Wednesday, March 5, 2014

Attacking Conjecture : "Ducky" the EDF Quad

I decided, against conventional "wisdom", to construct a quadcopter that uses Electric Ducted Fans (EDFs) for thrust.  There is conjecture all over the internet as to why this is a bad idea, but I was having a hard time finding any real science or performance data to back it up.  So, why not build one and see what happens?

What are EDFs?

EDFs are basically small, higher pitch, higher rpm propellers (sometimes called "impellers" in this application, to differentiate them from conventional propeller design) inside a cylindrical duct with a tapered inlet and as little clearance between the blades and the duct wall as possible.  They look more like jet engines than conventional propellers.  Tangent: Modern jet engines actually are turbine driven ducted fans, called "turbofans."

The primary benefit of a ducted fan is increased efficiency of the propeller by preventing (or reducing) the phenomenon of "tip vorticies" and allowing a greater range of operating speed without "propeller stall."  There are also some secondary benefits related to the air being forced through the duct.  (E.g. more output consistency in turbulence than an "open propeller" since the propeller is effectively shielded and moving faster.)

They spin at tens of thousands of RPM (as opposed to merely a few thousand RPM like conventional R/C props), but being completely contained means they pose less of a hazard to people or objects that the aircraft might collide with and take up less horizontal space along the plane of the propeller.

At any rate, if you tell the R/C community that you want to use an EDF for a multirotor application, you immediately get a few reasons why "it won't work" or "it's not a good idea."  I'll dissect those here:

R/C Community Myth Number 1:  "You can't use EDFs for static thrust applications."


Variations: 
  • "EDFs are less efficient at static thrust than open props." 
  • "EDFs don't produce full power until they get a bite on the air moving through the duct."

(That last one is my favorite, because it's a visual description of complete and utter nonsense.)

The conjecture is that EDFs really only shine when moving through the air at high speed.  What's odd about this conjecture is that the full-scale aerodynamics and aviation community says exactly the opposite about ducted fans:

  "... the ducted fan is more efficient in producing thrust than a conventional propeller, especially at low speed and high static thrust level." [http://en.wikipedia.org/wiki/Ducted_fan#Advantages]

 "When compared to an isolated propeller of the same diameter and power loading, ducted propellers typically produce greater static thrust."  [Abrego & Bulaga, "Performance Study of a Ducted Fan System", NASA Ames, 2002]

This would seemingly make them more ideal for multirotors than open props, as multirotors spend most of their energy on vertical lift (which is mostly static.)


R/C Community Myth Number 2:  "EDFs don't change speed fast enough for multirotor stability."


Variations:
  • EDFs have too much rotational inertia for the fine-power adjustments that the flight controller will need to make.
  • EDFs take too long to "spool up" (or down) to use for multirotors.

This one actually had me concerned.  Since EDF rotors spin at up to 10x the speed of conventional open props, it stands to reason that the inertia might be an issue.  Certainly, in the typical application of R/C EDFs - scale model jet fighters - it doesn't matter how fast the motor reaches a particular RPM or how much control "resolution" you have once it gets there.  So the performance characteristics in that regard are just largely unknown.

But that's just it.  They're unknown.  I could find absolutely no performance data about the power resolution or latency of R/C scale EDF systems.  It seems no one has measured it, yet many people are willing to state as canon that it's not good enough for this application.  The very definition of conjecture.

I decide the only way to find out is to buy some and try it.  After all, we're doing this for fun, right?  And what's more fun than potentially launching a completely uncontrollable aircraft with a massive LiPo battery on board?

R/C Community Myth Number 3:  "Without the adverse torque from the open propeller, you won't have any yaw ability."


I only saw this mentioned a couple of times, but Newtonian physics pretty much just disavows this one.  The opposing torque of each motor is proportional to the thrust it's producing.  I'm pretty confident there's enough torque there to yaw effectively.

Anyway... enough speculation.   I have plenty of reason to believe that this will work, in spite of the various opinions I've seen posted online that it won't.


First order of business:  Counter Rotating EDFs

I like the simplicity of quadcopters, but to build that I need counter rotating EDF rotors.  This turned out to be tricky, but I actually found a few.  I narrowed the ones I could find down to two likely candidates:
Both offered almost exactly the same amount of thrust at full power (4 pounds each!), but the Freewing ones are lighter and purport to draw slightly less power, so I decided to start with those.  Note: Both of the above are available from multiple vendors, so if you're shopping for them you might do a Google search on the names for comparison. 

