Traveling Yankee open source update

New version of the open source design has been uploaded on github.

Here's the main info about the project: https://github.com/syampolsky/yankee-cookies/wiki

Here's the list of changes from the initial release: https://github.com/syampolsky/yankee-cookies/wiki/Release-0.2-Notes

The main project page with all the file is here: https://github.com/syampolsky/yankee-cookies

Figured, I'd add the list of changes in the latest version of the design:

* Increased height of the fuse by increasing the height of the pipe tunnel by 0.25". The fuse side area and its distribution matches that of Dave Fitzgerald's Thundergazer. This also improved pipe tunnel clearance.
* Added wing drawing and ribs. The project now has DXF as well as the PDF version of the design.
* During test build of the wing it was noticed that the area around the flap mount at the tip was flexing/twisting too much. The design partial span flaps up to the last bay of the panel. Additionally, the flap is pivoted on two points: fuse and tip. Due to 2" rib spacing, the fixed section near the tip was only 2" which wasn't stiff enough to carry the flap. One solution is to add 1/2oz cloth to the last 2" of the wing or change rib spacing in order to increase the size of the fixed outboard section. Going with Plan B: change rib spacing from 2.5in to 2.0. The fixed part of the wing is now 4" wide instead of 2.5". This should improve the stiffness of the wing near the tip.
* Switched to rod take-apart hardware. H-mount take-apart is expensive to buy as well as difficult to make. The rod system is easier to make since it only requires a lathe. No need for CNC equipment or milling machine.
* Changed fuse sides from "1/32 ply+1/8 balsa" to "3/32 balsa + carbon veil + 3/32 balsa" sandwich. This approach has been first popularized by Randy Smith on this SV designs. The idea is to provide more "meat"(balsa) to help dampen the vibration. Balsa has a great ability to dampen small vibrations generated by the engine. Plywood on the other hand is a good vibration transmitter. When building a non-takeapart model, the largest balsa structure(ie. wing) is used to dampen these vibrations. In that case, using plywood makes sense: vibrations are sent to the wing where they are dissipated/absorbed by the balsa structure. In a takeapart, the wing is not glued into the fuse and only the mounting hardware is used to transfer vibrations to the wing. As a result, far less vibration is transferred to the wing. This is why we need the extra balsa in the fuse to help with vibration dampening.
* Removed square alignment holes in the fuse for formers to plug into. Originally this was an attempt to make formers self-align in the fuse, significantly simplifying construction of a straight fuselage. While this works extremely well, the outlines of the holes show through the finish. To help eliminate this, the holes were removed on the outer fuse skin. The balsa doubler(part of the balsa/veil/balsa sandwich) still has the holes to help with alignment.
* Modified engine crutch to remove sharp corners in the back of F1. The crutch now has a rounded taper in the back( when looking from above)
changed all formers from 1/8 balsa to 1/8 lite ply. In the past, we noticed that birch plywood(not lite ply) with lightening holes having a 1/8 border resulted in stronger formers that are just as light as 1/8 balsa formers. Will try to switch to lite-ply to see if we can still maintain the strength while saving weight.
* Added 1/64 ply face to the fuse halves at the split to protect the edges. Previously, the fuse halves would just end in edge balsa which can easily be damaged. We see this solution on many take-aparts and decided to give it a try to increase longevity of the model.
* Re-designed fuse take-apart ply mounts to save some weight and simplify construction.
* Added 1/64 ply shim/riser on the front half of the fuse to match the height of the back half mounts. We found interference between fuse mount plywood pieces. When the fuse is together, the mount of the front half is 1/64th too low and pushes on back half mount which bulges the fuse sides a bit. While 1/64th can easily be removed during sanding, the area will become weaker which something we want to avoid.
* Changed engine mount from -1 degree to 0 degree. This significantly simplified construction of the fuse. We may need to revisit this in the future once we have a better way to build the down-thrust more easily.
* Changed stab incidence from +1 degree to 0 degree to offset engine mount change.
* Modified rudder parts to use tabs and notches in order to self-align all pieces during assembly.
* Added a half former near the tail wheel mount pocket to strengthen the area. The will enable safe use of the launch stooges.

Collecting photos and videos of test construction here:

https://photos.app.goo.gl/QvEY8GgVkgKX5pJZ6

Does laser cutting release any stresses in the wood like cutting with a knife does?

Not that I can see. For example, I cut the stringers that goe inside the rudder frame and typically you cut them oversize, glue in and then sand down flush because then you cut them, the stress releases and they bow. I did not observe this at all during laser cutting. The stringers were cut to size and came out straight and true.

That said, I do not use balsa for spars or stringers. I typically use basswood or birch spars. In this project, I will try a laminated balsa/CF tow/balsa sandwich.

Nice work, Steve. How many watts is that laser?

-Chris

6W. A lot of decisions regarding materials and thicknesses are based on laser capabilities. For example, while it can cut 1/4” balsa, it’s more than double the time it would take to cut the same part in 1/8”. It is also pretty bad at cutting plywood. 1/8 lite ply is ok but that’s about the most it can do in reasonable amount of time. I am looking at using the fewest number of thicknesses and materials possible. So far it looks like 1/64 ply, 1/8 lite ply, 1/16 and 3/32 balsa. Still have a few pieces made out of 1/32 ply but am looking to simplify those parts out

Sounds smart Steve. Hey, I was just reading that the phenolic glue in aircraft plywood is more difficult to laser cut than the glue in regular lumberyard plywood. Probably one cause of the issue.

-Chris