4 ms·

An electric ducted fan (EDF) is highly inefficient, had they put similar wattage through a gearbox to say a folding prop, efficiency may have doubled (albeit an

An electric ducted fan (EDF) is highly inefficient, had they put similar wattage through a gearbox to say a folding prop, efficiency may have doubled (albeit an admittedly more onerous form factor). This is just the beginning of electric assist single passenger flight! Reference: plenty of experience pushing 8KW+ through 2KG RC gliders (F5B) and far slower EDF models.

patentatt 6y ago
Bit of an aside, but are you saying you are flying an EDF glider?
- CrazyCatDog 6y ago
  Haven’t bothered :) Only “toy” EDF models, performance from an 8KW system revving at 60K rpm at 10S (10 lipo cells in series) swinging a 19” folding prop is on par with a mini turbine in terms of performance (0-> 180mph+ in under 2 seconds). Granted this is for 2 meter gliders, but I would expect similar efficiencies in larger scale applications (the bmw EDF is not that much larger than RC).
  - generalizations 6y ago
    That's crazy. For a non-RC dude, how much thrust does that produce, and how much did it cost?
    - CrazyCatDog 6y ago
      The $ sounds high, but it’s nothing compared to the time that goes into building and preparing... https://youtu.be/oCS84I8vJK8 https://youtu.be/oCS84I8vJK8
thimkerbell 6y ago
If someone wanted to learn all this stuff in a "for dummies" class, what course would they look for, among the MOOCs?
- melvinroest 6y ago
  +1 I'm also open to the idea that a HN'er teaches us. Yes, I know what I'm asking. You don't have to do it ;-) It would just be really nice :)
- FabHK 6y ago
  I liked this course: Introduction to Aeronautical Engineering, by Technical University Delft in the Netherlands, on edX. It assumes basic physics and maths, but then gives a nice overview about all the fundamentals - aerostatics, aerodynamics, propulsion, materials, structure of aircraft, etc. https://www.edx.org/course/introduction-to-aeronautical-engineering-2 https://www.edx.org/course/introduction-to-aeronautical-engi... If you're specifically looking at autonomous flight of drones, then the Udacity Flying Car nano degree is not bad (it contains several mistakes, but takes you through quite some material with neat exercises). It presupposes a bit more maths and programming. I was free (for a month) during the early stages of the pandemic, now I think it's back to normal prices. https://www.udacity.com/course/flying-car-nanodegree--nd787 https://www.udacity.com/course/flying-car-nanodegree--nd787
dTal 6y ago
It's true that big, slow things are, all other things being equal, more efficient than small, fast things. Momentum is mv, but kinetic energy is 1/2 mv^2, so that squared term means it's cheaper to move a lot of mass(m) slowly than a little of it quickly. You can build big, slow ducted fans, but you do hit size limits much earlier than with free props because of the shroud. HOWEVER, efficiency is maximized when outlet velocity matches airspeed (or you have wasted momentum that will dissipate into heat, when your outlet stream mingles with the rest of the atmosphere). For your slow gliders, a small ducted fan with a high outlet velocity will indeed be quite inefficient. But this guy, traveling at hundreds of mph, needs a propulsion unit with a similarly high outlet velocity. A big, slow thing isn't going to cut it for that. A big, fast thing would, but would also blow their power budget. Within their design constraints, this is pretty close to the optimal solution.[1] There's also the small matter of free props being horrifically dangerous :) [1] They've helpfully given us enough information to work out some basic aerodynamics of their fans. At 7.5 Kw each, assuming 90% efficiency each of the motors and ESCs, and eyeballing the duct and hub diameters as 18cm and 8cm each respectively (could be off, here), we should expect an outlet velocity of about 230 mph. The guy reached an airspeed of nearly 190mph, or over 80% of the outlet velocity, so they aren't doing badly at all considering the drag.
FabHK 6y ago
I thought efficiency of a fan < 2 counterrotating stacked fans < ducted fan?
wombatmobile 6y ago
> had they put similar wattage through a gearbox to say a folding prop, efficiency may have doubled (albeit an admittedly more onerous form factor) They chose not to do this because the more onerous form factor results in amputation of the pilot's viscera and legs.
- h2odragon 6y ago
  > results in amputation of the pilot's viscera and legs. Granted, that could be an issue, but once the government contracts for production of a few thousand units such minor flaws can be addressed with a field upgrade package, barely tripling the unit cost.
usrusr 6y ago
"electric assist single passenger flight" If electric is assist, what would be the main power source?

patentatt 6y ago

Bit of an aside, but are you saying you are flying an EDF glider?

