the spar yesterday, the new 2.5mm 5 ply birch material arrives at lunchtime tomorrow.

Today, I am scarfing the last of the next set of laminations for the other spar. You can see the line on the scarf is pretty tight! My Master, Vet has taught is padawan well.


I think that if the wood wasn't a slightly different color, you might not see the line on this scarf of this bottom lamination at all...


I have also glued a few of these 1" thick pieces for the intercostals at the wing attaching points.


With the last lamination of the second spar drying on the table, those laminations waiting in the spar table to be glued, the spar on the floor waiting for the plywood skin, and the blocks being glued, you can see the basement is pretty busy,

[Jeff Brooks]

Hi all,
I was considering applying the 2.5mm thick 5 ply 90 degree birch on the spar in a 45 degree orientation. It is my understanding that, orientating the grain in this direction is the strongest method.


I know that aviation plywood manufacturers make plywood with the grain in a 45 degree orientation, but the advice in NACA TR 344 ( http://ntrs.nasa.gov/archive/nasa/ca...1993091413.pdf ) is specific about the "grain of alternate plies forms angles of +/- 45 degrees with the longitudinal axis of the beam" is twice as strong than any other orientation. That means using 90 degree ply and turning it 45 degrees.

If this is so, I plan to cut the plywood in diagonal pieces, and use a diagonal scarf joint.

As I have been building this thing by committee, I am looking for insight from the crowd here.

[hank jarrett]

All of our advice on this one (those right and wrong) don't count for as much as that one Fed who gets to accept or reject your work. I would ask HIM.
I have been right before with the FAA and it didn't matter a bit. They still rejected a perfectly good (and actually TESTED) part.
Hank

[baldeagle]

45 degree aircraft ply does have the laminations at 90 degrees to each other, they're just 45 degrees to the length of a 4x8 sheet, so you can get longer pieces out of it than if you take a sheet of 90 degrees and cut across it at 45. With the 45 degree ply you'll have less scarf joints. Look on the Wicks Aircraft web site, 45 degree ply is made exactly for box spars.

[Jeff Brooks]

Baldeagle, thanks for that info! It is very helpful.

I bought the 3 ply Birch/Poplar material for my project because I read that this 3 ply Birch/Poplar combo was structural, and I read in the British report that 3 ply birch was originally used, but it also said all three plys were probably Birch.

Recently, the strength of the poplar in the wood I was using was questioned, so I asked Achim what he is using, and he said 3 ply Birch in his plane & 5 ply Birch for his customers.

So, I have been debating sending the 3 ply birch/poplar back to Aircraft Spruce and buying the 5 ply Finnish Birch. This would cost me about $300 in shipping costs to return it, and order the right stuff. The Idea that I could cut the 3 ply at a 45-degree angle and still use it appeals to me financially!

But, I have been looking at the differences in weights of each material too. 5ply Weighs 4.82 lbs for a 4' x 4' piece, and 3 ply birch/poplar weighs 7.5 lbs for a 4' x 4' piece. It seems the 5 ply is stronger & lighter.

I guess I will sacrifice the money for shipping it back, and shipping the new stuff here.

[Dogtail2]

Jeff

Looking at your drawing, I would say that the grain direction should go from bottom left to top right, on the left side, and from bottom right to top left on the right side. In other words, a reverse of direction at the middle.