PA-18 wing geometry

[Msbm4220]

Working on a pa-18 wing that was partially built. I decided to take the wing apart and start over for a few reasons.

In measuring/locating holes for all things drag struts, butt hardware, strut attach points, rib locations and looking at the piper drawings and sturm's drawings I made a discovery which is baffling. I'm sure this confusing only because I don't know the whys....

From what I have seen (which is not much compared to others) quite a few people build wings on sawhorses. Ie, a FLAT resting place so the bottom of the front and rear spars would rest on the sawhorse. And some hieght adjustment would be made to the wing tip rear spar for washout. Makes sense.

Looking at drawing 10645 you can see where the rear spar sits quite a bit lower than the front spar even though this illustration is in a vertical presentation.

what I find confusing is, why the ribs are constructed with what appears to be a flat bottom and the voids where the rib slides over the front and rear spars are SQUARE to the bottom edge of the ribs leading me to believe the bottom edges of the front and rear spar should be on the same plane. Yet according to the drawings they are not on the same geometric plane.

Can anyone provide pics, explanations to this.

thanks in advance...

-MB

[Msbm4220]

After further review….I see that drag strut 5 has the same measurement from the bottom of both front and rear spars. The number 1 drag strut is 19/32 higher from the bottom of rear spar.

Just seems counterintuitive with respect to washout…

inquiring minds

-MB

[DENNY]

The best way I can explain it might to be designed with a slide rule, pattern built with a micrometer, fit with a hammer. If you look at piper drawings the have quite a bit of + or - built into the fuselage. Wing washout is also adjusted to fit each plane. Given all the possible angles each rib may be exposed to it is best to have some play in the ribs so they can adapt to what every washout is put in the wing, from a production standpoint that square was simple and fast. There is a old thread about washout that gos into pretty good detail and talks about the drag strut locations.
DENNY

[skywagon8a]

After further review….I see that drag strut 5 has the same measurement from the bottom of both front and rear spars. The number 1 drag strut is 19/32 higher from the bottom of rear spar.

Just seems counterintuitive with respect to washout…

inquiring minds

-MB

Think about this for a minute. In relation to the front spar the rear spar is higher at the tip than it is at the root. Thus the wash out is built it. Wash out = wing twist at tip nose down. Wash in = wing twist at tip nose up. Wash in = increased angle.

[Steve Pierce]

Assemble the wing flat up to trammeling. Once the wing is trammeled then add the washout block under the rear spar at the outboard aileron bay rib and install leading edges and false spar. I am just a dumb ass mechanic and that method has worked for me on more wings than I can count.