Aluminum wing spar to cabin floor attachment


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Jonny C

Jun 17, 2014
London, UK
Hi everyone, I'm doing some drawings for a conventional aluminum low-wing single seat plane that could be easily made with my CNC router.

For simplicity, I would like the center section of the wing to sit on top of the lower longerons. The issue of restraining the L angle of the lower spar cap then arises.

In the attached pic there's a sketch showing the spar (grey with green caps), lower longeron (pink) and a red formed hat section which is riveted to the floor skin and the lower spar cap. Does anyone know if this is a viable method for restraining the lower spar cap? Has any other design used a similar method?

The alternative would be to have a break in the longerons so the wing spar sits directly on the floor skin, with a splice section on the outside of the floor skin (shown in the drawing spar 2). Thanks for any thoughts on this.



Well-Known Member
Dec 9, 2004
Hamilton New Zealand
So this is a 3 peice wing, 2 sings and a built in centre section, I take it. I have only seen design that interrupts the longerons in GA for single engine GA, most designs don't tolerate the extra depth in the cabin.

There seem to be two solutions to the external doubler on the longeron,
  • one uses a hat section that goes all the way to the lower engine mounts, eliminating the internal longeron fwd of the spar allowing more fuselarge shaping / foot space
  • The other is basically your concept
There isn't much of a structural requirement to tie the spar to the lower skin normally, by the time one has designed for the the vertical shear loads to be picked up, there normally is sufficient capacity for the drag loads as well.


Super Moderator
Staff member
Log Member
Oct 18, 2003
Saline Michigan
What is the point in tying the spar to the lower skins? Usually, there is not much reason to do so...

The really big thing that is going on is the beam that is the fuselage is attached to the beam that is the wing. Since the fuselage beam is a really deep spar with rather modest stresses while the wing is a rather shallow beam with much bigger stresses, the wing spar is primary, the wing spar through the fuselage moves as it does. If the fuselage belly skin is attached to the spar it moves with the spar. The really big tie in between the fuselage and wing is made by reinforcing the fuselage walls and transferring load through that reinforce.

The common schemes include bridging from firewall to behind the spar. Look at the belly of the entire Cherokee to Seneca line. Four hat sections longitudinally below the belly. Then they have a beefy spar carry through built into the fuselage walls.

The scheme with the hat section tying the spar caps to the belly ... The hat section and bottom skin then become part of the beam. Revisit the parallel axis theory for computing I of the beam, computing the beam centroid for the neutral axis, and the y dimensions up and down from neutral axis. I am anticipating this hat section below the spar and the belly skin shifts the neutral axis, y's get quite a bit bigger, but I does not increase much, so peak stresses are greatly increased. UGH!

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