Why battery-powered aircraft will never have significant range

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

markaeric

Well-Known Member
Joined
Jul 31, 2015
Messages
346
Location
Wichita
Yeah, but believe it only once you see it. If you've ever followed sciencedaily.com, you'd find significant claims - all with at least some credibility - seemingly several times a week. That's not to say what you linked isn't an upcoming advancement in lithium battery design, or perhaps one of the many I've read about before, but afaik, none of them are in the commercial stage. It's also possible that there will be several advancements in the time between now and when the above goes commercial, so it won't appear to be such a massive leap from the previous generation. And that's ok. 5-8% increase in specific energy density per year is a safe bet. We'll get there.
 

Farfle

Well-Known Member
Joined
Dec 31, 2015
Messages
80
Location
Redmond, OR
Batteries are getting there. The new Zero motorcycles packs are almost 230 whr/KG at a pack level. More is possible, especially now that actual money is being put into the industry.
 

karoliina.t.salminen

Well-Known Member
Joined
Oct 7, 2008
Messages
407
Location
Finland
Old model S cells are 190 Wh/kg, the Gigafactory cells are better, and the new 200 kWh roadster would indicate that even much better. Because the model S is a heavy car and with its batteries the 200 kWh roadster would be almost three ton car, but it isn't. Something better is in the Gigafactory cells which has not been made public yet, they are not only few percents better, but a lot better. Tesla is not talking about it to not cannibalise model S and X.

Tesla roadster 2 battery pack could be a good one for practical electric airplane. Of course the plane needs to be designed to carry the load from the start, electric conversions are going to suck like they suck in cars vs Tesla.
 

saini flyer

Well-Known Member
Joined
Mar 12, 2010
Messages
487
Location
Dallas, TX
The smallest battery block with BMS and all the casing including from zero in 2018 model is 42 lbs and 3.6kWhr.
This is about 188.5 Whr/Kg which is the practical number to consider and not just the pouch level weight. All the packaging, wires, BMS, connectors etc add to the weight and are essential but it is indeed the best plug and play system available so far for us.

Also as I mentioned earlier, they have improved the capacity about 10% per year. I think in 2019 they will have a single battery block that will provide juice to generate 46hp which requires two battery boxes today. The entire weight of the system including motor, controller, charger, BMS, battery, harness etc will be just shy of 100 lbs for 46hp. This is very good for a UL or self launch motorglider as it is less than the weight of 2 cycle Rotax 582 but produces more torque.

Also hooking up two together yields a redundant 92hp for <200lbs with torque twice that of Rotax 912ULS. With high AR wings, a small Polini thor 250 based belt driven integrated alternator generator producing 20KW is ample to get a 2 seater to move at LSA speeds with the take off that can compete at Valdez!

We have so many threads that here on a budget friendly two seater that does not break the bank. Out of the 1/3, 1/3,1/3 cost distribution for the overall cost of the kit, the avionics part has shrunk and with novel material and manufacturing the hardware kit price might go down a bit too but the FWF is a killer and the electric hybrid should change that too(but it will take time).
Batteries are getting there. The new Zero motorcycles packs are almost 230 whr/KG at a pack level. More is possible, especially now that actual money is being put into the industry.
 

Dusan

Well-Known Member
Joined
Sep 15, 2014
Messages
162
Location
Canada
I compared 2 power-plants, similar power, electric vs internal combustion engine (ICE). The result I condensed in the following graph:

Electric vs ICE.jpg

The electric motor weight is only a fraction of the weight of the ICE. The graph sows that for short trips the weight of electric motor + batteries is smaller than for the ICE + gas. It seems that for aircraft specifically designed for short trips, under an hour, it makes sense to go fully electric.

I used values for real batteries you can buy and weight data from engine and motor datasheets. The future batteries I expect to be better in the specific energy department so it makes sense to start investing in an electric power plant. The fact that an electric motor does need only a fraction of the cooling compared to an ICE, so reduced cooling loses and the fact that cowling can be more aerodynamic, reducing drag is not taken into consideration, and only can help.

I did this for a study on personal VTOL aircraft that could be used for commuting. For a VTOL aircraft there is a big difference in how much power is needed in hover/transition versus cruising. In this case an electric motor have also the advantage to be highly efficient even when needs to output only a fraction of it's maximum power.

More details about my project here: aliptera.com
 

tspear

Well-Known Member
Joined
Feb 12, 2014
Messages
972
Location
Outside Boston
I compared 2 power-plants, similar power, electric vs internal combustion engine (ICE). The result I condensed in the following graph:

View attachment 68958

The electric motor weight is only a fraction of the weight of the ICE. The graph sows that for short trips the weight of electric motor + batteries is smaller than for the ICE + gas. It seems that for aircraft specifically designed for short trips, under an hour, it makes sense to go fully electric.

