This page is a blog post of sorts. It won’t be around forever. It’s just a way for Benjamin Crews to document and potentially share his thoughts about all this.
Why?¶
An immediate question, if you’re not the sort to get excited about python code that automates structural analysis, is “Why are you doing this…?” And the answer is pretty simple: I’m a huge aircraft nerd, and this is a passion project of mine. I love advanced analytical concepts, dusting off old tomes of papers and documentation, and making tools that can help people with advanced engineering topics. Additionally, I truly believe that the aerospace industry would be better off if there were more open-source libraries and utilities tailored to our unique requirements, written by experts, and hopefully validated against existing data or scientific research. I don’t think I’m an expert, and I’m definitely no scientist, but I’ll do the best I can!
So, why did I choose this exact implementation and tool to work on… is this really needed in the industry? Well… things start to get more complicated here. The Society of Allied Weights Engineers (SAWE) publishes many great papers, standards, procedures, and handbooks. I’m still very new to Weights Engineering (I became an SAWE member in 2024), so I have a lot to learn here, but I hope through this project I can grow into an expert! In the world of Weights Engineering, there are many procedures to estimate the weight of aircraft and aircraft components. Most of these procedures, however, are rooted in historical data and many assumptions of design practices. They often fail to answer questions about how design changes affect the weight predictions.
Basic Weight Approximations¶
As an example, consider the basic equations from Raymer. Raymer offers an Approximate Group Weights method, which gives very simple formulas based on linear regression of historical aircraft data. All we need is the wetted areas of geometry, and an idea of the aircraft’s mission role. For a fighter aircraft this could work out as follows:
So that’s a bit of math, but in the end we end up with an answer kinda close to the empty weight of an F-16 Block 50 (18,900 lbs). Clearly it’s off, but it’s also incredibly simple and straight-forward. This is great for the early conceptual design phase, and especially when the structural engineers aren’t yet involved.
Stepping up in fidelity Raymer proposes a statistical approach with many more factors and inputs. This method is theoretically closer to the true value, but again assumes typical construction methods and standard design practices. In the evolving world of weight-critical structures and searches for efficiency, this becomes much less useful, much earlier in the design process. As an example, take the F-16 Horizontal Stabilizer: A design study was conducted in 1982 on the weight impacts of (at the time) advanced composite materials [But82]. This study shows that a non-honeycomb metallic stabilizer weighed approximately 167 lbs (compare that to half our previous estimate for a single stabilizer), but with advanced materials it could be reduced to 158 lbs! That’s 18 total lbs saved on the vehicle, which really matters for an aircraft with the mission role of outmaneuvering every other combatant in aerial combat!
To my knowledge, most weight approximation methods follow a similar procedure, trading off increasing mathematical and statistical complexity for increasingly accurate estimates. Statistical methods have factors for things like advanced materials, or configuration changes. This is one of the duties of the Weights Engineer during the Design Process.
Digging Deeper¶
At a certain point in the design cycle, structural designers start asking questions like “Is a stringer or longeron system more efficient? How close do these frames really need to be? What’s the weight tradeoff of this wing being mid-fuselage mounted vs bottom-mounted? How much weight gets added to the fuselage when we add an active sweep mechanism into the wing?” These questions are all quite complicated to answer and often require a (team of) Stress/Structural Engineer(s) to conduct detailed analysis studies over a much longer period of time than a simple factor lookup and summation of line items. And yet, these can be very important questions to the overall design! Often, teams can rely upon the expertise of those that came before, but that’s not always correct and not always available.
And this scene is where SWEEP comes in.
… the risk is high of adopting a less than optimum basic configuration at the very inception of a program, adversely affecting system effectiveness throughout the life of the design.
SWEEP’s documentation is extensive. I haven’t even read through it all. The executive summary, however, is the most useful at, well… summarizing what it does and why [Asc74]. In general, structural weight is a huge driving function of total vehicle weight, and therefore effectivity. To use existing methods, you need derived statistical parameters, relevant to your mission and flight profiles. What if you start pushing the limits beyond the data collected? Fly higher, fly faster, pull harder g’s, or even fly into the realm where thermomechanical effects begin to drive the structural sizing. You need an analytical solution instead of statistical one; or perhaps you could iteratively build planes on the fringe of your dataset and proceed to collect more data each time.
Additionally, in modern times AI/ML and Large Language Models are commonly used in place of statistical derivations and factors. These have the same problem. To build something beyond the existing dataset, you inherently assume that the trends continue. Maybe they do, but a physics-based approach will give more confidence in the estimates.
… but SWEEP already exists…?¶
Maybe! I certainly don’t have access to it. Given it was written in 1974, in FORTRAN, for some specific mainframe hardware systems, it’s unlikely that even with the original source code we could execute it. Updating this into a modern language opens up the tools and methods for today’s, and the future’s, aircraft designers and engineers! So far, I haven’t been able to find a legible copy of the original source code. With the original code, perhaps AI could convert it into a full executable library, but I have my doubts. So, this leaves us with a proverbial mountain of method documentation, and very long list of work to cobble pieces off that mountain and build into some functional structure…
I selected python as the language for this project, because I’m fluent and love it. If python’s execution speed becomes an issue, such that languages like C# or Julia need to be considered, this project is already a huge success. Until that time, python should be fast enough and have enough features to execute what I need.