So now that I know the size, weight, and power requirements of the EDFs I'm using, I put together the following parts list for the rest of the aircraft:

Testing the EDF / ESC combination

Once I got the EDFs, I tested one by connecting it directly to the "Throttle" output of the RX and hooking up the Multistar 45A ESC and battery.  I tethered the EDF to a block of wood and ran it at various power outputs on my back patio.  Observations:
  • Holy %#& this thing is loud!  It sounds like a vacuum cleaner from hell.
  • It produces a LOT of thrust, just as advertised.
  • It changes speed between 25% -> 50% -> 100% and back as quickly as I can move the throttle lever.  There is no noticeable "lag time".
I decide to proceed with the build!


Mounting the EDFs to the Talon frame



I found that rotating the Talon's included "T mounts" 90 degrees made a good vertical mounting post for the EDFs.  Initially I mounted them simply by wrapping long PVC zip ties around the EDF body and through the mounting holes of the T mount.  

There were two problems to this approach:
  • It eliminated the possibility of mounting the Talon's included "landing gear", as it went on the bottom of the (now rotated 90 degrees) T mount.
  • While it seems fairly stable on the Z axis, it allows the EDF to rotate a bit which will cause alignment issues (and possible yaw control issues) in flight.

Still, I deemed it good enough for testing, so with that decided on I went ahead and mounted the rest of the wiring and electronics:

Ducky, fully assembled!

Test Flights

I did two tests prior to the filmed flight test you see below.
  • Tethered test:  I tethered the quad to the ground using some long bungee cords and some heavy weights so that it couldn't fly away.  In this configuration, I did some run-up tests, hovered it (within the length of the 36" tethers) and saw that it actually flies and seems to be controllable.
  • Hover test:  Gingerly, I removed the tethers and basically did the same flight.. I flew it up to about 3 feet AGL and tested that I have control of it before bringing it back down and shutting it down.
After these two successes, I decided to do the first "free flight" test.  In this test I'm running on only one of the two 35C Lipo batteries.  Note:  This reduces the weight but probably also means I don't quite have full power available, as the battery is not quite capable of delivering the full 160 - 180 amps of current that the combination of the four motors could possibly draw.


IT FLIES!!!!!

It actually flies pretty damned well, given that I haven't made any changes to the default MultiWii 2.3 PID settings.  That high speed climb was a little crazy, given that I knew I was at the output limit of the single battery... but I couldn't help it.

That's all for now, but I promise there's more to come...

7 comments:

  1. I suspect there are two independent issues regarding the "efficency" of an EDF.

    The first is, as you point out, the tip vortices, which are mitigated by the ducting. My understanding of why not all props have ducts is that, at some point, the drag due to the duct overcomes the advantage of the tip vortex reduction.

    The second is the fact that an EDF runs at higher RPM, has more blades, and the blades might be at a different AoA. I actually don't know how that plays out, but it'll be interesting to see your results. It may be that an EDF has such a high air speed going through it that the speed of the aircraft (static vs. moving) is irrelevant, kind of like a turbojet, so you may get more *consistent* performance.

    Anyway -- this is an awesome-flying machine. Good luck with it and keep us posted! :)

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  2. regarding myth 2, the edf changes speed faster because its blades have less mass. so its better.

    regarding myth 3, torque is Torque=Watts/(Rad/s). doesn't matter what prop is on it.

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  3. This looks really interesting, but the overall weight of the 4 EDFs and the 6S batteries make it impractical IMHO. Those things eat power like a starving wolf.

    At least you proved that it works, and that good, I guess.

    HD

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    1. Where exactly is the line between "practical" and "impractical" for an R/C aircraft?

      Certainly, it isn't a good candidate for some applications. In particular, any application where you want long flight times or are concerned with how much noise you are making. :)

      But, it flies great and gives me about 5 minutes of flight time on a full charge using the two batteries I initially spec'd for it. So I'm quite happy with it.

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  4. For what an old man's opinion is worth, well done Sir. Yes it may well consume more power but so do most jets. the vertical was fantastic, now all we need is for others to pick up the cause and refine it....just as quads have come such a long way over the last decade, the duct fan model has far to go. I have seen radio control go from vacuum tubes and reeds to amazing bits of technology ....having an insight to the dark nature of such things I worry about some of our toys but then I am old and maybe don't understand the good of things anymore...

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  5. I wanna say on thing to you ! I already buy all the pieces !

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    Replies
    1. Thank you ! I really like this idea.

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