 |
This is the wing tip portion of the Scratch One electric-powered RC sailplane design. Note that the three spars have been tapered down as they approach the tip. Although it implies in this photo that the front and rear spars were tapered to 3/16 inch in height and the middle or main spar to 1/4 inch in height, I actually tapered all three spars to 3/16 inch to keep it simple. That is the way it is stated on the full-size plans. |
Q-15: A reader questioned the tapering of the wing spars on my Scratvh-One design. He had read the plans to indicate that the front 1/16 X 5/16 (high) spar tapered down by 3/16 inch; that left only 1/8 inch of material on the spar at the tip.
A-15: I have to agree this came out a little confusing. Our reader actually read the plan notes incorrectly. The front 1/16 X 5/16 spar is suppose to taper down to 3/16 of an inch at the tip. That means I’m only reducing the height by 1/8 inch. There is a difference when reducing "by" and reducing "to". I E-mailed an extra photo to the reader and he thought that was helpful so I will include it here.
The entire idea of tapering the three spars was to be able to thin the airfoil as it approaches the wing tip. Had I left the three spars a constant height, the airfoil shape going towards the tip would have gotten progressively thicker.
You can see my hand notes in the photo. Originally I had the front spar tapering down to 3/16, the middle (main) spar to 1/4 inch, and the rear spar to 3/16 inch. On the final plans I kept it simple and just indicated all three spars should taper to 3/16 inch. The resulting tip airfoil is a little flat, but that is much better than having a thicker airfoil at the tip.
My reader friend also wrote that he couldn’t find any 1/16 x 5/16 balsa since it isn’t a standard size. It is easy to just cut out your own spars from sheet-balsa material using a long, steel straight edge and an X-Acto knife. You might mess up a few spars on your first tries, but I’ll bet you will quickly get the hang of it.