As many of you know, I recently returned to school to complete my engineering degrees; and simultaneously began researching small footprint turbofan engine technologies. As none of this in theory has anything to do with modeling (or fun), I haven’t had a lot of time to write. Modeling this engine is now on my mind, and so is the question of Top-Down or Bottom-Up?

# Top-Down or Bottom-Up?

We are nearing the end of the basic mathematical model. I have begun considering how I would bring this beast to life in a digital model; top-down or bottom-up. I am a controlled person and like designing within limits; top-down gives me precisely that. However, in this case I am not so sure that torturing myself by bringing an adaptable model to bear is a good idea, where the limits are still quite uncertain.

Image Courtesy of Stinging Eyes

# The Problem

Stage count is the fly in the ointment. For example, I think we will have 6 low-pressure (LP) fan stages (yes, 6 bloody stages), but am still trying to reduce it to 4. The High Pressure (HP) compressor is equally ugly, and the HP and LP turbines are dependent on the compressor models. If I do not know how many stages will be involved, I will have to constantly return to the master skeleton, and adjust the coefficients to adjust how much space I have to work with. Ugghh!!

However, if I let go of my notions about “how to design something properly”, I might find an easier way. Perhaps if I made my design sections as CAD models with named planes at either end, I can join the design sections end to end, in a bottom-up approach. It will be easier to keep all my section specific design constraints in the respective design sections. Later when I do the bypass and exterior skin sections, I can import key surfaces from each section to work from, and build outward.

# Conclusion

Top-down or Bottom Up is one of the first questions I ask prior to modeling. I rarely decide against a master skeleton approach. Moreover building outward is my big fear; since this entire project involves a very small cross sectional dimension limit, it seems counterproductive to work outward. That said, it also seems insane at this point to try and monkey-fart the parameters about in a skeleton file to eventually fit sub-structures that can perform as intended. After much consideration, I think keeping the mathematical design limits tight, and then building the structure outward from the HP compressor will allow the greatest degree of flexibility, and the easiest model to alter, which we expect to do a lot until all aspects of the engine design have been played out.