February 7th, 2018
In this post, we will discuss the three main methods and how they differ from each other.
This is one of the most common methods for VIP completions, business jets, some major commercial jet OEMs and other aircraft cabin integrators. In this method, a detailed stress analysis specifications and guidelines document is generally provided by the customer or integrator for the stress engineers to strictly follow.
This document may be known as the “Structural Design Criteria (SDC)”, or it may have other names such as “Stress Analysis Control Document (SACD)”.
This document will generally list:
Adhering to this document is required in order to satisfy the approving authorities and the customer’s internal procedures for compliance. Full-scale structural tests of the aircraft cabin are not required in this method. However, tested allowable loads are quite common. It is common for the customer or integrator to provide standard installations and allowable load test results on various approved components. Industry standard methods are used for analysis purposes. These are known as classical hand calculations.
Aircraft structure interface loads are extracted using the FEM models. Then classical hand calculations are used for writing safety margins for all the critical components along the load path to the aircraft structure, based on historically accepted reference materials or books such as Bruhn, Roarks, Niu, and MMPDS.
Various load and material or special uncertainty factors are accounted for in the margin calculations. Finally, everything is documented in the form of an approved report format and then submitted as part of the entire certification package.
A majority of the activities are similar in terms of building a finite element model and calculating aircraft attachment interface loads. The differences are as follows:
There will be cases in which the structural modifications to an existing unit or a new unit similar to a certified unit are not major enough to require testing.
In such cases, the previously certified unit’s report contents, tested component allowable loads, materials, and full-scale test results can be used to present a logical argument.
The substantiation for the changes can be done by comparison and by demonstrating that the new unit, or changes to an existing unit, do not alter the overall load path significantly. It is also necessary to demonstrate that they do not result in the overall weight increase compared to the certified weight.
Again, all analyses are documented in approved report formats and then submitted as part of the entire certification package.
If you are interested in reading more about FEA and stress analyses, you can download a free eBook here.
We also invite you to check out the Stress Ebook LLC. blog and learn more about aerospace engineering and FEA.
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