[Max]

So most of last night was spent cutting and fitting the missing rib sections.

I would cut a full rib, then cut that rib accordingly to make it fit into the sections that were missing. I dont think that there was any segment of the wing that actually need a full rib. Getting these partial ribs to fit into the existing structure took a little bit of work but it was not overly difficult. I did this while I was watching the Blackhawks and the Kings go at it. My glue of choice is white glue (weldbond). While CA is fast, I find white glue to be more forgiving of any gaps in the fit. I also feel the fillet effect provided by aliphatic glues provides a better shock-proof bond than CA, which tends to shatter rather than flex when impact forces are applied. But I digress....

I also replaced the trailing edge from the side that was missing.

On the other side, a segment of TE was missing near the root. The original TE on this plane was a built up assembly. The one I put on the side that was missing was a solid piece of TE stock. So I cut the other side off completely and replaced that one with a solid piece as well.

It no longer looks like a mangled wing and looks more like something that could fly!! Looking good so far!!

[Dave Holmes]

If you're not careful, you're going to end up with an airplane, or something closely resembling one!

[Max]

Originally Posted by Dave Holmes

If you're not careful, you're going to end up with an airplane, or something closely resembling one!

HA HA.. I'm trying . One thing is for sure. I haven't spent so much time building a woodie in so long. I forgot how enjoyable this part of the hobby is.
Of course, having some crapy weather every now and again that forces me into the basement helps!!

[OLD PRAIRIE RED NECK]

Originally Posted by Max

I haven't spent so much time building a woodie in so long. I forgot how enjoyable this part of the hobby is.

Shhh! Max! It's a very well kept secret these days!

[Max]

So now that the wing structrure is reasonably restored, I got to work on the motor mounts.

I started with the G10 motor mount for the dimensions. I found a 2"x2" mount would fit this perfectly.

After cutting 2 mounts, I glued them to the leading edge. I set the wing flat on the workbench and sat the mount flush on the bench. Then I used a square and a toothpick to set downtrust. As mentioned earlier, Downthrust was guesstimated using the TLAR process ("That Looks About Right"). I used CA to hold it against the leading edge.

Now that the mount was semi-held in place, I used a piece of wood temporarily glued to the the top to keep it rigid. Now I could backwards-engineer the side-walls of the motor-mount. To carry the thrust loads from the motor to the wing, the side-walls of the mount have to extend all the way back to the main spar.

Using paper and scissors, I cut out the templates to fit the mount. Then I used the paper templates to the the side-walls. I cut these from doorskin. I made 4 and then glued them into place.



I spaced the motors so that I had enough clearance for 11" props, if I ever have the need to go to that large a prop.
Notice in this picture where the mount is attached to an existing rib



When the mounts were dry, I reinforced each vertical corner of the mount where it met the side-frames with triangle stock.



I test fit the motors onto the mounts...Looking good!!!

[Max]

Last night was working on the wings and something caught my eye.


On both wings, main spar and rear spar, top and bottom looked like they were once broken and repaired.


Now for the life of me, I cant recall when or how these spars were broken and when it was repaired.

I dont know if my friend John had crashed and I repaired it for him and then the plane flew some more before its final crash OR, if this repair was done when I received the wreckage from him all those years ago and then I mothballed it.

Either way, the spars needed re-enforcing. Particularly on the bottom, where there is a higher tensile load on the bottom spars.

I happened to have some scraps of a really thick fiberglass cloth at home.
The weave on this cloth is about 1/4" wide!



I unwound a few strands from the cloth lengthwise, and ended up with several 4' long strands.

Using slow-cure epoxy, I laid 2 pieces on the bottom main spar and one peice on the bottom rear spar.



This should provide ample tensile strength to the lower main and lower rear spars.

[stardustertoo]

Either way, the spars needed re-enforcing. Particularly on the bottom, where there is a higher tensile load on the bottom spars.

Hi Max, your project is looking pretty good

One comment though in regards to the bit I quoted. I've done a bit of studying of some of these things over the years, and I recall a magazine article a few years back on structural failures. I dont recall all of the details, but basically what they were saying was that most all failures actually happen due to the top spar buckling under compression, not the bottom side being 'torn' by the tension. They backed this up with a bunch of testing. (Side note-The full size Citabria family of aircraft with older wodden spar wings have an airworthiness directive to inspect for compression cracks on the top edge of the spar, right at the outer strut attachment. This is an annual requirement)

In fact, if I recall correctly from my 'materials class' in full size aircraft maintenance school, most woods have a higher tensile strength than they do compression strength. As a result I have commonly built some sport planes with a spruce top spar and a balsa bottom spar to save weight. Never had a wing break unless it hit something

Now, that being said, with a prior break in the spar, there is definitely a weak spot there. What you have done with the glass fibers will certainly help the botom, but make sure you do something with the top. That weak spot on top is right where the spar will buckle under load, and fiberglass will not help that very much as it also does not have very good compression strength.

