A Guy Named Bernoulli and Ducted Propeller Designs

Want to learn why ducted propellers improve both thrust and efficiency? Well, we’ve got both the lesson and the teacher for you. [Bruce Simpson] isn’t exactly a household name, but we bet most of you already know who he is. He’s the guy from New Zealand that in 2003 set out to make a $5000 DIY Cruise Missile – and he claims that he would have succeeded if the NZ Government hadn’t got in the way. Now-a-days [Bruce] focuses most of his energy on his two YouTube channels, RC Model Reviews and XJet.

Back to the lesson at hand: In the video after the break, he does a wonderful job explaining how the walls of a duct work to stop the high pressure area of the propeller from moving to the low pressure area. You’ll see something similar on the wings of jets aircraft. There will be a small vertical fin on the end of the wing, and much like a duct, it’s job is to physically separate the two areas and prevent tip vortex effects. That in turn increases efficiency. [Bruce] has some other great RC/model type videos, so if you like what you see, you might want to subscribe. Or you can learn more about the DIY Cruise Missile saga.

50 thoughts on “A Guy Named Bernoulli and Ducted Propeller Designs

      1. Actually, he endured quite a bit of scrutiny by area residents since the publicity.
        His poor choice in words had real consequences at home… =/
        The state basically doxed him, and made it nearly impossible to enjoy an RC hobby.

    1. Continuing what ipaq said, the body that regulates RC model flying has gone out of its way to do everything possible to prevent him from flying, since he’s been critical of some of their policies and regulations. It’s really sad, and a large example of how an organizing body can really screw you and leave you with no repercussions.

      1. Honestly, that just isn’t the whole truth of it. He was caught flying model aircraft over an active runway. Was defiant about the whole thing, and here we are today…… Model Flying NZ has done it’s level best to advocate for a guy that tends to be his own worst enemy.

        1. yeh afraid he did, in as many words around 5:50 he’s comparing the length travelled by the two air streams and discussing how the top would have to travel faster to arrive at the same time, implying the particles need to meet again for some reason, this is pretty much the incorrect lift theory.

    1. Are you sure he’s wrong? I dunno. Did he say equal time?

      He did say that the top flow was faster. The article you linked itself says that airflow on the top stream is very much faster, faster than equal transit time would have predicted. Maybe there’s something wrong with the way he describes it.

      Also the demonstration with paper at the start was really neat.

      1. It was implied. The line over the top of the airfoil is longer than the line under the airfoil and therefore must travel more quickly. The false assertion here is that the particles must traverse this distance in the same amount of time.

        What’s funny about this to me is that if this phenomenon were true, airplanes would have a very hard time flying indeed! The predicted flow rates over the top would be only slightly higher than those on the bottom resulting in very little lift.

        This is a very common misconception. My undergraduate fluid mechanics professor who formerly worked for Boeing made this same mistake!

        1. It’s what I was taught at school, only 20-odd years ago. For a long while it seemed the correct explanation, I believe it was considered so by everyone, at one time. Just briefly, why DO planes fly, and how can an aerofoiled wing fly upside-down? And if it isn’t to do with transit time, why are aerofoils so shaped?

          1. 2-second explanation – planes fly because their wings push down on the air below them.

            Wings redirect air flows downward, which by conservation of momentum, means that the wing goes up. The shape of the wing isn’t terribly important for generating lift. Take a flat sheet of, say, thin wood and try and run with it tilted slightly upwards. It ends up wanting to take off. (It’s fairly obvious if you think about it – air runs into the flat sheet, ends up going downward, which means it exerted an upward force. Voila, lift.).

            Wings can fly upside down because the shape isn’t that important for generating lift. It’s the angle of attack. Are you forcing air down? Yes? Then you go up.

            Why are wings shaped like that? Because they redirect air really well with minimum drag shaped the way they are. Moving air downward necessarily induces a circulation – the wing-shape helps to form that circulation efficiently and without a lot of loss (unlike a flat sheet, which would be a mess).

  1. Too much conversation… Why you don’t take a measure and report the difference in % of the static trust?
    We must think that with a duct we have more weight to carry with the same propeller.

  2. Holy cow! I thought he was a goner.. I remember reading his site when I was in highschooler during the early 2000’s. Reading his material on pulsejets was the primary driver towards shaping my studies towards physics and then aerospace engineering.

    If you’re out there Bruce, Thank you!

      1. I have wanted to do ducted fans on my quad since I started building it. Even WITH a 3D printer it’s not that easy. The extra weight of the ducting can easily make it worse than without it. You also have to take into account new frame design to hold it all together.
        I am sure that a properly designed and built ducted fan multi-rotor would be amazing, sadly it’s beyond my ability to create.

  3. Main concern: in his theory yes it might give a little extra lift. But does this little extra compensate enough for the extra weight added by the ducts? Does it also compensate for the loss in his shown design the sprockets will give extra resistance to downward airflow. The good news : yes using these ‘ducts’ will make the quad or tri safer for when you bump into someone…
    IMHO having as much power to weight ratio is best for flying the little quads. Might be wrong for bigger ones, we’ll see…

    1. I would think that one of the main problems is that a multicopter is balancing and steering by adjusting the speed of some of the motors, and the added mass (inertia) on the rotor makes the system a lot less responsive, because it needs more force from the motor to break or accelerate a ducted rotor. (The problem is that the mass gets added far away from the center of rotation). If you would have a rotor with pitch control on the blades, something that could be done easyer on larger machines, the duct could probably be a viable solution. You could do the very quick adjustments for stability by shifting the pitch slightly, and would only need to accelerate or decerate the rotational speed for slower thrust corrections.

