5 ms·

For general aviation (small prop planes), it's because so much of the cost of running one comes from the engine. The most common piston engines are supposed to

For general aviation (small prop planes), it's because so much of the cost of running one comes from the engine. The most common piston engines are supposed to make it 1500-2000 hours between overhauls, and an overhaul runs around $16K, so you have to save $10-15 per flight hour in engine reserve to pay for the overhaul when it comes due and to cover any unexpected maintenance between overhauls. Plus, there are consumables that aren't cheap -- oil changes, spark plugs, and so forth. A brushless electric motor has a lot fewer moving parts, and a lot less vibration, so overhauls could be cheaper and with a larger interval between them, and regular maintenance would be cheaper as well. Plus, no leaded avgas to mess with, no worries about reduced power at higher altitudes where the air is thinner, no worries about carburetor icing or adjusting the air/fuel mixture. Sounds like a win to me.

TrueDuality 7y ago
Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against. This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor). That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well.
- dogma1138 7y ago
  I’m not entirely sure how you’ll cool the engines at high altitudes, combustion engines can use their exhaust to cool the engine. Using large radiators on an electric engine kinda defeats the purpose especially when they can’t be effectively be built into the body of the engine.
  - jandrese 7y ago
    I'm not sure why largeish passive radiators are a problem. If you're worried about the aesthetics you can put them inside of ducts and pass the air over them. This doesn't seem like it should be that big of an issue. Plus you'll probably want to recover some of that waste heat to heat the cabin.
  - xyzzyz 7y ago
    How exactly does one use exhaust to cool the engine? This is something I've never heard of in the context of cars of motorcycles, where exhaust is generally removed as soon as possible, and if it's used for anything, it's for running turbochargers that make the engine even hotter.
    - FabHK 7y ago
      Not sure what GP was referring to, but they might have had in mind the practice to run engines a bit too rich, so that fuel runs through the system with insufficient oxygen to burn, ie as a liquid, which cools the engine.
    - stevehawk 7y ago
      There are two strategies for keeping cylinders cool in cruise: push the red mixture knob forward for a rich fuel/air mixture setting that uses unburned fuel to lower cylinder head temperatures, or pull the mixture knob out to such a lean setting that an abundance of air and reduction in engine power and heat cools the cylinders. Running aircraft engines “lean of peak” (LOP) typically reduces airspeed about 5 percent in cruise while lowering fuel consumption about 20 percent—but not all engines are capable of LOP operations, and doing it wrong can cause permanent and costly engine damage. source: https://www.aopa.org/news-and-media/all-news/2019/may/flight-training-magazine/technique-lean-of-peak https://www.aopa.org/news-and-media/all-news/2019/may/flight...
  - yetihehe 7y ago
    Nope, combustion engines generate MUCH more heat than electric engines. Also they don't use exhaust to cool the engine, with all due respect, your argument is entirely backwards. It will be easier to cool electric engines and they will need smaller radiators (if any, have you seen radiators on engines in drones?). More probable is that you will need to heat batteries so that they don't freeze at higher altitudes.
- verelo 7y ago
  But no worse of a problem than it currently is for fossil fuel engines, right?
- iamtheworstdev 7y ago
  I don't math good, so I'll just ask : Does the RPM of the electric engine matter if the RPM of the propeller can't exceed a certain rate (because the tips of the prop will break the speed of sound, which you generally don't want for comfort reasons)? I mean this in terms of power generation and pushing air.
