# Why battery-powered aircraft will never have significant range

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#### BBerson

##### Light Plane Philosopher
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)
At what power level, normal or short boost that causes overheat?
What is the motor efficiency at overboost power compared with rated continuous?

#### Vigilant1

##### Well-Known Member
Electric power train is simple and light..might drop the ratio to 8/1.
Maybe, for a very unusual case where the fuel+ powerplant weight was a very high percentage of MTOW and/or if aircraft weight was a very big factor driving total fuel used in a typical flight (e.g powered lift aircraft, fuel used in a time-to-climb record attempt, etc). For typical GA aircraft with a typical (approx 75% ?) share of time spent in cruise flight, total weight isn't a big factor in determining fuel required (because induced drag is less significant than parasite drag)

#### tspear

##### Well-Known Member
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.
Again, over stating the delta between power sources. Go back to much earlier in the thread to the interview with the CEO of Pipistrel, he showed how battery density only needs to double or triple before electric planes will reach a tipping point and push avgas out of the way.
Next, go look at the performance difference between the Alpha Elektro and the Alpha Trainer, the avgas version has a range that is slightly larger than four times the electric version. (Yes the electric cruises slightly slower). But this is not even close to the 53 times multiple or the 17.9 times listed above.

Tim

#### Dan Thomas

##### Well-Known Member
In theory yes, but since electric AC has less cooling demand, we might be at 10/1 need.
Batteries and electric motors need cooling, too. In cold weather the cabin also needs heating, and that isn't free with electric as it is with gasoline.

#### Dan Thomas

##### Well-Known Member
Electric power train is simple and light..might drop the ratio to 8/1.
Simple and light until you add the batteries. Then it ain't so simple and light anymore.

#### Dan Thomas

##### Well-Known Member
Next, go look at the performance difference between the Alpha Elektro and the Alpha Trainer, the avgas version has a range that is slightly larger than four times the electric version. (Yes the electric cruises slightly slower). But this is not even close to the 53 times multiple or the 17.9 times listed above.
We discussed that before. They HAD to reduce the cruise speed to get the range to look less than horrible. And they HAD to limit the climb time for the same reason. If you start flying it like you can fly the avgassed airplane, the range will be truly awful.

#### tspear

##### Well-Known Member
We discussed that before. They HAD to reduce the cruise speed to get the range to look less than horrible. And they HAD to limit the climb time for the same reason. If you start flying it like you can fly the avgassed airplane, the range will be truly awful.
Agree, I am just pointing out the delta is not nearly as great as implied by the statements which focus just on the energy density of fuel vs battery.
I do not have the knowledge calculate what the difference in power draw should be with the speed change in cruise. No clue on how the other factors come into play either.

Tim

#### Sraight'nlevel

##### Well-Known Member
Simple and light until you add the batteries. Then it ain't so simple and light anymore.

That is true, but Cessna 172 has 1/8 of its mass fuel...where as Rutan Voyager had 3,5/1 ratio of fuel.

So I'd say electric aeroplane is doable...even for a long range flying.

Not cheap, but doable.

#### Dan Thomas

##### Well-Known Member
That is true, but Cessna 172 has 1/8 of its mass fuel...where as Rutan Voyager had 3,5/1 ratio of fuel.

So I'd say electric aeroplane is doable...even for a long range flying.

Not cheap, but doable.
The takeoff of the Rutan Voyager revealed what happens when you carry that much fuel. So did the takeoff of the Spirit of St. Louis. That much overload does not indicate that tons of batteries are the way to go. The average pilot has trouble enough flying a 172, never mind something with a huge wing loading and a high stall speed. The accident rate of light aircraft would go out of sight, and they would be fatal accidents.

The Voyager was a 10,000-pound airplane (gross) that maxed out at 122 MPH. It was a purpose-built airplane, for the round-the-world nonstop flight. No sane person would build an electric two-seat airplane as heavy as that just so they could fly a reasonably long distance. remember, too, that the Voyager's weight would fall dramatically as the fuel was consumed. An electric airplane's weight doesn't change. Starts heavy, stays heavy.

