Why battery-powered aircraft will never have significant range

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tspear

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That comes from a simplistic comparison to the energy density of gasoline that ignores conversion efficiency.

Gasoline has 13200 Wh/kg energy density.
Such a simple statement covers a multitude of aspects, however one part often missed is the amount of drag used for cooling used to offset the lack of thermal efficiency avgas has required.

Tim
 

Dan Thomas

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Such a simple statement covers a multitude of aspects, however one part often missed is the amount of drag used for cooling used to offset the lack of thermal efficiency avgas has required.

Tim
Around 50% of the total energy goes out the exhaust pipe. Another 20 or 25% is lost via the cooling system. But a well-designed system, such as the P-51's, can end up with no drag at all. Even some lightplanes such as the old Cessna 310 used exhaust augmentors. The exhaust stream was directed into a duct that pulled the cooling air through the engine compartment, making that waste heat work to cool the engine.

1641919841270.png
 

J.L. Frusha

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Not to mention that the belly-scoop with the radiator for the P-51 Mustangs also produced thrust with waste heat...

Using that same principle, the waste heat of an oil cooler might produce additional thrust, in the same manner.
 

tspear

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Around 50% of the total energy goes out the exhaust pipe. Another 20 or 25% is lost via the cooling system. But a well-designed system, such as the P-51's, can end up with no drag at all. Even some lightplanes such as the old Cessna 310 used exhaust augmentors. The exhaust stream was directed into a duct that pulled the cooling air through the engine compartment, making that waste heat work to cool the engine.

View attachment 120340
I believe you 50% number is high. From what I recall, it is closer to 60-80% is wasted (easiest is to take the BSFC number and subtract from 1 to get a good efficiency number, and avgas engines at best approach .38 I believe and as low as .80 on full ROP takeoff). The problem with all such designs. They all tend to have very specific reals of efficiency, and produce much higher drag in other operational realms.
This is why Mooney and Cirrus both tend to have small entrances for the engine intakes, and decent gaps for the exhaust heat and the exhaust itself. Neither tries to turn the heat into thrust, instead they have focused on minimizing drag across the full envelope.

(Please check my math above, I am way outside my knowledge base!)

Tim
 

Dan Thomas

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Not to mention that the belly-scoop with the radiator for the P-51 Mustangs also produced thrust with waste heat...

Using that same principle, the waste heat of an oil cooler might produce additional thrust, in the same manner.
Lots of debate as to whether it did nor not, but in any case the drag was pretty much eliminated. The Meredith Effect, I think they called it.
 

AeroER

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Such a simple statement covers a multitude of aspects, however one part often missed is the amount of drag used for cooling used to offset the lack of thermal efficiency avgas has required.

Tim
The conversion efficiency of ICE and electric motor propulsion must be considered to make a rational comparison.

Both are nowhere close to 100% efficiency, and include the propeller.
 

Dan Thomas

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I believe you 50% number is high. From what I recall, it is closer to 60-80% is wasted...
Straight out of the Aircraft Systems textbook I used in teaching my Aircraft Systems class. Depending on who you consult, the numbers are all over the map. Here's another take on it:

1641921679091.png


Now, that friction loss is just more heat and usually ends up in the cooling system losses. The 30% effective power is likely a bit high for the typical aircooled aircraft engine, meaning more exhaust and cooling losses.
 

WINGITIS

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Around 50% of the total energy goes out the exhaust pipe. Another 20 or 25% is lost via the cooling system. But a well-designed system, such as the P-51's, can end up with no drag at all. Even some lightplanes such as the old Cessna 310 used exhaust augmentors. The exhaust stream was directed into a duct that pulled the cooling air through the engine compartment, making that waste heat work to cool the engine.

View attachment 120340
Like a Lewis machine gun....

LewisGunParts1.jpg
 

Pops

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Common Caustic Lye, the basis for true soaps... At one time almost every household in America made its own Lye from wood ashes, then produced various grades of lye soap, depending upon the purpose of the soap.
During WW-2 my grandmother made soap for all the family. Soap was rationed along with sugar, coffee, gasoline, etc. I hated to take a bath with grandmothers soap. Told my mother that the dirt kept me warm.
 

J.L. Frusha

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5 Gallons of Gas = 167.05053 Kilowatt Hours of electricity.

2021 estimate is $157/kWh on a usable-energy basis...

NIO says it will be selling 150 kWh solid-state batteries for 2022... ~$26,219 and ~917 lbs
 

Vigilant1

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5 Gallons of Gas = 167.05053 Kilowatt Hours of electricity.
FWIW
167 kwh is the heat content ('lower value") of the gasoline. We can't get that much electricity from it.

Relevant to airplanes, converting fuel to thrust might be about 25% efficient.

If we start with electricity (from a battery), we might get about 70% efficiency total in making thrust.

Still, there's no doubt that, in general, we can store a lot more motive power per pound and per dollar as gasoline rather than in a battery.
 
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Dusan

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So 16 pounds of batteries (minimum) for 1 pound of gasoline?
Specific energy of gasoline is at about 12900 Wh/Kg and best lithium batteries have 240 Wh/Kg (wikipedia), so the theoretical ratio is about 53. Taking into consideration that only about 25% of that gasoline chemical energy is converted into mechanical shaft power, vs about 75% for the chemical energy stored by batteries, that reduces the practical ratio to 53*0.25/0.75=17.9. So for comparing for the same shaft power, you need about 18 kg of batteries for 1 kg of gasoline.

The thing is more complicated as the specific power (Power-to-weight ratio) of electric motors are well above 10 kW/kg (e.g. Emrax 268 P/W=11.56 kW/kg) and piston engines P/W are much lower (Rotax 912 P/W=1.3 kW/kg). This means if designed for short flight, the full weight of the electric propulsion system could be on par or even lighter than a conventional piston propulsion system. The break even is at about 20 to 40 minutes.
 
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