  4. There are simple proofs to show that the bernoulli effect is incorrect. If it was correct, then airfoils would work equally going reverse or forward, and to create a high lift airfoil all you would have to do is make the airfoil twice as tall. Experimentally neither of these work. The NACA airfoil database has some high lift airfoils, but if you are convinced of the Bernoulli effect, you’d never pick them out of a lineup.
    I haven’t tried it yet, but I suspect that if you cut a square hole in a piece of cardboard and tape a piece of of paper the width of the hole so that it just reaches the other side, then let it hang down, when you blow across the cardboard the paper will rise to meet the cardboard, but no higher, even though there is no airflow over the bottom side.

    1. No, the Bernoulli effect is absolutely correct – moving air definitely has lower pressure. At least it better, considering all Venturi pumps use it, and I’m pretty sure they work fine. What’s wrong is the “equal transit time” part – the idea that the air has to move faster across the top because it’s got a longer path. That part is nonsense, and leads to everything you mention above.

    2. The Bernoulli effect is real–it’s a simple result from conservation of energy. The error is when the argument of “equal transit time” is made to explain why flows in the low pressure region are moving more quickly than flows in the high pressure region.

      I don’t personally think it’s very useful in explaining to a layperson why airplanes fly–momentum theory, i.e. that the wing must turn the flow of air downwards to create lift upwards, is much more useful and easy to understand.

      As for ducted rotors, you’ll find they don’t often buy themselves on to many aircraft, large or small. There’s a considerable weight penalty to enclosing a rotor in a duct. To gain the benefits of reducing blade tip losses, the duct must sit very close to the blade tips. These gains are offset in part by the extra drag of the duct and the stators that support the rotor within the duct. Ducts have terrible performance at non-zero angles of attack, where flow separates off the duct walls and is ingested by the propellors.

      All in all, ducts make sense only when the rotor is moving at relatively high speed, is highly loaded, will not operate at high angle of attack, and serves another purpose like acoustic shielding or containment. Turbofans, which you can view as a kind of ducted fan, own this space. Other kinds of ducted fans are only seen in exotic applications, like the Bell X-22A, and the case for them even there is much weaker.

  5. Bruce is a Genius, Been down to see him a few times and got a demonstration of one of his pulse jet engines quite some time ago. Sadly our NZ government is renowned for throwing a spanner in the works of great projects.

    Anyone have access to some hydrophobic material, Be interesting to know if air being a fluid having one side of the paper coated in a hydrophobic coating and the other not what the changes would be.

      1. air flows similarly to water and that’s why aerodynamics testing can be done in a tank of water. Hydrophobic materials have highly textured surfaces at a microscopic scale and that’s what makes the water bead and flow so freely over them. With that said, it would be possible to reduce wind drag on a surface by treating it with a hydrophobic coating. the idea is like that of a golf ball. The dimples reduce drag.
        Disclaimer: I’m not a professional and I apologize for any incorrect information I have provided. I’ve only tried to explain things how I understand them

        1. The mythbusters got really incredible mileage improvements — around 10% — by adding a thousand fist-sized dimples to a car! I’m a bit mad now that I can’t buy a properly dimpled car (not just some crappy, hail-damaged car)!


          That said, most man-made hydrophobic surfaces are not structured, they are just coated with a material that has very low surface energy like a fluoropolymer or silicone. We can make structures that are hydrophobic, but they are very fragile and since they’re generally supposed to work outdoors to shed rain, the durability is almost never worth the cost. Plants, in contrast, do make extensive use of topology because it’s a lot easier for plants to grow little structures (and regrow leaves when the structures are damaged).

  6. I always thought (and still think) that ducted fans are only effective at high speeds and not at hovering. while at high speeds, the air rushing in at the front creates lift from the ring. but when hovering the fan must do all the work to suck in the air over the ring area, so when taking mechanical/electrical losses into account, a ducted fan in hovering mode is LESS EFFECTIVE than an open prop. So far thats also what i read in forums about mounting ducted fans on quads. maybe someone with experience can correct me oder support this.

    1. I have a couple toy quads, one has ducted fans and one doesn’t (One is the Air Hogs Helix and the other is some chinese generic that’s about the same size). the non EDF one is more responsive and quieter. EDFs have sick efflux speeds but smaller disc area. So they don’t work as well for hovercraft or WIG type models. Its kinda like the difference between impeller powered waterjet craft and props-it takes a bit more power to get the same results going at lower speeds.

  7. I thought he looked familiar! When I heard the NZ accent, I thought he was the pulse jet guy who was featured on an episode of Junkyard Wars (Scrap Heap Challenge.) I’d still like to know how to build my own cruise missile though. It would come in handy for sending cargo to my mates in Australia from the U.S.

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