  - elihu 7y ago
    Gas-powered aircraft don't have transmissions, and gas engines tend to only be efficient in a narrow band, so I'd guess that the RPM limitations of an electric motor (which can generally tolerate a wider range) wouldn't be an issue as long as it's a reasonably-engineered motor. Beyond a certain RPM range electric motors lose torque. Eventually you'd reach the limits of the bearings or the maximum centrifugal force that the rotor can withstand, but that's usually at a higher RPM than gas engines are made to survive. I would expect an electric motor designed for an aircraft would be optimized for maximum power/efficiency at whatever RPM band is optimal for the propeller, and the battery would be optimized for whatever voltage is optimal for the motor at that RPM range.
- dbcurtis 7y ago
  > Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against. Yes, but air-density-altitute problems are identical no matter what is spinning the prop, righ? > This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor). OK, so do the math for me here, I am a bit confused. I am not a motor whiz, but torque at the output converted to watts is going to be power at the input less i^2*r losses. The back-EMF will go up as the motor speeds up, so it may be at a somewhat less efficient point on the RPM-versus-torque curve, but the resistance of the actual wires doesn't change. > That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well. I think that is all true. Seems to me you design cooling around the cooling capacity of the air flow at max operating altitude. Is that really a huge design constraint?
- ericpauley 7y ago
  As another commenter noted, propeller speeds are limited by multiple factors, and generally complex aircraft use a constant-speed propeller that changes pitch. Because of this the increased electric speed likely isn't a factor.
- tlb 7y ago
  Cooling the motors isn't a big problem. Their heat output is around 10% of the shaft power, compared to combustion engines that put out 200% of shaft power as heat. While piston engines need a lot of cooling air flow and often have cowls that are mostly open at the front, an electric propellor drive can have a small cooling vent. The batteries may produce as much heat as the motor, and they have a more limited temperature range. Their service life decreases when run above 40 C. Tesla batteries are liquid-cooled.
  - Mountain_Skies 7y ago
    Could low temperatures especially at higher altitudes be a problem? Aircraft probably get exposed to higher temperature gradients and in much shorter periods of time than a typical Tesla would encounter. Not sure about Lithium Sulfur batteries but reduced capacity at lower temperatures is common for many battery types.
    - tlb 7y ago
      It's certainly not a problem if the batteries are in the climate-controlled fuselage. It might be tricky to keep them insulated inside the wings (where fuel typically is.)
blendo 7y ago
And the motor _should_ be a lot more reliable. But discouragingly, just last August, in Norway, a Pipistrel Alpha Electro lost power shortly after takeoff and crashed. https://aviation-safety.net/wikibase/228207 https://aviation-safety.net/wikibase/228207 Hopefully, these folks in Fresno, CA will get their electric aircraft flight school up and running sometime soon. But summertime in California's central valley is challenging: https://sustainableaviationproject.com/2018/10/28/we-hit-the-100-hour-mark-for-endurance-testing/ https://sustainableaviationproject.com/2018/10/28/we-hit-the...
squarefoot 7y ago
"A brushless electric motor has a lot fewer moving parts" Not to mention it makes a lot less weight to lift; the difference is huge.
- simonebrunozzi 7y ago
  But the battery would be much heavier than a fuel tank. You can't have an electric engine without a battery, so...
  - FabHK 7y ago
    Well, there are fuel cells (that turn fuel into electricity).

