Parts sent off for manufacture!


After I showed a few fan gurus my previous post on iteration pU (and after i was nearly done nesting the section for lasercutting), I was given a few suggestions, which really have the potential to make the design a lot better.

-Banana blade: one of my designs had the centre of gravity behind the pitch axis of the blade and i had an almost 30% higher root bending moment which is not good. This is because of the centrifugal forces- imagine spinning around holding onto the handle of something long like a gold club- if you try bend it out of position it will resist that. For iteration pU I moved the spar of the blade back as far as practical (because the blade gets thinner further back, and you need thickness to resist bending), and this reduced the root bending moment a lot. I forgot you can also use this force to your advantage- for instance if you put dihedral on the blade (bend it forwards) then the aerodynamic and centrifugal forces start to cancel each other out! Its not quite something for nothing though, because it means you have to make the blade a more complicated shape. Its not mega obvious in the side by side CAD comparison, but this small change reduced the root stresses significantly ~20%. Which is definitely worth a little wait! The lower the stresses, the less likely any catastrophic failures will be, and the longer the fan will last.

-Blockage effects: the aerodynamic code i am using to design the ducted fan system was never fully finished by the authors, i think because some US government program was cancelled or something (not that im complaining because 10 years later anyone can download it for free). But anyway it doesnt take one effect into account which can result in torque over estimates around 3-5% depending on how many fan blades you have and how thick they are (worse error for more thicker blades- ie high solidity). The basic point is that my blades may be too wide and with not enough angle of attack by a small margin especially near the root. The reason is that the code assumes the blades are flat sheets with no volume, but in reality they have a finite thickness, and as a result they constrict the airflow as it passes through the fan. Imagine a venturi, where the air flows quicker through the tightest part. This venturi effect is very small at the tips, where the blades are very thin compared with the area they travel through, but close to the root where the blades are thicker, and the diameter that they travel through is smaller, the effect may be significant.

Since the pitch is adjustable on my blades, this would be easy to correct, however it means that the blade wont be running quite as efficiently as designed.

Posted in: Fan design, General on December 11th by admin


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