Lilium EVTOL

[AZjeff]

Back in 2016 someone posted about this and caused a little stir and the thread was locked. It was called a paper airplane by Astro14! Here it is flying around in Spain. There are probably 100 things to hate about it but still kinda neat. Lilium website

 

[Cujet]

It is neat that they can accomplish this using ducted fans and digital control. But as always, there is not enough energy available to make a practical case for such a vehicle.

 

[goingplacesanddoingstuff]

It is neat that they can accomplish this using ducted fans and digital control. But as always, there is not enough energy available to make a practical case for such a vehicle.

To put this in perspective, here’s some rough back of the napkin math.

The energy density of li ion is about 0.6 MJ/kg. Assuming a gas engine is about 40% efficient at converting gasoline (46 MJ/kg) into work, then a gas power source is about 18 MJ/kg equivalent, or 31x more power dense.

I presume the advantages of lower cost and noise are driving some of the unmanned drone advances we’re seeing lately.

 

[TumblrDucks]

Neat! They must put a lot of effort in writing fly control.

 

[Cujet]

To put this in perspective, here’s some rough back of the napkin math.

The energy density of li ion is about 2.5 MJ/kg. Assuming a gas engine is about 40% efficient at converting gasoline (13 MJ/kg) into work, then a gas power source is about 5.3 MJ/kg equivalent.

There’s a lot more that could be calculated and accounted for here but broadly speaking battery power is about half the energy per unit of weight. It’s worse, but it’s not like it’s so dismal it’s not worth doing. There are some advantages like no loss of power at altitude, quieter, cheaper, etc.

Battery power has the advantage of being converted into useful work at high efficiencies. Unfortunately, that is where the good news ends.

Your math is not accurate. Gasoline/jet fuel is 46 mj/kg. Also of note, lithium batteries are about 0.5 mj/kg, of which 70% are of practical use.

But even more interesting than the enormous disparity in energy between even the best batteries and hydrocarbon fuels, is the fact that the heat of combustion is used for more than just turning a turbine to drive a fan. That "waste" heat is used to raise the speed of sound in the "duct" of a fanjet. Allowing much higher discharge velocities and therefore, high cruise speeds. The hotter the exhaust, the higher the stagnation point (speed at which the airflow can not exceed).

Faster duct discharge equals more thrust, for a given airflow.

This is why modern airliners get 120+ passenger MPG. And fighter jets with 2400MPH discharge velocities, can achieve Mach 2++

 

[goingplacesanddoingstuff]

Battery power has the advantage of being converted into useful work at high efficiencies. Unfortunately, that is where the good news ends.

Your math is not accurate. Gasoline/jet fuel is 46 mj/kg. Also of note, lithium batteries are about 0.5 mj/kg, of which 70% are of practical use.

But even more interesting than the enormous disparity in energy between even the best batteries and hydrocarbon fuels, is the fact that the heat of combustion is used for more than just turning a turbine to drive a fan. That "waste" heat is used to raise the speed of sound in the "duct" of a fanjet. Allowing much higher discharge velocities and therefore, high cruise speeds. The hotter the exhaust, the higher the stagnation point (speed at which the airflow can not exceed).

Faster duct discharge equals more thrust, for a given airflow.

This is why modern airliners get 120+ passenger MPG. And fighter jets with 2400MPH discharge velocities, can achieve Mach 2++

You’re right, my sources were wildly off. That’s what I get for looking things up before I’m truly awake. On double checking it does look like some batteries can push 0.7 MJ/kg these days.

I will point out, though, that the weight of the engine itself probably needs to be part of the calculation. Since combustion engines tend to be heavier, I’d argue that this should be factored in as well.

 

[Cujet]

I will point out, though, that the weight of the engine itself probably needs to be part of the calculation. Since combustion engines tend to be heavier, I’d argue that this should be factored in as well.

Yes engines can be heavy and power to weight is a very real issue. Especially the common diesel engine, which does see some aviation use today. And today's electric motors can be beyond amazing with regard to maximum output.

I was looking at the Stark Varg electric motocross bike the other day. The motor makes an unreal (claimed) 80HP, which is stunning for a 220 pound dirt bike. Dyno results show the RWHP to be about 66-67HP, which is still stunning. The drive motor actually makes 70 as the dyno can account for drag and that's about where it comes up. My point is, this is very high output in such a small package.

Yeah, that round thing, that's the 80, er, 70HP motor. (a little joke, still impressive as can be)

And just like the Tesla Plaid, it's darn near impossible to beat in a drag race.

 

[pilottravis]

It is neat that they can accomplish this using ducted fans and digital control. But as always, there is not enough energy available to make a practical case for such a vehicle.

I've been to multiple aviation industry conferences and these things are the latest push for "the next great thing". All the different companies telling everyone that this technology is right around the corner if airports invest in the landing and charging facilities. All sounds fun until we ask the question of how much power is required to charge several of these at the same time. Always get a few sneers when they say 5-7 megawatts. At the very least a substation next door, and more likely a generating station next door.

 

[Cujet]

. Always get a few sneers when they say 5-7 megawatts. At the very least a substation next door, and more likely a generating station next door.

Many people are unaware that today's turbofan engines "thrust" rating is darn near the "HP" rating. If a GE90 makes 115,000 pounds of thrust, it also makes about 115,000 HP!

Another thing people miss is that 1000W (1KW) of battery power is required to make 1 real world HP, this is due to controller, wire and motor losses.

So, we can easily know just how much in the way of KW and KWh is required to power, and the numbers can be staggering if we are talking about large aircraft.

Put another way, I drive by a power plant that uses 2ea GE, aero-derivitive engines to drive the generators. Just how many EV "jets" can this thing charge with just 2 jet engines? Answer, 1.