TrueDuality 7y ago

Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against. This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor). That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well.

dogma1138 7y ago
I’m not entirely sure how you’ll cool the engines at high altitudes, combustion engines can use their exhaust to cool the engine. Using large radiators on an electric engine kinda defeats the purpose especially when they can’t be effectively be built into the body of the engine.
- jandrese 7y ago
  I'm not sure why largeish passive radiators are a problem. If you're worried about the aesthetics you can put them inside of ducts and pass the air over them. This doesn't seem like it should be that big of an issue. Plus you'll probably want to recover some of that waste heat to heat the cabin.
- xyzzyz 7y ago
  How exactly does one use exhaust to cool the engine? This is something I've never heard of in the context of cars of motorcycles, where exhaust is generally removed as soon as possible, and if it's used for anything, it's for running turbochargers that make the engine even hotter.
  - FabHK 7y ago
    Not sure what GP was referring to, but they might have had in mind the practice to run engines a bit too rich, so that fuel runs through the system with insufficient oxygen to burn, ie as a liquid, which cools the engine.
  - stevehawk 7y ago
    There are two strategies for keeping cylinders cool in cruise: push the red mixture knob forward for a rich fuel/air mixture setting that uses unburned fuel to lower cylinder head temperatures, or pull the mixture knob out to such a lean setting that an abundance of air and reduction in engine power and heat cools the cylinders. Running aircraft engines “lean of peak” (LOP) typically reduces airspeed about 5 percent in cruise while lowering fuel consumption about 20 percent—but not all engines are capable of LOP operations, and doing it wrong can cause permanent and costly engine damage. source: https://www.aopa.org/news-and-media/all-news/2019/may/flight-training-magazine/technique-lean-of-peak https://www.aopa.org/news-and-media/all-news/2019/may/flight...
- yetihehe 7y ago
  Nope, combustion engines generate MUCH more heat than electric engines. Also they don't use exhaust to cool the engine, with all due respect, your argument is entirely backwards. It will be easier to cool electric engines and they will need smaller radiators (if any, have you seen radiators on engines in drones?). More probable is that you will need to heat batteries so that they don't freeze at higher altitudes.
verelo 7y ago
But no worse of a problem than it currently is for fossil fuel engines, right?
iamtheworstdev 7y ago
I don't math good, so I'll just ask : Does the RPM of the electric engine matter if the RPM of the propeller can't exceed a certain rate (because the tips of the prop will break the speed of sound, which you generally don't want for comfort reasons)? I mean this in terms of power generation and pushing air.
- elihu 7y ago
  Gas-powered aircraft don't have transmissions, and gas engines tend to only be efficient in a narrow band, so I'd guess that the RPM limitations of an electric motor (which can generally tolerate a wider range) wouldn't be an issue as long as it's a reasonably-engineered motor. Beyond a certain RPM range electric motors lose torque. Eventually you'd reach the limits of the bearings or the maximum centrifugal force that the rotor can withstand, but that's usually at a higher RPM than gas engines are made to survive. I would expect an electric motor designed for an aircraft would be optimized for maximum power/efficiency at whatever RPM band is optimal for the propeller, and the battery would be optimized for whatever voltage is optimal for the motor at that RPM range.
dbcurtis 7y ago
> Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against. Yes, but air-density-altitute problems are identical no matter what is spinning the prop, righ? > This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor). OK, so do the math for me here, I am a bit confused. I am not a motor whiz, but torque at the output converted to watts is going to be power at the input less i^2*r losses. The back-EMF will go up as the motor speeds up, so it may be at a somewhat less efficient point on the RPM-versus-torque curve, but the resistance of the actual wires doesn't change. > That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well. I think that is all true. Seems to me you design cooling around the cooling capacity of the air flow at max operating altitude. Is that really a huge design constraint?
ericpauley 7y ago
As another commenter noted, propeller speeds are limited by multiple factors, and generally complex aircraft use a constant-speed propeller that changes pitch. Because of this the increased electric speed likely isn't a factor.
tlb 7y ago
Cooling the motors isn't a big problem. Their heat output is around 10% of the shaft power, compared to combustion engines that put out 200% of shaft power as heat. While piston engines need a lot of cooling air flow and often have cowls that are mostly open at the front, an electric propellor drive can have a small cooling vent. The batteries may produce as much heat as the motor, and they have a more limited temperature range. Their service life decreases when run above 40 C. Tesla batteries are liquid-cooled.
- Mountain_Skies 7y ago
  Could low temperatures especially at higher altitudes be a problem? Aircraft probably get exposed to higher temperature gradients and in much shorter periods of time than a typical Tesla would encounter. Not sure about Lithium Sulfur batteries but reduced capacity at lower temperatures is common for many battery types.
  - tlb 7y ago
    It's certainly not a problem if the batteries are in the climate-controlled fuselage. It might be tricky to keep them insulated inside the wings (where fuel typically is.)

blendo 7y ago

And the motor _should_ be a lot more reliable. But discouragingly, just last August, in Norway, a Pipistrel Alpha Electro lost power shortly after takeoff and crashed. https://aviation-safety.net/wikibase/228207 https://aviation-safety.net/wikibase/228207 Hopefully, these folks in Fresno, CA will get their electric aircraft flight school up and running sometime soon. But summertime in California's central valley is challenging: https://sustainableaviationproject.com/2018/10/28/we-hit-the-100-hour-mark-for-endurance-testing/ https://sustainableaviationproject.com/2018/10/28/we-hit-the...

squarefoot 7y ago

"A brushless electric motor has a lot fewer moving parts" Not to mention it makes a lot less weight to lift; the difference is huge.

simonebrunozzi 7y ago
But the battery would be much heavier than a fuel tank. You can't have an electric engine without a battery, so...
- FabHK 7y ago
  Well, there are fuel cells (that turn fuel into electricity).