The ideal shape for a multirotor ‘ducted-fan’ that can dramatically improve thrust and flight time.

2016 update: David has produced a great customizable OpenSCAD file based on the study referenced in this post. The file can be found over at thingiverse here (thing:1535549).

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In a previous post I discussed how the addition of a well designed duct can reduce hovering power consumption of a rotor by over 60% or alternatively nearly double the thrust if power consumption is kept at the same level as the unducted rotor. These claims are based on a scientific study conducted on this very topic which can be found here (Mirror here) (Jason L. Pereira 2008).

If you want to have a go at testing these claims for yourself then this post will hopefully give you an idea of what kind of shape that duct needs to be and also suggest some possible ways to construct it.

The perfect duct for hovering

Below is an cross-sectional view of a duct taken from the reference above. It outlines the critical parameters that affect performance: Diffuser included angle (θd), diffuser length (Ld), inlet lip radius ( r lip), blade tip clearance (δ tip).

The parameters that affect performance: Diffuser included angle (θd), diffuser length (Ld), inlet lip radius ( r lip), blade tip clearance (δ tip)

The parameters that affect performance: Diffuser included angle (θd), diffuser length (Ld), inlet lip radius ( r lip), blade tip clearance (δ tip) (Credit: Jason L. Pereira 2008)

According to the study the optimal optimal configuration found experimentally includes a δtip = 0.1%Dt,  rlip = 13%Dt, θd = 10and Ld = 50% to 72%Dt. I have used sketchup to mockup what such a duct would look like for a 5″ rotor as seen below. Note that I went with Ld = 50% to try and keep the overall length to a minimum and also made it a single surface by using the inner wall only.

Single Rotor Duct Design

Single rotor duct design

The same duct can also be extended slightly for a contra-rotating setup and shown below.

Double rotor contra-rotating version (left image made transparent)

Double rotor contra-rotating version

The dimensions for a single rotor duct are shown below (larger views available) for a 5″ rotor. Scale accordingly for a rotor of your choosing, but be sure to measure your rotor first as it may not be quite what you expect.

Top view with dimensions

Top view with dimensions

SIde view with dimensions.

SIde view with dimensions.

You can find a copy of the sketchup model here.

Construction

The most difficult part about making a duct for a multirotor will be keeping it light enough that you do not offset the respective gains. Thankfully, the literature suggests that when done correctly the additional thrust gained is so large (~90%) that you would need to nearly double the weight of your multirotor to cancel out the gains.

Remember though, if you only want to bench test the thrust or efficiency improvements (as I hope to do eventually) then the duct can weigh whatever you want, it doesn’t matter. The following is a list of suggestions for how you could achieve a lightweight duct.

  • Carbon fiber – The ideal solution: In an ideal world you would construct your duct out of a single piece of 0.5mm thick pre-impregnated vacuum bagged and autoclaved carbon fiber. If have access to this equipment then look no further.
  • Fiberglass reinforced foam – The realistic option: A more DIY solution would be to purchase some styrofoam blocks  and carve them into the desired shape. For added rigidity you could also layer fiberglass on the surface like you would for a RC boat hull or model plane wing.
  •  Cheapest possible or just for testing – If you just want to test out the idea then really anything will do the job, balsa wood, cardstock, old drink bottles, paper mache, gap filling foam etc.

The most difficult part of the build would be getting the blade tip clearances as close as possible, as this has a big impact on efficiency but also requires very tight tolerances. One way around this problem is to build the duct quite roughly and then line the inside of the duct where the rotor will be placed with a material that can be scraped away by the rotor as it turns. Something like expanding gap filling foam may be perfect for this. Once the rotor scrapes away the excess foam you should be left with blade clearances measured in fractions of a millimeter.

If you do end up trying something let me know as I would be really interested to see what you came up with and how you went.

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About Richard

I am a PhD candidate in Materials Engineering located in Melbourne, Australia.
This entry was posted in Multirotor Projects and tagged , , , , , , , , , , . Bookmark the permalink.

28 Responses to The ideal shape for a multirotor ‘ducted-fan’ that can dramatically improve thrust and flight time.

  1. Hans says:

    Nice !
    Reading this makes me crave for spare time 🙂

  2. Glenn West says:

    Richard, I actually am 3d printing PET/Tglaze Plastic. Its commonly used in Soda Bottles.
    I find in incredible tough, and its perfect to print your ideal shape, and also include motor mount as well. Let me know if you want to work togeather to do this. As I can help u get them printed.

