• J3-Cub.com is the largest community of J3-Cub pilots, owners and enthusiasts. With over 1000 active members, we have fostered a vibrant community and extensive knowledge base. J3-Cub.com hosts a library of over 13 years of technical discussions, J3 data, tutorials, plane builds, guides, technical manuals and more. J3-Cub.com also hosts an extensive library of J3-Cub photos.

    Access to the J3-Cub.com community is by subscription only. Membership is only $49.99/year or $6.99/month to gain access to this community and extensive unmatched library of knowledge.

    Click Here to Become a Subscribing Member

    You will also get two J3-Cub decals as well!

Mistery of cubs wing design details (J-3, PA-11)

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

byunghun54

Active Member
Joined
Feb 3, 2009
Messages
40
Reaction score
0
According to Piper drawings (for wooden spars and about same way for aluminum spars too), the
compression strut attach point at the front spar at the wing root is 1 29/32 hign
from the bottom of the front spar. For the rear attach point of the compression strut is 2
1/2 from the bottom of the rear spar. Rear spar is slightly raised (about 9/32") from the ref line to fit the design of the wing ribs due to concave bottom line of the ribs.
Therefore compression strut attach point at the rear spar is 2 25/32 high from the ref
line horisontally extended from the front spar bottom.

That means that the struts are raised 7/8" in elevation at the rear spar root mearsured from
the rib ref line which is vertical to both spars.

I examined all the compression fitting drawings and rib drawings so I am aware
that both spars and compression strut joined squarely and my description and
dementions are from the original factory drawings (some of them I myself scanned
from microfilm I obtained from Smithonian Museum) However anyone can discover this easily by carefully examining one of those PA-18 spar drawings.


I just wondered if anybody noticed such an unexpected location fo compression
strut during the (re)assembly of the original piper wings and thought about the
design philosophy of the wing.

Of course my initial thought was that Piper engineers intentionally put compression strut at staggering elevation on the spars to give a proper wing washout to creat the twisting moment at the root ( FYI at the wing tip the compression struts' elevation on front and rear spars are same if the 9/32 of concave feature of the wing is ignored).


However further examination of spar attatch points location at front and rear spar to the fuslage reveals that the above twisting moment created by compression strut is effectively nullified as these two spar butt to fuselage points are almost parellel to rib ref line instead of the compression struts' 7/8" offset ( 1.657 degrees off from the rib ref line) !!

Therefore, I may not ever found out the philosophy of the wing design but let me know if anybody had ever found this and given a deep thought about this.

 

Latest posts

Back
Top