I am wondering if it would work to have a fan with two or three blades which have short-ish span and very long chord length, like half a rotation for each blade. Would a shroud (that is molded on the tip of the blades) create/solve any problems? I am also thinking about tandem blades, but that would only work with shorter chord lengths, correct?

The main objective is to keep noise to a minimum.

I have no background in aerodynamics, just been reading up about it for a while. So please try to dumb it down where possible.

Edit: this is what I came up with, taking all the advice I've received so far. https://www.thingiverse.com/thing:4069048 Any ideas on improvements?

  • $\begingroup$ You should transform the title in a question as this is standard in this Q&A website. $\endgroup$ – Manu H Dec 22 '19 at 21:11
  • $\begingroup$ Related: aviation.stackexchange.com/q/67177 $\endgroup$ – Koyovis Dec 22 '19 at 23:15
  • $\begingroup$ Early jet fan blades are just like that, long and narrow, but it turns out they are fragile and vulnerable to bird strike, so the industry moved to wider blades. $\endgroup$ – user3528438 Dec 23 '19 at 17:51
  • $\begingroup$ @Rudi, what is your overall objective? (Besides noise reduction) Are you designing a jet engine? $\endgroup$ – Michael Hall Dec 23 '19 at 18:23
  • $\begingroup$ @Michael Hall, I am designing a quiet desktop fan blade that still moves a descent amount of air. I plan to power it either with a 2 pole (2900 rpm) or 4 pole (1450 rpm) ac induction motor, depending on what speed I need. $\endgroup$ – Rudi Dec 24 '19 at 9:29

The main objective of the engine is to generate required power $\text{P}$, which is:

$$\text{F} \times \frac{D}{T}$$

where $F$ is force, $D$ is distance, and $T$ is time.

Power generated by modern jets through continuous combustion of fuel dwarfs their piston predecessors.

Next up is efficiency. As in wings, longer, thinner blades do a better job converting this power. A typical measurement is the thrust-specific fuel consumption:

$$\frac{g}{(s \cdot kN)}$$

which is fuel flow (fuel over time) per thrust.

Piston engines went from 2 to 3 to 4 blades, enabling more thrust output, but the "wake", or turbulence generated from one blade lowers the efficiency of the next one following it. 1 blade is most efficient, but to get more thrust from the more powerful engines, some efficiency is sacrificed. Fan jets follow this trend with many blades.

But short blades with a very long chord would be hopelessly inefficient for generating thrust in air. A very good example of this thinking is the modern windmill design, with fewer, longer blades than the old style mills. The faster you turn, the more you want to be like the modern mill. The turboprop makes the most of propeller efficiency and turbine power but:

All types of props or ducted fans reach a speed limit where their own drag makes them less efficient than low bypass turbojets. Propellers reach this limit before fans do, which is why we see fans on longer range airliners.

So we did not get to noise reduction with this concept, which is relegated to desk top fans.

Modern jets do have some success with noise reduction by controlling their fan blade tip speeds, and by improving the design of their containment housings (nacelles).

  • $\begingroup$ Thanks for all the info. So I now understand that a long chord would not be worthwhile in this situation. Would it work if I tried something like this: encrypted-tbn0.gstatic.com/… I keep thinking that the shroud would help a lot with noise reduction and this is my biggest objective. $\endgroup$ – Rudi Dec 23 '19 at 14:10
  • $\begingroup$ @Rudi certainly worth a try, it may improve thrust output as well (acting as a running blade (airfoil) tip "fence"), and thanks to ymb1 for edits. $\endgroup$ – Robert DiGiovanni Dec 23 '19 at 16:24
  • $\begingroup$ @Rudi especially if you could recess the ring into a groove in the nacelle airflow intake, which could effectively eliminate the "gap" between the blade tips and the intake wall. Stay on it. $\endgroup$ – Robert DiGiovanni Dec 23 '19 at 16:34
  • $\begingroup$ This is what I came up with so far thingiverse.com/thing:4069048 $\endgroup$ – Rudi Dec 26 '19 at 19:41

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