  3. Ghost says:

    Another solution would be to use the Styrofoam block as a “lost shape” (sorry, I don’t know the exact term in English).
    You carve the Styrofoam block to the desired shape, then use it as a support for fiberglass or carbon and epoxy resin. Once the resin as fully polymerised, you can dissolve the Styrofoam shape using acetone.

  4. Hi,
    I am working professionally with autonomus vehicles in Sweden together with Royal Technical University and Swedish Transportational Agency. (Amongst other things) This ducted thing has started to intrest me because we made a underwater propulsion using inverted technology. That means we used the outer ring as the motor and had the propellers carry the magnets. like this: http://www.eraurobotx.org/pics/boat/motors.png

    I see a lot of benefits using this kind of design even in air, since we get δtip down to 0. And we risc no wear due to very small tolerances between the duct and the propeller. Since we get a multipole design we will get much more torque so we can keep the inductors in the motor extremely small an light weight. The only issue can be to get the speed of the motor high enough, but it shall be possible with a customized driver. It also gives the benefit of extremely accurate control of the rpm.

    What do you think?

    Best regards,
    Mikael Sundberg
    Electronical engineer and product developer

    • Richard says:

      Hi Mikael

      What an interesting idea! Are you aware of any studies that have applied this technology to propellers for aircraft?

      I can see why this design would have many benefits for watercraft where weight is not critical. However, for aircraft the biggest challenge with this design would be overcoming the weight penalty that comes from using the shroud as the stator. It would be necessary to scale up the rotor diameter considerably for it to be effectively in air and I suspect that this design would quickly become impractical without becoming overly heavy. You could increase the rotational speed and have very high propeller loading to try and overcome this to some extent but I suspect this will come with its own efficiency drawbacks. I have no formal education in aerospace engineering so I am definitely not the right person to comment on this though.

      One area I do have some knowledge in is that of brushless motor design. Typically when constructing a brushless electric motor for use at high rotational speeds you try and maintain a rotor diameter as small as possible. This is because the centrifugal forces placed on the rotor at high speeds are enormous. For an electric motor ‘rotor’ that is the size of a conventional aeroplane propeller as needed for this design the maximum rotational speed would be severely limited by the strength of the materials it could be constructed from. For comparison consider the amount of engineering that goes into fabricating turbine blades that need to operate at high speed. Once you attempt to add the bearing surface, permanent magnets and magnetic core material as well into those blades it becomes a near impossible challenge. Thats not to say it couldn’t be done on a much smaller scale, such as for hobby sizes RC planes. Its just a question of if the advantages outweigh the drawbacks.

      Its an interesting questions and something I will no doubt give some more thought!

      Cheers,

      Richard

  5. Hi Richard,

    Nice post, it is just what I need!

    Just a question: on the double rotor design, what is the optimal Ld for both propellers? I don’t understand why the top propeller is not in the same position as the one in the single rotor design.

    Thanks!

  6. I’ve been thinking about the advantages of this design myself. One advantage it has over the ducted fan with a motor is that the moment arm of the motor and the turbine are one and the same rather than having a much smaller “crank” of the motor turning a much larger rotor. I think you are saying the same thing in different words.

  7. CoiLLoTe says:

    Thanks a lot for this article, this is really help about what I’m considering to customize today (an Hubsan X4 107L with a duct made of SLA 3D printed)
    δtip = 0.1%Dt Damned !
    this is so short clearance !
    Have you any data or source about the increasing clearance effect on efficiency ?
    I’m shure that I can never get this as so close because propeller are really cheap.
    In fact grinding a smooth material with the propeller is the key. I’ll try.

  8. Janne says:

    Hi Mikael,
    Did you perform real life tests on customer multirotor applications like on DJI machines or similar?
    I have two guys from Chalmers working on a Xjobb and I think that ducted propellers could be very beneficial for their application.

    Cheers
    Janne

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  10. David Harlock says:

    Hi Richard,

    Thanks for your work on this subject. I’ve designed a customizable OpenSCAD file based on the study that you referenced. I’d be interested in any feedback you might have on it – it can be found at the following link if you’re interested in taking a look: http://www.thingiverse.com/thing:1535549

    Regards,
    David.

