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Does anyone have enough aerospace knowledge to comment on the design? Comments on the article seem skeptical on placement of the propellers, but I have absolute

Does anyone have enough aerospace knowledge to comment on the design? Comments on the article seem skeptical on placement of the propellers, but I have absolutely no expertise here to make any assessment myself.

csours 7y ago
I have no special knowledge myself, but as I understand it, the little propellers along the wing greatly reduce takeoff and landing speed and distance - they basically produce airspeed without groundspeed. Note - this is based on a video I watched one time, if I got this wrong, please educate me.
- gnramires 7y ago
  That's correct, essentially you can apply Newton's law (F = dp/dt): the propellers over wings can produce a strong downward momentum flux (air flux) that by reaction generates lift (upward force). Without this flux being directly controllable by the wings (or steerable in other ways), you need to rely on the vehicle's airspeed to generate sufficient airflow on the wings to produce lift (so you'd expect greater takeoff speed/distance necessary). You might wonder also about efficiency. Generally the energy efficiency of an aircraft is dictated by 1) friction losses 2) outgoing airstream velocity. The lower the friction and the slower you can push an air stream the more efficient the aircraft ((2) is because the kinetic energy imparted on the air is quadratic on speed, while the momentum, and force, is only linear in speed). That's why high-bypass turbofans are now the norm (more air at slower speeds). And why you want a large propeller surface of low speed (I expect at very large propellers you get diminishing/negative returns due to additional drag), which can be few large props or many smaller props (which have the previously discussed advantages).
starpilot 7y ago
There's no better place to put them if your goal is redundancy with lots of small props. Puller props are generally more efficient because they're not in the wake of the wing, but it's a tradeoff and you can Google as for the pros/cons of push vs. pull props. The larger tip props are really interesting. Larger, slower rotors are more efficient. Putting them far outboard puts them in the least-disturbed flow while knocking out the tip vortices. This is only possible with lightweight electric motors vs. shrouded turbines, otherwise the wing/wing box would be quite heavy to support such a long moment arm.
oliveshell 7y ago
Here’s a NASA paper on the propeller arrangement: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160010105.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201600... Apparently the small propellers’ primary function is “to act as a high-lift device [1]” rather than to generate thrust. They allow the wings to have a higher aspect ratio (i.e., be ‘long and skinny’) for reduced drag. Without them, such a wing would give undesirable low-speed handling characteristics and a higher stall speed. Also, their speed can be varied independently and quickly by computer to compensate for turbulence and wind gusts. Looks like quite an interesting design that plays to the strengths of electric motors. [1]: https://en.wikipedia.org/wiki/High-lift_device https://en.wikipedia.org/wiki/High-lift_device
- eternauta3k 7y ago
  Do you mean "lower stall speed"?
  - bathtub365 7y ago
    A higher stall speed means you need to go faster not to stall and is an undesirable property. Lower means you can go slower before stalling
  - numpad0 7y ago
    Stall speed = minimum takeoff/landing speed(without margins) Sort of the “speed you have to drive on the ramp on a highway”, you don’t want to have to merge at 250mph
Gibbon1 7y ago
The tip propellers reduce losses due to wing tip vortexes. From memory about half the drag from a wing comes from those. So potentially there is some savings there. The reason no ones built an conventional aircraft with tip props before is because mechanical limits, put a piston engine at the tip the wing and like a big weight on a spring. And losing an engine would mean real problems flying the aircraft. Electric motors are simple and have a higher power to weight ratio. So probably workable. And electric aircraft reducing drag is really important because of the low energy density of batteries compared to aviation fuel.

csours 7y ago

I have no special knowledge myself, but as I understand it, the little propellers along the wing greatly reduce takeoff and landing speed and distance - they basically produce airspeed without groundspeed. Note - this is based on a video I watched one time, if I got this wrong, please educate me.

gnramires 7y ago
That's correct, essentially you can apply Newton's law (F = dp/dt): the propellers over wings can produce a strong downward momentum flux (air flux) that by reaction generates lift (upward force). Without this flux being directly controllable by the wings (or steerable in other ways), you need to rely on the vehicle's airspeed to generate sufficient airflow on the wings to produce lift (so you'd expect greater takeoff speed/distance necessary). You might wonder also about efficiency. Generally the energy efficiency of an aircraft is dictated by 1) friction losses 2) outgoing airstream velocity. The lower the friction and the slower you can push an air stream the more efficient the aircraft ((2) is because the kinetic energy imparted on the air is quadratic on speed, while the momentum, and force, is only linear in speed). That's why high-bypass turbofans are now the norm (more air at slower speeds). And why you want a large propeller surface of low speed (I expect at very large propellers you get diminishing/negative returns due to additional drag), which can be few large props or many smaller props (which have the previously discussed advantages).

starpilot 7y ago

There's no better place to put them if your goal is redundancy with lots of small props. Puller props are generally more efficient because they're not in the wake of the wing, but it's a tradeoff and you can Google as for the pros/cons of push vs. pull props. The larger tip props are really interesting. Larger, slower rotors are more efficient. Putting them far outboard puts them in the least-disturbed flow while knocking out the tip vortices. This is only possible with lightweight electric motors vs. shrouded turbines, otherwise the wing/wing box would be quite heavy to support such a long moment arm.

oliveshell 7y ago

Here’s a NASA paper on the propeller arrangement: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20160010105.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201600... Apparently the small propellers’ primary function is “to act as a high-lift device [1]” rather than to generate thrust. They allow the wings to have a higher aspect ratio (i.e., be ‘long and skinny’) for reduced drag. Without them, such a wing would give undesirable low-speed handling characteristics and a higher stall speed. Also, their speed can be varied independently and quickly by computer to compensate for turbulence and wind gusts. Looks like quite an interesting design that plays to the strengths of electric motors. [1]: https://en.wikipedia.org/wiki/High-lift_device https://en.wikipedia.org/wiki/High-lift_device

eternauta3k 7y ago
Do you mean "lower stall speed"?
- bathtub365 7y ago
  A higher stall speed means you need to go faster not to stall and is an undesirable property. Lower means you can go slower before stalling
- numpad0 7y ago
  Stall speed = minimum takeoff/landing speed(without margins) Sort of the “speed you have to drive on the ramp on a highway”, you don’t want to have to merge at 250mph

Gibbon1 7y ago

The tip propellers reduce losses due to wing tip vortexes. From memory about half the drag from a wing comes from those. So potentially there is some savings there. The reason no ones built an conventional aircraft with tip props before is because mechanical limits, put a piston engine at the tip the wing and like a big weight on a spring. And losing an engine would mean real problems flying the aircraft. Electric motors are simple and have a higher power to weight ratio. So probably workable. And electric aircraft reducing drag is really important because of the low energy density of batteries compared to aviation fuel.