Why battery-powered aircraft will never have significant range

[tspear]

Catching up on random reading came across a nice summary of electric flight:
https://www.airspacemag.com/flight-today/electricity-in-the-air-180969510/

It is interesting how many efforts are justa few years away, and do not appear to depend on huge increases in battery technology.

Tim

 

[ypsilon]

Maybe I could have stated it better.
I was thinking in reverse. You want a plane to fly at set true air speed; not at a specific L/D ratio.
At 5K MSL in almost all aircraft this takes more power then at 15K or 25K. The reason is as you state; lower drag. This means less energy.

Tim

Since the required lift is fixed, for lowest drag (and therefore lowest energy consumption) you fly at highest L/D ratio (Straight stationary flight). You reach that L/D at at a higher TAS when flying higher, but you don't safe any energy (i.e. you won't go farther just by going higher).

 

[Sockmonkey]

Thinking about how electric motors can have such a broad effective speed and torque range makes me wonder about prop length. Generally you want it just short enough that the tips stay at least a hundred knots below supersonic yes?
Yeah, I'm grossly approximating here. Just yell at me later. If you could freely trade off RPMs for torque without needing a fat gearbox, how long would you want the prop on an LSA type plane? Yeah, ground clearance is an issue too, but in hypothetical though exercise land we can ignore that.

 

[DonEstenan]

Since the required lift is fixed, for lowest drag (and therefore lowest energy consumption) you fly at highest L/D ratio (Straight stationary flight). You reach that L/D at at a higher TAS when flying higher, but you don't safe any energy (i.e. you won't go farther just by going higher).

Umm, with fixed L/D, you spend the same amount of energy (kW) - but when flying higher, you are flying faster. Hence, if you are total-energy limited (batteries/but also fuel tanks), you spend the same time in the air (ignoring the energy needed to go higher - can be partially recovered when going down) but are going faster = you do go farther: distance = time x speed, time is the same, speed is higher.

 

[bmcj]

Has anyone mentioned dirigibles? Seems like a dirigible offers a great opportunity for electric because your lift is provided by buoyancy, leaving only forward thrust for the electrics to deal with. Also, it has an enormous surface for covering with solar panels.

 

[henryk]

Has anyone mentioned dirigibles? Seems like a dirigible offers a great opportunity for electric because your lift is provided by buoyancy, leaving only forward thrust for the electrics to deal with. Also, it has an enormous surface for covering with solar panels.

https://www.youtube.com/watch?v=xOJvtaiNp5c

=please !


https://www.researchgate.net/profile/Evgeny_Sorokodum/publication/265849627_Unmanned_aerial_vehicles_flying_wing_with_oscillating_propulsor/links/541ec3550cf2218008d3cfbb/Unmanned-aerial-vehicles-flying-wing-with-oscillating-propulsor.pdf

=moore complex solution=keywords=DRAG ANIHILATION, ENERGYEFFECTIVE OSCILLATING PROPULSOR...

 

[ypsilon]

Umm, with fixed L/D, you spend the same amount of energy (kW) - but when flying higher, you are flying faster. Hence, if you are total-energy limited (batteries/but also fuel tanks), you spend the same time in the air (ignoring the energy needed to go higher - can be partially recovered when going down) but are going faster = you do go farther: distance = time x speed, time is the same, speed is higher.

kW is power, not energy. Think about it the following way: Without engine your plane (glider) would sink in still air. The higher, the faster. Your engine has to put exactly the amount of potential energy into the system, you'd be losing without an engine (i.e m*g*h). That means that when going faster (at same L/D), you need to spend more energy (per second) (i.e. power). Flying higher (and therefore faster) means more energy per second but the same amount of energy per meter (in an ideal world, not talking about Re numbers, coffin corners and air breathing engines).

 

[Bill-Higdon]

This article requires you register (free) to read it but it's interesting http://aviationweek.com/future-aerospace/liquid-battery-promises-safe-energy-dense-power-electric-aircraft

 

[Doggzilla]

kW is power, not energy. Think about it the following way: Without engine your plane (glider) would sink in still air. The higher, the faster. Your engine has to put exactly the amount of potential energy into the system, you'd be losing without an engine (i.e m*g*h). That means that when going faster (at same L/D), you need to spend more energy (per second) (i.e. power). Flying higher (and therefore faster) means more energy per second but the same amount of energy per meter (in an ideal world, not talking about Re numbers, coffin corners and air breathing engines).

Don is correct. Your statements are conflicting.

Your previous statement agreed with him when you said a given L/D was faster at higher altitude, while using the same power setting.

Now you're saying it requires more power. It does not. The additional speed is allowed by the reduction in drag by the reduced air density.

No additional power is required to take advantage of that.

 

[ypsilon]

Don is correct. Your statements are conflicting.

Your previous statement agreed with him when you said a given L/D was faster at higher altitude, while using the same power setting.

