Monday, 24 April 2017

Lilium jet electric aircraft – update



Credit: Lilium/Wired

Although the Lilium aircraft is called, a “jet”, it would be more usual to describe its propulsion as, “ducted fan”.1

Advantages of ducted fans compared to conventional propellers1

● more, shorter blades allow higher speed of rotation & more thrust, without the problems caused by the tips of the blades approaching the speed of sound

● reduced noise – especially when driven by an almost-silent electric motor
● surrounding shroud makes ducted fan safer
● occupies less space

In urban environments, where Lilium sees many of it's aircraft operating, these advantages are particularly valuable.

Conventional aircraft generate lift by using relatively few engines to produce thrust (jet &/or propeller) to drive the aircraft forward. Forward speed produces lift due to the motion of the wings through the air.2

A small number of engines is a limitation imposed by internal combustion engines. In an electric aircraft, many small motors can be used.2 Illustrations show the Lilium with 36. Many motors allow lift to be generated at low aircraft speed (even zero) because they can drive air over the wings at a speed independent of the aircraft’s forward motion.

Update – Other advantages of multiple engines include:2

● shorter (front-to-back & side-to-side) wing is lighter & generates less drag

● wings can be optimised for cruise, rather than take off & landing, improving overall efficiency


In an aircraft with few engines, the most difficult & dangerous parts of a flight are taking off & landing, due to the direct dependance of lift on speed. Wings are optimised for these purposes.

Multiple motors, by disconnecting lift & speed, increase safety at take off & landing. Wings can be optimised for cruise, increasing overall efficiency – wings are shorter & narrower & so lighter, with less drag. Check the image above & the video.

Such a configuration will have disastrous glide characteristics. Electric motors are very reliable, & 36 of them provide plenty of redundancy, but battery failure is a problem which does not appear to have been addressed. This is not surprising in a prototype with only one unmanned test flight to its credit. Two solutions which spring to mind are battery redundancy (which may already have been allowed for in the existing battery configuration) or a Cirrus Airframe Parachute System.3

Lilium has its ducted fans pivoted vertically, to direct thrust at a range of angles, including straight down, for vertical take off.

Multiple motors can be run at different speeds to turn the aircraft, eliminating the drag created by a vertical tail.

Drag is further reduced by a lifting body design4 – the body of the aircraft is shaped to produce lift – thus the wings can be smaller & lighter for the same total lift.

Lilium has backing from the European Space Agency.



Engadget – “Lilium's electric personal jet manages a vertical takeoff”
https://www.engadget.com/2017/04/21/lilium-electric-jet-vertical-takeoff-landing/

Futurism – “A Real Life, All-Electric Flying Car Just Took Off”
https://futurism.com/a-real-life-all-electric-flying-car-just-took-off/

Manufacturer’s web site
https://lilium.com/
__________

1 Just to complicate things, almost all current jets are of the turbofan type, which is a ducted fan, where the fan is driven by a gas turbine – that is to say, a jet engine.

Wikipedia: “Ducted fan”, https://en.wikipedia.org/wiki/Ducted_fan
“The most common ducted fan arrangement used in full-sized aircraft is a turbofan engine, where the power to turn the fan is provided by a gas turbine. Turbofan engines are used on nearly all airliners, fighters, & bombers.”

2 HitchHiker’s Guide 2 Tech: “LEAPTech — many more propellers for better performance”, https://hhg2tech.blogspot.com/2015/04/leaptech-many-more-propellers-for.html


3 Picture this: complete blackout conditions at 5,000 feet – engine fails – it is not possible see the ground, let alone look for a landing site – pilot survives – video at the link is definitely worth a look

Cirrus Aircraft: “Cirrus Airframe Parachute System”,
https://cirrusaircraft.com/innovation/airframe-parachute/


4 Wikipedia: “Lifting body”, https://en.wikipedia.org/wiki/Lifting_body

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