OK, I'm ready to join Bart in building the version 2 of the DO-24K. After what we learned on the version 1, Bart has done a super job in redesigning the airplane for an easier build and a big reduction in the anticipated ready to fly weight. Bart has started on the tail for his build, and although I had planned to start with the fuselage, I now have decided to start with the N-strut and mid-wing. There's a lot going on in this part of the airplane and it's probably the most complex, so here I go:

I'm starting with the N-strut. It's a good design but I decided to add some strength to this component by incorporating thin CF sheet into the assembly. Bart's design is a good bit of engineering. It will mount the mid-wing with a simple latching mechanism and is locked with a simple locking part. More on that later.

Jim

Don't worry, I don't build this fast. I'm just doing a catch up posting of what I had been able to squeeze in between other projects and trips.

The secret to making a straight and true wing assembly is to ensure that the structure is firmly jigged or supported during the initial phase of the build. The elegant ribs are pretty flexible by themselves so the first phase is to ensure that no warps or twists will be built in. One thing that I might mention now is that Bart's CAD design is precision work. That, with the great job that Charlie Manzano did with the laser cutting, really makes for parts that fit exactly right. I try to dry fit each assembly before the glue is used. If a part doesn't seem to fit right, it's probably the wrong part. What I'm doing now is to get that first skin sheet on the frame to lock in the structure as it sits on my bench.

Jim

Love those ribs..love them. No really.

How were the CF pieces cut?

Very nice Jim , your on a good start now. More fun to come.

Cheers,

Bart

A superb design job by the master, and what looks like a great build Jim...I'll be watching.

Originally Posted by mistairjoe

How were the CF pieces cut?

I wish that I could say that I just sent the pattern over to a friend with a CNC
cutter but I just cut the parts using a scroll saw and a lot of file work. Other parts
That I intend to use CF sheet for will be very much easier to make, thank goodness.

Jim

I did a little more work on the midwing today. The top skin just behind the LE is on and I also got the cove skin on in the area just above where the flap will retract into. Getting these two skins on right now will keep the frame from developing any twists or warps. The cove skin is especially stiff since it is a curved surface. I used one of my favorite tools for jigging up a structure to keep it straight. I use aluminum angle strips. These work great because they are very stiff and the 90 degree angle acts a leg to keep the structure off the work surface. I found the best way to put the cove skin on was first to wet down the curved area of the skin and then clamp it in place until it's dry. once it's dry, it will hold the shape and doesn't apply any stress to the glue joint. For gluing it in, I tack it into place using thick CA and a kicker to hold the skin up against the curved portion on the rib. Once that is done, I then come in with a filet of slower drying glue to add strength to the bond. I use Titebond II for most work and Titebond III for its slower drying time, allowing a longer working time on larger structures, too. I also opted for a full length wing joiner spar. Bart did a good job designing the midwing for maximum strength with minimum weight but I was still a little nervous about the strength of the midwing. Based on my experience with the version 1, and the fact that the bending load on the midwing is concentrated along a very narrow area at mid span, I decided to go for the full span CF tube. It's probably overkill but it only added one ounce to the overall weight and I'm thinking that one ounce is worth several pounds in comfort!
The build goes pretty fast at this point but will slow down later, as the details pile up.

Jim

This will be a great companion build to Bart's. Very nice work Jim. It sure looks like it's moving along very well.

J

"its modelling Jim, but not as we know it".......!

Looking at that wing reminds me of pictures inside zeppelins. Remarkable.

What scheme are you working towards?

All the best

Alec

Thanks, John. There's no way that I can build as fast as you do so I expect that mine, and Bart's, too, will be posting for quite a long time.

Jim

Alec, it will be the same scheme as I did on the version 1 prototype, the Dutch aircraft used in the Dutch East Indies, circu 1942.

Jim

Since there was no provision for the full length wing joiner, how are you attaching it. Seems to be floating.

It doesn't have to be supported at every rib. I added support where needed. I don't want to give the impression that I am not entirely satisfied with Bart's design of the midwing. His design is more than adequate for any flight loads that the airplane will be subjected to and he was able to do it without adding any excessive weight.
What I wanted to add was extra strength to protect against excessive loads imposed on the wing due to a hard landing. I'm pretty easy on airplanes but sometimes things happen. As an example of the loads expected, here is what I mean: take a balsa stick and try to break it over your knee. It will break easily. Do the same with a 1/2" CF tube and it will be extremely difficult. Now, on the DO-24, the midwing is supported on a very narrow fulcrum, with heavy weights, the outer motors, concentrating their loads on the outer portions of the midwing, with the bending moment concentrated at the fulcrum. The version 1 DO-24 was much heavier, of course, but I noticed that on a hard landing, cracks were happening on the top wing skin due to the slight bending of the midwing causing stress fractures in the skin. Adding a full length CF joiner tube will stiffen up the wing to prevent this. The tube only needs to be supported at the fulcrum, and at the points that it already is, in the wing joining area. The wing struts will provide additional support for flight loads, because they are in tension, but provide less support for the landing loads due to their ability to flex, even with the small amount that they are capable of.
Without going into a complete stress analysis on this thread, I hope that this will be sufficient.

Jim

I was.. hoping for a complete stress analysis but this should suffice for now.