    • Richard says:

      Looks great! I can’t add much more than that as it has been quite some time since I have looked into this topic. Hopefully someone can use your 3d model to produce some parts and do some real word testing. It could be worth trying to bring this to the attention of group like these guys to see what they can do with it.

      Anyway, I will quickly update my post on the topic to include a link to you work.

      Cheers,

      Richard.

  11. Dan Allen says:

    This is the most well thought diagram I ever seen for a ducted fan. I’m definitely going to test this out because I’ve seen unsuccessful attempts everywhere.
    So my question is, is there an ideal prop for this concept? I would assume a 2-bladed blunt tipped prop would get an enormous amount of thrust. Normally the blunt tip would create turbulence, however the duct may do away with that. Is that feasible?

  12. rfengfpv says:

    In case you’re not already aware of it, see Andy Shen’s ducted quad design based on this work at http://www.shendrones.com/liberty-ducted-quad.

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  14. Fraser Steen says:

    Excellent article that finally has some of the practical information I need on dimensions and how to build this.

    How does this kind of ducting impact on blade design? Surely bullnose type props will be preferable since the ducting mitigates the issues associated with tip vortices?

    • Richard says:

      Thanks.

      Yes, I believe the closer the tollarence between the blade and the duct the better (think turbofan). From memory the paper referenced in the post has more details so maybe take a look there as well.

      Cheers

  15. Brad says:

    The ducted fan has great thrust advantages over an open blade system. The big thing you have to look out for (and the reason you don’t see them in VSTOL aircraft) is the transition between vertical flight and horizontal flight. You will stall the blades on the leading side of the ducts as the air moves over the ducts. This stall causes the engines to loose lift and controllability. Maybe on a small drone with lots of excess power this won’t be as big of a problem but in normal aircraft this issue has cost lives.

  16. Ben says:

    Where the bottom begins to widen – does this need to account for the height of the propeller? Should you basically design a straight cylinder from the top of the propeller to the bottom of the propeller and then begin the widening after the bottom of the prop? Or should the widening just begin at the top edge of the prop regardless of prop height (i.e. immediately after the inner lip of the duct)? This would mean that by the bottom of the propeller, the prop tip would be slightly farther away from the duct edge. The image with the dual motors setup implies that the ducts walls should be straight for the height of the prop … Trying to design a micro quad with at least a portion of this design. I’m not sure it will really help all that much as I’ll need to clip a lot of the duct away. Using the full duct would basically quadruple the weight of the quad. But I’d still like to get as close as possible.

  17. Jeremy says:

    Hi great work

    Is there any guide on the best shape design to hold the motor in position ?

  18. Caleb says:

    Does it matter if you are using a 2 or 3 blade prop?

    • CoiLLote says:

      yep, I think it matter, assuming the duct reduce drain at poppeler end, adding one should add lift, it should add a bit current consumption too.
      2, 3 then 4 blade could be nice.
      more should drain more power than adding something positive.
      this need a try !

  19. Randy Steck says:

    Very interesting site and details. I am just starting to get interested in quadcopters but have dealt with aerodynamics in rocketry and as a pilot for a long time. Your results correspond to a very interesting exhibit that I took note of a few years ago at the Hiller Aviation museum. The concept of the “flying platform” also found that the upper edge of the cowling was responsible for 40% of the the lift generated. http://www.hiller.org/event/flying-platform/

    I will definitely need to put this on my list of things to 3D print and try out.

    Also, I’ve done high power rocket fins to precise aerodynamic shapes using a 3D printer to print the mold for the airfoil. The construct is then layed up in the mold with carbon fiber and fiberglass to achieve very low weight and very precise surfaces. I’m sure this can work with such a mold also.

  20. Bruno Eye says:

    Hi,

    I’m designing a 3″ quad and would like to use ducts, after watching a few videos (https://www.youtube.com/watch?v=Cew5JF8q6eY and https://www.youtube.com/watch?v=0stl1U9evzU&t=6s) on ducts I am unsure how stong the negative effect of using the annular wing is on multirotors where the axis upon which the motors lie are different and also the size. I can imagine it causing something like light flexing to instability, if anything at all. This would help me in designing my frame a lot, but maybe you do not know either. I so I will try to make two frames one with and one without an annular wing on top of the duct and compare (I am somewhat unwilling to do this as my budget is tight but think it is more worth it then investing in some overpriced motors).

    Regards,
    Bruno.

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