Cool project from the forums; Ducted Mini Tricopter

Kevin_eric on the forums is on a mission; To increase performance and efficiency of his Mini Tricopter. He’s doing this by adding 3D printed ducts around the propellers.

His method goes a little like this; 1. Design/modify ducts 2. Test results 3. Evaluate results 4. Go back to 1.

After making 7 revisions of the 5 inch version, the design generates at least the same amount of extra thrust as the ducts weigh. Which is a very promising results.

To achieve better efficiency and more thrust he’s now moving to a 6 inch propeller setup.

But why on earth does ducts generate more thrust from the same power setup? Mainly 2 reasons. When a propeller spins around the airfoil on the propeller creates lift by generating low pressure on the top of the blade. Nature hating differentials the high pressure air on the bottom wants to flow to the top of the blade and equalise the system. This results in a spiral vortex forming at the tip of the blade which induces drag and ruining lift. By adding a shroud/duct with a very small clearance between the propeller and the duct wall the high pressure on the bottom can’t flow to the top which almost completely kills the vortex at the tip of the blade. The second reason for the additional lift is the lip of the duct. This is designed to have a soft edge that rolls over the and down the outside of the duct. In fact it’s shaped like an airfoil! The air being sucked into the propeller rushes past this aerofoil lip, which generates a low pressure zone just like a wing, which we know generates lift! Pretty clever stuff.

Be sure to check out his work in this thread for the 5″ version and this thread for the 6″ version.

Keep up the great work Eric!

6 thoughts on “Cool project from the forums; Ducted Mini Tricopter

  1. Thanks David!

    A few things of note here…

    -I’m still tinkering just having posted anything in some time due to the holidays. Besides, I’d like to build a Tri-Baby to use as the testing platform as it’s a much more compact system and would likely see the best use of a Ducted system. Until this is done, I’m not likely to post much unless folks ask for something specific.

    -Biggest problem I’ve had is printing the Ducts as most methods either have warping or layering issues. (PLA & ABS) Thus far SLS Nylon is the best material to print with but it’s also pretty expensive. (100-500 Euro for a set) Making a solid design that is functional, impact resistance and yet light is difficult. Hence all of my work has been with PLA or ABS which costs around 10-20 Euro per duct to have printed. Until a really really good 5″ design has been finalized it’s not worth messing with 6″ Ducting simply due to the added cost and complexity.

    Note: If a design were finalized it could be possible to mass produce the ducts for a fraction of what they’re printed for.

    -Design wise I’ve found that micro winglets within the duct just aren’t viable as the winglets slow the mass mass down to much. Beyond this what I have discovered does work is to invert to motors / props and have the duct built around this concept. The basic idea being that the airflow is free from all obstructions with the motors hub being open to the airflow entering the duct. This should improve cooling, protect the propellers / motors and allow the tri-copter to physically land on the ducts. Granted this would require reworking the tricopter a bit to balance out the weight distribution but this is 100% doable. I’ve modified my Tri-mini to run it’s front motors / props inverted and I\m seeing a nice increase in flight times due to the reduced drag.


    Kevin & Erik

  2. @ Tomas

    Primarily Ducts are used to safe guard the blades / motors. Effectively allowing you to fly through light brush / tall grass with little to no effect. A good branch can still snag you but at least your not very likely to lose the Prop. Beyond that the Copter sounds signifigantly muted and looks rather cool for bystanders as the Propellers are not visable.

    Ducts do work great if designed / installed well…

    First off the blades have to meet the wall clearance requirements to eliminate the tip-vortices. This gap increases as the Ducts size increases. Typically for 5 or 6″ propellers you’ll see 1mm gap. I know this sounds small but its totally doable if you use shims the space the propellers within the Duct, then torque down the motor mounts. This all assumes that the Duct has been sized proportional to the Propeller used. Thus the Ducts Lip and Defuser have to meet the correct size and shape profile to be effective. Now for some reason, should the blades flex during flight they can move outside of the “sweet spot” and most of the Ducts benifits maybe be lost. Because of this, it’s best to use very ridgid Props that provide minimal flex during high loads / RPMs. Ideally we want to reach the break even point so that the Ducts added mass is counter acted by the additional lift being created. This does act apon the copter by adding to the inertial mass but IMO this is trivial for Copters having 4:1 or greater thrust.

    One weird quirk of Ducts that you may not know of is that they’ve a self-leveling effect. For sort of a loose example, imagine your flying forward. Copter is tilted over slightly effecting forward flight, which also tilts the Ducts. The leading edge of the Ducts Lip, begins to generate more lift than rear of the Ducts Lip. This uneven lift forces the Ducts to attempt to balance out. Further more, at higher speeds the Lip acts very much like an Aircrafts-Wing. Thus allowing the Copter to pitch over significantly while still maintaining controllable lift.

  3. I was following with interest this development. Now there is an incentive here in my area to use ducted props: the local Brisbane (Australia) city council has made the progressive step to create designated drone flying areas in park in the city. It is a trial at the moment, no doubt the result of some very positive lobbying from some quarters.

    There is a sign posted at the parks stating the rules. I am happy with all the rules, except the one that says that drones should have prop guards installed. I may have to go Kevin’s way in future.

    As a side-note, I am quite happy with the local aviation authorities in the way they consult and the relative (to other authorities) pragmatism. The hobby will continue to flourish in Australia if CASA maintains their attitude towards model aviation.

    • @BigRC

      I am more than happy to work with you and support your needs.
      Please shoot me specifics about what motor, prop and tri-copter combination your using so I can see what I can do for your build. Odds are that I may already have something but just in case….


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