Multi stage EDF

[Pnichols96]

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]

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]

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.

 

[PsyBorg]

Why not just run multiple fan sets on planetary gears and make a high compression super fan drive... gear ratios area wonderful thing.

 

[Corbarrad]

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...

 

[rrc1962]

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.

 

[Đ⁴zıruš Đıąbľų§ Ńøçłəřņvś]

What if the second fan had more pitch tuned enough..or properly

 

[quorneng]

rrc1962 is correct.
An EDF works on a 'cold' cycle so by far the best way to increase thrust is to increase the mass flow. Bigger diameter fan is best, more blades is less efficient. Raising the pressure with more stages requires the same energy to be used in each stage but for only a modest (about 30%) thrust gain.

In a combustion cycle the temperature of the gas is dramatically increased which increases its volume many times over resulting in a similar increase in the exhaust velocity. In this way the energy used to compress the gas in the first place is rewarded by a many times over increase in thrust, but only if you get it all exactly right!

 

[earthsciteach]

Fans staged in series won't realize a benefit until atomized, flammable liquid is inserted into the train and ignited. Its the expansion of the gasses that gives jets their power.

There is something to be said about dumping raw fuel into the exhaust stream of an edf and igniting. David Windestal did it way back in the day. But, there was this group of kids at Flite Fest this year (9th-10th grade I'm guessing) that built a large FT Viggen with a propane injection ring in the thrust cone. I was standing by with a fire extinguisher for a ground fire test. It was impressive. Sounded like a huge blow torch! That was the first time I've ever used a fire extinguisher!

Apparently, they stayed up all night replacing the 3D printed fan duct (it was a big ducted fan) with stove pipe. They flew it early Sunday morning according to the one kid's dad. He guessed at the increase in power to be around 30%.

Now, take that and tune the exhaust shape and length...

 

[PsyBorg]

You can easily work around the drag and vacuum issues by adding a variable venturi at each stage to act as "fuel" thus adding more volume to add to the prior stage to get compressed even more. Simple naca ducts might even be sufficient. 1st stage BIG inlet slow turning low pitch to move air mass, next stage higher gearing and larger pitch with its own inlet, maybe even a third stage with even bigger gears and more pitch again with its own inlet. Dial in the thrust tube properly and I am sure you could produce more thrust then just a single fan engine.

Counter rotating fans at each stage using a planetary gear set would work nicely. The drawback is size and weight to produce a viable prototype. Then would the gains be worth that effort over just a single fan.

 

[cyclone3350]

I liked this one. Not sure if it did anything, but God bless him for trying and making a video.

 

[quorneng]

Do the turbo fans on airliners use more than a single stage cold air fan?
I am sure that with the millions spent on engine development if there was an advantage there would be multistage turbo fans by now.

 

[PsyBorg]

I liked this one. Not sure if it did anything, but God bless him for trying and making a video.

That did nothing for speed if the second fan wasnt spinning faster or had bigger pitch blades then the first. I found it funny when the firt thought I had was.. all that top mount motor is going to do is pull it vertical. At the end what did he say.. When he hit the switch the plane started to rise up.. He just confused that with added thrust not the fact he was pulling the plane straight up haha.

I would have liked to see a speed gun on that with both situations to actually compare.

 

[cyclone3350]

Do the turbo fans on airliners use more than a single stage cold air fan?
I am sure that with the millions spent on engine development if there was an advantage there would be multistage turbo fans by now.

I guess U can say yes. For GE, the smallest airliner engine is the CF34 (Regional Jets) and I think their largest is the GE9X for the 777. They range from 10-14 stages of compressing ambient air to a combustion chamber. Once fuel and highly compressed air is ignited, it basically explodes and is directed out as exhaust. That energy is directed over several turbines which is used to drive the compressors and the main forward fan unit itself. For today's airliners, the main force of thrust is that big front fan U see when U look inside a jet engine. It is nothing more than a prop that blows air just as the fan disk does on an EDF. The big difference is, a jet is using a single power source (kerosene) to drive all those fans efficiently. To multi stage an EDF would require multiple motors and the batteries to power them. I guess the closest way to do this, would be a coaxial motor system with contra rotating fans. This would allow two EDF's to occupy a single narrow space. It probably won't provide the same thrust as two EDF's side by side, but I would like to see it done just for the coolness factor. Sounds like @Mid7night tried this.

 

[quorneng]

cyclone3350
I can assure you it has been done quite few times but with poor results. Virtually doubling the planes weight for about 1/3 more thrust is a poor trade.
The best way of increasing the thrust for a given diameter is by increasing the fan blade count. It used to be as little as 3 and is now up to 11 but with careful design could go higher. For example the GE90-115B fan in the Boeing 777 has 22 blades.

 

[Mid7night]

FYI: If I didn't mention it before, yes; the contra-rotating-fan idea has also been tried and failed. I didn't try that one, but I read about it on RCGroups.

Also remember my note: My "twin inline EDF" experiment actually produced LESS thrust than a single EDF would. It's not just worse than two side-by-side, it's worse than one BY ITSELF.

 

[cyclone3350]

FYI: If I didn't mention it before, yes; the contra-rotating-fan idea has also been tried and failed. I didn't try that one, but I read about it on RCGroups.

Also remember my note: My "twin inline EDF" experiment actually produced LESS thrust than a single EDF would. It's not just worse than two side-by-side, it's worse than one BY ITSELF.

Wow! I guess if there was an efficient way to do it, someone like Schubeler would have made one. I have been reading your post and find your stuff amazing. I am making a kind of sort of F-5 (that's what I am calling it for now) that will weigh in around 7.5lbs. Am I on the right track using two FMS 70mm 1900kv 6s?

 

[Mid7night]

Wow! I guess if there was an efficient way to do it, someone like Schubeler would have made one. I have been reading your post and find your stuff amazing. I am making a kind of sort of F-5 (that's what I am calling it for now) that will weigh in around 7.5lbs. Am I on the right track using two FMS 70mm 1900kv 6s?

Yeah, absent other information I'd say that's on the right track.