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Comment by _hypx | Hacker News Reader

All VTOL aircraft have the same issue: Insane energy consumption. There's a reason why quadcopters have flight times measured in minutes, and why helicopters never came close to displacing small aircraft. This will be no different.

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Quadcopters have short flight times because they need to use batteries. Commercial helicopters regularly have 3+ hours of flight time.

(Of course, Lillium's aircraft is battery-powered. But it's not fair to say that an aircraft capable of VTOL will always have a dramatically reduced endurance compared to a non-VTOL aircraft, especially since this one can transition to fixed-wing flight.)

That's the point. Lillium is also battery powered. It will have minutes of runtime in all likelihood.

They're saying 190 mile range and 170 MPH cruise speed. So I guess that's around 60 minutes.

https://en.wikipedia.org/wiki/Lilium_Jet#Specifications

More than that because FAA mandates at least a half hour of reserve fuel.

No, quadcopters have short flight times because they choose to generate thrust using multiple fast and small propellers instead of a larger and slower one. Quadcopters will always lose to a single rotor of the same overall diameter.

Multiple rotors do decrease efficiency and this flight time somewhat, but nowhere near as much as switching from fuel to batteries. The mass energy density of batteries is less than 0.1x fuel. There are some gas-powered quadcopters out there which much longer fight times than the electric ones. Compare an electric CP RC heli to nitro and nitro has much longer fight time. Of course, a combustion engine is usually heavier than electric motors too, so there are diminishing returns from switching to fuel as your fuel or battery to engine mass ratio decreases. But, the main consideration in this regard for flying things is almost always: fossil fuel is way, way more energy dense than batteries in a mass sense. See e.g https://www.batterypowertips.com/comparing-ev-battery-and-fu...

It is, it falls out of the physics of VTOL vs. fixed wing flight. It's so extreme that cargo helicopters have to fly forwards to take off at MTOW, because it dramatically lowers the power required to get the aircraft off the ground relative to taking off vertically. You can minimise the impact in the mission requirements, but there's a reason hover and VTOL are typically specialist requirements. They tend to be very costly in terms of endurance.

To put some numbers on this, down in the large quadcopter/fixed-wing size of aircraft, you can expect about 200W/kg of power required for the quadrotor/VTOL phase and more like 20W/kg once you’re in fixed-wing mode, assuming you have wings and speeds that provide a reasonable L/D ratio. Literally a factor of 10 (or more, if your fixed-wing design is more like a glider.)

I commented elsewhere here on the video of a test flight. In that video they achieved that the aircraft flew like a normal plane with lift provided by its wings.

This mode of flight is said to be ten times more efficient than the helicopter mode during VTOL. That is: for each minute that they can cut from flying like a helicopter during starts and landing, they gain ten minutes of normal flight time.

I think the energy argument does not apply that much more than for other small electric aircraft.

The key difference is that VTOL / hovering with its associated energy consumption will only be required for start and landing.

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