CrazyCatDog 6y ago
Haven’t bothered :) Only “toy” EDF models, performance from an 8KW system revving at 60K rpm at 10S (10 lipo cells in series) swinging a 19” folding prop is on par with a mini turbine in terms of performance (0-> 180mph+ in under 2 seconds). Granted this is for 2 meter gliders, but I would expect similar efficiencies in larger scale applications (the bmw EDF is not that much larger than RC).
- generalizations 6y ago
  That's crazy. For a non-RC dude, how much thrust does that produce, and how much did it cost?
  - CrazyCatDog 6y ago
    The $ sounds high, but it’s nothing compared to the time that goes into building and preparing... https://youtu.be/oCS84I8vJK8 https://youtu.be/oCS84I8vJK8

thimkerbell 6y ago

If someone wanted to learn all this stuff in a "for dummies" class, what course would they look for, among the MOOCs?

melvinroest 6y ago
+1 I'm also open to the idea that a HN'er teaches us. Yes, I know what I'm asking. You don't have to do it ;-) It would just be really nice :)
FabHK 6y ago
I liked this course: Introduction to Aeronautical Engineering, by Technical University Delft in the Netherlands, on edX. It assumes basic physics and maths, but then gives a nice overview about all the fundamentals - aerostatics, aerodynamics, propulsion, materials, structure of aircraft, etc. https://www.edx.org/course/introduction-to-aeronautical-engineering-2 https://www.edx.org/course/introduction-to-aeronautical-engi... If you're specifically looking at autonomous flight of drones, then the Udacity Flying Car nano degree is not bad (it contains several mistakes, but takes you through quite some material with neat exercises). It presupposes a bit more maths and programming. I was free (for a month) during the early stages of the pandemic, now I think it's back to normal prices. https://www.udacity.com/course/flying-car-nanodegree--nd787 https://www.udacity.com/course/flying-car-nanodegree--nd787

dTal 6y ago

It's true that big, slow things are, all other things being equal, more efficient than small, fast things. Momentum is mv, but kinetic energy is 1/2 mv^2, so that squared term means it's cheaper to move a lot of mass(m) slowly than a little of it quickly. You can build big, slow ducted fans, but you do hit size limits much earlier than with free props because of the shroud. HOWEVER, efficiency is maximized when outlet velocity matches airspeed (or you have wasted momentum that will dissipate into heat, when your outlet stream mingles with the rest of the atmosphere). For your slow gliders, a small ducted fan with a high outlet velocity will indeed be quite inefficient. But this guy, traveling at hundreds of mph, needs a propulsion unit with a similarly high outlet velocity. A big, slow thing isn't going to cut it for that. A big, fast thing would, but would also blow their power budget. Within their design constraints, this is pretty close to the optimal solution.[1] There's also the small matter of free props being horrifically dangerous :) [1] They've helpfully given us enough information to work out some basic aerodynamics of their fans. At 7.5 Kw each, assuming 90% efficiency each of the motors and ESCs, and eyeballing the duct and hub diameters as 18cm and 8cm each respectively (could be off, here), we should expect an outlet velocity of about 230 mph. The guy reached an airspeed of nearly 190mph, or over 80% of the outlet velocity, so they aren't doing badly at all considering the drag.

FabHK 6y ago

I thought efficiency of a fan < 2 counterrotating stacked fans < ducted fan?

wombatmobile 6y ago

> had they put similar wattage through a gearbox to say a folding prop, efficiency may have doubled (albeit an admittedly more onerous form factor) They chose not to do this because the more onerous form factor results in amputation of the pilot's viscera and legs.

h2odragon 6y ago
> results in amputation of the pilot's viscera and legs. Granted, that could be an issue, but once the government contracts for production of a few thousand units such minor flaws can be addressed with a field upgrade package, barely tripling the unit cost.

usrusr 6y ago

"electric assist single passenger flight" If electric is assist, what would be the main power source?