SWEEP is documented and archived by the Defence Technical Information Center. Search for the relevant technical number, or maybe the links below will help… [Asc74] [Hay74a] [Hay74b] [Hay74i] [Wil74a] [Wil74c] [Har74] [Cha74] [CHM74] [Hay74c] [Hay74g] [Hay74d] [Hay74j] [Gay74] [Hay74e] [Hay74h] [Hay74f] [Hiy74b] [Hiy74a] [Mar74] [All74] [Wil74b]
References¶
R Allen. A Structural Weight Estimation Program (sweep) for Aircraft. Volume IX - User's Manual. Appendix A. Technical Report, Rockwell International Inc., 1974.
L. Ascani. A Structural Weight Estimation Program (sweep) for Aircraft. Volume I - Executive Summary. Technical Report, Rockwell International Inc., 1974. URL: https://apps.dtic.mil/sti/pdfs/ADA002850.pdf (visited on 2025-01-21).
David N. Buther. Non-Honeycomb F-16 Horizontal Stabilizer Structural Design. Technical Report ICAS-82-2.4.3, General Dynamics, Fort Worth, TX, 1982. URL: http://www.icas.org/icas_archive/ICAS1982/ICAS-82-2.4.3.pdf (visited on 2025-02-04).
D Chaloff. A Structural Weight Estimation Program (sweep) for Aircraft. Volume V - Air Induction System and Landing Gear Mod- Ules. Part 1: Air Induction System Module. Technical Report, Rockwell International Inc., 1974.
D. Chaloff, R. Hiyama, and C. Martindale. A Structural Weight Estimation Program (sweep) for Aircraft. Volume V - Air Induction System and Landing Gear Modules. Part 2: Landing Gear Module:. Technical Report ADA002858, Rockwell International Inc., Fort Belvoir, VA, June 1974. URL: http://www.dtic.mil/docs/citations/ADA002858 (visited on 2025-01-21), doi:10.21236/ADA002858.
G Gayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Appendix F: Program Listings, Overlays (9,0), (10,0) and (18, 0). Technical Report, Rockwell International Inc., 1974.
H. Haroldson. ADA002856. Technical Report, Rockwell International Inc., 1974. URL: https://apps.dtic.mil/sti/pdfs/ADA002856.pdf (visited on 2025-01-21).
G Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume II - Program Integration and Data Management Module. Appendix A: Data Management Module Flow Charts and Fortran Lists. Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume II Program Integration and Data Manage- Ment Module. Part It Program Integration. Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Appendix A: Gen- Eral Information for Module Flow Charts and Listings. Appendix B: Program Flow Charts, Overlays (8,0), (14,0), (15,0), (16,0) and (17,0). Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Appendix D: Program Flow Charts, Overlay (18,0). Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Book 1: Technical Discussion, Sections I and II. Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Book 3: Technical Discussion, Section V. Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Pro- Gram (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Appendix C Program Flow Charts, Overlays (9.0) and (10,0). Technical Report, Rockwell International Inc., 1974.
G Hayase. A Structural Weight Estimation Pro- Gram (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Book 2: Technical Discussion, Sections III and IV. Technical Report, Rockwell International Inc., 1974.
G Hayase. ADA002853. Technical Report, Rockwell International Inc., 1974. URL: https://apps.dtic.mil/sti/pdfs/ADA002853.pdf (visited on 2025-01-21).
G. Hayase. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VI - Wing and Empennage Module. Appendix E: Program Listings, Overlays (8,0), (14, 0), (15,0), (16,0), and (17,0):. Technical Report ADA002862, Rockwell International Inc., Fort Belvoir, VA, June 1974. URL: http://www.dtic.mil/docs/citations/ADA002862 (visited on 2025-01-21), doi:10.21236/ADA002862.
R Hiyama. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VII - Fuselage Module. Appendix a; Module Flow Charts and Fortran Lists. Appendix B: Fuselage Module Sample Output. Technical Report, Rockwell International Inc., 1974.
R. Hiyama. A Structural Weight Estimation Program (sweep) for Aircraft. Volume VII - Fuselage Module. Technical Report ADA002867, Rockwell International Inc., Fort Belvoir, VA, June 1974. URL: http://www.dtic.mil/docs/citations/ADA002867 (visited on 2025-01-21), doi:10.21236/ADA002867.
C Martindale. A Structural Weight Estimation Pro- Gram (sweep) for Aircraft. Volume VIII Programmer's Manual. Technical Report, Rockwell International Inc., 1974.
P Wildermuth. A Structural Weight Estimation Program (sweep) for Aircraft. Volume III - Air- Loads Estimation Module. Technical Report, Rockwell International Inc., 1974.
P Wildermuth. A Structural Weight Estimation Program (sweep) for Aircraft. Volume XI - Flexible Airloads Stand-Alone Program. Technical Report, Rockwell International Inc., 1974.
P Wildermuth. A Structural Weight Estimation Pro- Gram (sweep) for Aircraft. Volume III - Airloads Estimation Module. Appendix A: Module Flow Charts and Fortran Lists. Appendix B: Sample Output. Technical Report, Rockwell International Inc., 1974.