6 ms·

This 'proposal' seems to assume that the rest of the aircraft would be almost unaffected by the change, which is very unlikely. Existing engines are designed t

by nickff 10y ago

This 'proposal' seems to assume that the rest of the aircraft would be almost unaffected by the change, which is very unlikely. Existing engines are designed to compress air, combust fuel, and recover energy from the exhaust to push air (to propel the aircraft) then compress more (to keep the cycle going). Once you are no longer combusting fuel, all of that compression is simply wasted energy. An electric aircraft is much more likely to have single-stage axial fans, with a much larger diameter than jets like the Concorde (because larger and slower fans are more efficient). This difference in propulsion will probably affect various other elements of airframe design. As an aside, Elon Musk has mentioned VTOL as a possible feature of electric aircraft, and this makes a lot of sense. If the fans can be tilted (to provide part of the 'lift' for takeoffs and landings), it means that the airplane could have very small wings, which are usually good for efficiency, but cause problems while landing conventionally.

scoot 10y ago
> it means that the airplane could have very small wings, which are usually good for efficiency This is all well and good under normal flight conditions, but the glide ratio in the event of motor failure would not lend itself well to commercial flight. Short wing aircraft are fine for military use where low occupancy and ejector seats provide emergency egress. Passenger aircraft - not so much.
- nickff 10y ago
  It depends on the reliability of the motors and how many of them there are. With electric aircraft, you could have many more engines spread along the wing (which has some benefits), so the redundancy may be enough to guarantee some power for landing. In addition, there is no 'safe' aircraft, and all of them can fall out of the sky; glide ratios don't help you if the wing breaks at the fuselage. If the motors are as reliable as the wings, then making them more critical seems reasonable.
etwigg 10y ago
> Once you are no longer combusting fuel, all of that compression is simply wasted energy. Not true. Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. And the efficiency with which that heat is turned into mechanical power will be dependent on the compression ratio, regardless of whether the thermal energy is coming from electricity or burning fuel. Thermal energy in chemical fuel and the potential energy in a battery or a raised weight have the same units, but there's actually a difference. You can turn energy in a battery or a raised weight into mechanical energy with 100% efficiency - not achieved in practice, but there's no theoretical limit. When you turn thermal energy into mechanical energy (via Otto cycle in a car or Brayton cycle in a jet), the peak efficiency you can achieve has a theoretical limit based on how much you can compress the working fluid (air) before injecting thermal energy (burning fuel or an electric arc). So if you've got electic energy that you can turn into mechanical energy in a fan with 100% efficiency, why would you take the thermodynamic losses of "burning" that electricity? Because we don't know how to go supersonic speeds with a fan or propeller - the only way we know how, for now, is with a jet running on the brayton cycle. And all the advances we've made in high compression fuel-burning jets would apply to electric jets just as well. If you're okay limiting your top speed to ~Mach 0.7, then you're correct spinning a large propeller using electricity is absolutely a better way to go, and you can make that propeller a lot bigger if you're able to tilt it. Karem Aircraft has done extremely interesting work on figuring out how to tilt rotors, and much of their research would apply to electric motors as well: http://karemaircraft.com/ http://karemaircraft.com/
- thom_nic 10y ago
  This idea of an electrical-arc jet engine is new to me. Can you point to any published material on such a design or prototype? Very intriguing.
  - nickff 10y ago
    Nobody who has taken a course in energy conversion (an second or third level thermodynamics course) would even bother to propose this. It is not coincidental that all mechanical engineers take this course, and mechanical engineers are also the people who design engines.

by nickff 10y ago

scoot 10y ago

> it means that the airplane could have very small wings, which are usually good for efficiency This is all well and good under normal flight conditions, but the glide ratio in the event of motor failure would not lend itself well to commercial flight. Short wing aircraft are fine for military use where low occupancy and ejector seats provide emergency egress. Passenger aircraft - not so much.

nickff 10y ago
It depends on the reliability of the motors and how many of them there are. With electric aircraft, you could have many more engines spread along the wing (which has some benefits), so the redundancy may be enough to guarantee some power for landing. In addition, there is no 'safe' aircraft, and all of them can fall out of the sky; glide ratios don't help you if the wing breaks at the fuselage. If the motors are as reliable as the wings, then making them more critical seems reasonable.

etwigg 10y ago

> Once you are no longer combusting fuel, all of that compression is simply wasted energy. Not true. Instead of burning fuel, you can use electricity (something like an arc welder) to generate heat. And the efficiency with which that heat is turned into mechanical power will be dependent on the compression ratio, regardless of whether the thermal energy is coming from electricity or burning fuel. Thermal energy in chemical fuel and the potential energy in a battery or a raised weight have the same units, but there's actually a difference. You can turn energy in a battery or a raised weight into mechanical energy with 100% efficiency - not achieved in practice, but there's no theoretical limit. When you turn thermal energy into mechanical energy (via Otto cycle in a car or Brayton cycle in a jet), the peak efficiency you can achieve has a theoretical limit based on how much you can compress the working fluid (air) before injecting thermal energy (burning fuel or an electric arc). So if you've got electic energy that you can turn into mechanical energy in a fan with 100% efficiency, why would you take the thermodynamic losses of "burning" that electricity? Because we don't know how to go supersonic speeds with a fan or propeller - the only way we know how, for now, is with a jet running on the brayton cycle. And all the advances we've made in high compression fuel-burning jets would apply to electric jets just as well. If you're okay limiting your top speed to ~Mach 0.7, then you're correct spinning a large propeller using electricity is absolutely a better way to go, and you can make that propeller a lot bigger if you're able to tilt it. Karem Aircraft has done extremely interesting work on figuring out how to tilt rotors, and much of their research would apply to electric motors as well: http://karemaircraft.com/ http://karemaircraft.com/

thom_nic 10y ago
This idea of an electrical-arc jet engine is new to me. Can you point to any published material on such a design or prototype? Very intriguing.
- nickff 10y ago
  Nobody who has taken a course in energy conversion (an second or third level thermodynamics course) would even bother to propose this. It is not coincidental that all mechanical engineers take this course, and mechanical engineers are also the people who design engines.