BUCKLING ANALYSIS OF AIRCRAFT WING

[bhagav_n]

Hi,
I am trying to perform buckling analysis of aircraft wing in MSC Nastran which is modelled in Patran. The wing consists of ribs, skin and spars. I divided lift load by planform area and applied on the wing upper surface as uniform pressure. The Root section nodes are fixed. While performing buckling analysis (SOL 105), Im getting negative buckling index. Can anyone clarify this?
The loading condition and results are attached here...

 

[bhagav_n]

The result image is uploaded here..

 

[SWComposites]

Yes, it means the structure buckles when the loads are reversed.