#### Sraight'nlevel

##### Well-Known Member
The takeoff of the Rutan Voyager revealed what happens when you carry that much fuel. So did the takeoff of the Spirit of St. Louis. That much overload does not indicate that tons of batteries are the way to go. The average pilot has trouble enough flying a 172, never mind something with a huge wing loading and a high stall speed. The accident rate of light aircraft would go out of sight, and they would be fatal accidents.

The Voyager was a 10,000-pound airplane (gross) that maxed out at 122 MPH. It was a purpose-built airplane, for the round-the-world nonstop flight. No sane person would build an electric two-seat airplane as heavy as that just so they could fly a reasonably long distance. remember, too, that the Voyager's weight would fall dramatically as the fuel was consumed. An electric airplane's weight doesn't change. Starts heavy, stays heavy.
Yes sure, but it's the only way with electrics without solar.

1/1 weight of the batteries and the craft.....just like in cars.

#### Aesquire

##### Well-Known Member
Any price per Killowatt hour you quote today is as irrelevant to the future battery in the future plane as the 1960 dollars used for decades to uh, let's say "inform Congress" about inflation of costs of government expenditures.

Frankly, it won't matter what electricity costs except to limit your flying like the cost of fuel. By the time you've spent double the cost of a gasoline powered airplane you're committed, unless you're in the tiny minority to whom money is unimportant to your whims because you have billion\$. For the richest people it won't matter if the gasoline or electricity or caviar is expensive or rationed or forbidden.

Btw , I base the "double the cost" statement on the few electric craft I can actually afford & order today and fly this spring. It's a Ultralight Trike, but it could be an electric paramotor for about the same cost ratio, double, with less range than a 5 gallon gasoline version. When you get up to Pipistrel sized and cost craft the price ratio may improve, since a Rotax 912 isn't cheap.

I don't have the budget for a Pipistrel, darn it. The electric Xenos conversion, maybe.

#### Dusan

##### Well-Known Member
At what power level, normal or short boost that causes overheat?
What is the motor efficiency at overboost power compared with rated continuous?
Overheat is created when the cooling system does not manage to remove heat produced. This could happen even if operating the motor under the rated power if operated at a regime of lower efficiency, (e.g. more torque, less rpm).

Overboosting is always on the downwards efficiency slope, so more heat is generated, see this performance graph:
This extra heat needs to be removed - and this works by making the cooling system more effective, but that works only to a point, so there is a time limit for overboosting, depending on the calorific capacity of the motor, the 'heat inertia' - how much heat can it soak in before overheating.

#### BBerson

##### Light Plane Philosopher
This extra heat needs to be removed - and this works by making the cooling system more effective,
Nice graph.
So where on that graph are most VTOL operating for best power to weight ratio (from your post 2220)?
I suspect they are operating in “max power” where the efficiency looks about 60%. And battery efficiency is also lower at that max power.

#### Dusan

##### Well-Known Member
Again, over stating the delta between power sources. Go back to much earlier in the thread to the interview with the CEO of Pipistrel, he showed how battery density only needs to double or triple before electric planes will reach a tipping point and push avgas out of the way.
Next, go look at the performance difference between the Alpha Elektro and the Alpha Trainer, the avgas version has a range that is slightly larger than four times the electric version. (Yes the electric cruises slightly slower). But this is not even close to the 53 times multiple or the 17.9 times listed above.

Tim
As I stated in my previous post, the electric propulsion performance is augmented a lot by the electric motor's large power-to-weight ratio. For the same shaft delivered power the electric motor and controller weight is about 1/4 the weight of the piston ICE, leaving a lot of room for the batteries weight, the break even now is between about 20 to 40 minutes. When the batteries specific energy (energy/weight - not battery density, which is volume/weight) doubles and triples, the breakeven with a conventional gasoline ICE will move somewhere between 1-2h and respectively 1.5-2.5h Will this be enough to 'dethrone' the conventional ICE? I don't know, but it's a much better option.

#### Dusan

##### Well-Known Member
Nice graph.
So where on that graph are most VTOL operating for best power to weight ratio (from your post 2220)?
I suspect they are operating in “max power” where the efficiency looks about 60%. And battery efficiency is also lower at that max power.
The efficiency at max power of any motor is 50%, and no motor can operate at that point without burning out, regardless of time and cooling. Usually this max power is not stated in the datasheet, but 'peak power' which is less, but achievable for a limited time. This 'peak power' seems to be the power used for best power-to-weight ratio presented on wikipedia, see the datasheet: 268 (200kW | 500Nm) - EMRAX. The 'peak power' efficiency drops to 93% from the 96% continuous. At peak power the generated heat is 14 kW vs. 4 kW continuous power.

