Building Tips and Tricks 2 of 4

by Tandy Walker

Layout and SAM Site Integration and Editing by Web Master Ned Nevels

(material provided by builder Tandy Walker)

For the required 10 oz wing loading, the minimum weight for the 414.55 sq.in. Class A Bomber is 28.29 oz. Or 816.15 gms (my Acculab scale resolution is one gram). With the Bomber completely assembled and after installing an APC 9 X 4 prop on the Shilen Old Timer .19 with a Fox solid aluminum spinner, it only weighed 787 grams, which was under weight by 29 grams. I measured out 29 grams of lead strip and placed it on the engine. The weight was exactly the required 816 grams, but the bad news was that the C.G. was too far aft. There was only one thing for me to do at this point. Move the engine forward and that is what caused all of the delay.

As most of you know, moving the engine forward to get the C.G. dialed in at 50 % at the minimum 816 gram weight is an time consuming iterative procedure and there is no good place to start! To spare you the details of all of the iterations I did during yesterday and this morning, I will cut to the chase and give you the bottom line. I used plywood to make up a laminated 1/2" spacer with a 6 oz lead insert inside the spacer. The spacer's total weight was 19 grams (see pictures 2 and 3 below). I used a second 10 gram Shilen radial mount against the firewall to mount the landing gear, which with the spacer weight provided for the required additional 29 grams of weight to achieve the desired total weight of 816 grams. This arrangement moved the engine forward the width of the second Shilen radial mount plus the 1/2" spacer, which magically trimmed the C.G. at the desired 50 % point. The last picture below shows the trial fit of the plywood spacer sandwiched between the two Shilen radial mounts.

Now, I will coat the plywood spacer with epoxy and let it set up over night. God, what a Kabuki Dance this has been!.

You got me, at least on the .01 degrees! I calculated the tip dihedral to be 23.52 degrees. I wanted the dihedral rib to split this angle, so the center section tip rib needed to inclined 11.76 degrees (a calculated number). Taking 90.00-11.76, you get 78.24 degrees. I went to the bottom of the wing plan and drew in an angle of 78.25 using my K&E protractor (see Picture 1). Your right, the best I could do was 78.25 degrees, by splitting the two division lines, which is .01 degrees too much. Then I cut a 78.25 degree template out of 1/4" balsa (see Picture 2), I used this template to jig the angle that the tip rib was glued in at. Once this was dry, I trimmed the spar, L.E., and T.E. tip over hang and sanded the edge ribs until every thing was flush and rechecked the tip rib angle with the template. (see Picture 3).

The truth of the matter is that the resulting angle is probably somewhere between 11.6 and 11.8 degrees, which is close enough for gluing purposes. My requirement was to get the angle of the ends of the spar, L.E., and T.E. as close as practical. When the tip panel is glued to center panel, angular measurement will not be used. Instead, a jig will be built to elevate the wing tip to the correct vertical or rise dimension. What little angular differences there are in the end joints will compensated for in the gluing process.