Now you're saying it requires more power. It does not. The additional speed is allowed by the reduction in drag by the reduced air density.

No additional power is required to take advantage of that.

No, my statements aren't conflicting. Let's just not mix up power and energy.

Just think of a sailplane you launch at 10000m. We all agree that it will glide down on a straight path (i.e. the L/D will stay the same until it hits the ground at sea level). Energy available corresponds to the alitude of the plane (E=m*g*h). The power dissipated equals the sink rate (P=E/t). It is clear that when the L/D stays the same and the airspeed is higher, the sinkrate grows by exactly the same proportion. IOW: If you measure your "energy loss" in Joule/second (which how power / a "Watt" is specified), you see, that the higher you fly the more power is required to keep the plane on it's gliding path. If you measure the efficiency of the glide (i.e. Joule/meter) then you'll see that it doesn't change with altitude. You alway spend the same amount of energy for a given distance.

hope that makes it clearer now.

 

[Andy_RR]

Ypsilon has just given you the why. Nice...!

 

[pictsidhe]

Thinking about how electric motors can have such a broad effective speed and torque range makes me wonder about prop length. Generally you want it just short enough that the tips stay at least a hundred knots below supersonic yes?
Yeah, I'm grossly approximating here. Just yell at me later. If you could freely trade off RPMs for torque without needing a fat gearbox, how long would you want the prop on an LSA type plane? Yeah, ground clearance is an issue too, but in hypothetical though exercise land we can ignore that.

Optimum tip speed depends on forward speed and prop size. A bit under sonic is an extra limit if the ideal velocity gets higher than compressibility and shock effects will allow.

Well, if it is hypothetical, an infinite prop is best. But if we consider Re let alone the hassle of making it, it isn't. In reality, if revs are free, the largest prop that won't hit the ground is optimum on a slow plane.

If you haven't played with Javaprop yet, you should.

TN-212 from NACA is good one to look at.

 

[Sockmonkey]

Optimum tip speed depends on forward speed and prop size. A bit under sonic is an extra limit if the ideal velocity gets higher than compressibility and shock effects will allow.

Well, if it is hypothetical, an infinite prop is best. But if we consider Re let alone the hassle of making it, it isn't. In reality, if revs are free, the largest prop that won't hit the ground is optimum on a slow plane.

If you haven't played with Javaprop yet, you should.

TN-212 from NACA is good one to look at.

Ok, that makes sense.
Have you given any more thought to that jet-prop thing we were talking about?

 

[BJC]

Interesting presentation here: https://m.youtube.com/watch?v=JELNfk-_0Dg&feature=youtu.be


BJC

 

[pictsidhe]

Ok, that makes sense.
Have you given any more thought to that jet-prop thing we were talking about?

A little. A turbofan with a jet cat type gas generator feeding a fan with tip jets seems possible. It would have better tsfc than a turbojet, but would still be very thirsty. Could be very light, though. Turbofan 103, anyone?

 

[Sockmonkey]

A little. A turbofan with a jet cat type gas generator feeding a fan with tip jets seems possible. It would have better tsfc than a turbojet, but would still be very thirsty. Could be very light, though. Turbofan 103, anyone?

My previous question about prop length puts me in mind of the Piaggio seaplane.

You could do something similar with a non-seaplane electric using separate props for takeoff and cruise to solve the ground clearance problem if it had self-feathering props.

Could you elaborate a little on how your version of the tip-jet fan would work? It sounds similar to what's described in this paper here. https://archive.org/details/DTIC_ADA279680

I still think my version of the jet-prop is viable for larger cargo plane types as the noise would be less of an issue.

 

[lr27]

If you were going to do that, it might make sense to drive the wheels and then jump, though that makes takeoff a bit hairy. The Piaggio wouldn't need any of this gadgetry if it had wheels instead of hydrofoils.

 

[Sockmonkey]

If you were going to do that, it might make sense to drive the wheels and then jump, though that makes takeoff a bit hairy. The Piaggio wouldn't need any of this gadgetry if it had wheels instead of hydrofoils.

Yeah, I wouldn't risk a purely jump takeoff. I'd use powered wheels as a low-speed assist for prop thrust for a plane optimized for cruise. Would be a way to get around the rule about fixed-pitch props.
Electrics don't really need that though since they have such a broad speed range.

 

[Doggzilla]

For those of you who have not heard, NASA, CAL Tech, and Honda announced a working Fluoride battery. These batteries have massive improvements in storage, even over 10 times higher.

In the past they required extremely high temperatures in order to function, but the new batteries require much more reasonable temps.

Depending on how well these batteries work compared to the high temp versions, this could very well make electric flight something more reasonable.

https://cleantechnica.com/2018/12/08/honda-nasa-caltech-claim-fluoride-battery-breakthrough/

 

[pictsidhe]

A 10 fold improvement sounds like BS to me.