4 ms·

There are other reasons for rotors to be overhead, as well. Locating the center of mass below the center of thrust makes a more stable system. Viewing the groun

There are other reasons for rotors to be overhead, as well. Locating the center of mass below the center of thrust makes a more stable system. Viewing the ground, which is something that many passengers would desire, is much easier when there isn't a bunch of machinery in the way. The further rotors are from the ground, the less likely they are to strike debris when landing and taking off. I'm trying to think of reasons it would be good to have low rotors... maybe something about noise experienced by the passengers?

rtkwe 7y ago
Getting them high enough up would require a larger super structure supporting them and would look pretty goofy to get them high enough up that a standing person wouldn't risk hitting them. These can also be stopped pretty quickly just by shorting all the motor leads together so stopping them in an emergency isn't very hard.
- jcims 7y ago
  I'll go with goofy looking and keep my legs. :)
  - rtkwe 7y ago
    The other thing is these are meant for operation over water so if you have crashed it will be into water and the blades will stop very quickly due to the drag.
- FabHK 7y ago
  I think there are extremely good reasons to keep the rotors on top, not below, with people around the craft or entering and exiting. One prop in front of single engine plane is enough of a safety hassle that one makes a big deal out of it in safety briefings, and yet people run into them again and again [1][2]. At any rate, the Volocopter [3] has the rotors on top, and while it might not look as sporty as the Kitty Hawk, I don't find it annoyingly goofy. [1] https://www.huffpost.com/entry/lauren-scruggs-model-plane-propeller_n_1382613 https://www.huffpost.com/entry/lauren-scruggs-model-plane-pr... [2] http://www.rightthisminute.com/video/graphic-man-hit-plane-propeller http://www.rightthisminute.com/video/graphic-man-hit-plane-p... [3] https://www.volocopter.com/en/ https://www.volocopter.com/en/
Retric 7y ago
> Locating the center of mass below the center of thrust makes a more stable system. That’s a common fallacy, early rockets placed the nozzle’s on top based on this assumption but it’s not useful. Ex: https://en.m.wikipedia.org/wiki/Rocket#/media/File%3AGoddard_and_Rocket.jpg https://en.m.wikipedia.org/wiki/Rocket#/media/File%3AGoddard... The issue is the thrust is in line with the angle of the craft so you get the same thrust vector from below or above the aircraft. Edit: https://en.m.wikipedia.org/wiki/Pendulum_rocket_fallacy https://en.m.wikipedia.org/wiki/Pendulum_rocket_fallacy
- jessaustin 7y ago
  The rocket situation is not clearly similar to this one. In the first place, these craft are controlled by varying the speeds of the different rotors. That means that the thrust is not "in line with the angle of the craft".
  - Retric 7y ago
    They are still attached to the aircraft so net vector rotates with the aircraft. Take a modern twin engine aircraft and they can turn by changing the thrust from each engine, but when the engines are outputting identical thrust they don’t add stability. In both cases rotating the aircraft also rotates the net thrust in an identical fashion. Which means there is zero restoring force from the engines to return them to the original orientation. PS: Fixed wing aircraft get stability from their airframes not their engines. https://en.m.wikipedia.org/wiki/Longitudinal_static_stability https://en.m.wikipedia.org/wiki/Longitudinal_static_stabilit... Though in fighter jets this may be aided by active control.

rtkwe 7y ago

Getting them high enough up would require a larger super structure supporting them and would look pretty goofy to get them high enough up that a standing person wouldn't risk hitting them. These can also be stopped pretty quickly just by shorting all the motor leads together so stopping them in an emergency isn't very hard.

jcims 7y ago
I'll go with goofy looking and keep my legs. :)
- rtkwe 7y ago
  The other thing is these are meant for operation over water so if you have crashed it will be into water and the blades will stop very quickly due to the drag.
FabHK 7y ago
I think there are extremely good reasons to keep the rotors on top, not below, with people around the craft or entering and exiting. One prop in front of single engine plane is enough of a safety hassle that one makes a big deal out of it in safety briefings, and yet people run into them again and again [1][2]. At any rate, the Volocopter [3] has the rotors on top, and while it might not look as sporty as the Kitty Hawk, I don't find it annoyingly goofy. [1] https://www.huffpost.com/entry/lauren-scruggs-model-plane-propeller_n_1382613 https://www.huffpost.com/entry/lauren-scruggs-model-plane-pr... [2] http://www.rightthisminute.com/video/graphic-man-hit-plane-propeller http://www.rightthisminute.com/video/graphic-man-hit-plane-p... [3] https://www.volocopter.com/en/ https://www.volocopter.com/en/

Retric 7y ago

> Locating the center of mass below the center of thrust makes a more stable system. That’s a common fallacy, early rockets placed the nozzle’s on top based on this assumption but it’s not useful. Ex: https://en.m.wikipedia.org/wiki/Rocket#/media/File%3AGoddard_and_Rocket.jpg https://en.m.wikipedia.org/wiki/Rocket#/media/File%3AGoddard... The issue is the thrust is in line with the angle of the craft so you get the same thrust vector from below or above the aircraft. Edit: https://en.m.wikipedia.org/wiki/Pendulum_rocket_fallacy https://en.m.wikipedia.org/wiki/Pendulum_rocket_fallacy

jessaustin 7y ago
The rocket situation is not clearly similar to this one. In the first place, these craft are controlled by varying the speeds of the different rotors. That means that the thrust is not "in line with the angle of the craft".
- Retric 7y ago
  They are still attached to the aircraft so net vector rotates with the aircraft. Take a modern twin engine aircraft and they can turn by changing the thrust from each engine, but when the engines are outputting identical thrust they don’t add stability. In both cases rotating the aircraft also rotates the net thrust in an identical fashion. Which means there is zero restoring force from the engines to return them to the original orientation. PS: Fixed wing aircraft get stability from their airframes not their engines. https://en.m.wikipedia.org/wiki/Longitudinal_static_stability https://en.m.wikipedia.org/wiki/Longitudinal_static_stabilit... Though in fighter jets this may be aided by active control.