I’ve been working on expanding my Turbomachinery Design Library, which is written in Python for Blender. I’m working on generating what some in the RC world would call a multi-stage ducted fan. In reality it is better called a multi-stage axial compressor.
There are no multi-stage axial compressors on the market for RC applications. The reason why, as I understand it, is simple: efficiency. The same amount of thrust can be achieved at higher efficiencies with a larger diameter ducted fan. However, I can envision a reason why one would want a multi-stage axial compressor over a single stage ducted fan.
An electric axial compressor is analogous to a Turbojet, while an electric ducted fan is like a Turbofan. The reason the airline industry today is powering their airliners with turbofans over the turbojets used in the 60’s is because the same amount of thrust can be generated more efficiently using the larger diameter fans.
The advantage of multi-stage axial compressors over single-stage ducted fans is the same thrust can be delivered in a smaller package, allowing smaller airframes, reducing drag and allowing higher flight speeds to be achieved. This is why most jet fighters today are turbojet powered. If efficiency isn’t your primary concern and speed and manoeuvrability is, a turbojet, or electric axial compressor, may be the solution.
Before the advent of 3D printers, designing and prototyping turbomachinery has been outside the grasp of the masses. I’m a very lucky engineer to be afforded such an important ability.
How many people will be interested in printing/using this? Probably not that many judging on my poll. But, it’s interesting enough for me to pursue out of personal interest.
You know I am in! Doing things with 3d printers that wasn’t possible before is an obsession. I have no skills like yours, but I’m down to print planes or quads that utilize this- efficiency be damned.
Is there a worlds speed record for a 3d printed plane?
I’m not sure how to best design a turbocharger. However, every turbocharger I’ve seen uses a mixed flow compressor which can achieve higher compression ratios per stage than an axial flow compressor. There’s likely a good reason behind their use.
I could quote you the flow rate this 64mm OD axial flow compressor is being designed for… Pressure isn’t much higher than atmospheric.
So, multi-copters are naturally inefficient, especially at hover where the advance ratio is zero. Some general rules that I understand is that the larger the prop diameter, or the more blades a propeller has, the more efficient it will be. Ducted fans tend to be less efficient compared to an equivalent thrust propeller, likely because tend to be very small diameter spinning at much higher RPMs. Also, unducted propeller have efficiency losses due to tip losses, tip vortices: http://upload.wikimedia.org/wikipedia/commons/1/1e/Tip_vortex_rollup.png The ducts for ducted fans and ducted propellers help reduce these tip losses, making the propellers more efficient.
If I was concerned about improving multi-copter efficiency, I’d probably look into larger diameter, lower RPM, ducted propellers.
That human powered multi rotor had super slow props. It would be fun to do a super efficient light weight quad with ridiculously slow props and see how long you could keep it in the air.
So if slower is better, at 100% efficiency, the props are stationary. Cool.
@Andre_Roy It wasn’t so much efficiency (although that would be a huge plus) as it was weight savings, but you bring up valid points. I may end up with a duct ala the HK’s from Terminator. However I still would like to try your design on more conventional aircraft.
That reminds me of a quad I saw that had a single motor driving the props via belts. The props were about the size you’d expect, but variable pitch (including reverse thrust) for acrobatics. For the price of that complexity, it seems one could design with larger props for efficiency, but maintain maneuverability.
The one I saw was proprietary. I know MIT has worked with variable pitch quads, but I have no idea if they open sourced any of it. I believe both use pitch control hardware from R/C helis.
Efficiency is dependent on many things. All I can tell you with any confidence is that if you are seeking efficiency this is likely not the solution.
Unfortunately, there is no the time or money for CFD. Maybe someday. If someone wanted to collaborate with me and provide that type of support, I’d be thrilled. For now, I’m just using a mean line analysis and will do your traditional bench top and in flight testing. I doubt most off the shelf RC ducted fans and props are ever analysed in CFD. Efficiency and optimal design isn’t normally the highest priority to RC aeromodellers.
