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For context, a gallon of gas carries about 33.4 Kwh, a Cessna 172 has a fuel capacity of 56 gallons. Topped up at take-off it's carrying the equivalent of 1.87

For context, a gallon of gas carries about 33.4 Kwh, a Cessna 172 has a fuel capacity of 56 gallons. Topped up at take-off it's carrying the equivalent of 1.87 Mwh. That will take you about 1,200 km. I expect short range electric commuters will come first - similar to cars.

wyldfire 4y ago
For the mass of 56 gallons of gas (~200kg) you could carry 240kwh of Li-air battery? Feel free to check my math, could've gotten it wrong.
tjmc 4y ago
What's the conversion efficiency with a Cessna 172's ancient engine though? Batteries don't have to get that much better for electrics to dominate the training GA market. There's a Pipistril Velis Electro[1] that shares a hanger with my flight school. It can only fly for about 30 minutes + 30 mins reserve with 2 relatively light pilots. If it had a battery which allowed for 2 hours of flight you've now tripled your range - 1.5hours + 30 mins reserve. That would cover all ab-initio training. A battery like the one in the article would expand the range further to cross country capability and/or higher useful load, with the complete reconstruction of a piston engine every 2000 hours gone from the equation. Can't wait. 1. https://www.pipistrel-aircraft.com/products/general-aviation/velis-electro/#tab-id-2 https://www.pipistrel-aircraft.com/products/general-aviation...
cameldrv 4y ago
The math is quite a bit better than those numbers would suggest. An aircraft piston engine is roughly 25% efficient, but an electric motor can be over 90%. A gallon of gas weighs about 2.7 kg, so the gas is about 12.3 kWh/kg. If we adjust this type of battery to account for the difference in efficiency, it is about 4.3 (equivalent) kW/kg. Ok, so that's still a factor of 3 worse. However, all the gas in the Cessna weighs about 336 lbs. The engine weighs about 250 lbs. A comparable electric motor is about 70 lbs. That gives you another 180 lbs, or over 50% more weight to fill with batteries. Now we're at about 50% of the useful energy of the Cessna. You can also factor in a little bit of gain from "regenerative braking" since the electric airplane can charge the batteries while descending. This may not make a huge difference in range for just point to point flying, but it does add some extra for flight training with lots of landings, and also provides some extra reserve in the case of a missed approach for example. All of that is to say that this doesn't bring the small electric airplane to parity with gasoline, but it's starting to much more credibly inch in on its turf.
- edrxty 4y ago
  50% is well within the engineering margin available too. Ultimately you can make 2 seaters that are a little bigger than existing ones but have the same range. Most Cessnas don't fly with full tanks all the time, people usually fill them to ~60% (tabs) for the vast majority of flying and training. Also, hyper efficient aircraft like the LongEZ (2000mi range on 52usg) become more interesting. Most legacy piston singles are horribly inefficient aerodynamically, to the point where just comparing them by frontal cross section is a completely legitimate strategy. A large amount of thrust is wasted just cooling the engine and the wings on most Cessna's are not flush riveted, seams not covered, etc. People often wonder why multi engine aircraft don't have 2x the performance; they have over 2x the cross section. Composite aircraft have the potential to allow for extremely competitive designs. I've done estimates before suggesting we don't even need 50% of the performance, but rather somewhere around 30% to start to be competitive. 1.2kw/kg is well beyond viable.

wyldfire 4y ago

For the mass of 56 gallons of gas (~200kg) you could carry 240kwh of Li-air battery? Feel free to check my math, could've gotten it wrong.

tjmc 4y ago

What's the conversion efficiency with a Cessna 172's ancient engine though? Batteries don't have to get that much better for electrics to dominate the training GA market. There's a Pipistril Velis Electro[1] that shares a hanger with my flight school. It can only fly for about 30 minutes + 30 mins reserve with 2 relatively light pilots. If it had a battery which allowed for 2 hours of flight you've now tripled your range - 1.5hours + 30 mins reserve. That would cover all ab-initio training. A battery like the one in the article would expand the range further to cross country capability and/or higher useful load, with the complete reconstruction of a piston engine every 2000 hours gone from the equation. Can't wait. 1. https://www.pipistrel-aircraft.com/products/general-aviation/velis-electro/#tab-id-2 https://www.pipistrel-aircraft.com/products/general-aviation...

cameldrv 4y ago

The math is quite a bit better than those numbers would suggest. An aircraft piston engine is roughly 25% efficient, but an electric motor can be over 90%. A gallon of gas weighs about 2.7 kg, so the gas is about 12.3 kWh/kg. If we adjust this type of battery to account for the difference in efficiency, it is about 4.3 (equivalent) kW/kg. Ok, so that's still a factor of 3 worse. However, all the gas in the Cessna weighs about 336 lbs. The engine weighs about 250 lbs. A comparable electric motor is about 70 lbs. That gives you another 180 lbs, or over 50% more weight to fill with batteries. Now we're at about 50% of the useful energy of the Cessna. You can also factor in a little bit of gain from "regenerative braking" since the electric airplane can charge the batteries while descending. This may not make a huge difference in range for just point to point flying, but it does add some extra for flight training with lots of landings, and also provides some extra reserve in the case of a missed approach for example. All of that is to say that this doesn't bring the small electric airplane to parity with gasoline, but it's starting to much more credibly inch in on its turf.

edrxty 4y ago
50% is well within the engineering margin available too. Ultimately you can make 2 seaters that are a little bigger than existing ones but have the same range. Most Cessnas don't fly with full tanks all the time, people usually fill them to ~60% (tabs) for the vast majority of flying and training. Also, hyper efficient aircraft like the LongEZ (2000mi range on 52usg) become more interesting. Most legacy piston singles are horribly inefficient aerodynamically, to the point where just comparing them by frontal cross section is a completely legitimate strategy. A large amount of thrust is wasted just cooling the engine and the wings on most Cessna's are not flush riveted, seams not covered, etc. People often wonder why multi engine aircraft don't have 2x the performance; they have over 2x the cross section. Composite aircraft have the potential to allow for extremely competitive designs. I've done estimates before suggesting we don't even need 50% of the performance, but rather somewhere around 30% to start to be competitive. 1.2kw/kg is well beyond viable.