For VTOL - hover operation, the conservative approach is to use the rated power, so hover can be maintained indefinitely.

Here are more details on peak power and torque from the emrax268 manual:

#### BBerson

##### Light Plane Philosopher
This 'peak power' seems to be the power used for best power-to-weight ratio presented on wikipedia,
More good info, thanks.
The discussion of power to weight ratio is vital. But I go further and state that VTOL designers use thrust to weight ratio. Which includes prop/rotor efficiency to get the thrust needed.
I asked one of the VTOL leaders (starts with J) about 5 years ago at the start of the evtol craze what his lift system thrust to weight ratio was. He replied that he didn’t understand the question.

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#### Dan Thomas

##### Well-Known Member
When the batteries specific energy (energy/weight - not battery density, which is volume/weight) doubles and triples, the breakeven with a conventional gasoline ICE will move somewhere between 1-2h and respectively 1.5-2.5h.
When it doubles or triples. If. Could. Maybe. Might. Possibly.

It all hangs on whether it's possible. Gasoline engines improved a lot in the first few decades of development, then they pretty much stalled. Toyota has achieved 38% thermal efficiency in an engine under development. Most ICEs of light-aircraft size are less than that, 25-35%, maybe. Huge slow-turning diesels can hit a bit over 50%.

The point is this: ICE efficiency (thermal efficiency, the measure of how much of the fuel's thermal energy is turned into useable power) rose quickly until it didn't rise anymore, and continued development makes only small gains.

The same thing is likely to happen with battery technology. It hits a ceiling sooner or later, and we might---might---be close to that now.

#### Dan Thomas

##### Well-Known Member
1/1 weight of the batteries and the craft.....just like in cars.
Just like in Teslas? Those heavy cars?

• 5,390 lbs – Model X Plaid
• 5,185 lbs – Model X Long Range
• 4,766 lbs – Model S Plaid
• 4,561 lbs – Model S Long Range
• 4,416 lbs – Model Y Long Range/Performance
• 4,065 lbs – Model 3 Long Range/Performance
• 3,582 lbs – Model 3 Standard Range Plus
• 2,723 lbs Gen. 1 Tesla Roadster
Those 4500-5000 pounders are in the neighborhood of the weights of those monstrous old Cadillacs of 1959. Great lumbering beasts that sure were smooth-riding, but don't expect good handling or mileage. And forget parallel parking

An airplane carrying that much battery is going to be a total dog, with its high wing-loadings, and so it needs more power, which means more weight, which means more power---

#### Sraight'nlevel

##### Well-Known Member
Just like in Teslas? Those heavy cars?

• 5,390 lbs – Model X Plaid
• 5,185 lbs – Model X Long Range
• 4,766 lbs – Model S Plaid
• 4,561 lbs – Model S Long Range
• 4,416 lbs – Model Y Long Range/Performance
• 4,065 lbs – Model 3 Long Range/Performance
• 3,582 lbs – Model 3 Standard Range Plus
• 2,723 lbs Gen. 1 Tesla Roadster
Those 4500-5000 pounders are in the neighborhood of the weights of those monstrous old Cadillacs of 1959. Great lumbering beasts that sure were smooth-riding, but don't expect good handling or mileage. And forget parallel parking

An airplane carrying that much battery is going to be a total dog, with its high wing-loadings, and so it needs more power, which means more weight, which means more power---

Yes 4 seat or six seat electric aircraft has to be pretty big. Not necessarily heavy in wing loading.

#### Dan Thomas

##### Well-Known Member
Yes 4 seat or six seat electric aircraft has to be pretty big. Not necessarily heavy in wing loading.
To get wing loadings down, you need bigger wings, which means longer wings, which means much stronger spars and fuselage carrythroughs and so on. None of it is weightless.

Everything in airplanes involves compromises, and going too far in any direction results in a poor airplane.