I used values for real batteries you can buy and weight data from engine and motor datasheets. The future batteries I expect to be better in the specific energy department so it makes sense to start investing in an electric power plant. The fact that an electric motor does need only a fraction of the cooling compared to an ICE, so reduced cooling loses and the fact that cowling can be more aerodynamic, reducing drag is not taken into consideration, and only can help.

I did this for a study on personal VTOL aircraft that could be used for commuting. For a VTOL aircraft there is a big difference in how much power is needed in hover/transition versus cruising. In this case an electric motor have also the advantage to be highly efficient even when needs to output only a fraction of it's maximum power.

More details about my project here: aliptera.com

Dusan,

Good luck, and have fun chasing the dream.
Am I reading it that you think 20HP is the target power?
Why does at 40HP the electric out weigh the Rotax from the start?

Tim
 

markaeric

Well-Known Member
Joined
Jul 31, 2015
Messages
346
Location
Wichita
Dusan,

Good luck, and have fun chasing the dream.
Am I reading it that you think 20HP is the target power?
Why does at 40HP the electric out weigh the Rotax from the start?

Tim
Unless I'm losing my mind, it looks like they're equal in weight around the 20 minute duration mark.
 

Dusan

Well-Known Member
Joined
Sep 15, 2014
Messages
162
Location
Canada
So at time 0 there is no batteries, no fuel, the weight of the electric is about 15Kg, the ICE is 66Kg. Comparing them at 40KW, the red and blue line, the electric is less until about 20 minutes. At 20KW the electric weight stays lower until about 40 minutes, yellow and green line.

As I mentioned before, this is relevant a small aircraft I'm doing the concept on, 40KW would be needed to hover and 20KW for cruise. What I'm saying, is that under a certain range, all electric propulsion would weight less than an internal combustion.
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,215
Location
Port Townsend WA
The minimum battery weight can't be close to zero. Depleting a battery faster than about 5 minutes will limit the battery life.
 

Dusan

Well-Known Member
Joined
Sep 15, 2014
Messages
162
Location
Canada
The minimum battery weight can't be close to zero. Depleting a battery faster than about 5 minutes will limit the battery life.
Sure thing, but who wants an aircraft that flies only 5 minutes anyway?
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,215
Location
Port Townsend WA
Sure thing, but who wants an aircraft that flies only 5 minutes anyway?
5 minutes is enough to launch a VTOL. But I haven't seen any electric system that is lighter than a two-stroke. The giant overpowered models that displayed enormous power at Oshkosh 2017 were two stroke. If electric was lighter they would use it.
 

Dusan

Well-Known Member
Joined
Sep 15, 2014
Messages
162
Location
Canada
From Electric flight - potential and limitations Hepperle, page 9-12 "Today it seems to be possible to build electric motors having a specific mass of about 2 to 4 kW/kg. This compares favorably with the specific mass of larger turboshaft and turbofan engines at cruise power" and see the graph from Figure 12; so definitely piston engines are left in the dust. Too bad batteries are not on par yet.
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,215
Location
Port Townsend WA
Interesting report. But motor weight isn't relevant. System weight is all that matters.
Or for a VTOL it is the lift/weight ratio.
 

Dusan

Well-Known Member
Joined
Sep 15, 2014
Messages
162
Location
Canada
I agree, the system weight is what matters in the end.

What I'm saying is because the electric motor weights so less than a typical piston engine, it makes sense to use a small battery for a short range aircraft; the electric propulsion weight, the motor + battery, could be smaller than for a piston ICE + fuel.

Designing an aircraft, even VTOL, that will be used only for 20-30 minutes, for fun or commuting, it definitely makes sense to go fully electric; not only from the weight perspective, but also for the increased benefit of better motor efficiency at a wide range of power settings. The ICE has it's best efficiency at limited power setting and cannot cope well with the wide range of power required for a VTOL aircraft. Also the cooling required for the electric propulsion is so much less, the drag losses associated with cooling are almost completely eliminated. The end result is an comparable weight aircraft, having less drag, with the drawback that today, it has shorter range. But it will be so easy to upgrade when better batteries are available.

For VTOL, I agree again, lift/weight matters. A typical fixed wing aircraft have a L/D ratio of about 10. The propulsion system of a VTOL aircraft of comparable weight needs to produce 10 times more thrust, needing much bigger engine and rotor. This is because the propulsion system of the fixed wing aircraft needs only to overcome drag, but the VTOL propulsion system needs to provide full lift as the wings are not producing lift in hover mode. This is where a wing that is capable of providing lift in hovering, comes in handy. There is a wing capable of producing lift in hover, it is the "lip wing", and produces lift by being exposed to the rotor's intake low pressure area. More details about the lip wing here: aliptera.com. The lift that is produced by the lip wing reduces directly the thrust and consequently the power needed to be produced by the motor, so the motor cam be smaller, lighter, consumes less power; the rotor also can be smaller, having less drag.
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,215
Location
Port Townsend WA
I don't get "lip wing". The Hiller duct is a duct, not a wing. The lip on the duct can increase lift somewhat, but usually the added duct weight is about the same so no gain.
 
Top