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Ducted fan motors vs propellers??

Posted by Rafi Nahmani on January 28, 2018 at 6:18pm

Ducted fan UAVs maintain a higher payload to stored volume ratio. The vehicle can carry 4x the payload of a similar footprint fixed- or open-rotor UAV. 

I have heard different things online about ducted fans vs open wing proppellers. Some say they are less efficient some say they are much more efficient!?

Ive heard of some people saying they are twice or even 4x more efficient. I'm thinking of designing a heavy lift drone and wondering if I shouls go with ducted fans..

Any input appreciated thanks!

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Replies

  • Sam Worthington February 3, 2018 at 3:48am

    Rafi, I'm not sure about the 2x or 4x efficiency claims, but you could expect a 30% or 40% increase in static thrust for a well designed ducted fan compared to an open propellor of an equivalent diameter.

    I'll keep meaning to work through the calculations again to compare the two, but never quite find the time.

    I suspect part of the difficulty with a ducted fan quad, is that the airflow gets complex as it moves from hover into forward flight. I don't think there is a lot of thought put into the aerodynamic design of a lot of quads and shrouding the rotors with the aim of increasing efficiency would require that extra effort.

    Have a look at the following two papers for some fact-based discussion:

    Rutkowski & Krusz "Design and Analysis of Ducted Fan for Mult-R...

    Abrego 'Performance Study of a Ducted Fan System'

    Sam

    Worthington Sharpe

    https://storage.ning.com/topology/rest/1.0/file/get/3702413921?profile=original
  • Rafi Nahmani January 29, 2018 at 12:38pm

    First and second article are really convincing

  • Rafi Nahmani January 29, 2018 at 12:37pm

    http://www.avidaerospace.com/blog/2015/3/24/the-case-for-ducted-fan...

    ???

    By reducing propeller blade tip losses, the ducted fan is more efficient in producing thrust than a conventional propeller of similar diameter, especially at low speed and high static thrust level (airships, hovercraft). ... Ducted fans offer enhanced safety on the ground.

    But there are so many online sources saying that its efficient.. I actually dont care about drag or wind resistance. Just lift. And payload compacity..

    If these articles are accurate I can achieve the same amount of thrust/lift with motors that are half the size? Am i wrong??

    The Case For Ducted Fan UAV —
    When it comes to safety, payload, and scalability, the ducted-fan UAV is incomparably better than quad-rotor UAVs.
  • Moderator
    Graham Dyer January 29, 2018 at 7:44am

    https://www.wired.com/story/the-physics-of-why-bigger-drones-can-fl...

    The Physics of Why Bigger Drones Can Fly Longer
    Bigger isn't always better—but when it comes to drones, it kind of is.
  • Andre K. January 29, 2018 at 3:05am
    Ducted fan is less efficient, and much less efficient if you need horizontal speed too, not only hover.
  • Moderator
    Graham Dyer January 28, 2018 at 10:43pm

    Ducted fans are less efficient, because a smaller mass of air is used for propulsion and additional friction drag and added weight from the shroud and support structure.

    The highest propulsion efficiency is a large, slow-spinning prop, not a small high revving fan.

    However, large ducted propellers could overcome the disadvantages. They are very seldom used though, even with the high budgets of some major companies.

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