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Multi stage EDF

#1
I know EDF's are not like a jet engine, but if you added more then one set of blades onto the driveshaft, would that increase thrust? My design is an internal motor with outflow going through 90 degree ducting so it will theoretically have more back pressure. If you connected two or more stages of blades in series to one driveshaft, would there be an advantage in an application with back pressure, or even open wing mounted designs?
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#2
I know EDF's are not like a jet engine, but if you added more then one set of blades onto the driveshaft, would that increase thrust? My design is an internal motor with outflow going through 90 degree ducting so it will theoretically have more back pressure. If you connected two or more stages of blades in series to one driveshaft, would there be an advantage in an application with back pressure, or even open wing mounted designs?
Adding a second set of blades will definitely increase the load on the motor and be effectively like fitting a single fan unit with far more blades. Try it if you wish but please keep a look out for smoke.

Tandem fan units, (one EDF unit mounted behind the first) can theoretically allow the second unit to run a higher pitch set of blades or similar at higher RPM because the pressure at the inlet is already high and pushing the blades at low rotational speed. Where tandem units are used the second motor needs to be significantly higher Kv to make use of the compression efforts of the first unit.

Have fun!
 

Mid7night

Jetman
Mentor
#3
Great question! And not surprisingly, not a new one actually. This was investigated by the guy at http://www.vasamodel.cz/, but I can't find links to his dual-fan stuff anymore. It also was done by a guy named Ralf Dvorak, example here: https://www.rcgroups.com/forums/showthread.php?262808-Electric-Ducted-Fans-November-2000

Short answer: The power generated by this complex setup was soon overtaken by the increase power of new brushless tech, and the complexities of a twin-fan setup were more trouble than they were worth.

I also tried this myself on my university senior project: We attempted to make a twin-inline ducted fan. And we did, and it flew! HOWEVER: The total measured thrust of our twin-inline EDF was only 1.5lbs! For TWO 65mm ducted fans! That's less than just one nowadays.

Now maybe we didn't design it properly (for sure), and maybe it could still be made to work (possible), but it will require quite a bit of trial and error to test and prove the optimal arrangement of blade pitch, motor RPM and the balance between the two. Just putting two fan rotors on a common shaft won't buy you anything.

Anyway, welcome aboard! Don't be scared off by all us grouchy naysayers who think we know it all cuz we've tried and failed...try stuff for yourself and maybe you'll find something others missed! Definitely learn from the mistakes others have made, but don't be afraid to try something different. :cool:
 

PsyBorg

Wake up! Time to fly!
Mentor
#4
Why not just run multiple fan sets on planetary gears and make a high compression super fan drive... gear ratios area wonderful thing.
 
#5
I'm not sure a geared setup would amount to much if anything in gained thrust.
Your first stage would have to move enough air to “feed“ your second stage or your second stage would have to work against the vacuum generated between both stages. At the same time overcharging the space inbetween your stages would net you less power because the first stage would need to work against the resistance of the second one...

I'm unsure if the fan part of a turbofan engine which is the bit you'd be emulating is multistaged...
 
#6
The idea behind multiple turbine stages is to boost pressure at each stage prior to the combustion chamber. Higher pressure into the combustion chamber = more thrust. Not sure if the same concept would apply to a EDF since thrust is not produced by burning jet fuel in a combustion chamber.

If you add more blades to the EDF to increase flow, you also increase parasitic drag through the unit, which means you need even more power to overcome that. Eventually you reach a stage where the battery required to drive the unit gets too heavy for the amount of thrust you generate. You're better off operating the EDF at it's max efficiency and if you need more thrust, add another EDF. Max efficiency will give you the best thrust vs battery weight.