A bigger prop spinning at lower RPM is more efficient in terms of static thrust than a smaller prop spinning at higher RPM. It has to do with the volume of clean air that the prop is moving through as well as the velocity. Also, a larger diameter prop with fewer blades is going to be more efficient than a smaller diameter prop with more blades. This is the reason helicopters have a large, relatively slow prop instead of a small fast prop, and typically a small number (2-5) blades. Also the reason why drones and gliders have a long narrow wing instead of a short stubby one. Adding a duct to a prop while keeping everything else the same is going to make it more efficient than the same prop in free air because of tip vortices. Again the analogy with airplanes is that if you add winglets, which reduce tip vortices, the efficiency of the wing increases.
So, if you built a drone with large diameter EDFs, you might get efficiency as good as or better than the same drone with props in free air if the efficiency gain of the duct is enough to offset the extra weight. Whatever you do, small EDFs with a large number of blades such as the ones in the picture are not going to be as efficient as larger, slower props in static hover. They are better suited for something like a jet where you have large forward speed, and you want a small rotor disk area to reduce drag at high speed.
Having said all that, it's not how much flight time you get, but how much fun you have flying, so Kudos to Josh for trying something new!
it is correct that efficiency drops with the cube of the speed, for a ducted fan or open propeller. there is nothing specific about a ducted fan that requires high speed.
I agree in order for drones to be used widely in society they need to be much safer. If you were to put a cowling around the prop you get both safety and a more directed air flow.
Comments
A bigger prop spinning at lower RPM is more efficient in terms of static thrust than a smaller prop spinning at higher RPM. It has to do with the volume of clean air that the prop is moving through as well as the velocity. Also, a larger diameter prop with fewer blades is going to be more efficient than a smaller diameter prop with more blades. This is the reason helicopters have a large, relatively slow prop instead of a small fast prop, and typically a small number (2-5) blades. Also the reason why drones and gliders have a long narrow wing instead of a short stubby one. Adding a duct to a prop while keeping everything else the same is going to make it more efficient than the same prop in free air because of tip vortices. Again the analogy with airplanes is that if you add winglets, which reduce tip vortices, the efficiency of the wing increases.
So, if you built a drone with large diameter EDFs, you might get efficiency as good as or better than the same drone with props in free air if the efficiency gain of the duct is enough to offset the extra weight. Whatever you do, small EDFs with a large number of blades such as the ones in the picture are not going to be as efficient as larger, slower props in static hover. They are better suited for something like a jet where you have large forward speed, and you want a small rotor disk area to reduce drag at high speed.
Having said all that, it's not how much flight time you get, but how much fun you have flying, so Kudos to Josh for trying something new!
i THINK the bigger EDF would be more efficient.
the studies if have read are all nondimensional, ie scaling does have an effect.
but that doesn't FEEL right.
the problem with 1 big one is the weight of the duct becomes ciritcal
brings me to a question multiple small motor vs few big ones
what about scaling is the bigger the EDF the more efficient?
I disagree with part of bGatti's statement.(
A properly designed ducted fan adds up to 60% lift.
that may not be offset by the added weight of the duct
(Singh, Shape Optimization of a Ducted Rotor System for Aerodynamic Performance, 2011)
Read More: http://arc.aiaa.org/doi/abs/10.2514/6.2011-1203
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it is correct that efficiency drops with the cube of the speed, for a ducted fan or open propeller. there is nothing specific about a ducted fan that requires high speed.
oh the noise of those screaming edf will surely be interesting.
speaking of safety heres a great use
http://www.ctvnews.ca/video?clipId=973500&playlistId=1.1387041&...
I agree in order for drones to be used widely in society they need to be much safer. If you were to put a cowling around the prop you get both safety and a more directed air flow.
hmmm. Air Hogs Elite Helix X4 ios very quiet. Ned repalce ducted fun - more blades more rpm and more closed tube =)
I've actually been thinking about 3D printing a duct for a 9x5x3 prop. It would be a fun project once I get some spare time.
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