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Old Jul 20, 2011, 06:41 PM
jackerbes is offline
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USA, ME, Ellsworth
Joined May 2008
18,080 Posts
Blu Baby 33 KFm3 Polyhedral Wing Build - Part 2

11 - The filler strip has been glued down in this photo and the KFm3 75% strip is laying bottom (bare foam) side up and will be glued down next. It will be aligned with the faintly seen line I drew earlier and that will form the 75% step.

12 - The contact area where the 75% strip will contact the wing panel got spritzed with water, two beads of PU were run the length of the back of the 75% strip, that was spread evenly with the flexible plastic spreader, and the 75% strip was brought into alignment with the line and pressed down to contact the wing panel.

Plenty of weight was added to keep things in contact as the glue foamed up and I kept a watchful eye open to make sure the strip did not move around in the first few minutes.

13 - I have some 2mm fibreglass rods that came out of kites that make for good leading edge reinforcements so I decided to drop one into the leading edge here instead of the 1/8" dowel. These rods will let you whack leaves off of trees and even trim the smaller new growth if you have enough speed. They are also handy for ringing trash barrels and administering punishment to dogs that are foolish enough to think that it is OK to catch airplanes.

Since I am going to bend the wing tips up for dihedral I cut the 2mm rods short enough (20" long in this case) to be only on the flat portion of the leading edge

The 3/16" dowel that is the main spar is also seen in this photo, it will be rolled back to the right and against the leading edge of the 75% strip when the wing is closed. The dowel and the 75% strip will both get some PU glue on them and foam will build up around them and create a second full span glue line to join the one formed by the filler strip.

14 - OK, both the strips are glued down and we can get ready to close the wing up. PU glue sticks best to bare foam. So the plastic skin needs to be removed from the top of the filler strip if it is still there.

Fold the KFm2 50% strip back again (I use that board again to help with that) and, while it is held down flat against the top of the 75% strip, a line is drawn across the wing right at the 50% line. Then let it fold up out of the way again.

15 - The line is faintly seen here, it marks the area where the 50% strip will rest on top of the 75% strip. Use the tip of the utility knife to make a very light cut along the line (just deep enough to cut through the skin) and peel the skin off of the area where the 50% strip will rest on the 75% strip. If you lift the skin up at a shallow angle as seen here it will come off in one piece, if you lift it too steeply the skin will tear when it encounters the pinholes in the skin and be a lot harder to remove.

16 - The final closing! There is a lot going on here now. The back of the folded up KFm3 50% strip will make contact with the top of the filler strip and the 75% strip when it is folded down and the leading edges will join again. So the back of the 50% strip needs to be spritzed with water, it is easiest to do all of it and even the area where the tape is at the leading edge.

Also spritz the dowel with water and spritz the area in front of the 75% strip where the dowel will lay.

The dowel has been cut to 20" long, centered on the wing, and marks made to center the dowel in the wing again. This is the point where I usually put a disposable glove on one hand (otherwise it takes several days to "wear out" the PU on your fingers and let them look "not dirty" again.

Spread a bead of PU on the filler strip, run a small bead of PU on the tape and 2mm rod up leading edge where the tape is, run a thin line of PU on narrow strip that was peeled of skin at the front of the 75% strip, put a bead of PU down in front of the 75% strip where the dowel will be up against it.

Use the plastic spreader and spread the PU on the filler strip, spread the bead of PU on the narrow exposed strip at the front of the 75% strip.

Use a small brush (I use disposable acid brushes) to spread the glue around in the leading edge joint and in front of the 75% strip. Wet the dowel and lay it in the glue that is in front of the 75% strip, roll it over to get it covered with glue. Bring the dowel back against the 75% strip and you're done in here!

17 - Weight it all down, look carefully under the edge of the board board and weights to make sure the three layers in in contact and add more weight or light clamping (the foam will crush easily) as needed. Give this a good hour or more to set up (with Quick PU) because it will want to spring open on it's own. For regular PU give it overnight or longer.

18 - Look at tips of the parts on the corners to make sure you have full contact. The end of the filler strip is seen here, the dowel and 2mm rod are down in there where they can't be seen.

19 - And there it is, a fully closed flat, KFm3 wing. You just wanted a good flat and sporty aileron wing, this would be a good stopping point on the wing build. Cut your ailerons out and finish it up that way. The wing is about 80 grams at this point, if I had left the 3/16" dowel and the 2mm fibreglass rod out it would be about 60 grams.

To be continued...

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