Occasionally we get to start a project with a blank sheet of paper. But, in reality most Aerospace companies fail to optimize correctly at PDR and we are left with changing configuration half way thru the program.
BACKGROUNDWe need to reconfigure the stringers, ribs and skin thickness. The ideal is to do this with a FEM, but first we need to know the new panel config to build a new weight savings FEM. We could build a smeared model to create flexibility, but at this stage in the program its not a necessity because we know the running loads at each of the stations.
The Non FEM methods can be very useful because we know the running loads in our panels and we know the total load in our stiffener configuration.
We can build a non FEM Hypersizer database that will provide solutions with a relatively good accuracy. The existing (Non optimized FEM) will most likely have two parameters in the output.
LOADS WE RECOVER FROM THE NON OPTIMIZED FEMSkin running load lb/ in (From shell elements)
Stiffener axial load (lb) (From bar or rod elements).
On occasion the stiffener is modeled with shell elements, but for a loads model this is NOT a good solution because it complicates the post processing task with NO net benefit for the stress engineer. I want to know the total stiffener applied load.
MAKE LIFE A LITTLE EASIERThe FEM outputs the stiffener total load (bar or rod element). A stress person is ALWAYS interested in knowing the TOTAL LOAD (lb) in the stiffener. If we look at the STRESS TAB on project sizing form the resultant panel internal loads are shown in the form of lb/in for each COMPONENT (Skin , Cap, web etc).
lb/in is appropriate for the skin. Stress engineers discuss running loads in skins in the form of lb/in and the FEM outputs loads as lb/in in the skin.
The stiffener needs a different approach. Stating loads in the web or cap in lb/in is kinda OK, nice to know, but not very useful. In reality we would prefer the axial load in the stiffener elements. We could quote load in the cap, axial load in the web and total load in the stiffener. I am only interested in moments on the whole section.
REQUESTCould Collier modify the stress TAB and show the total load in the stiffener ? This is the data a stress person wants.
The reality is we need three loads in the summary
Total panel load
Total skin load
Total stiffener load
