Cutout in lower wing skin: How can I hand calc this?

[Ab1869]

Hello,

Let's say I have this composite wing structure: center box spar, aft spar, ribs and a upper/lower wing skin all bonded together. I want to make a rectangular cutout on the lower wing skin and either show this is good or size an externally bonded "donut" doubler around the cutout, all via hand calcs.

I used Example A5.12 in Bruhn to generate the attached shear force and bending moment diagrams about different stations of the wing (Mx is about the chord and My is about the spanwise axis of the wing). These are resolved about an axis 40% of the chord. From here, I'm not sure how to proceed.

I'd assume the skin in this area would be experiencing some combined loading: in-plane tension due to bending, as well as some shear due to torsion.

Would I go to A15 in Bruhn (shear flow in closed-thin walled section) and analyze the stresses prior to the cutout in this cell? But once I introduce the cutout, it's no longer a closed-section right? How would I compare the pristine to the cutout cell? Or is this a plates/shells problem?

This is all theoretical and none of this is going to fly. I'm just working on this out of personal curiosity and possibly a project to show on a portfolio. I went to school for mechanical engineering so roleplaying as an aero engineer here. I appreciate any guidance you could provide!

 

[rb1957]

I'm sure a repair could be designed, but it would be extensive to analyze, and I doubt a hand calc could do it ...not without massive assumptions.

1st, what are the ply orientations and lay-up of the wing skin ?

2nd, what are the original loads, even the internal loads for a couple bays around this cut-out (which may be easier to get (or to use) than the external loads on the wing ?

3rd, your doubler orientations and lay-up ? I would have something like 3x the size of the cut-out (length and breadth) tapering plies to the edges. I would have an internal doubler and the cut-out cover would attach to this (smooth OML).

4th, wet wing ?

5th, you could design the cut-out cover to be effective (quite hard to do) and that would minimise the doubler (to a local splice). Recognise I don't know why you're doing this, what you're trying to achieve with the cut-out ...

6th, DTA ??

7th, that's just a start !!

 

[Ng2020]

wing torsional stiffness would be severely impacted, as the aft cell is no longer a closed cell for this bay. this would affect aeroelastic characteristics of the wing involving the torsional mode (divergence, ctrl reversal, flutter analyses need to be revisited).

Torque in this bay would be carried by the forward cell, and differential bending of the forward cell and aft spar. So the forward cell and aft spar margins of saftey would need to be recalculated for the revised internal loads.

The external doubler would never restore the original strength and stiffness margins, and any significant external doubler is not likely to make the aerodynamicists happy.