RF Vs MS - Part 2
Background:
In my previous article, I tried to highlight how stress analysis stands as a cornerstone of structural integrity in the airframe design and how the two giants (A-Company and B-Company) ensure that their aircrafts can withstand the rigors of flight. ?
Distinct Approaches to the same set of Analyses
As I hinted in my previous article, to delve deeper into the intricacies of stress analysis, we can look at the two distinct classes of problems addressed by both Airbus and Boeing - Static and F&DT (fatigue & damage tolerance). Using their own system of units, whether it is a static or F&DT stress check, both the A-Company and B-Company arrive at their RF and MS respectively. These critical safety calculations are derived from their historically developed in-house methods, processes and tools which are validated via rigorous testing.
These are not mere calculation methods, but structural engineering “philosophies” . They are collected in repositories of company-specific stress analysis handbooks, as revered as a Bible, which I will try to compare and contrast in this part of the article. I would certainly keep in mind the proprietary boundaries of each company while doing this (a definitely tight-rope walk)!
Static Analysis:
So, as it turns out, a Bible for the A-Company to refer to for a typical static strength calculation is the “Handbuch Struktur Berechnung (HSB)”, that is simply German for “Structural Calculation Manual”. The B-Company’s Bibles have the more familiar English titles, like the “Boeing Design Manuals (BDM)”, backed-up by their multitude of in-house tools like the GK-Joint, Lestab, etc;
领英推荐
If we look at these handbooks from a stress analyst’s perspective who is methods-agnostic, we may find an overwhelming barrage of deliberately encrypted names (due to the competition, perhaps?) used for almost everything that goes into a calculation. For example, both companies tend to have customized and often patented material names (whether metallic or composite), a number of safety-factors for allowable strength of those materials (or to be used during calculations), customized bolts/rivets/fastener types,? etc; all of which are in turn governed by a set of “holy commandments” (read design-rules), fancy charts, tables and all that jazz until you arrive at the final RF or MS.
F&DT Analysis:
A similar mystery surrounds Fatigue and Damage Tolerance. The A-Company deploys methodologies documented in Manuals like M2841 and in-house software tools like the Spectrum generation Modules for assessing the fatigue life or crack propagation modules within their ISAMI suite of tools (notoriously known as “The black-box” by the stress fraternity!). The B-Company’s calculations are based on the methods and tools documented in their cryptically named “Book-2”, for instance.
Airbus calculates an AFI (Airbus Fatigue Index) based on a DSG (Design Service Goal) which is nothing but the expected number of GAG (Ground-Air-Ground) flight cycles its aircraft should survive during its life. They have what I call the “fancy factors” F1, F2, F3, …, F7 to be combined with the stress concentration factor Kt and arrive at the final AFI. On the other hand, Boeing takes a different approach, utilizing methodologies from their "Books”, to calculate their? FRFs (Fatigue Rating Factors) and DFRs (Detailed Fatigue Ratings), combined with their “less fancier” factors A, B, C, D, E, U which all finally end up in a Fatigue MS. Boeing's F&DT methodology, in my view, exudes a lot of redundancy in terms of employing safety factors that often seem to exceed regulatory requirements.
Food for Thought
All these intriguing conundrums add an extra layer of complexity to the comparison of methods, sparking my curiosity further to find out if these seemingly divergent approaches can lead to a comparable safety metric between the two. Maybe, I should delve even deeper into the topic next time, by picking up one example problem each from the Static and F&DT analysis and find out the corresponding RF as per A-Company and MS as per B-Company. That should tell how close they are,? qualitatively as well as quantitatively.
In Conclusion:
As I try to take this panoramic view of the world of aerospace stress analysis, the contrast between Airbus and Boeing methodologies clearly sparks curiosity. While their approaches may diverge, they share a profound commitment to the core principles of aerospace engineering which is like a marriage of art and science.? So, the next time you fly in or simply gaze upon the majestic flight of A-Company or B-Company, try to appreciate the brilliance of these stress analysis methodologies/philosophies and the extraordinary minds behind them that ensure one common goal in the end which is of paramount importance: safeguarding the lives of those aboard their aircraft.