What I would do for the top spar is cut a slot in the rib right in front of the spar and glue in a 1/16 or 1/8 light ply 'splint', spanning about 2 inches beyond the crack on each side. Taper the ends of the splint so there is a gradual reduction in thickness, this will prevent creating an abrupt 'stress riser' which would create a weak spot right at the ends of the brace. You could even add another one to the back side of the spar, but make that one about an inch shorter on each end so that it does not end at the same spot as the front one.

[Max]

That is an interesting perspective on spar failures, Ron.

I have to agree 100% that under stress, a top spar will compression fail before the bottom spar tensile fails

It is common knowledge that any material is (usually) a lot stronger in tensile load than compression.

Just for fun I looked it up for balsa.

Balsa

Balsa wood Properties Guide
Density163±10 kg/m³
Compressive Strength
low density 4.7MPa
medium density 12.1 MPa
high density 19.5 MPa
Tensile Strength¤
low density 7.6MPa
medium density 19.9 MPa
high density 32.2 MPa
Elastic Modulus –Compression 460 ± 71 MPa
Elastic Modulus -Tension1280±450 MPa

Low Density =75kg/m³(0.0027lb/in³);
Medium Density= 150 kg/m³(0.0054lb/in³);
HighDensity =225kg/m³(0.0081lb/in³)

Under normal conditions, I agree that the top spar will fail first based on the fact that balsa under compression is 1/2 as strong as balsa under tension.

I would therefore agree that when a wing and spar is being designed, the failure mode of the spar in compression should be of greater consideration.
These considerations would affect how the spar is engineered.

But in my case, the spar has already been engineered and presumed adequate.

My reasoning for strengthening the underside is this:

Given that the design is airworthy with the existing spars, the compressive loads of the spar is adequate (by design).
In this case, there are 2 1/2Sq spars, one top and one bottom, strengthened by shear-webbing on both sides creating a box-structure

For illustrative purposes I am going to over-simplfy the physics but readers should be able to get the gist of it;

Lets say for this example, the model is designed to enure 7 G's.
AUW for this model is 6 pounds, so under maximum G's the model weighs 42lb before catastrophic failure (And I will agree the failure mode is the top spar fails under compression)

Assuming the top spar is designed to withstand 42 pounds of compressive force (6lb @ 7G), the bottom spar being the same dimensions as the top spar should endure about 84 pounds of tensile load before it ever fails

Given the designed box-structure spar for this model, a broken top spar when re-glued would have the same compressive strength as the original. (42 pounds)
But a broken bottom spar when re-glued would not have the same tensile strength as the original. In the example I used above, the bottom spar should endure 84lb of tensile load, but the re-glued joint might only endure 24 lb of tensile load**. So now my plane would only be able to endure 4G's before the lower spar fails. In reality, an unbroken lower spar should never fail (should to to 84 lb before it ever did).

Anyway, that is my logic. My fear is the re-glued lower spar would have lost a considerable amount of its tensile strength.

Nevertheless, your point is well taken and I'll probably add a re-enforcement to the spars just to be extra sure!



** Caveat: Strength of glue joints vary depending on the joint and adhesives used.
In most cases the glue joint fails. In special cases, the join/adhesive is stronger than the original material In my case, I am choosing to err on the side of caution, since I have no idea how the spars were originally repaired.

[stardustertoo]

Makes sense to me Max

Interesting side note related to your 'fine print' at the bottom relating to the glue joint and glue. In that same test they also revealed that a box spar setup with the shear webs like the Kadet wing, is only as strong as the glue joint holding the shear webs to the spar. They tested different styles of shear webs, ('C' beam, 'I' beam, etc) and in every case, the spar failed when the spar caps on the top of the wing delaminated from the web, and then failed in compression. In almost every case the shear webs stayed nicely attached to teh bottom spar cap. They tested different ways to glue the webs as well to try and prevent that delamination from happening, and once they figured that out they had an unbelieveably strong spar........especially when they added a bit of carbon strip to the spar cap. If I remember right they ended up with a spar that they were loading upwards of 250 lbs.....and still small and light enough to use in a sport plane!

[Max]

Hey Ron. These are interesting facts. Thanks

Originally Posted by stardustertoo

If I remember right they ended up with a spar that they were loading upwards of 250 lbs.....and still small and light enough to in a sport plane!

Wow. That's like 20 G's !!